Channels

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6 Deepwater-Reservoir Elements:Channels and Their Sedimentary Fill

Introduction

Deepwater channels have received considerable attention in the petroleum industry dur-ing the past decade, because of (1) the important discoveries that have been made in severaldeepwater basins in which reservoir performance was critical to development decisions andstrategies (e.g., Campos Basin, Brazil, offshore Angola, Nile, Mahakam Delta, northern Gulfof Mexico, West of Shetland Islands, and offshore mid-Norway); (2) the ability of 3D seismicto increasingly image the complex internal geometries of channel systems (especially thosethat are sinuous); and (3) the need to avoid shallow flow problems while drilling channel-fillsediments.

Many slope systems in passive margins are extremely muddy. The importance of chan-nels as sand conduits for bypass to the basin floor probably was not fully appreciated untilabout 15 years ago, when large volumes of sand were recognized to occur downdip of muddyslope systems (e.g., Angola and the northern Gulf of Mexico). Many slope channels aremarked by evidence of sediment bypass (coarse-grained lags, traction deposits, heterolithicdeposits of fine-grained tails, and fine-grained levees, in some instances).

Channels and their fills have been studied for many years, from different perspectives,and using multiple data sets, including data from the modern seafloor, from the shallow sub-surface (shallow seismic for shallow-hazards drilling surveys), from deeper-explorationseismic, from reservoirs, and from outcrops. In this introductory section, we present a fewgeneral, qualitative concepts that are widely accepted concerning channels and their fills.

Because of the large number of published studies, and because most channel fills have atleast some unique characteristics, we only provide here a range of examples of channelsystems.

Definitions

A certain amount of ambiguity exists in the geoscience community about the meaning ofthe term “channel” in a deepwater context. Part of this lack of clarity stems from definitionsderived from different types of data (high-resolution bathymetry, deep seismic, and outcropobservations) examining different geologic settings (present sea bottom, subsurface reservoirs,and outcrops). Channels are defined as “elongate negative-relief features produced … and/ormaintained by turbidity-current flow... [Channels] represent relatively long-term pathways forsediment transport. Channel shape and position within a turbidite system are controlled bydepositional processes … or erosional downcutting … channel relief can be dominantly ero-sional or depositional in origin or can result from a combination of both processes” (Mutti andNormark, 1991).

The word channel has become a catchall term used for bathymetric depressions on theseafloor, irrespective of their size or origin. The term channel is suitable for characterizingmodern seafloor depressions. However, the term is often misused in referring to a channel-likefeature on seismic or in outcrop. In fact, it is the channel fill or channel form that is imaged orobserved. We use the term “channel fill” throughout this chapter to refer to the sediments thatwere deposited within the depression.

Environments of deposition

Channel systems can develop on the slope, at the base of slope, and on the basin floor(Figures 6-1, 6-2). On the slope, they develop in confined settings, such as intraslope basins oras submarine canyons. Sometimes the canyon head occurs in shallow shelf environments.

Deepwater Reservoir Elements: Channels and Their Sedimentary Fill

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Figure 6-1. Shaded bathymetry map of offshore Nigeria. Superposed are late Pleistocene slope channels of different ages. The channels are erosional inthe upper slope and become less erosional downdip, with aggradational channel-levee or channel complexes downdip. After Mitchum and Wach (2002).Reprinted with permission of the Gulf Coast Section SEPM Foundation.

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Figure 6-2. Coherency map superposed on a 3D perspective of the seafloor that shows the outer shelf to basin plain, Kutei Basin, eastern Borneo. Mostcanyons are straight to slightly curvilinear; one canyon has a sinuous bend downslope of the cycle 3 lowstand delta. Sediment waves (Chapter 7) arepresent on the upper and lower slope. Base-of-slope toe thrusts have bathymetric expression. After Saller et al. (2004). Reprinted with permission ofAAPG.

Cycle #1 Lowstand Deltas

5 kmN

Sediment Wavesand Ridges

Slump-relatedTruncation

Cycle #3 Basin-Floor Fan

Youngest Toe-Thrust Anticline

SinuousChannel

Sediment Waves

Cycle #3 (P3) Lowstand Delta

Modern Reefal Shelf Edge& Cycle #2 (P2) Shelf Edge

~2000 m

~100 m


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In tectonically active areas, such as the southern California continental borderland or the coastof Angola, the canyons extend extremely close to the shore and capture sand that moves along-shore by longshore-drift processes. On the basin floor, channels tend to be shallower andexhibit a distributary pattern. Sinuous channels can occur on the slope and on the basin floor.

Downslope change in general characteristics

The geometries of open channels and filled channels change in response to changes ingradient, from a single deep feeder channel, to shallower and broader channels and sets ofchannels in a more unconfined setting (Figure 6-3). In the down-current direction, the smallerchannels can either stack vertically or spread laterally, depending on the degree of confinementto which the channels are subjected. The result is a downdip change in the aspect ratio (theratio of thickness to width) of channels, from 10:1 (updip) to 50:1 (downdip), although aspectratios can be higher for some channels (Figure 6-4). These aspect ratios are markedly differentfrom those of sheets (1000:1) (Chapter 8).

A key feature of channels and channel fills is that, regardless of the size of the overallsystem, the aspect ratios are similar (within an order of magnitude), whether the channels arepart of the largest modern submarine fan channels (e.g., Bengal, Indus, Amazon) or are onlysmall channels that are resolvable in outcrops or with extremely high-resolution seismic data.

Figure 6-3. Generalized variations in channel forms. (a) An erosional channel, which may be deep and have arelatively low aspect ratio (Fig. 6-4). (b) In a confined setting, such as a salt minibasin, channels may stack ver-tically (i.e., be multistory). (c) In a less-confined setting, channels tend to migrate laterally, giving rise to multi-lateral amalgamation and a higher aspect ratio for the entire deposit. Reprinted with permission of KevinPickering.

Isolated sediment conduitBoth sand bodies have

comparable and low

width depth values

Multistory channel

amalgamation

[vertically stacked]

Multilateral channel

amalgamation

(offset lateral stacked)

?a b

c

Introduction

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Figure 6-4. Log-log graph showing the relationship of channel width to depth (aspect ratio). Data points arederived from a number of published and unpublished examples (Clark and Pickering, 1996). Note that this isa log-log plot, so that any single data point on one axis can correspond to an order of magnitude numericalrange on the other axis. Reprinted with permission of Kevin PIckering.

Origins

Deepwater channel fills have been classified into three broad categories: (1) those thatoriginated by erosion (including slumping) of underlying substrate and that have no, or few,associated overbank-levee deposits, (2) those that originated by aggradation of levee-overbankstrata to give an intervening depression in which channel and levee strata interfinger (the ori-gin of levees is discussed in detail in Chapter 7), and (3) those that form by a mix of erosionaland depositional processes, either contemporaneously or during separate phases of evolutionof the fill (Figure 6-5) (Mutti and Normark, 1987, 1991; Clark and Pickering, 1996; Morrisand Normark, 2000). The fill of erosional channels is sometimes referred to as amalgamatedchannel sands or large channels, and the fill of aggradational channels is sometimes referredto as leveed channel fill or low-relief channel levees (Mayall and O’Byrne, 2002; Saller et al.,2004). Channels are more erosional updip because they have higher flow velocities as a resultof higher slope gradients and less accommodation. Farther downdip, channels become mixederosional-depositional and/or aggradational.

Channels can develop during various positions within relative stages of sea level. Somechannels are interpreted to have formed during periods of a relative lowstand of sea level,when large volumes of coarse-grained sediment could erode the slope and pass through to thebasin floor (Chapter 3). Erosion of channel walls also destabilizes them, which leads to slides

1000

100

10

1

0.11 10 100 1000 10000 100000

Depth

(m)

Width (m)


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Figure 6-5. Cross sections of three general classes of deepwater channels: (a) those that are theproduct of submarine erosion on the seafloor, (b) those that are the product of a mixture of ero-sional and depositional processes, and (c) those that are the product of levee deposition, whichgives rise to an intervening seafloor depression (a channel). After Clark and Pickering (1996).Reprinted with permission of AAPG.

into the channel. Besides filling the channel with slide blocks, back-filling of the channel isinterpreted to occur during a relative rising stage of sea level, when the locus of depositionmigrates progressively landward, and less sediment reaches the basin floor.

In addition, large submarine canyons may also form during highstands of sea level, byrapid seaward progradation and oversteepening of shelf margin deltas (the “slope readjust-ment” of Ross et al., 1994) or by headward erosion by physical and biogenic processes (Mayand Warme, 2000). Slope readjustment might also occur during periods of faulting or rapidtectonic subsidence along a shelf margin.

Channel shape: Vertical and plan view

The shapes of channels vary, from elongate, relatively straight channels to those that arehighly sinuous, in much the same manner as the shapes of fluvial systems vary from elongate-braided-distributary to meandering. Although the origin of channel sinuosity is debated (wewill discuss this below), general observations are that the degree of sinuosity is inversely pro-portional to slope gradient, and that finer-grained, lower-energy channel fills tend to be moresinuous than do coarser-grained, higher-energy fills.

Within one depositional sequence (Chapter 3), commonly the style and shape of channelsystems change vertically, from more areally widespread, erosional to mixed erosional-aggra-dational distributary channels near the base, to smaller, aggradational channels with prominentlevees near the top (Figure 6-6). This predictable vertical stacking pattern occurs because

Erosional

Erosional/Aggradational

Aggradational

(a)

(b)

(c)

Introduction

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Figure 6-6. Schematic cross sections illustrating variations in channel fill in an erosionally confined area.(a) Sequential development of channel-fill facies. At the base, a large erosional surface is overlain by thin sand-rich lag, which is overlain by shale-rich debrite, which is overlain by thick, amalgamated channel-fill deposits(high net:gross), which are overlain by leveed-channel deposits (lower net:gross values). Inset figures show(left) an interpreted gamma-ray log through the deposits, and (right) the shape of the channels in plan view(higher sinuosity = channel-levee; lower sinuosity = sand-rich channel fill. (b) Repeated erosion and fill is acommon feature in channels. Older channel-fill sediments are preserved as erosional remnants in youngerchannels. After Mayall and Stewart (2000). Reprinted with permission of Gulf Coast Section SEPM.

channels normally backfill during a relative turnaround and early rise in sea level, when theenergy, grain size, and volume of flows are all diminishing and the depositional axis is step-ping progressively landward (Chapter 3).

Internal fill, stacking, and hierarchy of channels

The nature of channel-fill deposits varies greatly. Deposits can include gravel, sand,mud, and mixed fill, depending on a number of factors, including tectonics, climate, and sedi-ment supply (Reading and Richards, 1994; Richards et al., 1998; and Chapter 1 and Chapter 5in this volume). Channel-fill sediments may consist of a variety of sediment gravity-flowdeposits, from turbidites to debrites and slide blocks, coupled with hemipelagic suspensionfallout. Grain size of the fill generally decreases upward, in accordance with the upwardchange from more distributary-like channels to smaller leveed channels (Figure 6-6). Inter-nally, several stacking patterns are possible depending on the width and depth of the masterchannel, position within the channel, and internal sedimentary processes (Figures 6-6, 6-7).

A hierarchy of channel fill can be recognized in most channel strata. Table 6-1 sum-marizes three hierarchical classifications of channel fill by Gardner and Borer (2000), Navarreet al. (2002), and Sprague et al. (2002). Sprague et al.’s (2002) classification is illustratedin Figure 6-7. These three classifications are based on the particular outcrops and reservoirs

Lower N:GChannel-leveecomplex

High N:GStackedChannel fill

Slumps anddebris flows(local or longdistance?)

HighSinuosityChannel

Sands andGravel

(Bypass phase) Lower SinuosityChannel. orientationoften controlledby faults, e.g.

gamma ray

20

m

Debrite

Lag

Channel fill

Re-incisionand fill

Low relief ch-levee

High sinuosity ch-levee

100 m3 km

a

b


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Figure 6-7. Schematic cross sections illustrating the different hierarchies of channel fill. (a) Multiple channelfills and intervening shale from levee deposits create significant heterogeneity, with many potential flow barri-ers and baffles. After Mayall and Stewart (2000). Reprinted with permission of the Gulf Coast Section SEPMFoundation. (b) Confined channel hierarchy from a single channel element, through a complex of elements,through a complex set of elements, and finally to a complex system. After Sprague et al. (2002). Although com-plex, this hierarchical scheme is recognizable on seismic-reflection profiles and on some large outcrops.Reprinted with permission of the AAPG.

1Gardner and Borer (2000), 2Navarre et al. (2002), 3Sprague et al.(2002); in Abreu et al., 2003)

Mudstones

Slump / Debris flows

Mudclast conglomerates

Levee deposits

Channel margin

Stacked channel sands in axis

Coarse channel lags

50m

1km

100m

2km3o 4o 5o 6o

Confined Channel Complex System

Channel Complex Set

Channel Complex

Channel

Incre

asin

gC

om

ple

xity

Confined Channel Hierarchy

a

b

Table 6-1. Comparison of hierarchical classifications of channel fill.

Type of channel fill Thickness (m) Width (m)

1Geobody No data No data2Facies association No data No data1Single-story channel 7 202Channel phase 10–15 2501Channel complex 25 8002Channel story 30–40 250–5003Channel 40 3001Submarine-fan conduit 60 1000–20002Channel complex 110 1000–20003Channel complex 40 20002Depositional system No data provided No data1Submarine-fan conduit complex 300 2000–30002Megasequence >200 No data3Confined-channel-complex system 100–200 1000–5000

Introduction

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that the authors studied. There is general agreement among the classifications regarding thick-ness and width, but there is some overlap in names applied to different hierarchical compo-nents (Table 6-1).

What is perhaps most significant about these classifications is that, although the internalfill normally is quite complex, channel fill can be subdivided into an organized, recognizablepattern. Future studies of channel fills likely will routinely employ some level of hierarchicalclassification of channel fill. In several outcrop examples below, this hierarchy is noted, but itis more difficult to recognize in multifold seismic.

Equilibrium profiles in channels

A final important relationship to introduce is the changing nature of channels along theircourse. Interpretation of 3D seismic and seafloor images has enabled workers to recognize animportant pattern in the evolution of many deepwater channels: the constantly changing posi-tion of erosion and deposition along a channel. A channel’s profile is usually concave up in alongitudinal profile (Figure 6-8) (Pirmez et al., 2000). The channel is in equilibrium with thelocal slope when the sediment discharge is carried through the channel, and there is minimumaggradation or degradation (Figure 6-8). Deepwater channels appear to develop a smooth lon-gitudinal profile that is balanced between erosion and deposition. When a change in gradientoccurs along the profile, the channel system tends to smooth the bathymetric irregularities byerosion and deposition.

Where there is a disruption to the equilibrium profile (when the rate of sediment dis-charge changes or the rate of tectonic movement exceeds the rate of sediment influx), channelsystems respond in a variety of ways (Figure 6-8). These include downcutting of the thalwegand of the meander cutoffs updip of the knickpoints, development of distributary channels andsheets as the system becomes aggradational, and damming of the channels and redirection offlow, in association with the development of folds and faults.

The implications of Figure 6-8 include the following. (1) Entrenchment and wideningcan affect an entire length of the channel. (2) The deepest erosion along a channel occurs inareas near the crest of underlying structural features (e.g., the margin of an intraslope basin) ornear the site of avulsion of a channel-levee system. (3) Downdip of these points, the basin wid-ens and has gentler gradients. The resulting deposits, either leveed-channel systems or sheets/depositional lobes, are controlled by the proportions of sand and mud within the flows.

Pirmez et al. (2000) concluded that the timing of erosional and filling events is primarilycontrolled by the processes that impact the frequency of sedimentation in deep water and thenature of the sediment load. In addition, it is essential that we understand the mechanics ofequilibrium profiles, if we are to understand the distribution and type of channel reservoirs.Commonly, the location of the zone of erosion relative to deposition/aggradation migrates bas-inward, landward, and/or laterally during the evolution of one channel.

With this general background in mind, the remainder of the chapter is devoted to(1) case examples of regional- to development-scale characteristics of channels and their fill,(2) a summary of the sedimentary processes that probably are unique to leveed-channel depos-its, and (3) a series of case studies of channel-fill reservoirs.


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Figure 6-8. Schematic diagram summarizing the depositional processes associated with the development of an equilibrium profile along a submarinechannel. (a) Equilibrium processes are summarized at different-size basins along a continental margin: basin scale, one intraslope basin, and one singlechannel avulsion. (b) Cross section and map view of how a channel equilibrium profile adjusts as the result of changes in erosion and in depositional ratesof turbidity currents (times 1 and 2). After Pirmez et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Fouundation.

Final profile

Initial profile

Net accumulationprofile

eros

ion

depo

sitio

n

Longitudinal profilealong channel

Intra-slopebasin*

Levee-ch.avulsion

Basinscale

Idealized mapview

shelfcanyon

upper fanmiddle fan

lower fan

equilibriumpoint perched fill (time 2)

ponded fill (time 1)erosion of older fill (time 1)

channel-levee (time 2)

depositional lobe (time 1)downcutting of parent channel (time 1)

channel-levee growth (time 2)

Erosion rate

Deposition rate

x'

x

x

x'

localbaselevel

channel-levee growth (time 2)

shelf edge

spillpoint

basinfloor

avulsionsite

spillpoint

Distance (not to scale)

Dep

ositi

onal

sys

tem

scal

e

time 1time 2

time 1time 2

Equilibrium profile development in submarine channel systemsa

b

Regional-scale characteristics

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The geologic literature is now replete with spectacular examples of deepwater channelsthat have been imaged with side-scan-looking systems for the surface and with 3D seismicdata for both the surface and the shallow to deep subsurface. These images have been essentialin our understanding of the morphology, evolution, and processes associated with deepwaterchannel systems.

Seafloor images

Images of channels on the seafloor illustrate three important concepts for explorationgeoscientists: (1) channels act as conduits connecting the shelf to the basin floor and allowingcoarse-grained sediment to bypass the mud-dominated slope, (2) channels can have linear tosinuous geometries, and (3) a transition zone from aggradational channels to depositionallobes exists and is called the “channel-lobe transition zone.”

Most modern deepwater channel systems have not been active sediment conduits sincethe latest Pleistocene, with only a mud drape having been deposited during the Holocene.When surficial fan channels are cored, they commonly penetrate 1–3 m of hemipelagic muds(e.g., Nelson et al., 1992; Piper et al., 1997). A few exceptions do exist, such as some Califor-nia systems and the Zaire Fan, offshore Angola, which remains active with sediment gravityflows today (Babonneau et al., 2002).

Slope conduits

The late Pleistocene slope and basinal channels on the Nigerian continental slope are agood example of a modern inactive system located along a divergent margin (Figure 6-1). TheNigerian margin has large volumes of Cenozoic sediments that drained from central Africa.Syndepositional tectonics—in the form of growth and reverse faults—and shale diapirismboth developed in response to the extensive sediment loading (Chapter 15). For each channelsystem that is distributed across this broad area, the depositional patterns tend to be the sameas the channels extend from the shelf, upper slope, lower slope, and basin plain. Simplisticallyput, channels tend to be erosional updip and tend to change downslope to leveed channels(aggradational).

Channel morphologies

The variations in surficial channel morphologies are illustrated in Figures 6-2 and 6-9.In the offshore Kutei Basin (Figure 6-2), most of the canyons and/or erosional channels on theslope are straight to slightly curvilinear; one erosional canyon has sinuosity. Aggradationalleveed channels emanate from the canyons onto the basin floor as channel sinuosity increasesdowndip.

An amplitude extraction map of the seafloor offshore Trinidad illustrates the downslopechanges in the shapes of channels (Figure 6-9). Two channels are present that have been activerecently (the northern and southern). Both have thin levee deposits with low-amplitude sedi-ments. In the upslope environment, the southern channel is nearly straight, 1 km (0.62 mi)wide, and incised into the slope (Figure 6-9b). About 10 km (6 mi) downslope, the southernchannel’s sinuosity begins to increase and the channel width decreases to 0.5 km (0.3 mi)(Figure 6-9c). Farther downslope, the sinuosity of the northern channel has increased(Figure 6-9d), as the channel passes between the mud volcanoes. The channel has incised intothe slope, and hanging oxbow loops have developed. Later, the channel filled with fine-grained sediment.


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Figure 6-9. Amplitude extraction maps of seafloor and upper slope, offshore Trinidad. Diagonal striping pat-tern from upper left to lower right is a remnant of the seismic acquisition (“footprint”): (a) seafloor image andlocation of detailed images; (b) image of updip portion of straight to slightly sinuous channel, mud volcanoes(orange areas), and recent mudflows; (c) downslope example of same channel—note the mudflows emanatingfrom the mud volcanoes; and (d) farther downslope, a different channel has become highly sinuous, with low-amplitude fill. Erosional terraces and meander cutoffs associated with an earlier phase of incision are clearlyvisible. Modern and older channels meandered around the mud volcanoes. Modified from Brami et al. (2000).Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Damuth et al.’s (1983) landmark publication, which summarized the highly sinuouschannels that extended across much of the modern Amazon Fan, changed the geoscience com-munity’s perception about deepwater channels (Figure 6-10). Prior to their publication, mostworkers considered submarine channels to be straight to slightly curvilinear. In addition, tur-bidity currents were not considered capable of producing sinuous (and probably meandering)channels. Three-dimensional seismic imaging of the seafloor now routinely detects sinuouschannels in many different tectonic settings (Figures 6-9 through 6-12). Sinuous channels are achannel form that has received considerable research during the past decade for (1) the sedi-mentary processes that give rise to their geometry and (2) their importance as reservoirs inseveral discoveries (e.g., Girassol and Dalia, offshore Angola: Navarre et al., 2002; Abreu etal., 2003) (see below for further discussion).

In 2003, a spectacular, 400-km long by 300-m deep, highly sinuous submarine channelwas discovered and mapped in 3000 m of water off the Mauritania coast (Krastel et al.,2004). Although this channel, named Cap Timiris Canyon, is unusual in that it is offshore ofthe Sahara Desert (unlike other major submarine channel systems which head near major riversystems), shallow, buried incised valleys on the outer shelf have been detected seismicallywhich presumably fed the Cap Timiris Canyon during more humid periods. In its distalreaches, the channel system is 100 m deep and 3 km wide, with well developed levees. Cores

d

b

c

a

10 km


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within the channel contain 50% thin-bedded sand/mud turbidites, with the remaining 50%being interbedded, fine-grained hemipelagites. The channel is deeply incised into underly-ing, high-amplitude sheet sands a few 10's of km wide. Seismic data suggest that the sheet wasfirst cut by small distributary channels, which later self-organized into a single main channelsystem.

Sinuous channel forms can also be imaged in outcrop at about the same scale as they canin modern seafloor images. Sinuous deep-marine channels are exposed in the Ordovicianstrata of central Algeria (Figure 6-13) (Hirst et al., 2002). In this setting, the channels are sand-stone, and the surrounding strata are dominantly overbank and slope shales. Differentialweathering has exposed the channels, so that they can be imaged from aerial photographs forlong distances.

Figure 6-9b. Closer view of image of updip portion of straight to slightly sinuous channel, mudvolcanoes (orange areas), and recent mudflows.

Figure 6-9c. Closer view of downslope example of same channel—note the mudflows emanatingfrom the mud volcanoes.

b

4 km

c

4 km


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Figure 6-9d. Closer view of farther downslope, where a different channel has become highly sinu-

ous, with low-amplitude fill.

Channel-lobe transition zone

The downfan transition from channels into lobe/sheet deposits is called the channel-lobe

transition zone by Mutti and Normark (1987, 1991) and is best studied with seafloor images

(Figures 6-14, 6-15). This transition zone marks the area where the aspect ratio of the deposi-

tional elements begins to change from the lower values for channels (30:1 to 300:1) to the

higher values of sheets (>1000:1; Chapter 8).

Wynn et al. (2002) summarized the key seafloor characteristics of one modern channel-

lobe transition zone using side-scan sonar and high-resolution, shallow-penetration seismic

profiles (Figures 6-14, 6-15). A spectrum of erosional and depositional bed forms can be

imaged on the seafloor. Moving downfan from the channel toward the lobe, erosional linea-

tions change downfan to large amalgamated scours and then to large isolated scours. Small

isolated chevron- and spoon-shaped scours are present along the edges of this transition. Far-

ther downfan toward the lobe (sheet) deposits, sedimentary structures evolve into depositional

bed forms, including sediment waves. The erosional scours commonly are filled with mud

from the overlying flows, which is considered a diagnostic feature of this zone in outcrop

(Mutti and Normark, 1987, 1991).

d4 km


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fi

Figure 6-10. Images of the Amazon Fan channel. (a) Bathymetry of the youngest channel, between 250 and 450km. Image is illuminated from the east. Also shown are the location of five ODP coring sites (930, 935, 936,939, 940) and detailed images in (b) and (c) (red boxes). (b) Detailed bathymetric image showing the incisedmeanders, hanging meander cutoff loop, east of the main channel. Image is illuminated from the west.(c) Detailed bathymetric image of the channel, with low sinuosity and evidence for incised meanders. Image isilluminated from the east. (d) Seismic profile across the channel, showing the location of the ODP site 936.Time-based logs (gamma ray, sonic) are shown. Location of profile is shown in (b). After Pirmez et al. (2000).Reprinted with permission of the Gulf Coast Section SEPM Foundation.

ODP 936

100 msSGR DT

936

930

935

940

939

a

b

d

936

939

c


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Figure 6-11. Chair seismic display of an upper Pleistocene sinuous channel-levee system (ca. 0.6 Ma), Missis-sippi Fan, northern deep Gulf of Mexico. Note the slide scars within the inner levees adjacent to the channel onthe horizontal display. In the vertical profile, prominent lateral migration and aggradation of the high-amplti-ude reflections are intepreted to be migrating channel-fill deposits. After Morton (2001). Reprinted with per-mission of Chris Morton.

These erosional features on the seafloor reflect the erosional processes associated withthe zone of hydraulic jump. As gravity flows reach a change in slope, they are interpreted toexperience a hydraulic jump associated with deceleration of the flows. The flows then begin tospread out and form the more sheetlike deposits (Chapter 8). In the example illustrated inFigures 6-14 and 6-15, the channel-lobe transition occurs across a 15- to 20-km zone; insmaller systems, the transition may occur across a 2- to 5-km-wide zone.

Seismic stratigraphic and wireline log expression

Shape and size

The shapes of channel fill in the subsurface are quite similar to those imaged on the sur-face and are as highly variable. Where channels have incised into the underlying slope, theshape of the erosional channels tends to be linear (Figure 6-16), although there are someexamples of sinuosity of the erosional surface. The shape of the aggradational channel-filldeposits can vary from straight to sinuous (Figures 6-16 through 6-19).

In gravel-rich and sand-rich deepwater systems (sensu Richards and Bowman, 1998),channels are straight to slightly curvilinear; they can stack and be offset from one another. Inplan view, they can appear to be braided. The Forties field in the North Sea consists of a sand-rich system (high net:gross values) in which the producing channels are straight to slightly

Laterally migrating channel-fill deposits

Slide scars


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Figure 6-12. GLORIA II side-scan sonar image of the late Pleistocene Indus Fan channels. Sinuosity values ofthe channels are greater than 3.5 in many courses along the channel. After Kenyon (1995). Reprinted with per-mission of Kevin Pickering.

Figure 6-13. Aerial photograph showing the UpperOrdovician sinuous deepwater channels in central Alge-ria. Note that the scale of the channel-fill strata isapproximately the same as it is for those channel-fill fea-tures in seafloor images shown the Figures 6-2, 6-9, and6-10. Desert exhumation of the slope shales has pre-served the channel-fill strata as topographically elevatedfeatures. See Beuf et al. (1971) and Hirst et al. (2002) fora detailed summary of the channel-fill facies.

Paleochannels

1 km


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curvilinear (Figure 6-17). In mixed mud- and sand-rich and mud-rich systems, sinuous chan-nels have now been documented using 3D seismic data in many different basins (Figures 6-18through 6-20).

The width of deepwater channels is also highly variable. Some of the larger modern sys-tems have channels greater than 5 km (3 mi) across, whereas in the buried systems, channelscan be hundreds of meters in width. Channel fill can vary in width along the course of a chan-nel (Figures 6-1, 6-2, 6-9, 6-19, 6-21). In general, channel width is largely controlled by thevolume and energy of the flows and the size of the receiving basin.

Figure 6-14a. Two interpreted seafloor images ofchannel-lobe transition zone (CLTZ). A side-scansonar image from the middle of the Agadir FanCLTZ, offshore Canary Islands, shows a varietyof erosional features (lineations, irregular scourzones, complex amalgamated zone) and reworkedsediment waves. A high-resolution, shallow-pene-tration seismic profile shows the smooth surface(right) changing to the irregular surface of a com-plex zone of scour and erosional lineations andamalgamated scours.

Figure 6-14b. A side-scan sonar image from theRhone Fan CLTZ (offshore southern France) showsa variety of large and small erosional scours andpossible sediment waves. Individual scour surfacesare as much as 1.5 km long and 1 km wide. A high-resolution, shallow-penetration seismic profile showsan irregular surface of small and large erosionalscours with as much as 10 m of relief. After Wynn etal. (2002). Reprinted with permission of AAPG.

a

Complex zone of

amalgamated scour Erosional

lineations

Reworked

sediment waves

Erosional

lineations

Smooth sea floor

Irregular scour floor

with erosional remnants

Proximal scarp of

large amalgamated scour

Flow direction

Complex zone of scour

and erosional lineations

Cross sectionProximal scarp of

large amalgamated scourSmooth sea floor

1 km

4300 m

4275 m

TOBI Image

TOBI vehicle track line

TOBI interpretation

b

MAK1 vehicle

track line

Small

scoursMAK1 interpretation

Small

scours

Complex

scour zone

Large

scour

Possible

sediment waves

Large erosionol

scours

Flow direction

Small scours

500m

10m

MAK1 profile

500 m

Large erosional

scour


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Figure 6-15. Schematic diagrams illustrating the channel-lobe transition zone in different end-member deep-water systems. (a) Cross sections through low- and high-efficiency systems. Low-efficiency systems are charac-terized by low-volume, sand-rich turbidity flows with low mud content. Both erosion and deposition occur atthe channel mouth at the break in slope, leading to the development of an attached lobe, and consequently,there is no channel-lobe transition zone. In a high-efficiency system, there are high-volume sand-rich flowsand a higher proportion of mud. Sediment bypasses down fan leading to the development of the channel-lobetransition and a detached lobe. (b) Cross section and map showing the distribution of sedimentary structuresthat develop in a high-efficiency system. From the mouth of the channel, progressing downfan, large-scaleamalgamated scours change to isolated scours, which change to sediment waves and mixed sand, whichchange to a sand-rich depositional lobe. After Wynn et al. (2002). Reprinted with permission of AAPG.

Low-volume, sand-rich flows

with low mud content

Erosion and deposition both

occur at the channel mouth,

leading to development of an

attached lobe and no CLTZ

LOW-EFFICIENCY SYSTEMLOW-EFFICIENCY SYSTEM

HIGH-EFFICIENCY SYSTEM

High-volume, sand-rich

flows with high mud content

Sediment bypassing leads to

development of a CLTZ and

detached lobe

Small break of

slope angleCut-and-fill

scoursAttached lobe

Sediment

waves

Detached

lobeErosion

scours

Large break of

slope angle

Sandy

proximal

lobe

Sediment waves

and mixed sand

deposition/reworking

Small isolated

chevron and spoon

shaped scours

Large

isolated

scours

Large

amalgamated

scour

Break of

slope

Sediment waves

and mixed sand

deposition/reworking

Sandy

proximal

lobe

Large

isolated

scoursLarge

amalgamated

scour

Break of

slope

Diagram not to scale

Small isolated

chevron and spoon

shaped scours

Erosional

lineations

Channel

levee

Channel

Channel

levee

CHANNEL CHANNEL-LOBE TRANSITION ZONE LOBE

b

a


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Figure 6-16. Amplitude extraction of the offshore western Nile margin, illustrating the distribu-tion of the producing channel trends (hot colors) and locations of discoveries (Sequoia, Saffron,Scarab, Serpent, Simian, Sinbad, and Sienna). Channels have incised into the mud-dominatedslope (green color) and have a fairly straight course. Channel-fill reservoirs are gravel and sand-rich. After Samuel et al. (2003). Reprinted with permission of AAPG.

Figure 6-17. Spectral decomposition attribute map of the channel-fill reservoirs of the upperPaleocene Forties field, Central Graben, North Sea. Channel-fill sediments consist of high-net:gross reservoirs that are slightly curvilinear in plan view. After Leonard et al. (2000).Reprinted with permission of the Gulf Coast Section SEPM Foundation.


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Figure 6-18. Amplitude extraction map (a 30-ms gated window) of the highly sinuous channel systems,

Miocene slope, offshore Angola. Note that the blue areas (low amplitude) correspond to a mud-dominated

slope. Diagonal striping pattern from upper left to lower right is a remnant of the seismic acquisition (“foot-

print”). Two sinuous channel systems (a, b) have about the same sinuosity values, although the channel-fill

facies varies in amplitude along the course of the channel, suggesting there were different grain sizes of chan-

nel-fill sediments. Southernmost channel (c) has a straight course. After Mayall and Stewart (2000). Reprinted

with permission of the Gulf Coast Section SEPM Foundation.

Importantly, channel shape and size change downfan. Posamentier et al. (2000) showed

an example of a late Pleistocene channel that changes downfan, from a fairly wide and slightly

sinuous form to one that is much narrower and is highly sinuous (Figure 6-21). This downfan

change in channel width and shape was attributed to decreases in the basin gradient and to the

response of the sedimentary flows to this change.

The thickness of channel-fill deposits is highly variable between different fan systems.

Channel-fill thickness decreases downfan. Single-story channel fill can be as thin as a few

meters, whereas the more complex channel fill (Table 6-1) associated with some large modern

submarine fans can be as much as 500 m (1500 ft) thick (e.g., the Amazon, Bengal, and Indus

modern fans). Channel-fill thickness depends primarily on the duration that the channel is

active, the volume of the flows transported through the channel, and whether the channel

avulses or remains dominantly aggradational. Channel-fill thickness also depends on the

degree of confinement and whether the channel is entirely filled prior to abandonment (i.e.,

whether there is remaining accommodation in the channel—whether it is underfilled).

a

c

b

4 km


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Figure 6-19. Composite display showing downslope changes in the evolution of channel fill within one sinuous channel. Flow was from the top to the bot-tom of the image. (a) Amplitude extraction map of one channel with highly sinuous, laterally migrating channel fill. Locations of vertical seismic profilesb-e are shown. Seismic profiles illustrate the changes in the channel-fill pattern. Profiles b and e show an aggradational and offset fill pattern. Profile cshows a dominantly aggradational channel-fill pattern. Profile (d) shows about 2 km of lateral migration of the shingled channel fill to the left (north-west). Reprinted with permission of Mike Mayall.

a

ed

cb

d

bc

e

100 m

500 m

100 m

500 m

100 m

500 m

100 m

500 m


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Figure 6-20a–c. Three seismic profiles from the Kutei Basin, southeastern Borneo, illustrating the downslopechanges in the upper Pleistocene channel-levee systems. Location of profiles (a)–(c) are shown in parts (d) and(e). The base of the system consists of a low-amplitude, chaotic interval (below P4), which is overlain by later-ally continuous parallel reflections (between P4 and P3.2). These reflections are interpreted to be a distribu-tary channel system. In turn, these deposits are overlain by a channel-levee system. Channel-fill deposits arehigh-amplitude reflections that show aggradation and some lateral migration in profiles (a) and (b). Levees arelow-amplitude reflections that dip away from the axis of the channel. Profile (c) shows that the levee and chan-nel have thinned considerably from updip. After Saller et al. (2004). Reproduced with permission of the AAPGand WesternGeco.

P4

P4

P4

P3

P3

P3

P2

P2

P2

P3.2

P3.2

P3.2

Channel Levee Complex

Channel-Levee Complex

a

b

c

G

F'

Lower Fan

Lower Fan

Lower Fan

100 ms

100 ms

2 km

Chaotic,DebrisFlows?



H-H'

'

H-H'


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Figure 6-20d, e. Coherency maps of the channel-levee systems illustrated in (a)–(c). Map (d) illus-trates the lower interval 16 ms below the 3.2 horizon in profiles (a)–(c). Several updip channelsbifurcate downdip. (e) Coherency map of 4 ms below the P3 horizon in profiles (a)–(c). The highlysinuous, meandering channel passes downdip to the unconfined sheets. Sediment waves arepresent outside meander loops (green arrows). After Saller et al. ( 2004). Reproduced with per-mission of the AAPG and WesternGeco.

d

e

a b c

a

a

a b

b

b

c

c

c

a


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Figure 6-21. Azimuth attribute map of a late Pleistocene channel, deepwater Makassar Straits, offshore east-ern Borneo. Note the downfan changes in channel morphology, from the channel being 1.0 km (0.6 mi) wideand having low sinuosity to having higher sinuosity and being 0.4 km (0.25 mi) wide. After Posamentier et al.(2000). Reprinted with permission of the Gulf Coast Section SEPM.

Edge relations

Updip, erosional channels can first develop in less than 100 m (330 ft) of water, wheresubmarine canyons have cannibalized into the shelf primarily by slides (Figures 6-1, 6-2, 6-9).Aggradational channels first develop in a few hundred meters of water, within the confinederosional system.

The nature of the lateral relationships of channels can be evaluated because channel-fillsediments commonly are more heterolithic than are the sediments in the adjacent levee oroverbank area. As a consequence, there is a greater range in seismic reflectivity in the channelfill, and this allows one to evaluate the edge of the container. In plan view, the edge of a chan-nel generally has a sharp boundary at any one point in time (Figures 6-16 through 6-18). Withlateral migration and meandering, the boundary can appear to be diffuse in some displays thatare interval-averaged (Figure 6-19).

Change in channel morphology


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In vertical profiles, the position of a channel edge changes during the evolution of a sin-gle channel or depositional sequence (Table 6-1; Figures 6-19, 6-20, 6-22). Where the chan-nel-fill sediments were deposited within an erosional container (i.e., in confined channelcomplexes), the sediments onlap against the erosional surface. If the channel is aggradational,the channel fill terminates against the levee. On seismic profiles, channel-fill reflections termi-nate against a different amplitude facies (Figure 6-18). Slides that originate from the innerlevee and fail into the channel can be imaged both in horizontal and vertical perspectives, insome settings (Figure 6-10).

In the downdip direction, channels can extend for long distances—in some cases, formore than 4000 km in modern fan systems (Bengal Fan, Emmel and Curray, 1985; Indus Fan,Kenyon and Millington, 1995; northwestern Atlantic mid-ocean channel; Hesse et al., 2001).Ultimately, channels pass into the depositional lobe/sheet deposit (Figures 6-14, 6-15; andChapter 8). These deposits can terminate in confined settings, such as an intraslope basin, or inunconfined settings.

The change from channelized to nonchannelized (sheets) occurs over variable distances.Aspect ratios change from 50:1 to as much as 1000:1 across fairly short distances. On horizon-tal views, this transition can be imaged accurately with high-resolution, shallow seismic. Inburied systems, the details of the erosional features in the channel-lobe transition zone cannotbe imaged.

Although most channels transition downfan into a depositional lobe with sheet deposits(Chapter 8), one unusual example of a channel terminus is found in the upper Paleocene Taysystem of the North Sea (Figure 6-23) (Jennette et al., 2000). The 3D seismic and the isopachmap clearly indicate that the channel terminates as a distinct linear feature. No lobelike orsheet deposits exist at the end of the channel. Examination of cores through this deposit indi-cates sand-rich debris flows. Jennette et al. (2000) interpreted these to be hybrid deposits. Theuniqueness of this feature is unknown. No other examples have been cited in the literature.

Figure 6-22a. Seismic profile from block 16, offshore Angola. Highlighted sequence consists of amass-transport deposit (labeled “sandy debrite”) at the base consisting low-amplitude chaotic,mounded, and hummocky reflections, overlain by amalgamated channel-fill and channel-leveesystems.

Channel / levee

Baselap

Sand debrite

S Na

Amalgamated channel sands

Sinuous meander forms


6-197

Figure 6-22b. Wireline log through the sequence shown in (a). The base of the sequence consists offine-grained sediments, corresponding to the MTD (see Chapter 9). Overlying sediments consistof sandy debrite and amalgamated channel-fill sand. Finer-grained sediments correspond to theleveed-channel systems. After Sikkema and Wojcik (2000). Reprinted with permission of the GulfCoast Section SEPM Foundation.

2050

2100

2150

2200

2250

2300

2350

Claystone

Slumped turbidites

(shale/sands)

Sandy debrite

Amalgamated

channel sands

Thin beds

Mudstone

b


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Internal reflections

Many of the channel-fill types illustrated in Figures 6-3, 6-6, and 6-7 are resolvable onmultifold seismic data, especially where the channel-fill strata are shallow, thick, and/or thesufficient frequency of seismic data to resolve them. Channel-fill patterns can vary, from lat-eral accretion to dominantly aggradational to aggradational and offset packages. Straightchannels have channel-fill reflections that are dominantly aggradational. In contrast, the chan-nel fill within one sinuous channel complex can vary dramatically across short distances alongits thalweg. A seismic-amplitude extraction of a highly sinuous channel from offshore Angolais shown in Figure 6-19. In plan view, the channel has a highly sinuous form; the channelmigrated laterally, with several discrete positions of channel-fill deposits. Four seismic profilesillustrate the progressive changes downdip in the channel fill in association with the channel-evolution processes. The most updip profile (line 1) illustrates aggradation with some lateralstacking. About 1 km (0.6 mi) downdip, line 2 shows dominantly aggradational channel fill.Down channel about 0.5 km (0.3 mi), line 3 illustrates well-developed lateral accretion of thechannel fill. In contrast, line 4 (0.5 to 1 km [0.3 to 0.6 mi] down channel) illustrates lateral-off-set stacking.

Other important relationships can be demonstrated from the 3D attribute maps. Thespectacular 3D images illustrate sinuosity and possible variations in the grain size of channelfill, based on amplitude strength (or whatever attribute is being used for display)(Figures 6-18, 6-19). In these examples, the channel fill varies considerably along the channel.The different amplitude strengths (i.e., different colors) are interpreted to reflect qualitativelyprimary differences in the grain size and lithology of the sediments.

In deeper buried-channel systems, the channel-fill strata can correspond to one seismicreflection (Figures 6-23 through 6-25) or may not be imaged at all. In such cases, many of thechannel-fill deposits are subseismic in resolution; thus, their internal geometries can only beresolved with wireline logs, cores, and reservoir-performance information.

As we described above, within one depositional sequence, commonly a vertical transi-tion in the style of channel fill occurs from distributary channel complexes to the singlechannel-levee system within one depositional sequence (Table 6-1; Figures 6-6, 6-22, 6-26).The single aggradational channel generally has the higher sinuosity values (Figures 6-4through 6-6). This also reflects a decrease in net:gross values.

Additional outstanding seismic-stratigraphic examples of channel-fill strata in both theshallow and deep subsurface include the following: offshore Angola (Mayall and Stewart,2000; Prather et al., 2000; Sikkema and Wojcik, 2000; Kolla et al., 2001, Mayall and O’Byrne,2002; Navarre et al., 2002; Abreu et al., 2003; Prather, 2003); offshore Nigeria (Pirmez et al.,2000; Deptuck et al., 2003; Fonnessu, 2003; Prather, 2003; Morgan, 2004); offshore Nile(Samuel et al., 2003); West of Shetlands (Lamers and Carmichel, 1999; Leonard et al., 2000);the North Sea (Leonard et al., 2000; Jennette et al., 2000); offshore Mahakam Delta (Posamen-tier et al., 2000; Guritno et al., 2003; Fowler et al., 2004; Saller et al., 2003, 2004); the Gulf ofMexico (Prather et al., 1998; Varnai, 1998; Jennette et al., 2003; Posamentier and Kolla,2003); offshore Trinidad (Brami et al., 2000); and the Campos Basin, Brazil (Bruhn, 1998,2001).

Wireline-log to seismic response

The wireline-log response of channel-fill strata is highly variable, which indicates thebroad spectrum of stratal grain sizes and also the complex and variable styles of channel fill(Figures 6-3, 6-6, 6-7). Grain size and net:gross values vary between the different systems; thesystems can be gravel rich, sand rich, mixed mud-sand rich, and mud rich (Richards et al.,1998; and Chapter 1 and Chapter 5 in this volume). Consequently, their wireline-log responsesare also highly variable.


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Figure 6-23. (a) Isochron map (twtt) of the Tay Fan system, Central Graben, North Sea. The areal distribu-tions of sequences 1 and 2 are outlined in white. Sequence 2 has a distinct fingerlike trend of the basinwardchannel as it extends to the southeast. Locations of parts (b)–(d) are shown. (b) Isochron map of the terminuschannel of sequence 2. Locations of seismic profile and well in (c) are shown. (c) Seismic profile illustratingdistinct external mound shape to the sequence, and location of well A. (d) Wireline log and cores from well Athrough the Tay sequences. Cores indicate that sequence 1 consists of thin bedded, shaley sandstone with con-torted and deformed facies and an angular mudstone-floating clast. Sequence 1 is interpreted to be mud-richsandy debrites with minor-thin-bedded turbidites. After Jennette et al. (2000). Reprinted with permission ofthe Gulf Coast Section SEPM Foundation.

a

d

c

b


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Figure 6-24a. Seismic profile across the Schehaillion channel-fill reservoir, offshore Shetland Islands, Scotland.Reservoirs correspond to a single, high-amplitude reflection. These attributes were used to geosteer for thehorizontal wells in the field’s development.

Figure 6-24b. Seismic-amplitude maps from preproduction (1993) and early postproduction (1999). Theincrease in amplitude in the 1999 map is caused by water injection into a closed reservoir compartment. Theschematic map shows the different reservoir compartments in the field. After Leonard et al. (2000). Reprintedwith permission of the Gulf Coast Section SEPM Foundation.

a

16 mmb

13 mmb

9 mmb

9 mmb

5 mmb

14 mmb

C03

W07

C04

5 mmb

Sweep from

W07 probable

38 mmb

C03 support from C04:

path uncertain

support from C04?

Seg 1

Seg 4

Pre-Production (1993) Post-Production (1999)

Infill Producer

b


6-201

Figure 6-25b. Wireline log from the Magnus field. The primary reservoir consists of Lower Cretaceous high-net:gross stacked channel-fill (“channelized sheets”) strata in the upper part, separated from the MSM Asandstone by the B shale. After Leonard et al. (2000). Reprinted with permission of the Gulf Coast SectionSEPM Foundation.

Figure 6-25a. Seismic profileacross the Magnus field,northern Viking Graben,North Sea.

SE

base Cretaceous

top B shale

top Heather

top Brent

top LKCF

100ms(c. 150m)

300m

a

2900

2950

3000

3050

3100

3200

3250

3300

3350

3400

3450

MSM GMSM F

MSM E

MSM C

B shale

MSM A

LKCF

UpperMSM

RHOBNPHI

Magnus type log

b


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Figure 6-26. Vertical seismic profile and horizon amplitude extractions across one channel-levee system. (a) Vertical seismic profile showing the entirechannel-levee system. High-amplitude, laterally continuous reflections at the base change upward to more-isolated high-amplitude reflections of thechannel fill. Parts (b)–(e) show a seismic profile and an amplitude extraction of successively younger horizons illustrating the change in channel-fill sys-tems. Parts (b) and (c) show a distributary channel-fill system. Parts (d)–(f) show increasing sinuosity in the channel fill, with a decrease in the distribu-tary pattern. Figures are courtesy of Henry Posamentier.

a b c

d e f

Development-scale characteristics

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Channel systems in two major submarine fans were cored in cruises of deep-sea scien-tific drilling programs: the Mississippi Fan was cored by DSDP Leg 96 (Bouma et al., 1985,1986) and the Amazon Fan was cored by ODP Leg 155 (Piper et al., 1997). Both of these stud-ies were important because they established the overall grain size of the channel-fill sedimentsand they correlated these sediments to the high-amplitude reflections (HARs) that characterizethe seismic profiles. DSDP Leg 96 cored the channel-fill sediments in the middle fan of theMississippi Fan at two sites (621 and 622). Both sites recovered primarily interbedded sand,gravel, and silts in the channel-fill facies. The coarser sediments corresponded to the high-amplitude reflection present in the channel-fill facies. ODP Leg 155 cored the Amazon Fanchannels at 10 sites (Figure 6-9). The most common facies is thick-bedded, structureless sand.Beds commonly include large mud clasts and consist of poorly sorted fine to coarse sand. Sandbeds range to as much as several meters in thickness. Medium- to thick-bedded organizedsands are also common in the channel-fill facies. Normal grading is the dominant verticaltrend, with sands at the base and clay at the top. Channel-axis deposits had net:gross values of50–70%, and the flanking levees are dominantly mud (Chapter 5).

In subsurface data sets, typical wireline-log patterns include blocky sands that thin andfine upward or are a combination of these separated by shales (Figures 6-22, 6-25). Individualchannel-fill reservoirs are 5–15 m (16–50 ft) thick; where these sands are vertically amalgam-ated, the reservoirs can be several tens of meters thick (Table 6-1; Figures 6-25, 6-27). In manybasins, a marked contrast in grain size occurs between the channel-fill sediments and the adja-cent slope system; reservoirs are quite coarse-grained (with gravels and/or coarse sand),whereas the overbank sediments are mud-dominated (Zafiro field: Humphreys et al., 1999;Dalia field: Abreu et al., 2003).

As we discuss in Chapter 8, distinguishing channel-fill sands and sandstones from sheetsands and sandstones using a single wireline log can be extremely problematic. One-dimen-sional criteria, such as coarsening- and thickening-upward patterns, although commonly cited,are not reliable indicators for unequivocally differentiating a channel-fill sand or sandstonebody from a sheet, because the latter can also exhibit a thinning- and fining-upward pattern.Without multiple logs or good seismic data, this kind of interpretation can be a daunting task.

There are several unpublished examples of fields in different basins in which definitiveinterpretations were made on the basis of one wireline log and 3D seismic interpretation. Onceproduction was established or additional wells were drilled, however, the interpretation had tobe modified to fit a more complex reservoir distribution. Therefore, we urge caution when oneis interpreting deepwater elements on the basis of the wireline-log signature from only a fewwells. RFT data can help identify laterally continuous shales that are more characteristic ofsheets than of channels. There is often a predictable change in net:gross values from the chan-nel axis (blocky sands with higher net:gross) to the margins (lower net:gross, possiblethickening-upward beds). Thus, if the data set includes several wells that show differences inlog patterns and net:gross values across short distances, the element is likely to be a channelfill (Chapin et al., 1994).


This section summarizes the development-scale features of channel-fill reservoirs thatcan be imaged or observed in outcrops, cores, and borehole images. Channel-fill depositsexhibit considerable complexity and variability; most of this heterogeneity occurs at a subseis-mic scale and can significantly impact reservoir performance. Below, we describe and provideexamples of the many variations that can be imaged with different data sets, and we show howthese can be applied to reservoir issues. At the appraisal and development stages, key factors


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Figure 6-27. A conventional resolution versus a higher-resolution seismic profile of Girassol field, offshoreAngola. The high-resolution seismic has provided a considerably better image of the hierarchy of channel fillwithin the reservoir unit, as shown by the schematic lithofacies interpretations. After Beydoun et al. (2002).Reprinted with permission of the SEG.

that control reservoir volumetrics and performance include the distribution of net:gross valuesdown and across the channel, the amounts of large- to small-scale connectivity and continuity,the presence of coarse channel lags that can selectively channel water or hydrocarbons, and thepresence of shale continuity that acts as baffles or barriers to production.

Conventional

High resolution

100 ms

100 m

Seq a

Seq b

Seq c Seq d

WellConventional versus high resolutiona

B3 Seq.aB3 Seq.bB3 Seq.c

B3 Seq.d

B1 Up

B2 Up

B2 Low

S3 S2B1 Low

S6

High resolution

S1S2

S3S4

S6

B1 B2

B3

Conventional

Enhanced interpretation with high resolutionb


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Outcrop expressions of canyons, erosional channels, and channel-fill strata

Many published studies report on deepwater channel-fill deposits in outcrop; thus,choosing representative outcrops for this book is a daunting task. For many years, fining- andthinning-upward stratigraphic intervals were considered to be diagnostic of channel fill, incontrast to coarsening- and cleaning-upward intervals, which were thought to be diagnostic ofprogradational lobes. However, using 1D data to interpret the 3D architecture of any type ofdeposit, whether from outcrop or subsurface logs, is fraught with uncertainty and should beavoided.

In outcrops of deepwater strata, differentiating between channel types, or even betweenchannel fill and other architectural elements, may be extremely difficult because (1) the depos-its of such elements may exhibit similar properties (e.g., amalgamated- and layered-sheetsands within channel fill), and (2) most outcrops are of a smaller scale than that of moderndeepwater channels (Morris and Normark, 2000). To interpret unequivocally channel-fillstrata in outcrop requires identification of the channel’s bounding surfaces and the strati-graphic relations between channel strata and associated extra-channel strata. In addition, thereare a variety of channel-like elements, such as channel thalweg/scour deposits, crevasse-splaydeposits, slide scars, and megascours/megaflutes, that can be misinterpreted as channel fea-tures if the outcrop is smaller than the dimensions of the element. Finally, the straight to highlysinuous pattern of modern channels and of their fill observed on seismic-reflection records isnormally neither observable nor mappable in 2D outcrops; yet, the 2D stratal relations can beextremely complex if a sinuous channel bends within the outcrop. In addition, aspect ratios canvary dramatically from one position to another within a channel fill (e.g., Figure 6-18).

In the following sections, outcrop examples have been selected to illustrate the range ofcomplexity that can occur within channel-fill successions. The fundamental subdivision weuse is erosional channels, mixed erosional-aggradational channels, and aggradational chan-nels (Figure 6-5) (Clark and Pickering, 1996). Secondary classification is based on the natureof the channel fill: gravel-, sand-, mud-prone, or mixed systems (Reading and Richards, 1994).Table 6-2 lists the principal reasons for selecting each particular outcrop; Figure 6-28 showsthe location of each outcrop. These outcrops provide quantitative information on the nature ofthe channel fill, although the information is not necessarily the same for each outcrop. Most ofthe information is characterized from 2D outcrops because of an unfortunate global lack of 3Doutcrops of deepwater channel fills. For each outcrop, we discuss its significance for the petro-leum geoscientist and compare and contrast it for reservoir applications. We also presenttables that summarize the key aspects of each outcrop. Please note that these tables list differ-ent aspects, because different studies have emphasized different things. Where appropriate, weinclude examples of seismic profiles of some analogous feature described in outcrop.

Erosional channels (including submarine canyons)

Erosional channels are major, long-lived features that have incised into slope or basinalstrata and that generally are not bordered by levee deposits. Large erosional channels arepoint-source conduits through which sediment is transported from shallow to deep water(Reading and Richards, 1994).

Point Lobos Submarine Canyon, California, U.S.A.: A pebble/sand-prone system

The Carmelo Formation at Point Lobos State Reserve in central California is an exampleof a submarine canyon that incised into Cretaceous granodiorite and filled with five sandstone-rich packages of Paleocene strata (Cronin and Kidd, 1998). Both sides of the canyon, which


6-206

are spaced more than 1 km (0.6 mi) apart, can be observed in outcrop, and the greater than170 m (510 ft) of fill can be traced from one end to the other in an oblique strike orientation(Figure 6-29). Each succession fines upward and is composed of (1) a basal unit of conglomer-ates that are stacked and channelized, which is overlain by (2) channelized turbidite sandstonesinterbedded with discontinuous shale-clast conglomerate debrites, which in turn are capped by(3) slumped and disturbed mudstones and shales. Strata are composed of 23 different lithofa-cies grouped into five categories (Table 6-3).

Conglomerates and sandstones dominate this canyon fill in a predictable, fining-upwardpattern for each package. In terms of performance of an analog reservoir, this stratificationstyle is important, because each potential reservoir unit (conglomerate and sandstone) will beisolated vertically by a mudstone and/or shale unit that extends across the length of the can-yon. Cronin and Kidd (1998) also reported the key observations that (1) slides and debris-flowprocesses are more important along the margins of the canyon, so that sediment is transportedlaterally rather than in the downcurrent direction; and (2) the presence of large bed formsattests to within-channel bar accretion and migration during canyon filling.

Miocene Capistrano Formation, California, U.S.A.: A pebble/sand-prone system

The Capistrano Formation is an example of a pebbly/sandy channel or canyon fill thatconsists of two separate successions. It crops out along beach cliffs at San Clemente StateBeach in southern California (Figure 6-30a). The formation consists of a series of laterallyaccreted and amalgamated, sandy and pebbly channel fills that form an oblique-to-dep-ositional-strike outcrop belt that is 1.4 km (0.84 mi) wide. The formation was first interpretedby Walker (1975) as a series of eight individual channel fills deposited on a basin floor. Busbyand Camacho (1998) later recognized the formation as part of a confined-channel system,incised into bioturbated, diatomaceous, slope mudstones and filled with laterally accreted,amalgamated

Table 6-2. Outcrops discussed in the text. Mud-rich canyons are from the subsurface.

Type of channel, Formation name

Grain size

Example shows

ErosionalCarmelo Formation Gravel-sand Complex, but predicable stratigraphy; shale barriers; hierarchyCapistrano Formation Gravel-sand Lateral channel migration; no shale barriers; hierarchyJackfork Group Sand-mud Small- to large-scale discontinuities; predictable stratigraphy Meganos Canyon, etc. Mud Shale-filled canyon

Mixed erosional-aggradationalRosario Formation Gravel Coarse-grained fill with leveesJuniper Ridge Gravel Coarse-grained fill of straight channel, with levees; hierarchySkoorsteenberg Sand Variety of sandy fills, and down-channel variationsBrushy Canyon Sand Down-channel variations and hierarchy

Aggradational

Cerro Toro Formation Gravel-sandAlternate hypotheses for origin; Coarse-grained fill with finer grained thin-beds; nature of channel margin

Grès d’Annot Sandstone Sand Sheet-sandstone-appearing channel-fill

Mount Messenger Formation Sand-mudFine-grained channel-fill (and levee) facies characteristics from behind-outcrop wellbores

Lewis Shale Sand-mud Channel complexity; behind-outcrop wellbore characteristics


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Figure 6-28. Map showing locations of outcrops of channel-fill sandstones (orange dots) and producing fields (yellow dots) discussed in the text. The num-bers of the outcrops correspond to those numbered in Table 6-2.

Outcrop Examples

Field Examples

(10) Gres D'Annot

(3) Jackfork

(9) Cerro Toro

(7) Skoorsteenberg

(11) Mount

Messenger

(12) Lewis

(8) Brushy

CanyonIndian Draw

Girassol

Gulf of Mexico

(6) Juniper Ridge

(5) Rosario

(2) Capistrano

(1) Carmelo

(4) Meganos

FortiesAndrew


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Figure 6-29. Internal stratigraphy of the Point Lobos submarine canyon fill, California. Five Paleoceneconglomerate-sandstone intervals are separated from each other by mudstone intervals, forming five distinctfining-upward sequences. The canyon, carved into granodiorite, is about 1 km across. Modified from Croninand Kidd (1998).

beds of sandstone and pebbly sandstone (Figures 6-30, 6-31). Campion et al. (2000) groupedthe eight channel fills into two channel complexes (Table 6-4: Channel fills 1–4 and 5–8). The1.4-km (0.84-mi) coastal cliff section reveals both of the margins of the master canyon, theonlap of channel-margin sandstone onto the canyon walls, and the individual amalgamatedchannel fills (Figures 6-30, 6-31). The axes of the lower channel fills 1–4 are located progres-sively toward the northwest (Table 6-4). The axes of the younger channels 5–8 are oriented

about 30o to those of the older complex.

Grain size, bed thickness, and net sandstone increase upward within both channel com-plexes (Table 6-4). In each case, this is because the channel margin forms the basal channel fill(channels 1 and 5), and the capping fill is oriented more toward the channel axis (channels 4and 8) (Table 6-4).

Campion et al. (2000) provided a 2D forward seismic model of the channel system,using zero-phase wavelets of 30- and 35-Hz frequency and densities and velocities typical ofsands and gravels (Figure 6-30b). Some, but not all, of the internal channel bounding surfaceshave been imaged in the model, demonstrating that not all heterogeneity that is important tofluid flow will be imaged on seismic profiles.

Weston Beach

Hidden Beach

Punta de losLobos Marinos

NW

CA

NY

ON

MA

RG

IN SE

CA

NY

ON

MA

RG

IN

SEQUENCES

5

4

3

2

150 m

0 375 m

Conglomeratedominant

Sandstonedominant

Muddominant


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Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.: A mixed sand/mud-prone system

The Jackfork Group of central Arkansas and eastern Oklahoma contains a series of out-crops and quarries that have been used for many years as a local (U.S.A.) training ground forgeoscientists and petroleum engineers working on deepwater reservoirs. The Big Rock Quarrysection of the Jackfork Group, near Little Rock, Arkansas, provides a continuous, 1-km-wideby 60-m-thick (3000-ft by 200-ft) exposure that is orthogonal to the paleocurrent direction of amixed sandstone/mudstone channel or canyon fill that has been incised into thinly laminatedand often slumped mudstones (Figure 6-32) (Slatt et al., 2000). This section is part of a larger,

Table 6-3. Twenty-three lithofacies from the Carmelo Formation, Point Lobos submarine canyon (from Cronin and Kidd, 1998).

Group, TypePercentage of

total strata

Conglomerate 28.9 1. Disorganized 9.3 2. Organized (reverse graded) 6 3. Organized (normal graded) 1 4. “Patchy” organized 11.6 5. Continuous 1

Conglomerate-sandstone mix 23.3 6. Sandstone breccia 2.2 7. Classic couplet (reverse to normal grading) 3.8 8. Couplet variation (significant floating clasts) 3.4 9. Couplet variation (high-angle foresets) 1.210. Couplet variation (dense basal lags) 0.911. Couplet variation (water escape structures) 0.312. Nested lens packet 4.213. Composite bed (traction carpets) 2.414. Couplet variation (mixed features) 415. Mixed sandstone with cobbles or pebbles, rip-up clasts 0.9

Conglomerate-mud mix 1.516. Pebble mudstone 1.5

Sandstone-mud mix 13.817. Sandstone without cobbles or pebbles, but with rip-up clasts 0.318. Thinly interbedded sandstones and shales 13.5

Sandstone variations 32.419. Nonmassive sandstone 0.920. Pebbly sandstone 3.421. Massive sandstone 1622. Low-density turbidite 0.823. Normally graded sandstone 2.9


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Figure 6-30. (a) Location of Miocene Capistrano Formation channel fill. (b) 2D seismic model of the Capist-rano channel fill using a zero-phase 30-Hz wavelet. (c) Line drawing of a section of the channel fill. Channelboundaries 4–8 are shown. (d) Outcrop photomosaic showing Channels 4–6. Arrows point to channel bound-aries. M = mud drape; C = boulder bed. Modified from Campion et al. (2000).

lenticular channel or canyon complex that is estimated to be at least 9.6 km (6 mi) wide and16–24 km (10–15 mi) long. Both lateral continuity and net:gross values vary along the lengthof the quarry wall because of a complex erosion-depositional history (Figure 6-33). This out-crop sits within the updip portion of the Jackfork Group and is interpreted to have been afeeder channel or canyon that fed the downdip basinal Jackfork strata (Slatt et al., 2000).

The facies proportions are: turbidite sandstones: 66%; debrites: 15%; interbedded thinsandstone/shale: 9%; and shale: 10% (Cook et al., 1994). Most of these facies occur as len-ticular rock bodies, with lenticularity at a variety of scales (Slatt et al., 2000). The entiresuccession at Big Rock Quarry thins upward (Figure 6-32) and can be subdivided into twobroad intervals. The lower two-thirds of the quarry wall is composed largely of thick-bedded,laterally discontinuous, structureless to graded, very-fine- to fine-grained, shale-clast-bearingsandstones (Figure 6-34b) interpreted to be a series of lenticular, laterally migrating and ver-tically accreting, high-energy channel-fill deposits (Cook et al., 1994). Shales mainly occur asthin drapes filling erosional depressions on the tops of sandstone beds, and as shaley debrites

Intersta

te5ParkBoundary

San Clement e

PACIFICOCEAN

Los Angeles

San Francisco

GreatValley

Sierra

Nevada

San Diego

San

Andreas

Faul t

San Andrea s

Faul t

N

Garloc kFaul t

Ventur a

0 160 Km80

Monterey

Willow s

Study Area

PACIF

ICOCEAN

San Clement e

0 0 .5 km

San Clement e

State Beach

Parkin g

CalaflaAve.

A.T.&S.F.Railroad

Capistrano Formation

Outcro p Belt

Locationof Channels

1-8

(afterWalker , 1975)

8

54

6

7

Connect s to south

side of footp ath

C

M

M

Channel 5

Channel 6

Channel 4

NW SE

d

c

a B

200

300

400

500

600

700

800

900

1000

Tim

e(m

se

c)

7 17 27 37 47 57 67 77 87 97Trace Index

0.130.110.090.080.070.050.040.030.020.01

-0.01-0.02-0.03-0.04-0.05-0.07-0.08-0.09-0.11-0.13

+inf

-inf

1.5 km

Architectural Model of Capistrano FM.

Capistrano Formation

Channel 4-5 Boundary

b


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Figure 6-31. Photomosaic and line drawing of the southeast margin of the Capistrano Formation’s channel fill (red line) and channel boundaries 1–4. Onthe photomosaic, the contact between channels 3 and 4 is shown by the solid black line and arrows. The base of channel 4 is marked by a mudstone drape(M). Modified from Campion et al. (2000).


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Figure 6-32. Pennsylvanian Jackfork Group in Big Rock Quarry, Little Rock, Arkansas. (a) East quarry wallshowing the locations of three outcrop gramma-ray logs, Arco logs #1, 2, and 3. (b) Line drawing of the stratig-raphy along the quarry wall. Location of the three logs are the same as in (a). (c) Outcrop gamma-ray log #1and core of the same stratigraphic interval drilled behind the quarry wall. Note the abundance of discontinui-ties throughout the stratigraphic interval and the finer-grained upper part of the channel fill in (b). Modifiedfrom Link and Stone (1986); Cook et al. (1994); Slatt et al. (2000).

Table 6-4. Characteristics of channel fill in the Capistrano Formation (Campion et al., 2000).

Channel number

Ave. grain size (mm)

Bed thickness

(cm)

Net:gross (%)

Dominant turbidite

faciesEnvironment

Remnant width (m)

1 <0.06 0.1–10 45 Tde, Tc, Tb Channel margin 1002 0.12 0.1–20 55 Tde, Tbc, Tc Channel margin 603 0.25–0.5 2–120 80 Ta, Tb, S3 Channel off axis 604 0.5 6–145 90 R1, R3, S3, Tb Channel axis 505 0.12–0.06 0.1–6 45 Tde, Tc, Tb Channel margin 406 0.12 0.1–40 60 Td, Ta, Tab, Tbc Channel argin 407 0.25 1–150 60 Tab, S3, Tc, Td Channel margin 408 0.5–1.0 4–200 95 S3, Tb, Tcd, Td Channel off axis 650

NW Channel fill

1.0 100–200 S3, S1, R1 Channel axis

Sand

Debris

Thin Beds

Mud Arco Log #3

Arco Log #2Arco Log #1

300

240

180

120

60

00 550 1100 1650 2200 2750

Ele

va

tio

n(f

ee

t)

Lateral Distance (feet)

N S

20

30

40

50

60

70

80

90

100

80

70

60

50

40

30

20

10

0

CHANNEL 1

CHANNEL 2

CHANNEL 3

50

100

Depthsin feet

Clastic RocksSand Mud

MFVF Th

ick-

ne

ss

(ft) Interpreted

Facies

Shell Big Rock quarry corehole

(After Link & Stone, 1986),and GR Log of nearby outcrop

BurrowedZone

? ? ?

3 21


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Figure 6-33. Lateral variations in thickness (gross, net sand, and bed), percent sand, and number of differentbed contacts across the Big Rock Quarry’s east wall (Figure 6-32). At about 2200 ft (315 m), there is a dra-matic change in these properties. After Cook et al. (1994). Reprinted with permission of Gulf Coast SectionSEPM.

(Slatt et al., 2000). The thalweg of one channel, and the adjacent collapsed channel margin, arewell displayed in the quarry wall (Figure 6-34c).

The upper one-third of the quarry section is composed of thinner and smaller, lenticularchannel sandstones and adjacent thin interbeds of stratified sandstones and breccias, inter-preted as lateral accretion packages of leveed channels deposited during the waning phase ofcanyon filling (Figures 6-34a, b) (Cook et al., 1994; Slatt et al., 2000; Abreu et al., 2003).

In summary, as an analog reservoir, these two styles of stratification would result in dif-ferent reservoir performance scenarios. A possible reservoir analog is the early MioceneBaudroie Marine and Baliste fields, offshore Gabon (Wonham et al., 2000).

Paleocene-Oligocene strata, California, U.S.A.: A mud-prone system

Large shale-filled channels or canyons are either not preserved or are difficult to recog-nize in outcrop. Several shale-filled canyons occur in the lower Cenozoic strata in thesubsurface of the Sacramento Valley in central California (Figure 6-35). These shale-filledcanyons include: Markley Canyon (130 km × 12 km × 760 m), Meganos Canyon (80 km × 13km × 760 m), and Martinez Canyon (33 km × 12km × 360 m) (Dickas and Payne, 1967;Almgren, 1978; Garcia, 1981; Boyd, 1984). Some deepwater sandstones also occur within thepredominantly shale fill. Older, shallow-marine sandstones that are truncated by the canyonproduce gas where the canyon fill is an updip or lateral shale seal (Figures 6-35, 6-36;Chapter 15). Shale-filled canyons, such as the Yoakum Channel (83 km × 17 km × 1 km), havealso been identified and mapped in the subsurface Texas Gulf Coast (Hoyt, 1959; Galloway etal., 1991).

Gross ThickNet Sand Thick Ave Sand Thick

Ave Shale Thick

Sand Shale ContactShale Shale ContactSand Sand Contact

Percent Sand

240

220

200

180

160

140

120

100

800 500 1000 1500 2000 2500 3000

Th

ickn

ess

(feet)

18

16

14

12

10

8

6

4

2

00 500 1000 1500 2000 2500 3000

90

85

80

75

70

65

600 500 1000 1500 2000 2500 3000

100

80

60

40

20

00 500 1000 1500 2000 2500 3000

Pe

rce

ntS

and






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Figure 6-34. Jackfork Group strata exposed at Big Rock Quarry, Arkansas. (a) The upper third of the quarrywall shows well-developed lateral accretion packages from thin leveed-channel strata. (b) The entire verticalsuccession, exposed in the quarry over a lateral distance of 100 m (300 ft). The upper third of the succession isshown in (a). (c) Close-up of the lower-right part of the quarry wall, showing a cross section through a small,internal leveed-channel deposit. The light colored, thick sandstone is interpreted as a channel thalweg, with achannel-margin slump to the left, then a thin-bedded levee-wedge that pinches out farther to the left. (a) isafter Abreu et al. (2003); (b) and (c) are after Slatt et al. (2000). Reprinted with permission of the Gulf CoastSection SEPM Foundation.

Mixed erosional-aggradational channels

The definition of erosional-aggradational channels is somewhat ambiguous, because (1) allchannels and their fill exhibit some degree of erosion and deposition, and (2) different workershave used the term differently. Mutti and Normark (1987, 1991) suggested that “mixed chan-nel fills” in ancient deposits exhibit facies that are diagnostic of erosional conditions followedby facies that are diagnostic of depositional conditions. This erosional/depositional cycle maybe repeated during different evolutionary phases within an ancient fill (Table 6-1). By contrast,modern mixed channels are believed to form primarily under depositional conditions that latertransform to erosional conditions (i.e., by downcutting of previously formed deposits; Clarkand Pickering, 1996). Furthermore, an erosional channel can change downdip to a depositionalchannel, as we discussed above. Clark and Pickering (1996) also point out that,

Channel axis Channel margin Lateral accretion package

100 m

Levee wedge

Slumped channel margin

Channel thalweg

a

b

c


6-215

Figure 6-35. (a) Map of northern California, showing the location of various subsurface, shale-filled subma-rine canyons. The Meganos Canyon occurs to the east of San Francisco (SF). (b) Close-up of Meganos Canyonand related oil and gas fields. (c) Locations of gas fields in relation to the Meganos Canyon fill. The shale actsas a trap for gas that occurs in sandstones adjacent to the fill. Modified from Almgren (1978).

within an intraslope minibasin, channel fills with depositional characteristics can be connectedby inter-basin erosional channels. Thus, there have been several usages of the term mixed ero-sional-aggradational channels.

With the following examples, we use the classification of mixed erosional-aggradationalchannels that refers to depressions that have been incised into submarine fans or slopes, thatmay or may not be bordered by thick, extensive levee deposits, and that contain simple or com-plex fill.

Upper Cretaceous Rosario Formation, Mexico: A gravel-prone system

The Rosario Formation in Baja California, Mexico, was deposited in a fore-arc basin dur-ing the Late Cretaceous. Different parts of this depositional system crop out in different areas ofBaja California. In the San Carlos area, the formation comprises deep marine and slope depositsand an incised submarine canyon and its fill (Morris and Busby-Spera, 1988). Here, the RosarioFormation unconformably overlies coarse-grained fluvial deposits of the El Gallo Formation. Inthe Arroyo San Fernando area, the Rosario Formation also unconformably overlies the El GalloFormation. The outcrops comprise a submarine-fan-valley deposit that is defined as “the largestchannel-like feature laterally bounded by thick, extensive levee deposits” (Morris and Busby-Spera, 1990). Here we consider it to be a mixed erosional-aggradational channel fill because ofthe erosional basal contact (though only poorly exposed), the abundance of erosional featureswithin the channel fill, and the associated aggradational levees.

AREA SHOWNBY MAP

SACRAMENTOVALLEY

CALIFO

RNIA

Gre

at

Valley

N

Mt. Diablo

PRINCETONSUBMARINE VALLEYSYSTEM

Sutter Buttes

MEGANOS

MARTINEZ

MARKLEY

S

SF

CO

AS

TR

AN

GE

S

SIE

RR

AN

EV

AD

A

PacificOcean

0

0

30

50

mileskm

SETTING OF TERTIARY CANYONSSACRAMENTO VALLEY, NORTHERN CALIFORNIA

SANFRANCISCO

OAKLAND

Vollejo

Martinez

Mt. Diablo

Livermore

Tracy

Midland fault

STOCKTON

RioVista

Brownwood Oil& Gas Field

Walnut GroveGasField

Thornton Gas Field

Meganos SubmarineCanyon ('Channel')

In Outcrop

0 5 10 15 m

0 10 20 kmN

LOCATION MAPMEGANOS SUBMARINE CANYON AND OUTCROP BELT

MEGANOS

CHANNEL FILL

HAMILTON

DOMINGAME

CAPAY

DUTCH SLOUGH GAS KNIGHTSEN GAS E BRENTWOOD GASN S

-4000'

-6000'

-8000'

-10000'

0 1 2 3 4 miles

1ST MASSIVE

2ND MASSIVE

3RD MASSIVE

HEWETT

SUBMARINE CANYON-TRUNCATION RESERVOIR


6-216

Figure 6-36. Six-fold hierarchical classification of Hickson and Lowe (2002) for the Upper Cretaceous JuniperRidge Conglomerate, California. The 300-m- (900-ft-) thick Juniper Ridge channel fill (fifth order) liesbetween the Lower and Upper Waltham Shale; the complete succession is 1000 m (3000 ft) thick (sixth order).The Juniper Ridge Conglomerate is composed of sandstone and conglomerate, divided into different fourthorder fining-upward sequences of conglomerate to sandstone. A series of second order, fining-upward sand-stone-shale intervals is grouped into a third order class. Individual Bouma intervals comprise the first order.

The channel fill is more than 670 m (2010 ft) thick and 5.5–7.5 km (3.3–4.5 mi) wideand is perpendicular to the paleocurrent direction. The adjacent levee deposits are more than500 m (1500 ft) thick and interfinger with the channel fill. Two general types of deposits com-prise the channel fill. The first type consists of clast-supported (and some matrix-supported)conglomerates interbedded with thick- to medium-bedded, massive to normally graded sand-stones. This type of deposit is interpreted to be the fill of the channels (Table 6-5). Cross-stratification in some of the beds records lateral migration of channels. The second type ofdeposit is medium- to thinly bedded, Bouma Ta-Tc sandstones interbedded with mudstone.This type of deposit is interpreted to be either the upper parts of channel fill or interchanneldeposits. Both types of deposit occur as single, fining-upward successions 10–50 m (30–150ft) thick and 0.25–2 km (0.15–1.2 mi) wide, and as amalgamated, fining-upward to blockystrata 45–210 m thick and as much as 5 km (3 mi) wide.

Low

er

Waltham

Shale

Junip

er

Rid

ge

Conglo

mera

teU

pper

Waltham

Shale

10

00

m

30

0m

10

0m

50

m

0.5

m

Channel-le

vee

syste

m

Levee

facie

s

Turb

idite

6th-order

5th-order 4th-order3rd-order

2nd-order

1st-order

TeTd

Tc

Tb

Ta

Conglomerate Sandstone Mudstone


6-217

Upper Cretaceous Juniper Ridge Conglomerate, California, U.S.A.: A gravel-prone system

The Juniper Ridge Conglomerate, which crops out near Coalinga, California, is inter-preted to comprise a coarse-grained braided plain deposit with straight channels and sandy tomuddy levees (Hickson and Lowe, 2002). The Juniper Ridge outcrop is composed of at leastnine separate conglomeratic units. Two-dimensional outcrops at one particularly well-exposedsection allowed Hickson and Lowe (2002) to develop a six-fold facies hierarchical scheme(based on 2D geometry), with the first order being the smallest-scale lithologic unit and withan open-ended, larger scale (Figure 6-36). Their focus was on the third- and fourth-order ele-ments. The third-order elements consist of IIImd (mud rich), IIIsd/md (interbedded sandstoneand mudstone), IIIss (thick-bedded sandstone), and IIIcg (pebble and cobble conglomeratewith <5% thick-bedded sandstone) units (Table 6-6). These sediments were deposited from:(1) low-concentration, low velocity, mud-rich turbidity currents (IIImd); (2) high concentra-tion, high-velocity flows (IIIss/md) (3) either collapsing, very high concentration sandy flowsincluding tractional flows (Kneller and Branney, 1995), or deposits of sandy debris flows(Shanmugam and Moiola, 1995) (IIIss), (4) bedload associated with high-velocity turbiditycurrents upon a multiple-channel, straight submarine braidplain (IIIcg).

Table 6-5. Lithology and sedimentary features of the Rosario Formation.

Features Valley fill Levee deposits

Channel deposits Interchannel deposits

Predominant lithology

Conglomerate and sandstone Sandstone and mudstone Mudstone and sandstoneBase—cg:ss ratio, 1:1; ss:ms ratio, 20:1

Ss:ms ratio, 3:1 to 1:10, average 1:1

Ss:ms ratio; average 1:1

Fining-upward sequence—-cg:ss ratio, <1:7; ss:ms ratio, 20:1 to 3:1

Geometry of sedimentary sequences

Channelized—10–50 m thick, 0.1 to 2 km wide. Fining-upward sequence—5–15 m thick

Nonchannelized, several kilometers wide

NonchannelizedLarge slumps, several (0.08 by 0.5 km)

Turbidite facies (sensu Mutti and Ricci Lucchi, 1972)

Base—Facies A, B, and F Fining-upward sequence—Facies C, with lesser amounts of D and B

Facies D and G Facies D, E, and G

Sedimentary features

Graded and ungraded conglom-erate-sandy matrix and clast-supported, lesser mud matrix and matrix supported. Massive sandstones with and without mudstone clasts; normally graded sandstone with granules common at the base. Fining-upward sequence—very thick- to medium-bedded, planar-laminated sandstones, Bouma sequence with and without the basal Bouma division, uncom-mon massive sandstones

Medium- to very thin-bedded sandstone, planar laminated, Bouma sequences with and without the basal Bouma division; very thin sandstones, ripple laminated.

Thick- to very thin-bedded sandstone; sedimentary structures similar to those of interchannel deposits except that medium- to thick-bedded sandstones are more common


6-218

These elements are regularly stacked to form 4th order, fining-upward elements consist-ing of a basal IIIcg, overlain by IIIss, then IIIss/md, interpreted as a channel (IIIcg-IIIss)-levee/overbank (IIIss/md) deposit. Clark and Pickering (1996) developed an equation for sinu-osity = (width-0.4) and sinuosity = 3.16 (depth-0.15). Using this equation for these outcropssuggests the Juniper Ridge Conglomerate channels were straight to moderately sinuous.

Table 6-6. Third-order architectural elements of the Juniper Ridge Conglomerate exposed at the Maloney Ranch section, Coalinga, California, U.S.A. (Simplified from Hickson and Lowe, 2002).

Classifi-cation

Lithofacies composition and characteristics

Lateral continuity

Thickness and lateral

extent

Two-dimensional geometry and bounding-surface

architecture

III md>80% muddy, rippled silt-stone beds grading upward to structureless mudstone

Sandstone beds: 2 to tens of meters long. Mud-stone beds not traceable because of cover.

For intervals ~300 m thick, lateral extent: ~30+ km

2D: Tabular3D (?): Sheets

III ss/md

30- to 50-cm-thick, fine to medium sandstone beds with T a–d divisions; 5- to10-cm-thick, fine sandstone beds with Bouma Tbcd divisions; 5-cm-thick, silt to very fine sandstone beds with muddy Tcde divisions

Thicker sand-stone beds trace-able as far as 100 m; some are len-ticular. Thin sand-stone beds (8–15 cm thick) appear tabular and more continu-ous

For intervals ~150 m thick, lateral extent: >2 km

2D:Tabular, with occasional lenticular sand bodies and soft-sediment folds3D (?): Sheetlike and small-scale channelized sand bodies

III ss

99% 0.5 to >3 m thick, medium, sandstone beds; massive and amalgamated; occasional cross-stratifica-tion, planar laminate, and mud clasts near bases and tops of beds. Basal con-tacts are typically flat and nonerosional

Individual beds are difficult to correlate due to cover. Thick (3–5 m), amalgam-ated sandstone packages are traceable over 300 m.

For intervals 10–100 m thick, lateral extent: 2 to >5 km (?)

2D: Tabular sand bodies confined within low-aspect-ratio channels that may show transition to unconfined sheets 3D (?): Confined and unconfined sheets

III cg

95% 1 to >5 m, well-organized, clast-supported and sandy matrix-supported conglom-erate with pervasive imbrication. Inverse and ungraded beds are com-mon. Some beds show low-angle cross-stratification.

Individual beds are difficult to con-rrelate because of coarse texture and amalgam-ation. Sandstone lenses traceable for 50 m

For intervals 50–100 m thick, lateral extent: 3–4 km

2D: Lenticular with erosional base and flat top3D (?): Channelized, with ubiquitous cut and fill


6-219

Skoorsteenberg Formation, South Africa (sand-prone system)

The Skoorsteenberg Formation and the Brushy Canyon Formation (discussed below) areprobably the two best outcrops in the world for demonstrating updip to downdip transitions indeepwater systems. These outcrops have fairly continuous 3-dimensional exposures, allowingfor detailed studies of the lithofacies.

The Skoorsteenberg Formation provides an example of a variety of types of channel-fills in a sand-prone system. Sheet deposits within the Skoorsteenberg Formation are discussedin Chapter 8. Johnson et al. (2001) identified five geometric types of fill that are interpreted tooccur in different parts of a fan system, thus they are all considered fan channels or valleys(Table 6-7).

In terms of architecture, channels in a proximal fan setting typically have a low aspectratio (30:1 to 80:1), high net:gross values (75–90%), excellent vertical connectivity, and lowlateral continuity (as seen in Figure 6-37, of the Skoorsteenberg Formation) (Sullivan et al.,2000). Individual channel fills are less than 400 m (1300 ft) wide and are 6–12 m (20–40 ft)thick. The channel fill is typical of the erosional types listed in Table 6-7. Massive sandstonesextend from margin to margin as a result of high-concentration turbidity currents completelyfilling the channels. There is no internal lateral facies change that could be misinterpreted assheet deposits, indicating that any overbanking processes produced a distinctive interchannelfacies dominated by low-concentration, Bouma Tb and Tc turbidites. In such a case, it is pos-sible that at a wellbore, the channel sandstones could be misinterpreted to be sheet sandstones.

In a medial fan position, the channel-fill deposits differ from those of a proximal fan.There are fewer erosional contacts, the aspect ratio of the channel-fill strata is higher, thenet:gross values decrease to 65–80%, and horizontal connectivity increases (Figure 6-38).Individual channel-fill deposits are as much as 1000 m (3270 ft) wide and 8–13 m (26–42 ft)thick. These are the “depositional channels” listed in Table 6-7. In these channels, the bedlengths of the mudstones are much shorter than the bed lengths of the sandstones, so that verti-cal connectivity is relatively low. Eighty percent of the mudstones are less than 0.3 m (1 ft)thick and have bed lengths shorter than 200 m (655 ft); they would be baffles to vertical con-nectivity in an analog reservoir. The 5–10% of the mudstones that are longer than 700 m (2295ft) would tend to be barriers in an analog reservoir and also could be mistaken for sheetsandstones.

Table 6-7. Channel types and their properties, Skoorsteenberg Formation (Johnson et al., 2001).

Channel Type Complexity Axis Margins Location

Erosionalmultiple channels thick-bedded turbidites

mid to upper fanupper fill thick beds thin-beds

Erosional multiple channels thick-bedded turbidites upper fan

Depositional simple sandy heterolithic mid-fan

Erosional simple thin-bedded turbidites upper slope

Mixed multiple channelslower fill is uniform proximal basin

upper fill is heterolithic floor/slope fan


6-220

Figure 6-37. Proximal-fan, channelized architecture of the Skoorsteenberg Formation, South Africa. (a) Linedrawing from photomosaic (b) of a 0.6 km (0.4 mi) section of channel fill showing a number of discrete, lentic-ular fills. (c) Expanded line drawing of the same section of the fill. (d) Two-dimensional, 30-Hz seismic modelof the area shown in (c). Note the irregular character of the seismic amplitudes. After Sullivan et al. (2000).Reprinted with permission of Gulf Coast Section SEPM.

8 95

10

31

46

2

775

50

25

0

75

50

25

0

FE

ET

FE

ET

0 0.5 miles 1 miles

WEST EAST

WEST EAST

WEST EAST

605040302010

0

6050403020100

FE

ET

FE

ET

0 1000 FEET 2000

Skoorsteenberg Formation(Proximal Fan Architechture)

Proximal fan channel architecture illustrating the compensationalstacking of these narrow, erosionally based channels

Detailed photopan showing channel architecture

Detailed channel architecture

mud clastconglomerate

amalgamated,massive sandstone

amalgamated,low concentrationturbidites

non-amalgamated,low concentrationturbidites

laminatedmudstones

Seismic Response 30 Hz

40

60

80

100

120

140

160

Tim

e(m

s)

a

b

c

d


6-221

Figure 6-38. Transition zone from the proximal to the medial channelized fan of the Skoorsteenberg Forma-tion, South Africa. Upper picture is the outcrop and the lower picture is a line drawing of the outcrop fromaxis to off-axis facies. The boundary between channel fill and thin-bedded facies is erosional. Net sand of the20-m (60-ft) section shown decreases away from the channel axis. After Sullivan et al. (2000). Reprinted withpermission of Gulf Coast Section SEPM.

Sullivan et al. (2000) constructed 2D forward seismic models from proximal-fan chan-nel data of the Skoorsteenberg Formation (Figure 6-37). The models were used to quantifyamplitude variations for the lithofacies, which were then built into the seismic models. Theseismic-lithofacies models were compared with conventional 3D seismic data in Diana Subba-sin, northern deep Gulf of Mexico, for 3D reservoir modeling and simulation.

Permian Brushy Canyon Formation, Texas, U.S.A.: A sand-prone system

The Permian Brushy Canyon Formation of west Texas is a well-established field site forexamining variations in channel forms. It contains sandy fill that was transported from anupdip feeder canyon to downdip basin-floor channels and associated sheets (Figure 6-39a)(Beaubouef et al., 1999; Gardner and Borer, 2000).

Fine, sandstone-rich turbidites comprise more than 90% of the Brushy Canyon Forma-tion, and debrites and hemipelagites each comprise less than 5% (Beaubouef et al., 1999).Updip, a series of slide scars is interpreted to have coalesced to form the original slope feedercanyon (Figure 6-39b), which then funneled sand to the basin floor during the main phase of arelative lowstand in sea level.

Various sedimentary features of channel-fill complexes have been documented progres-sively down the depositional axis (Table 6-8). In that direction, individual channels andchannel complexes become wider but decrease in thickness; these changes reflect a downdipchange from vertical to lateral accretion and channel spillover as the channel margins becomeshallower and less confining (Figure 6-39a). In the downdip direction, net:gross valuesincrease significantly, and the internal complexities that might promote stratigraphic compart-mentalization in an analog reservoir decrease. Massive sandstones are the principal type ofsediment gravity flow throughout the system (Table 6-8).

off-axis

west east

axis

605040302010

0

Feet

6050403020100

Feet

B

mud clastconglomerate

amalgamated,massive sandstone

amalgamated,low concentrationturbidites

non-amalgamated,low concentrationturbidites

laminatedmudstones

Skoorsteenberg Formation (Transitionfrom Proximal to Medial Fan)

channel off-axis association channel axis association

0 500 feet 1000


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Figure 6-39. (a) Schematic paleogeographic reconstruction of the Brushy Canyon channel-fan system, fromupdip feeder canyon to downdip, channelized fan. The location of the outcrop belt from which the interpreta-tion was made is shown (from Beaubouef et al., 1999). Reprinted with permission of AAPG. (b) Three-dimen-sional line drawing of the Brushy Canyon Bench channel-fill section, derived from a series of closely spacedmeasured stratigraphic sections. Net sand is very high, but there are many erosional sand-on-sand contacts.After Prather et al. (2000). Reprinted with permission of Gulf Coast Section SEPM.

* Bouma facies.** Other facies are: Bouma sequences, nodular sandstone, and plow and fill.

TG

CDZ

BM

CC

Relict

CarbonateSlope

Slope Siltstones

SlopeSiltstones

TOE-O

F-S

LOPE

BASIN

FLO

OR

PROXIMAL CHANNELIZED FAN:BRUSHY MESA (BM)

CHANNEL TO SHEET TRANSITION:COLLEEN CANYON (CC)

SHEET COMPLEXES:CORDONIZ CANYON (CDZ)

APPROXIMATE LOCATIONOF OUTCROP BELT

SCALE

1 Km

1 Mile

-N-

Schematic Paleogeographic Map

8600 8000 7000 6000 5000 4000 3000 2000 1000 60

Strike SectionPinch-out Amphitheater and

Salt Basin Overlook

Dip SectionIndian Cave Overlook

Strike SectionGuadalupe Canyon

Thin-beds w/Bouma sequences

master incision

top of outcrop

1st-order sub-seismic architecture

1

23

4

56

7

8

9

10

11

12

1314

10

11 1213

Vertical Exaggeration = 13.3

1°

10°15°30°45°

9000

8000

7000

5000

3000

2000

1000

0

6362

5600

Pinch-outAmphitheater

Salt Basin Overlook Campsite

Indian Cave Overlook

Guudalo

pe

Canyo

n

58106000

4000

Brushy Bench memberstation location

0 1000 2000 feet

100 feet

50

20

10

00200400800feet

a b

Table 6-8. Characteristics of updip-to-downdip strata in the Brushy Canyon Formation (condensed from Gardner and Borer, 2000, their Figures 12 and 13).

Facies tract

Fan #Net:gross average %

Area (m × m)

Aspect ratio (w/d)

Distance from shelf edge (km)

Amalg. massive

Non-amalg. mas-sive

% channel–channel-margin facies

avg. inside outsideTb-Tc*

SltstClast-rich

Oth-er**

Upper slope, slump scar confined (Gardner and Borer Fig. 39b)

7 50 80 201000 ×

4025 7 48 11 12 29

Lower slope (BM)

4 63 83 43200 ×

306.7 10 27–3 13–12

3–19

17–56

17–024–10

Base of slope 4 76 90 61600 ×

3513.7 25 34–26 6–12

11–11

10–38

6–033–23

Basin floor (CC)

2 93 95 91250 ×

2516 30

Basin floor (COZ)

2 94 99 88150 ×

207.5 32 87–74 2–2

1–12

7–03–12


6-223

Aggradational leveed-channel systems

Many of the published interpretations of leveed-channel systems are based on 3D seis-mic images; stratigraphic interpretations have often been made with little or no well or coreinformation (although more cores and borehole-image logs are now being acquired in thesereservoirs). In outcrop, leveed-channel systems are difficult to identify, particularly those thatare fine-grained and that easily weather at the ground surface. Also, because of the size of lev-eed-channel systems in relation to the size of common outcrops, identifying the coupledchannel-fill and levee strata usually is not possible. Many leveed-channel systems appear sinu-ous on seismic-amplitude extractions, thereby adding a level of complexity to outcrops inwhich the plane of the outcrop might be perpendicular to the long axis of the channel, so thatsinuous portions of the channel might wind in and out of a 2D outcrop (Bracklein, 2000). Inthe following sections, emphasis is placed on the channel fill of leveed-channel deposits.Chapter 7 provides more detail on the stratigraphic and sedimentologic nature of levees andtheir reservoir performance and potential.

Upper Cretaceous Cerro Toro Formation, Chile: A pebbly/sandy system

The Cerro Toro Formation has been studied in detail because of its excellent outcropexposures and facies interpretations (Figure 6-40). The formation is a 2000-m- (6500-ft-) thicksuccession of deepwater strata that were deposited in the Magallanes fore-arc basin during theLate Cretaceous (Winn and Dott, 1979; Beaubouef, 2004). Exposures are excellent, and out-crops greater than 300 m (1000 ft) thick often can be traced laterally for distances of 5–10 km(3–6 mi). Two alternate hypotheses about the origin and evolution of the formation have beenproposed—an aggradational leveed channel versus an erosional channel (Figure 6-40). In thischapter, we review the formation using the aggradational channel-fill interpretation (Figure6-40b).

Figure 6-40. (a) Cut-fill-spill model for the Cerro Toro Formation, Chile. According to this model,basinal, thin-bedded sandstones are incised by a channel, then backfilled with the Lago SofiaConglomerate, which also spills over the top of the channel. (after Coleman et al., 2000).Reprinted with permission of Gulf Coast Section SEPM Foundation. (b) Leveed-channel modelfor the Cerro Toro Formation, showing the complex internal distribution of channel and associ-ated levee facies within a master channel, and the levees that are associated with the master chan-nel. Synthetic wireline logs are superimposed on the model, showing the various log shapesassociated with different parts of the leveed-channel system. Modifed from R. T. Beaubouef (per-sonal communication, 2004). Modified from Beaubouef (2004). Reprinted with permission ofAAPG.

12

3 4 56 7

LEVEE/OVERBANK FACIESOVERLYING CHANNEL FILL

OFFSET, STACKEDCHANNEL COMPLEXES

COMPOSITECHANNEL MARGIN

STACKEDLEVEE COMPLEXES

b. Simplified back-filled, erosional channel model


6-224

Winn and Dott (1979) first proposed a leveed-channel origin for this formation. Theirinterpretation was supported by later work (DeVries and Lindholm, 1994; Beaubouef, 2004).Beaubouef (2004) suggests that the Cerro Toro Formation comprises a series of offset-stacked,leveed-channel-fill strata that were deposited within a master channel and that form a beltapproximately 5 km (3mi) wide and as much as 300 m (900 ft) thick. Levee beds are associ-ated with offset-stacked channel fill both within and outside of the master channel. The LagoSofia Conglomerate forms the channel-fill strata (Figure 6-41). It is composed of a wide vari-ety of coarse-grained strata (Table 6-9). Individual channel fills, which are continuous on thescale of kilometers, are on the order of 30 m (100 ft) thick (Figure 6-41). Channel complexesare several kilometers in width and as much as 250 m (820 ft) thick. Individual beds are highlyamalgamated and vertically connected, and there are numerous erosional bases. At the channelmargins, beds are made up of somewhat finer-grained and thinner-bedded, amalgamated Tabsandstones and pebbly sandstones.

The stratal relationships at the margin of the master channel are complex; channel-fillstrata commonly are not in continuity with adjacent thin-bedded deposits (Figures 6-40b,6-41). The same relationship is true for the smaller channel-fill deposits that fill the masterchannel. Channel-fill facies overlie erosional surfaces that cut into the older, adjacent thinbeds. This erosional contact between channel fill and adjacent thin beds led Coleman (2000) tooffer a build-cut-fill interpretation similar to that applied to the Brushy Canyon Formation

Figure 6-41. Outcrop photograph of Cerro Toro Formation, showing a close-up and line drawing of the LagoSofia Conglomerate and its erosional contact with adjacent thin beds. After Coleman et al. (2000). Reprintedwith permission of Gulf Coast Section SEPM Foundation.

~ 7.6 m

channel-fill conglomerate thin- to medium-bedded sheet sandstones

view to northwest


6-225

(Gardner and Borer, 2000), in which thin distal sheet sandstones (the levee beds of Winn andDott, 1979; DeVries and Lindholm, 1994; and Beaubouef, 2004) were deposited in a basinalsetting. This was followed by erosion of these beds and deposition into the erosional channelof coarse-grained bypass phase lags (Figure 6-40b). According to this interpretation, the finalfilling of the channel occurred during a reduction in current flow capacity; any sediment grav-ity flows that occur after the channel has filled spill over onto the adjacent seafloor, formingconstructional levees and some crevasse-splay sheet sands.

This outcrop example demonstrates the diversity of interpretation that can be applied toan association of thin-bedded and thick-bedded strata, even when the bounding surfaces can beobserved. On one hand, if the formation is a leveed-channel complex, it indicates that leveesand channels are not deposited contemporaneously. In contrast, if this formation represents aperiod of basinal deposition of distal layered-sheet sands, followed first by a period of down-cutting and incision into these beds and next by a period of channel backfilling with coarse-grained sediments, then either (1) a more complex sea-level and depositional history isimplied, or (2) the equilibrium profile along the channel that fed this system changed. Thisissue is discussed further in Chapter 3 and Chapter 8. Using either interpretation, the two com-ponent elements—thick-bedded channel facies and thin beds— likely would not be in pressureor fluid communication in an analog reservoir because of the complex channel margin.

Eocene-Oligocene Grès d’Annot System, southern France: A sand-prone system

The Grès d’Annot system in southeastern France provides an example of (1) the diffi-culty in differentiating channel fill or sheet sandstones if only a vertical succession (as in awellbore or core) is available, and (2) evaluating whether channel fill and levee developmentare coeval. These differentiations are important for predicting both sandstone body orientationand fluid flow paths in a reservoir.

The Grès d’Annot is dominated by sheet sandstones (Hurst et al., 1999; Lomas et al.,2000) (Chapter 8). However, channel-fill successions do occur in certain outcrop areas (Clark

Table 6-9. Channel-fill facies characteristics, Cerro Toro Formation (Beaubouef, 2004).

LithofaciesN:G (net sand %

of gross)Thickness of

beddingContinuity of bedding

Primarily clast-sup-ported conglomerates with matrix-supported conglomerates (debris flow deposits) and high-density turbidites

Very high in axial positions to intercutting channels, and limited drape facies

Conglomerates on the scale of several meters thick in axial areas; sandstones on the scale of meters in marginal areas

Continuity of individual beds is highly variable because of erosion in channel axes

Axial fill characterized by graded, inversely graded, and massive conglomerates

Values decrease (but still high) toward channel margin because of inter-bedded nature of marginal fill

Individual channel fill packages (bedsets) on the scale of 30 m; channel complexes on the scale of hundreds of meters

Width/continuity of channel-fill packages (bedsets) on the scale of kilometers; channel complexes on the scale of several kilometers

Marginal fill character-ized by bedded-amalgamated Tab sandstones

Amalgamated nature of fill results in a high degree of vertical connectivity between beds, channels, and channel complexes

Amalgamated nature of fill results in a high degree of lateral connectivity between beds, channels, and channel complexes


6-226

and Gardiner, 2000). The channel-fill strata occur at a variety of scales, with channel dimen-sions ranging from 0.9–4 km (0.56–2.4 mi) wide and 14–110 m (45–360 ft) deep. Thechannel-fill deposits are moderately to highly amalgamated, planar-bedded sandstones. Therelatively low relief and planar stratification make them difficult to distinguish in outcrop,unless the channel margins are exposed. Individual channel fills exhibit erosional bases andare filled with thick-bedded, amalgamated, medium- to granular-size, thick-bedded, fining-upward successions. Erosional scour features are common, as are Ophiomorpha and Thalassi-noides trace fossils. One channel fill can be traced laterally for as far as 4 km; on the basis ofpaleocurrent indicators, this orientation is representative of channel width. The channel-fillstrata are 110 m (330 ft) thick, giving an aspect ratio of 36:1. Aspect ratios of other channelfills in this area range from 36:1 to 200:1.

In addition, some bedding surfaces can be traced from the channel-fill strata to the adja-cent thinner-bedded deposits, suggesting that levees aggraded contemporaneously with filling ofthe channel. Where exposed, the channel margins show a complex architecture resulting fromnumerous erosional events. If the channel fill and levee beds are continuous in an analog subsur-face reservoir, there could be pressure and fluid-flow continuity across the channel margin.

Miocene, Mount Messenger Formation, New Zealand: A mixed sand-mud system

The upper Miocene Mount Messenger Formation crops out over a 20 km (12.5 mi)length of coastline along the west side of New Zealand’s North Island (Browne and Slatt,2002). The outcrop consists of an approximately 600 m (1960 ft) thick progradational succes-sion that includes numerous fourth-order (100,000- to 150,000-yr) sedimentary cycles(Browne and Slatt, 2002; G. T. Browne et al., personal communication, 2004) superimposedon a third-order relative fall and rise in sea level.

In addition to detailed outcrop studies along a portion of the cliff face (Figure 6-42;Table 6-10), two shallow boreholes were drilled, logged, and cored 100 m (300 ft) inland fromthe studied cliff face, and a high-resolution seismic line was acquired nearby to help correlatethe outcrop studies to subsurface expression (Figure 6-43a) (Browne and Slatt, 2002). The out-crop face is composed of channel fill, and proximal- and distal-levee strata (Figure 6-43b, c).Characteristics of the levee deposits are discussed in detail in Chapter 7; only the channel-fillarea is described here.

Two-dimensional channel morphologies vary from single to multiple scour-and-fill bod-ies. Aspect ratios of individual channel-fill strata range from 13:1 to 47:1 (Browne and Slatt,2002). Only one side of the largest channel-fill deposits crops out, where the contact betweenthe channel margin and adjacent proximal-levee beds is exposed (Figure 6-42). This channel-fill deposit is floored by a 2 m (6.5 ft) thick conglomerate bed that includes abraded gastropodand bivalve shells, as well as pebbles and cobbles, within a sandy matrix. The channel marginis sharp and contains slide deposits (Figure 6-43b). The remainder of the channel fill is com-posed of mudstone and sandstone beds in varying proportions that exhibit a characteristicupward decrease in dip magnitude (on borehole-image logs obtained from the wells behind theoutcrop) (Figure 6-43c; Table 6-10), and that onlap and overtop the channel margin(Figure 6-43b). Sandstones are predominantly Tb and Tc, and finer-grained beds are predomi-nantly siltstone. The percentage of sandstone varies laterally as well as vertically (Table 6-10).

This example of a leveed-channel complex within a mixed sand-mud system showsmany of the features described for coarser-grained systems and suggests that the processes aresimilar, irrespective of the type of sediment input. Channel margins are complex; channel-fillstrata, in this case mainly siltstone and fine sandstone, onlap the channel margins but are notobviously connected across the margin to adjacent levee beds. Also, these outcrops provide anexample of a systematic, upward decline in the dips of beds within a channel fill. This upwarddecrease is not as systematic in coarser-grained channel fills; it might be a result of lower lev-els of depositional energy and less erosive ability of finer-grained fill, so that depositionalpatterns remain intact during channel filling.


6-227

Figure 6-42. (a) Outcrop photograph of the Miocene Mount Messenger Formation’s leveed-channel deposits,Pukearuhe Beach, New Zealand. The horizontal surface just beneath the vegetated ground surface is the baseof a Pleistocene raised terrace deposit; the Mount Messenger Formation is beneath this surface. Note thewaterfall. The beach is a black sand (magnetite-rich) deposit; the outcrop sitting at the beach level to the left ofthe waterfall is a conglomerate that is the basal portion of a channel fill. (b) Line drawing of the outcrop,showing the base of channel and individual thinning-upward, thin-bedded intervals. After Browne and Slatt(2002). Reprinted with permission of AAPG.

VegetationQuaternary Deposits Quaternary

DepositsVegetation

Waterfall

Channel Base

BeachBeach

onlap

onlap

thinningupwardcycle

shellymassive

sandstonechannel-filling in traformational& basement clast conglomerate

(Pukearuhe Conglomerate)

Bedding077/08SE10 meters

thinningupwardcycle

thinningupwardcycle

NE SW

a

b

Table 6-10. Lithofacies associations, Mount Messenger Formation: Pukearuhe Beach.

Associa-tion

Sand (%)

Dominant sandstone

type

Bedding geometry

Dipmeter and borehole-image-log

faciesComments

Channel 25–80%Tb or Tc (can include conglomerate)

Large-scale scour with multiple scour and fill

Upward-decreasing dip magnitude and variable orientation

Fill drapes and onlaps base; basal portion often dominated by siltstone

ProximalLevee

20–80%

Tc (climbing ripple) domi-nant; Ta and Tb sandstone less common

Medium-scale erosional surfaces common

meter-scale upward decreasing dip with vari-able magnitudes and orientation

Scour surfaces often lined with siltstone

DistalLevee

15–50%

Thin Tb and Tc sandstones; Ta sandstones rare

Small-scale erosional surfaces common

Low-angle dips with little variation in dip magnitude or orientation; little variation in gamma

Siltstone-dominated; relatively thin sand-stone beds


6-228

Figure 6-43. (a) Map view of Pukearuhe Beach, northwest coast of the north island of New Zealand. Green is aflat, raised terrace underlain by Pleistocene gravels (Figure 6-42), beneath which the Mount Messenger depos-its occur. Yellow is the beach (Figure 6-42). The two red dots show the locations of two boreholes drilled behindthe outcrop. (b) Close-up view from beach level of the boundary (lower red line) between thin-bedded channel-fill and thin-bedded proximal-levee facies. Note the waterfall, which also is shown in Figure 6-42 for reference.(c) Dipmeter logs from (a) the central well, showing the upward decrease in dip magnitude of the channel fill(also visible in the outcrop by the onlap pattern of the fill against the channel wall) and the low-angle dips ofunderlying distal-levee beds.

176 E 178 E

37 S

39 S

41 S

Drill siteN

orth

Isla

nd

New Plymouth

Auckland

Wellington

gaspipelines

farmtrack

20-50m

high

cliff

metres

0 100

conglomerate outcrop(Pukearuhe Conglomerate)

waterfall

PUKEARUHEBEACH

accessonto beach

To NewPlymouth

PukearuheRd

gaspipelines

NORTHERNHOLE

CENTRAL HOLE

04

07

03

Seism

icLine

Pleistocenefluvial Channel-fill

Proximal levee

b c

0 10 20 30 40 50 60

0 10 20 30 40 50 60

Degrees

Degrees

20.0 120.0GR

20.0 120.0GR

5m

Channel-fill

Distal Levee

Proximal levee

a


6-229

Upper Cretaceous Dad Sandstone Member, Lewis Shale, Wyoming, U.S.A.: A sandy/mud-prone system

The lower Maastrichtian Lewis Shale and its associated Fox Hills Sandstone have beenstudied since the late 1990s, because theirs is one of the few outcrops of fine-grained, delta-fedleveed-channel systems that can be used as a scaled analog to fine-grained, leveed-channelreservoirs (Pyles and Slatt, 2000). These strata were deposited in the foreland basin of theWestern Interior Seaway. Several geological and geophysical techniques have been applied todocument the 2D and 3D architecture and facies distribution of this system for application toanalog reservoirs (Young et al., 2003). Pyles (2000) and Pyles and Slatt (2000) have proposeda detailed third- and fourth-order sequence stratigraphy of the Lewis Shale–Fox Hills Sand-stone. Based on correlations of shallow- to deepwater clinoforms, minimum (uncompacted)water depths in the basin were no more than 500 m (1640 ft) but deepened with time (Pylesand Slatt, 2000; Gonzalez, 2003).

An outcrop, informally named Spine 1, provides a cross-sectional view of a leveed-channel complex similar to those imaged from 3D seismic (Figures 6-44, 6-45). Spine 1 pro-vides a cross-sectional view of nine channel sandstones and their intervening thin-beddeddeposits, forming a 0.5 km (0.3 mi) wide outcrop belt interpreted to be a leveed-channel com-plex (Figures 6-45 and 6-46) (Witton-Barnes et al., 2000; Van Dyke, 2003). The fill of thelowermost channel (Channel 1 in Figures 6-46 and 6-47) is 150 m (500 ft) wide in outcrop andrepresents an oblique strike section through a sandy channel fill. This cross-sectional fillexhibits an asymmetric distribution of sedimentary facies, with interbedded shale-clast con-glomerates and high-density, sandy turbidites or debrites on one side, and cross-bedded,traction current sandstones and massive sandstones on the other side (Figure 6-47). Clasts

Figure 6-44. 3D perspective of a deepwater channel in the northeastern deep Gulf of Mexico. The channel hasa sinuous course and raised channel fill caused by differential compaction (Chapter 5). Note the scallopededges of the outer channel margin, suggesting sliding of levee strata into the channel. After Posamentier andKolla (2003). Reprinted with permission of Henry Posamentier.

Note small slump scars

Note raised channel fill due to differential compaction


6-230

Figure 6-45. Depositional model for the Dad Sandstone Spine 1-2 area. Two sheet sandstones thatoutcrop there are shown as two yellow bars underlying the leveed channel deposits. Figure modi-fied from Mayall and O'Byrne (2002) and Beaubeouf (2004) to fit the Dad Sandstone model.

Figure 6-46. (a) Topographic map showing the location of Spine 1 of the Dad Sandstone Memberof the Lewis Shale. Nine channel sandstones are shown by the black areas. The beds dip approxi-mately 12o to the southwest, perpendicular to the orientation of the spine. Location of the CSMStrat Test #1 well is also shown; it was positioned to drill through the dipping channel strataexposed in outcrop. (b) GOCAD model of Spine 1 (green), showing the distribution, in 3D space,of the nine channel sandstones. (c) The northwest face of Spine 1, showing six of the nine channelsandstones. Note the southwesterly dip of Channel Sandstone #1 beds. Channel sandstones arevertically separated by thin-bedded mudstones/fine sandstones. After van Dyke (2003).

Conceptual diagram modified from Beaubeouf (2004) and Mayall and Stewart (2000)

1km

70m

Scales of heterogeneity Dimensions Applications to drilling

(I) Leveed-channel system 1-2km wide Channel complexes separated by

levees; not connected.

(II) Leveed channel complex 0.5km wide; Channel sands not connected.

50-70m thick

(III) Leveed-channel sand <0.5kwide Internal variability in facies

3-12m thick and reservoir quality.

p

6800Spine 1

Spine 2

0.80 km

6725

N

Location MapSection 25, T16N-R92W

NW

1 2 3 4 5 6

1234

6

78

910

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Figure 6-47. (a) Close-up of Channel Sandstone #1 (Figure 6-46), showing the base of channelsandstone (based on shallow boreholes, ground-penetrating radar, and electromagnetic inductionimaging). c = south side of the channel fill, which is composed of cross-bedded and massive sand-stones. d = north side of the channel fill, which is composed of interbedded shale-clast conglomer-ates and turbidites/sandy debrites. (b) Location of Channel Sandstone #1 (yellow) in 3D space. (c)Cross-bedded sandstone. (d) interbedded shale-clast conglomerates and turbidites/sandy deb-rites. The conglomerate clasts are composed of thin-bedded mudstones/fine sandstones. After VanDyke (2003).

within the shale-clast conglomerates are composed of interbedded, thin sandstones and shalessimilar to those of associated levee deposits, indicating erosion and sliding of blocks of origi-nal levee beds from the steep side of a channel into the channel, as has been imaged in subsur-face leveed-channel analogs (Figure 6-48). A seismic image of a probable subsurface analogmay display the same two facies as those that are seen in outcrop (Figure 6-48). Reflections onthe steep side of the channel wall dip into the channel and may represent slide blocks of leveestrata. By contrast, the lower dip angle of reflections on the shallower-gradient side of thechannel may represent cross-bedded to massive sandstones.

Ground-penetrating-radar (GPR) lines acquired behind the outcrop, where it dips intothe subsurface, reveal lateral accretion surfaces corresponding to the cross-bedded sandstones,which are interpreted to be laterally migrating, in-channel bars (Figure 6-49a) (Young et al.,2003); lateral accretion packages also are commonly observed at the multifold seismic scale inother fine-grained, leveed-channel systems (Figures 6-11, 6-19, and 6-49b) (Abreu et al.,2003). GPR has revealed the presence of erosional remnants of thin-bedded levee strata withinthe fill of Channel 1, which can also be seen in nearby outcrops (Minken et al., 2004)(Figures 6-49a, 6-50a); this remnant is capped by onlapping and overtopping channel sand-stones. Similar features also can be observed on exploration seismic-reflection records(Figure 6-50b) (Minken et al., 2004).

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Figure 6-48. (a) Seismic-reflection profile of a leveed-channel interval. (b) Channel Sandstone #1(Figure 6-47). The steeply dipping reflections near the top of the right side of the channel fill onthe seismic line are interpreted as slide deposits and perhaps are analogous to the shale-clast con-glomerates on the “cutbank” north side of the outcrop. The shallow-dipping reflectors on the leftside of the channel fill, near its top, are interpreted as possible cross-bedded to massive sand-stones on the “point bar” side of the channel. Note the similarity in horizontal scale between theoutcrop channel fill and the channel on the seismic line. Seismic line from the western Gulf ofMexico.

The asymmetric distribution of facies (Figure 6-47), the asymmetric profile of Channel1 (Figures 6-47a and 6-51b), and the image of an apparently sinuous bend to the channel sand-stone (using electromagnetic induction; Figure 6-51c) all indicate that Channel 1, andpresumably the eight overlying channel-fill sandstones in this succession, are sinuous innature, perhaps within a master channel (Figures 6-40b and 6-45). In outcrop, only two of thechannel fills appear to be amalgamated (channel sandstones 4 and 5 have been combined tochannel sandstone 4 in Figure 6-47b), with the remainder being separated by fine-grained,thin-bedded strata that are interpreted mainly to be levee beds arranged in a manner similar tothat shown in Figures 6-40b and 6-45.

At a nearby outcrop, informally named Rattlesnake Ridge (Bracklein, 2000), the leveebeds are separated from adjacent channel-fill sandstones by a significant truncation surface(see Figure 6-52a for the orientation of this outcrop with respect to channel fill and leveebeds). A GPR line reveals thin levee beds in slide deposits overlying the base of the channeland channel-fill sandstones onlapping the channel margin (Figure 6-52). This relationshipindicates that the levee beds were deposited and had sufficient time to become partially lith-ified and then slide into the channel before the channel was filled. As we discussed above, a

~150 m

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Figure 6-49. (a) Ground-penetrating radar (GPR) line across Channel Sandstone #1, Spine 1,showing the base of the channel fill (red dashed line), an erosional remnant of muddy levee strata,onlap of beds onto the erosional remnant, a slide block, and lateral accretion packages. AfterYoung et al. (2003). (b) Lateral accretion packages from a subsurface seismic line. After Abreu etal. (2003).

significant time gap between levee aggradation and channel filling has also been proposed forthe Cerro Toro Formation (Beaubouef, 2004).

Similarities between the Dad Sandstone Member’s outcrops and sinuous leveed-chan-nel-complex reservoirs indicate that a similar degree of vertical compartmentalization ofindividual channel sands, lateral variability of internal channel fill, and possible compartmen-talization between channel fill and adjacent levee beds, can be anticipated in the reservoir.Lack of communication between channel and levee sands in Gulf of Mexico reservoirs hasbeen noted by Kendrick (2000) in the Tahoe field (Chapter 7).

Characteristics and processes of the formation of sinuous leveed-channel systems

In the following section, we discuss some of the generalized but diagnostic features andinterpreted processes of leveed-channel formation, because they are unique to this type ofdeepwater system.

Morphology and origin of sinuosity

Sinuosity in deep-marine channels has been recognized routinely from 3D seismic andoutcrops for the past decade (Figures 6-2, 6-9, 6-11, 6-18, 6-26, 6-44, and 6-53). Various the-ories have been proposed to account for this common feature. Several factors are probably

Lateral accretion surfaces

Slump

Onlap

Erosional remnant of levee beds20 nanoseconds

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a

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Figure 6-50. (a) Part of the Spine 2 outcrop (Figure 6-46a), showing a thin-bedded erosional rem-nant with onlapping channel-fill sandstone, similar to that imaged on Spine 1 (Figure 6-47a).(b) Seismic-reflection line from the Gulf of Mexico seafloor, showing a leveed-channel depositcontaining erosional remnants at its base. After Minken et al. (2004).

Figure 6-51. (a) Northwest face of Spine 1, showing the position where Channel Sandstone #1ends (red rectangle); to the southwest is a thin-bedded facies juxtaposed against the sandstone.(b) Flattened and unflattened GPR line, in the vicinity of the red rectangle, which shows the baseof the channel and the location of the pinch-out of the channel sandstone. (c) Horizontal electro-magnetic induction image, showing a right-angle bend in the sandstone (dark) as it dips into(toward the southeast) the outcrop at the point where the sandstone pinches out in (b). Theseimages demonstrate the sinuous nature of Channel Sandstone #1, from a northeast-southwesttrend to a southeast trend.

Inline 4915

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Figure 6-52. (a) Channel sandstone at Rattlesnake Ridge. The sandstone ends abruptly where itforms an erosional contact with thin-bedded levee deposits (area of P5 on the figure); the depositsare not exposed because of a thin soil cover. The location of GPR line 26 is shown as a red dashedline. (b) GPR line 26 showing—from left to right—horizontal, transparent radar reflections rep-resentative of the thin-bedded levee deposits, transparent radar reflections dipping (slumped) tothe right (into the channel), and near-horizontal reflections representing the boundaries betweenindividual channel-sandstone beds. The sandstone beds are clearly onlapping the dipping,slumped levee beds, indicating deposition of the levee beds followed by slumping and then fillingof the channel with sandstone. (c) Fresh contact between the channel sandstone and the gray-col-ored levee beds. Just beneath the erosional contact, the levee beds dip toward the lower right, asdepicted on the GPR line. After Young et al. (2003).

Figure 6-53. Photograph of the upper Miocene Solitary Channel, Tabernas Basin, southernSpain, showing lateral-accretion channel-fill deposits. Photograph is draped over a laser-gener-ated terrain model. Figure is courtesy of David Jennette.

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responsible for sinuosity, including bathymetric irregularities on the seafloor, the gradient ofthe seafloor, the sediment concentration, the nature of the delivery system (ignitive versusnonignitive flows), the volume of the flows, and the frequency and velocity of the flows thattravel through the channel (Clark and Pickering, 1996).

At present, two general theories have been proposed concerning the origin and migra-tion patterns of sinuous channels. One theory considers sinuous submarine channels to beanalogous to fluvial channels, even though the former are deposited in a different medium(seawater) from the latter (freshwater) (Abreu et al. 2003). The other theory considers sinuoussubmarine channels to differ significantly from fluvial channels in terms of their formativeprocesses (Peakall et al., 2000).

Abreu et al. (2003) suggest that sinuous channels migrate both laterally (termed“swing”) and in the downcurrent direction (termed “sweep”). Their conclusion is based par-tially on the presence of lateral-accretion packages, which have been documented in severaloutcrops and appear to be shingled, and on downlapping seismic reflections on the insidebends of sinuous channels (Figure 6-49b) (Mayall and Stewart, 2000). Accordingly, Abreu etal. (2003) believe that channels form by the systematic erosion of outer channel banks anddeposition along inner banks, during both lateral and down-current migration of the channel,much the same as is the case in fluvial meandering channels. Seismic images of channel fillalso are part of this interpretation. Once a bend is initiated, sediment from channel walls slidesinto the channel thalweg (Figures 6-11, 6-44, 6-47, and 6-48), and that would tend to block anddivert flows within the channel, thereby leading to sweep and further bend development. Thislatter process probably requires the presence of levee sediments while the channel is open tobypassing sediment flows, which indicates that deposition upon levees and within channels isnot synchronous. The outcrop characteristics of the Lewis Shale, described above, support thisinterpretation, as do slump scars on the outside bends of shallow subsea images (Figures 6-11,6-44).

By contrast, Peakall et al. (2000) claim that sinuous channels do not migrate down-stream but instead migrate laterally until an equilibrium point is reached, at the channel bendbeyond which lateral migration ceases, and the channel position stabilizes. Analysis withclosely spaced 3D seismic time slices of a sinuous channel system offshore West Africa sug-gested that there was neither significant lateral migration of bends nor downcurrent migrationas occurs in meandering fluvial systems. Peakall et al. (2000) proposed a three-stage model forbend development: (1) an initial point-bar develops in association with bend growth (swing),and lateral accretion deposits are preserved because of the lack of sweep, next (2) equilibriumdevelops, whereby the channel aggrades and neither swings nor sweeps, and finally (3) thechannel is abandoned and either fills with fine-grained sediment or remains open. Stage-onesweep occurs about a nodal point that remains in the same position throughout benddevelopment.

These two different interpretations of the origin of channel sinuosity illustrate that thereis no unanimity on the issue. Clearly, more work needs to be done to better understand the ori-gin of this complex pattern. Whether these two interpretations are compatible or incompatibleremains to be seen.

Sedimentary processes within sinuous channels

Unique processes associated with the development of leveed-channel systems give riseto significant internal complexity, both within the channels and outside the channels. Becausethis complexity may profoundly affect reservoir performance, we discuss the formative pro-cesses here.


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Levees form by overbanking or overspill of a portion of channelized sediment gravityflows as they move down the channel axis. “Flow stripping” is a process that has been calledunique to sinuous submarine channels (Piper and Normark, 1983; Peakall et al., 2000; Posa-mentier et al., 2000)—the process of separating the components of a sediment gravity flow asit travels within a sinuous channel (Piper and Normark, 1983). This process occurs along theoutside bends of sinuous channels, where turbidity-current flows accelerate, similar to theflow in fluvial channels. High-velocity sediment gravity flows are unable to negotiate thebend, thereby causing levee breaching and sediment deposition in the form of splays on theoutside of the levee, immediately downcurrent from the bend (see Chapter 7). Flow velocitynormally would be insufficient for all sediment to overtop the levee bend, so the coarser-grained fraction of the flow remains within the channel and the finer-grained sediment isstripped out and transported to an extra-channel or levee location.

Although flow stripping accounts for formation of deposits along the outer bends ofchannels, this process does not account for the formation of levees throughout their entirety,including alongside straight stretches of channels. Peakall et al. (2000) have suggested thatlevees can form more uniformly by a process of overspill. An overspill flow is a more spatiallycontinuous fluid wedge or cloud of fine-grained sediment that is suspended over the coarser,channel-confined sediment as it moves in the down-channel direction. This wedge spreads lat-erally as it moves down channel, depositing levee beds on both flanks of the channel.Confinement of the coarser-grained fraction within the channel limits mixing between the sed-iment gravity flow and ambient seawater. As a result, in-channel flows are better able tomaintain their density contrast with seawater; they therefore travel farther basinward andtransport more sediment than they would if they were unconfined. Recent experimental workby Keevil et al. (in preparation) has demonstrated the overspill process in a submerged, artifi-cial, sinuous channel. A strong secondary circulation pattern (oblique to downcurrent flow)was observed which creates signficant upwelling and overspill at the outer bend of the channelmargin. This sense of flow rotation is opposite to that found within meandering river channels.One result of overspill is the formation of sediment waves (Normark et al., 2002) atop theoverbank surface.

Observations of variable seismic amplitudes on 3D seismic horizon slices (Figure 6-19)suggest that sedimentary deposits within sinuous channels are not continuous. Instead, discon-tinuous mounds or pods of sediment occupy the floor of sinuous channels at any one timeduring their formation (Kolla et al., 2001). Peakall et al. (2000) stated that these mounds orpods occur preferentially along the outside bends of the channels, and the authors suggestedtwo possible reasons for this occurrence. The first reason takes into consideration the depth-averaged velocity equation of a turbidity current U2 = kCh, where flow velocity = U, sedimentconcentration in the flow = C, height or thickness of the body of the flow = h, and k is a con-stant. Applying this equation to the flow-stripping process dictates that flow velocity willdecrease as sediment overtops the outside levee bend and as sediment concentration (C) andthickness (h) both decrease. Decrease in velocity promotes deposition of the basal sedimentsas mounds or pods within, but preferentially alongside, the outer bend of the sinuous channel.Deposition of lateral accretion packages requires a reduction in current velocity as a flownegotiates the bend in a channel, so that net deposition occurs on the inside of the bend (Abreuet al., 2003).

However, Peakall et al. (2000) claim that in experimental flows, both C and U are strati-fied, so that the depth-averaged equation is too generalized to explain depositional processeswithin sinuous channels. Thus, Peakall et al.’s (2000) second, and favored, explanation is thatthe mounds are slides resulting from frequent failures of outer channel-margin strata(Figures 6-11, 6-44). Channel-margin failures may be concentrated on the outer channel bendsbecause of higher shear stresses and steeper slopes there. Slides of this nature have also been


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observed from a submersible vessel in the modern Coronado Submarine Canyon, offshoreCalifornia (J. Warme, personal communication, 2000).

An unresolved problem is that sediment gravity flows within leveed channels have longbeen thought to be short-lived, sporadic flows. Elliott (2000) has suggested that hyperpycnalflows generated during lowered sea level provide a means of transporting large volumes ofsediment to the deepwater environment in a quasi-continuous manner. Sediment eroded fromthe outer bends of channels provides an additional source of internal sediment within the chan-nel. However, Mulder et al. (2003) state that insufficient sediment concentration and a lowvertical concentration gradient, coupled with small grain size (smaller than medium sand) andlow flow velocity, probably preclude most hyperpycnal flows from being significant to theflow-stripping process (i.e., the entire flow remains in the channel).

The nature of the channel margin

The origin and timing of channel filling relative to levee development in leveed-channelsystems are not well understood. As we described above in this chapter, some levee beds arelinked to channel strata and others are not. Linkage provides evidence of contemporaneouslevee development and channel filling. The Rosario and Grès d’Annot Formations do showevidence of linkage, as do strata in the Bell Canyon Formation of west Texas (Dutton and Bar-ton, 1999) and Fan 5 of the Skoorstensberg Formation of South Africa (Kirschner and Bouma,2000). However, in these cases, usually only one or a few beds are visibly linked. By contrast,nonlinkage is documented from (1) outcrops of the Cerro Toro Formation, the Mount Messen-ger Formation, and the Dad Sandstone Member described above, (2) high-resolution seismicprofiles of the Indus Fan (Arabian Sea) and Benin-major (Niger Delta slope) channel-leveesystems (Deptuck et al., 2003), and (3) seismic-reflection profiles and production tests of sub-surface leveed-channel deposits (Kendrick, 2000; Posamentier and Kolla, 2003). In theseexamples, the channel-margin transition zone is complex, consisting of slump scars, slides,and mud linings, all of which can prevent connectivity between channel fill and levee beds.

To summarize this discussion, both linkage and nonlinkage of levees and channel fillseem to be possible and seem to be related to processes associated with levee and channeldevelopment in sinuous systems. This conclusion also raises the question of whether levee andchannel-fill depositions are contemporaneous. Where the two facies are linked, deposition isclearly synchronous. Where the two facies are not linked, a long time interval is apparentlyrequired for the walls of channels, which are composed of levee strata, to slide into the chan-nel. The other possibility, proposed by Gardner and Borer (2000) and Coleman (2000), is thatfor systems that lack linkage, the thin beds are not levee deposits, but instead represent deposi-tion in an unrelated setting and time period prior to incision (Figure 6-40a).

Reservoir implications of outcrop characteristics

Lateral continuity

As is the case with sheet sands and sandstones (Chapter 8), thickness and lateral changesin thickness of individual beds and packages of beds are critical parameters in the performanceand ultimate recovery of petroleum from a channel-fill reservoir. The greater the thickness andcontinuity of reservoir sandstones, the larger the drainage area that can be covered by a singleproducing well and the fewer the wells that will be required during primary and secondaryrecovery. Amalgamation of individual sandstone beds also can significantly improve verticalconnectivity and thus the economy of producibility.

The lengths (or widths) of channel sandstones are difficult to determine in outcrop, butthey are easier to determine than those of sheet sandstones (Chapter 8). This is because of the


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lenticular nature of channel-fill beds, such that outcrops need not be as laterally extensive to beable to encompass the width of an entire channel sandstone, including its margins and its rela-tion to adjacent extra-channel strata. Because of the pronounced effect of erosion on theultimate length and width of channel sands at the various scales, it is not particularly beneficialto calculate the rates of changes of bed thickness and continuity, in contrast to the case of sheetsands and sandstones.

This relationship has been demonstrated in several of the outcrop examples describedabove. Nevertheless, the orientation of the outcrop with respect to the sediment transportdirection often is not known or reported. An outcrop section that is oblique, rather than per-pendicular to the axis of sediment transport, will expose an oblique channel section that iswider than would be the case if the section were truly perpendicular to the channel axis. In thisregard, 3D seismic horizon slices can be particularly useful in determining true dimensionsand orientations of seismically resolvable channel features.

As we discussed in the examples above, there are a number of hierarchical levels ofchannel deposits and a wide range of lithofacies that fill the channels. This hierarchy hasimportant implications for predicting reservoir performance and optimizing well placementand orientation. Thus, identification of the hierarchical levels in both the subsurface and inoutcrop ultimately is important for reservoir development. If outcrops are not sufficiently longor thick, it may be difficult to accomplish this.

The ratio of width to thickness, or the aspect ratio, varies among the various outcrop andsubsurface examples presented above. This is because of a combination of factors, including(1) the hierarchy of the scale measured, (2) the extent of erosional truncation of the flanks ofchannel sandstone bodies, (3) the width of low areas between compensationally stacked, olderchannel fills, and (4) the dimensions of underlying topography, including slide remnants, thatproduce master channels. The aspect ratio and lateral continuity resulting from these, andother factors, are complex and not very predictable, mainly because of the extent of scour-and-fill processes in this environment. In pebbly and sandy channel-fill systems without muchshale, this is probably not important, because contacts between separate sands or sandstonesare amalgamated. However, the more mud that is present in the system (mixed sand-mud tomud-prone), the more serious becomes the problem of lateral continuity. The extent of thisproblem is difficult to evaluate in the subsurface, when only limited data are available.

Vertical connectivity

Vertical connectivity is another critical factor that affects production performance ofchannel-sand and sandstone reservoirs. As the above examples show, channel sands and sand-stones may be amalgamated at the bed and channel scales, thereby giving rise to good verticalconnectivity, or they may be separated by discontinuous shale drapes and more extensive late-stage muddy fill.

The presence of shales is most important because of its effect on the amount of verticalconnectivity of coarser-grained beds within channel fill (i.e., in the proportion of sand-on-sand[amalgamated] contacts). Various types and scales of shale beds are associated with channel-fill deposits. In pebbly and sandy systems, the proportion of sand-on-sand contacts is usuallyquite high. However, the more shale there is within the system, the fewer such contacts therewill be, and lateral trends along bed boundaries probably will be quite variable.

At the larger scale, shales that cap coarser-grained strata may be late-stage channel filleither because of a reduction in sand supply in the source area or because of channel abandon-ment (for example, the Point Lobo Canyon and Jackfork examples discussed above). If thechannel fills, and there continues to be no significant sand supply, laterally continuous shaleswill be deposited over and beyond the channel fill. Although these laterally extensive shales


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may form a minority of shales within a channel-fill system, they are the ones that can compart-mentalize a reservoir vertically. This is particularly important if the shales are greater than thereservoir is in areal extent.

At the smaller scale, shales infill scour depressions on the tops of sandstone beds andmimic the lenticular or irregular geometry of the surface upon which they were deposited (forexample, the Jackfork channel fill described above). These shales might be deposits from thefine-grained tails of turbidity currents and/or the product of hemipelagic suspension settling.Although the shale beds are discontinuous, they still might have a pronounced effect on fluidflow by acting as baffles and creating tortuous lateral and vertical fluid-flow paths, particularlyin oil reservoirs.

Textural, compositional, and structural characteristics of channel-fill sands and sandstones

As the selected outcrop examples show, a variety of grain sizes can occur within channelfill. There is no apparent relationship between grain size and the dimensions of the channelfill, as is evidenced by shale filling both large and small channels and sands filling both largeand small channels. On the basis of outcrop studies, several authors (Slatt et al., 1994; Slatt etal., 2000; Witton-Barnes et al., 2000) have suggested that an abundance of shale rip-up clastswithin sandstones can be a useful criterion for distinguishing channel fill from sheet sand-stones. Shale clasts are present in channel-fill reservoirs, such as that in the Pliocene A-50Sand (Sullivan and Templet, 2002), the 8500-ft Sand in the Garden Banks area (Fugitt et al.,2000), and some of the outcrops described above. However, as our other examples show, somechannel-fill sandstones are composed of thick-bedded, laterally continuous, massive to BoumaTa amalgamated beds that can be mistaken for sheet sandstones in an outcrop in which the lat-eral termination of the channel fill cannot be observed, and in the subsurface seen viaborehole-image logs or cores.

Other facies that are prevalent in channel-fill sandstones include (1) slide blocks thatoriginated from the channel margins and are composed of thin erosional remnants of beddedlevee strata, (2) in situ erosional remnants of levee beds, and (3) debris-flow deposits in whichclasts are also composed of thin-bedded levee strata.

A long-recognized and common feature of channel fill is the vertical fining-upward suc-cession of strata. This feature appears to occur at the various hierarchical scales, as the outcropexamples demonstrate.

Identifying channel-fill sands and sandstones in cores and borehole-image logs

The sedimentary features reviewed above can all be used to identify channel-fill sandsand sandstones in cores and borehole-image logs. However, it is usually more difficult tointerpret channel-fill deposits unequivocally, in contrast to sheet deposits (Chapter 8), becauselateral variations and stratigraphic relations cannot be observed in logs the way they can inoutcrops. Often, such features are also beneath seismic resolution. Criteria that distinguishchannel-fill sands and sandstones from sheet sands include (1) the existence of erosional basesof sand beds (Figure 6-54), (2) the presence of shale clasts in channel-fill sands (Figures 6-55through 6-58), (3) the interbedding of debrites and shale-clast conglomerates with turbidites(Figure 6-55), and (4) the abundance of chaotic-appearing features (as opposed to the morelayered nature of sheet sands (Figure 6-56) (Slatt et al., 1994; Witton-Barnes et al., 2000;Browne and Slatt, 2002).


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Figure 6-54. Borehole-image log of a portion of the the Mount Messenger Central well(Figure 6-43), along with two core pieces at the boundary between channel fill and underlying dis-tal levee beds. Note the sharp change in dip angles at this boundary, from relatively high-angledips above the erosional surface of the channel base to almost horizontal dips beneath the sur-face. Core photographs provided by G. Browne.

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Figure 6-55. Close-up photograph of interbedded shale clast conglomerates (recessed) and sandy turbidites/debrites (lighter color) along the north side of Channel Sandstone #1 (Figure 6-46). Borehole image character-istics of similar strata of the Dad Member, drilled in a well approximately 13 km from the outcrop. Debritesexhibit shale clasts (black) in a sandy-muddy matrix, and sandy turbidites/debrites are more uniform andlighter in color. After Witton-Barnes et al. (2000). Reprinted with permission of the Gulf Coast Section SEPMFoundation.

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Figure 6-56. Gulf of Mexico Pliocene deepwater deposits interpreted as (a) sheet sandstones and(b) channel-fill sandstones. The different characteristics on these FMSTM logs are interpreted asbeing diagnostic of sheet and channel sandstones, respectively, in other deepwater deposits. AfterSlatt et al. (1994). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

a b


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Figure 6-57a. Well-log interval for the Diana 3 well, Diana Subbasin, illustrating core lithologies, interpreteddepositional setting, and well-log response.

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Bed TypeTa = massive sandstoneTb = planar-stratified sandstoneTc = current ripple-stratified sandstoneTd = planar-stratified shaly sandstoneTe = planar stratified to massive mudstone

R3 = normally graded conglomerateR2 = inversely graded conglomerateSdf = sandy debrite/slurry flowMdf = muddy debrite

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Figure 6-57b. Plain and ultraviolet light of the cored interval at 11,015–11,045 ft shown in Figure 6-57a. Thecored interval is composed of sharp-based, upward-fining channel-fill strata, including channel-axis, channeloff-axis, and channel-margin associations. After Sullivan and Templet (2002). Reprinted with permission ofthe Gulf Coast Section SEPM Foundation.

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DIANA 3 CORE 3: 11015.00 to 11045.00 (ft)11015.00 11020.00 11025.00 11030.00 11035.00 11040.00

11020.00 11025.00 11030.00 11035.00 11040.00 11045.00

b


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Figure 6-58a. Well log and (b) cored intervals in the “lower” Above Magenta in the MC 809-1 well, Ursa field,Gulf of Mexico. Sands in this interval are visualized on seismic data as a discontinuous event with evidence ofshingling. The sands in the “lower” unit display blocky gamma-ray and resistivity patterns on well logs, andthey are not easily correlatable over short distances between wells. The cores show the sands to be massive tograded, with evidence of internal scouring and erosion. These data suggest the “lower” Above Magenta reser-voir is composed of amalgamated sandy channels. From Meckel (2002). Reprinted with permission of the GulfCoast Section SEPM Foundation.

a

16740

16760

16780

16800

16820

16840

16860

Above Magenta

Cored intervals

depth corrected

Cored intervals

reported depths

DRES_CMP

.2 OHMM 20

GR_CMP

20 GAPI 120

MC809 -1


6-247

o

Figure 6-58b. Cored intervals in the “lower” Above Magenta in the MC 809-1 well, Ursa field, Gulf of Mexico.

16785

16786

16787

16788

16789

16797

16798

16802

16801

16800

16799

16791

16792

16796

16795

16794

16793

16790

16803 16797 16791

b


6-248

Examples of channel-fill reservoirs

This section summarizes the performance of eight fields or discoveries that have chan-nel-fill reservoirs. Our examples are taken from the better-documented fields in the literature.For each field, we describe the basic setting of the field, the trap, and the rates of production;we show an example of a seismic profile and log data; then we discuss the important pro-duction history of the channel fill. Note that most of these fields have multiple reservoirs—wedescribe only the channel-fill reservoir. A key point is that channel-fill reservoirs have widelyvarying performance as a result of several factors, including the various levels and types ofcomplexity described for outcrops. In nearly every case, the initial models of the reservoirwere fairly simplistic, but production revealed more complexity.

Forties field, blocks 21/10, 22/6a, North Sea, U.K.

Key references

Wills (1991); Leonard et al. (2000); Carter and Heale (2003); Rose et al. (2005).

Location

The Forties field is located 180 km (112 mi) east-northeast of Aberdeen, Scotland, pri-marily in the U.K. license block 21/10.

Significance

The Forties field was discovered in 1970 and brought on stream in 1974 via four fixedplatforms, with a fifth platform added 10 years later. During the early stages of field develop-ment, conventional, near-vertical production wells were used. Waterflood was begun early inthe life of the field. Although the presence of shales was recognized, the possibility of themforming reservoir compartments was not considered. But, soon after production commenced,it was recognized that the western end of the field was depleting more rapidly than the rest ofthe field. This was found to be the result of a large channel sandstone on thewestern side, which is separated from the rest of the field by a thick shale. Also, later in thefield development stage, after an effective waterflood sweep of the high-net:gross channelsandstones, production relied more on thinner-bedded facies that occur along the margins andon top of the high-net:gross channel sandstones (Figure 6-59). Oil and water were found to becomplexly interlayered, and a more comprehensive management plan had to be implementedto target these unswept sandstones and to allow for selective perforation.

Trap

The trap is a large compactional drape structure with four-way dip closure over highnet:gross channel sandstones.

Size

The field area is approximately 93 km2 (23,000 acres), with a gross rock volume of 6032million m3 (64,382 ft3).

Age and depositional environment

The sands are Paleocene in age. The field constitutes a minor part of a larger Paleocenesubmarine fan system consisting mostly of stacked sandstones derived from a shelf area to the


6-249

Figure 6-59. Schematic cross section of the Forties field, Central Graben, North Sea, showing the distribution of reservoir lithofacies. After Leonard et al.(2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

channel base - layered debris flow- low potential for baffle

upper channel- massive sand- largely swept

channel top - layered sand-shale - oil by-pass potential

channel margin - collapse debris- potential vertical barriers

channel levee - thin-bedded turbidites (well connected on km scale)

- opportunity for infill

abandonment - hemipelagic shale- high seal potential


6-250

west and deposited in a sand-dominated ramp depositional system. The sands were depositedin elongate, topographically controlled fairways during relative lowstands of sea level.

Hydrocarbons in place, production history, and well/field rates

The field has been producing oil since 1975. STOIIP has been calculated at 4.2 billionbbl, and original reserves were estimated at 2.5 billion bbl, for a recovery factor of 57%. As of2000, 2.4 billion bbl had been produced. Plateau production of 500,000 BOPD occurred in1981, and by mid-1989, production had declined to about 250,000 BOPD. The first 16 wellsproduced 1 billion bbl. Thirty-seven wells were then required to produce the next billion bar-rels. As of the year 2000, 61 wells were producing, only one of which was from the earlydrilling program.

Seismic and well-log expression

On seismic profile, the Forties field comprises externally mounded deposits that havesome internal downlap. On 3D seismic, the channels can be imaged clearly (Figures 6-17 and6-59). Wireline logs show a blocky gamma-ray pattern for most amalgamated channel-filldeposits. Channel-margin deposits appear as finer-grained and interbedded sandstone andshale patterns on logs. In the later stages of field development, different seismic attributes wereused to help locate the bypassed pay (Figure 6-60).

Thickness, lateral continuity, and aspect ratio

Gross thickness averages 354 m (1161 ft) and ranges from 199 to 469 m (653 to 1539ft). Lateral bed continuity is generally good.

Vertical connectivity and net:gross values

Net:gross averages 65% but is higher in the central parts of channel sandstones. Verticalconnectivity is generally good. Complexity, in the form of thinner beds and more shaleincreases toward the top and along the flanks of the reservoir.

Sedimentary texture, composition, and structures determined from core and bore-hole-image logs

Channel sandstones are composed mainly (80%) of fine- to medium-grained, massivesandstones. They are generally poorly lithified, with a low detrital clay content and only minorcement.

Reservoir quality

Porosity averages 27% and ranges from 10 to 36%. Permeability averages 700 md andranges from 30 to 4000 md. Porosity and permeability generally decrease with a decrease ingrain size—coarser-grained sandstones have porosities averaging 27% and permeabilitiesgreater than 300 md (and often >1000 md), whereas finer-grained sandstones have porositiesof 23–25% and permeabilities of 100–200 md.

Drive mechanism

Water injection commenced in 1976 and continues to the present. There is also a naturalbottom-water drive.


6-251

Figure 6-60. Two seismic attribute displays of the Forties field, Central Graben, North Sea, illustrating bypassed oil in reservoirs. (a) Lithologic imped-ance, and (b) fluid impedance. After Leonard et al. (2000). Reprinted with permission of the Gulf Coast Section SEPM Foundation.

Fluid impedanceLithological impedancea b


6-252

N Sandstone, Ram-Powell field, northern Gulf of Mexico, U.S.A.

Key references

Lerch et al., 1997; Kendrick, 2000; Craig et al., 2003.

Location

Ram-Powell field is located 220 km (135 mi) southeast of New Orleans, Louisiana, inthe eastern Gulf of Mexico continental slope, in Viosca Knoll block 956 (Figure 6-61).

Significance

The Ram-Powell field was discovered in 1984, and production began in late 1997. It was oneof the earliest discoveries in the northern deep Gulf of Mexico. Until recently, the N sand wasconsidered to be the deepest of the Ram-Powell reservoir intervals (Figure 6-62). Earlyappraisal wells encountered high net:gross values. Reservoir modeling suggested a single oil-water contact and reasonable continuity (Figure 6-63), indicating that there would be satisfac-tory sweep efficiency and pressure support. However, the first three development wellsencountered multiple fluid contacts updip of the presumed oil-water contact, even thoughsome degree of pressure communication was noted between wells. This indicated a high levelof discontinuity and compartmentalization of the reservoir interval. Thus, a horizontal drillingstrategy was employed to improve production (Figure 6-64).

Figure 6-61. Map of the various reservoir sands comprising Ram-Powell field. Stratigraphic position of thesands is shown on the type well log. The stratigraphic position has been reversed for each of the sands on themap, to better image their 2D distribution. After Craig et al. (2003). Reprinted with permission of the AAPG.

LOUISIANA

NEW ORLEANS

-3700

-350

0

-3900

-4100

-410

0

-3900

-3700

-3500

J

L

N

A6

A4

A3

A5

A1

VK 867 VK 868

VK 912 VK 913

VK 957

1000 m

RAM POWELLVIOSCA KNOLL 8 BLOCK UNIT

113 km

114

km

23GAS

29GAS

17OIL

25OIL

MID

DLE

MIO

CE

NE

VK

957

A-2

8T

2V

K912-2

VK

956-1

GR RES

Jra "HYBRID": AMALGAMATED SHEETAND CHANNEL-LEVEE(OIL AND GAS)

L CHANNEL-LEVEE(OIL AND GAS)

M CHANNEL-LEVEE(OIL)

N AMALGAMATED CHANNEL(OIL)

30 m

RAM POWELL VIOSCA KNOLL 956 FIELDCOMPOSITE TYPE LOG

NET PAY


6-253

Figure 6-62. Seismic-reflection profile of the Ram-Powell field, showing the individual reservoir sands (J, L,M, N; Figure 6-61) as separate seismic reflections. After Kendrick (2000). Reprinted with permission of theGulf Coast Section SEPM Foundation.

Figure 6-63. Ram-Powell N sand reservoir, showing the seismically defined area of the sand and adjacent shaleand the presumed oil-water contact. In 1989, the three wells shown were drilled, and all were oil-saturated.Additional wells were drilled until 1999, and almost all had oil-water contacts at different structural eleva-tions, indicating compartmentalization of the reservoir. After Kendrick (2000). Reprinted with permission ofthe Gulf Coast Section SEPM Foundation.

Ram/Powell FieldWest East

J sand L sand

M sand

N sand

4.0 s

4.5 s

Tertiary Cretaceous Unconformity

3363 ft

TLP

PR

O OTEN L N ACT A TI OIL WATE C T

1989

956-3

956-2ST2 957-1

TLP

PO OTEN L N ACT A TI OIL WATER C T

1989

A-5

A-2ST1

956-3A-2

A-8ST1

957-1956-2ST2

4000’


6-254

Figure 6-64. Vertical wells and a horizontal well drilled into the Ram-Powell N sand. Note that the horizontalwell penetrated three isolated, lenticular channel sandstones. After Craig et al. (2003). Reprinted with permis-sion of the AAPG.

Trap

The reservoir sands are encased in, and capped by, slope shales, and they pinch outupdip to form excellent stratigraphic traps on the flank of a subtle, south-plunging nose. Thereservoirs truncate against a salt diapir approximately 6.5 km (4 mi) downdip, to the south.

Size

The N sand exhibits a north-south, elongate geometry reflecting the canyon into whichthe sands were deposited (Figures 6-61, 6-63). Canyon dimensions are approximately 6 km x2.5 km x 50 m (3.6 mi x 1.5 mi x 164 ft).


Unlike fields in the western and central Northern Gulf of Mexico which are located inponded salt-minibasins, the middle Miocene Ram-Powell reservoir sands were deposited onthe slope and toe-of-slope environment within a lowstand systems tract. The N sand wasdeposited as amalgamated to nonamalgamated channel deposits within a preexisting subma-rine slope canyon (Figure 6-62). Significant topographic relief at the base of the canyonprovided for thicker accumulations of sand in topographic lows. Water trapped within sands inthese low areas could not be displaced by oil that migrated into the reservoir later, thus givingthe multiple fluid contacts.

A-5 A-5ST2 A-2ST1 956-3

18 m

12 m

6 m

0 m0 m 100 m 200 m 300 mSHELL DEEPWATER PRODUCTION

Amalgamated Channel FaciesA-5 A-2ST1 956-3

RAM POWELL VIOSCA KNOLL 956 A-5 HORIZONTAL WELLWELLS PROJECTED TO SUBSEA DEPTH


6-255

Petroleum in place, production history, and well/field rates

Total Ram-Powell reserves in 1997 were reported to be 250 million BOE, of which 30%

are in the N sand. The N sand is an undersaturated oil reservoir. Cumulative production to the

year 2000 was 12 million BOE. Because of the risk of compartmentalization by isolated chan-

nel sands, the trajectory of horizontal wells was designed to penetrate multiple sands (Figure

6-64). The strategy was to use two wells for stratigraphic control when drilling a horizontal

well. The two initial horizontal wells penetrated both oil- and water-bearing sands, making

them unsuitable as producers. A third well drilled between these two—a 725-m (2380-ft) hor-

izontal well—produced at a peak rates of 11,681 BOPD.

Seismic and wireline-log response

The N sand consists of one seismic reflection (Figure 6-62). The N sand reservoir is dif-

ficult to image on seismic because of overlying reservoirs and similarities in acoustic

impedance between water-bearing sands and capping mudstones (Figure 6-62). Wireline log

characteristics are also quite variable but often are expressed as blocky to slightly fining-

upward sand units separated by shales (Figure 6-63). Channel-fill sand units vary in thickness.

In some wells, they are amalgamated with few shales; in other wells, they may vary with from

one to four cycles of blocky to upward fining on gamma-ray log curves separated by shales.


Sand thicknesses are reported to be between 8 and 47 m (25 and 140 ft), with abrupt lat-

eral variations. Continuity is also quite variable, as is typical of channel-fill sandstones. Aspect

ratios for the reservoir are not reported.


Vertical connectivity is quite variable. Channel sands appear to be amalgamated in some

areas of the reservoir and not in other areas. Net: gross values also vary.


Sedimentary features of reservoir sands are not reported.

Reservoir quality

Pressure-transient analysis permeability is estimated at 300 md.

Drive mechanism

Aquifer support is negligible.


6-256

Garden Banks 191, northern deep Gulf of Mexico

Key references

Fugitt et al., 2000.

Location

The field sits in 230 m (750 ft) of water in Garden Banks Block 191, northern Gulf ofMexico, about 257 km (155 mi) southwest of Lafayette, Louisiana, U.S.A.

Significance

The field was discovered in 1977 by drilling through the 4500-ft sand (a sheet-sand res-ervoir discussed in Chapter 8). Gas production began in 1993 from the 4500-ft sand. It was notuntil 1990 that a well tested a deeper seismic amplitude anomaly, and the 8500-ft sand was dis-covered. This field provides an example of excellent gas production mainly because of goodvertical connectivity of individual reservoir sands, even though laterally discontinuous shalesare common. However, because of these shale breaks, recovery efficiencies would have beenconsiderably reduced if the reservoir were producing oil.

Trap

The trap has been formed by updip shale-out of sands.

Size

The field is about 2830 m x 1670 m (8500 ft x 5000 ft) in areal extent.


The sands are early Pleistocene in age. They were deposited during sea level lowstand asan upper slope channel complex in an intraslope basin created by salt withdrawal. The sandswere fed by shelf-edge deltas 16–24 km to the north.


During the period 1993–2000, the 8500-ft sand had produced 126 BCF from four wells:the A1, A2, A3, and A7. The combined flow rate of 150 million ft3 of gas per day has declinedsteadily through time. Production performance has indicated that the connectivity and continu-ity of sands is quite variable. For example, RFT pressures show that three individual sandstoneintervals (Members 3, 4, and 5, described below) are vertically connected and have behaved asa single tank. The uppermost two members (Members 1 and 2) are mutually connected but areseparated from the lower members.


The reservoir interval exhibits high-amplitude, dipping reflections, and a strong, flatgas-water contact (Figure 6-65). Well logs show clean sands at the base, which become thin-ner-bedded and shalier upward (Figure 6-65c).


6-257

Figure 6-65. Garden Banks 191 gas field. (a) Seismic-reflection profile showing the dipping beds, horizontal gas-water contact, and three wells drilledinto the reservoir. (b) Schematic diagram of the interpreted seismic line, showing the original gas-water contact and the various sand bodies comprisingthe reservoir. Note the multiple fluid contacts after production. (c) Five reservoir intervals in the field. After Fugitt et al. (2000). Reprinted with permis-sion of the Gulf Coast Section SEPM Fouundation.

-8500

-9000

1

2

3

4

5

1

2

3U

3M

3L

4

5

A18500 Sand

c

8500

9000 90009000

8500

12

3

4

5

G/W Aug 1998

Orig. G/WOrig. G/W

20

0’

400’

A7 A1 A3C C ’

Swept Zone

a

b


6-258


The 8500-ft sand is a 274 m (900 ft) thick succession that, overall, fines upward, but thatis subdivided into a number of intervals or members separated by shale breaks. Shale breaksare not thought to be continuous across the field.


The 8500-ft sand is informally divided into five members, on the basis of shale breaksand perched water contacts (Figure 6-65). From the top, downward these are termed Members1–5. Member 3 has been further subdivided into upper (U), middle (M), and lower (L)members based upon perched water levels (Figure 6-66). A variable pattern of water influxover time indicates that, overall, sands are in good vertical communication, even though Mem-bers 3–5 are separated from Members 1 and 2. Most shales are believed to act more as bafflesthan as barriers to vertical communication.


The lower part of the channel fill (Members 3–5) is dominated by 0.9- to 3.7-m (3- to12-ft-) thick, massive, fine- to medium-grained sands, based on core and borehole-image data.Rip-up clasts are concentrated at the bases of individual beds; the presence of these clastsresults in a shalier log response than is characteristic for these sands, and therefore the log-based reservoir quality tends to be underestimated. These beds are separated by periodic lami-nated to homogenous, thinner sandstones and siltstones. The upper part of the channel fill(Members 1–2) consists of thinner-bedded and finer-grained beds, which are interpreted to belate-stage leveed channel deposits (Figure 6-65).

Figure 6-66. Five reservoir intervals in the Garden Banks 191 gas field and a cross section through interval 3,which has been subdivided into 3U (upper), 3M (middle), and 3L (lower). Different reservoir pressures wereencountered in 3M, between wells A1 and A2, and different gas-water contacts were encountered betweenwells A2 and 2. These features all indicate that the 3M interval is compartmentalized, presumably by the len-ticular nature of the channel sandstones. After Fugitt et al. (2000). Reprinted with permission of the GulfCoast Section SEPM Foundation.

-8500

-9000

1

2

3

4

5

1

2

3U

3M

3L

4

5

A18500 Sand

-8500

-8500

-9000

-9000

A1 A2 2

3U

3M

3L

4

Different Pressures Different Gas/Water

Gas

Water GasWater


6-259

Reservoir quality

Thicker-bedded sands have porosities and permeabilities that are similar to the sheetsands described in Chapter 8; that is, they have 17–34% porosity and 1–2500 md permeability.Thinner-bedded, finer-grained sands have lower porosities and permeabilities, but still reach24–29% and 100–550 md, respectively.

Drive mechanism

The reservoir has produced from a combined pressure-depletion/weak-water-drivemechanism. Production performance has been excellent because of good vertical connectivity.

Andrew field, U.K. Sector, North Sea

Key references

Leonard et al., 2000; Jolley, 2003; Jolley et al., 2003.

Location

Andrew field is located in UKCS blocks 16/27a and 16/28 North Sea, 230 km offshorefrom Aberdeen, Scotland.

Significance

The field was discovered in 1974, approved for development in 1994, and broughtonline in 1996. It is small by 1980s North Sea standards, so the long time between discoveryand development was the result of a number of economic and technical uncertainties that wereminimized only after development of horizontal well technology. Drilling of 11 horizontaldevelopment wells (down from the 24 originally planned conventional wells) with high flowrates (averaging 10,000 BOPD) and low drawdown pressures (100 psi [690 kPa]), locatedfrom the crest outward to the flanks, provided the incentive to proceed in developing the oilrim. The field is an example of success achieved entirely with a carefully monitored, horizon-tal well development plan.

Some of the Andrew field’s success can be attributed to careful investigation of the fourcategories of potential heterogeneities: (1) variations in channel geometry; (2) shale barriersand baffles; (3) high permeability streaks; and (4) faults. An example of this investigation ismentioned here for shale barriers and baffles. The performance of a horizontal well throughthe oil column was simulated under scenarios of the presence and absence of a laterally contin-uous shale that might occur across the field (Figure 6-67). In the upper example, no shalebarrier is present, so there is significant gas coning into the heel of the well. In the lowermodel, the horizontal well has penetrated a continuous shale, which provides a sufficient sealto prevent early coning of gas into the wellbore. The results of the modeling provided the basisfor well design. Production logging of individual wells indicated that selective water wasunderrunning into high-permeability (500-md) sandstones (another of the potential heteroge-neities listed above) (Figure 6-68).


6-260

Figure 6-67. Two horizontal-well simulation models for the Andrews field, showing the effects ofthe absence (Sensitivity 1) and presence (Sensitivity 2) of a shale barrier between reservoir sands.The horizontal well is drilled into a relatively thin oil rim. The absence of a shale barrier resultsin drawdown of gas and early gas breakthrough into the wellbore. The presence of the shale bar-rier retards drawdown and allows unaffected oil production from the well. After Jolley et al.(2003). Reprinted with permission of the AAPG.

Figure 6-68. Schematic cross section along the A3 horizontal well in Andrews field, showing waterentry (blue) points along high-permeability (500-md) sandstones, interpreted from an RST logrun. The water has underrun the A3 shale in the B1 sandstone. Even though the shale barrier hasprevented gas drawdown (Sensitivity 2 in Figure 6-67), different permeabilities in underlyingsandstones result in uneven water breakthrough. After Jolley et al. (2003). Reprinted with per-mission of the AAPG.

Aquifer

Aquifer

Oil

Oil

Gas

Gas A1

A1

A2

A2

A3

Modeling Gas and Water Cones

Sensitivity 1:

A3 shale absentVertical gas cone at heel

Sensitivity 2:Continuous A3 shaleHeel protected by A3 shale,under-run at toe

A3

2300

2400

2500

2600

Northeast flank

Heterolithic unit

Perforations

Oil inflow points from 1996 production logging

Unknown fluid inflow points from 1996 production logging

Water inflow points from 1998 production logging

Gas cap

Oil column

Water cone

Continuous shale barrier

Discontinuous shale

Base of major channel unit

A1 sandA2.1, A2.2 low net-to-gross sands

Initial GOC 2496 mGOC Jul. 98 2506 m

Water coming throughB2 sandstone Water underrunning A3 shale

in B1 sandstone

Intra B2 shale B2s

OWC 2443 m TVDss

Crest


6-261

Trap

Hydrocarbon trapping is by a four-way dip closure over a simple dome structure abovean underlying Zechstein salt diapir.

Size

The field has a vertical relief of 140 m (460 ft) and an area of 13 km2 (5 mi2).


The reservoir is in the Paleocene Andrews Formation. Leonard et al. (2000) called it a“submarine fan sandstone with channelized geometries” ; these geometries were influenced bydepositional topography related to the underying salt diaper.


Original estimates of 262 million standard bbl original oil in place were later revisedupward to 315 MMB. As a result, reserves were also revised upward from 132 MMBO in1996 to 154 MMBO in 2002. Gas is estimated at 280 BCF. More than 70 MMB were pro-duced in 2.5 years while the field was at maximum flow rates, with an increase in rate from54.5 to 75,000 standard BOPD, and an increase in peak production of 1.5 years. Individualhorizontal well rates average 10,000 BOPD with reserves per well of 13 million bbl of oil. Therecovery factor also increased from 46% in 1996 to 49% in 2002 and is expected to rise to53% by additional drilling options.

Seismic and well log expression

No information is provided.


There is not much published information on these parameters. However, as we men-tioned above, it was anticipated that shales could be laterally continuous across much of thefield. Such shales range from 25 cm (0.8 ft) to 5 m (16 ft) thick, and account for less than 10%of the reservoir volume. The two main reservoir sandstones, A and B, are separated by a thickshale, called A3 (Figure 6-68). Several thinner shales subdivide the A and B sandstone (Figure6-68).


Net:gross is generally greater than 85% and as much as 97% in the main reservoir zone.There is excellent vertical connectivity of individual sands.

Sedimentary texture, composition, and structures determined from core and borehole-image logs

High-permeability (400- to 800-md-, presumably coarse-grained) sandstones occur atthe bases of channels as a lag or as loosely packed sandstones toward the top of the channelfills. These zones provide preferential zones for water movement through the reservoir (thiefzones).


6-262

Reservoir quality

Average porosity is 20% and average permeability is 200 md. Kv/Kh varies from0.00001 to 0.8.

Drive mechanism

Drainage is by horizontal wells.

Indian Draw field, southeastern New Mexico, U.S.A.

Key references

Phillips, 1987; Clark and Pickering, 1996.

Location

Indian Draw field is located in the southeast part of Eddy County, New Mexico, U.S.A.,within the petroliferous Delaware Basin.

Significance

The field was discovered in 1973. Erratic distribution of oil and water in the “A3” paysand is similar to that in other Delaware Basin fields and indicates that sandstones are complexand compartmentalized. Although this is a small, mature field, the significance here is thatdetailed sedimentologic and dipmeter analysis provided excellent documentation of the lentic-ular, discontinuous style of compartmentalization within this deepwater channel reservoir(discussed below).

Trap

Regional structural dip is toward the east-southeast, and sandstone stratigraphicallypinches out updip to the west.

Size

The total channel fill within Indian Draw field is approximately 2.5 km (1.5 mi) long,1.6 km (1 mi) wide, and 67 m (220 ft) deep.


The middle Permian (Guadalupian) Cherry Canyon reservoir sandstone at Indian Drawfield is one of several deepwater sandstones deposited in the Delaware Basin of west Texasand New Mexico, including the Brushy Canyon (discussed above) and Bell Canyon Forma-tions. Cherry Canyon sandstones in Indian Draw field are located about 16 km (10 mi) fromthe approximate downdip edge of the paleoshelf, which was a carbonate reef front. Variationsin thickness both within and outside the field boundaries indicate that the sands were depositedinside an erosional channel. It has been interpreted that high-discharge turbidity-current flowseroded the underlying basinal muddy substrate during a sea level lowstand, and later filling ofthe channel occurred from lower-discharge, more-sinuous flows.

The first sediments to be deposited on the channel floor were pelagic carbonates (“BigDolomite”) that blanketed the erosion surface. This was followed by deposition first of thelower “A1” sandstone, then of a zone of bioturbated sandstone and shale (“A2”), and finally of


6-263

the “A3” sandstone. Figure 6-69 illustrates the original, simplified, fieldwide correlation of thethree sandstones and a net-sand isopach of the A3 sandstone.

Detailed analysis of dip patterns (described below) led to subdivision of both the A3 andA1 sands into a series of discrete lenses that, in map view, form a curved pattern (Figure 6-70).The western margin of the channel is mapped as being steeper than the eastern side, as wouldbe the case for a cross section of a sinuous channel. Within the A3 sandstone, three individualsandstone lenses, each on the order of 5–10 m (15–30 ft) thick, occur progressively toward thewestern, outside bend of the channel, indicating that they are lateral accretion packages or in-channel bar forms (Figure 6-71b) (Abreu et al., 2003). Within the A1 sandstone, four individ-ual lenses, each on the order of 3 m (10 ft) thick, were mapped. They become progressivelyyounger from the north toward the south end of the meander bend.

These patterns are analogous to what we described earlier for the channel-fill deposits ofsinuous leveed-channels. Although Phillips (1987) describes the sandstones as having beendeposited within an erosional channel that was carved into basinal mudstones, it is possiblethat levee beds actually comprise part of this channel system and have not been recognizedbecause of a lack of core or borehole-image logs beyond the confines of the field.


Oil production is from the A3 sandstone, where porosities exceed 20%. Initial produc-tion from individual wells varied from 15 to 126 BOPD and as much as 31 BWPD.


Seismic data are not available from this field. Conventional well logs reveal sandstonesseparated by shale beds (Figure 6-70). Variability within the A3 and A1 sandstones is apparenton dipmeter logs and provides an additional means of subdividing and mapping the sandstone(Figure 6-72), as we discussed above. Comparison of core with dipmeter patterns revealed sev-eral diagnostic dip trends. (1) An upward decrease in dip above a major dip change (reflecting

Figure 6-69. Indian Draw field, Texas, with (a) a fieldwide cross section and (b) a net sand map of the produc-tive A3 sandstone. In the cross section, the A3 sandstone is correlated as a relatively continuous deposit. AfterPhillips, 1987). Reprinted with permission of the SEPM.

20

20

0

30

40

30

40

0 4000 ft

1000 m0

12 8

13 17

24 19 20

T22S

A3 Net Sand

b

31

00

33

00

35

00

32

00

34

00

36

00

32

00

34

00

36

00

32

00

3400

36

00

DATUM “2-Finger Limestone”

“A3”

“A2”“A1”

“Big Dolomite”

Champlin Wilson 1 Amoco Unit 19

IP : 151 BOPD + 20 BW IP : 108 BOPD + 5 BW

N A A’ S

a

Amoco Unit 20Amoco Unit 1


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Figure 6-70. SP log of the A3, A2, and A1 reservoir sandstones in Indian Draw field. Various dip patterns areshown for the different sandstones, as well as for different parts of the same sandstone. After Phillips (1987).Reprinted with permission of the SEPM.

3250

3300

3350

3400

Resistivity

“A3”

INDIAN DRAWZONE

“A2”

“A1”

Big

Dolomite

10 20 30 40 0 5DIP

Amoco Unit 21

N=12Ma=316°


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Figure 6-71. (a) Interpretation revised from that shown in Figure 6-69a, now illustrating the internally discon-tinuous nature of the A3 sandstone. (b) Isopach maps of the individual lenses determined by the detailed corre-lation strategy. After Phillips (1987). Reprinted with permission of the SEPM.

Figure 6-72. Gamma-ray and dipmeter logs of Member 5 of the A3 sandstone, showing the variability in dippatterns, which provided a means of subdividing the sandstone into five separate intervals (a–e). Reservoirfluid flow will be very complex in this sandstone because of the variable dip magnitudes and orientations ofbeds. After Phillips (1987). Reprinted with permission of the SEPM.

AU 14 AU 10 AU 7 AU 1 AU 9

B B ’N S

0 50 100GAMMA

100 85 70 55SONIC

32

00

33

00

34

00

32

00

33

00

34

00

34

00

33

00

32

00

33

00

34

00

34

00

33

00

Two Fingers

“A3”

“A2”

“A1”

TG Limestone

Big Dolomite

e

cd

f

b

IP : 100 BOPD + 22 BWIP : 126 BOPD + 7 BW

IP : 81 BOPD + 31 BWIP : 108 BOPD + 5 BW

IP : 91 BOPD + 15 BW

a

0

15

0

0

20

30

20

12 8

13 17

24 19 20

gf

e

drape

slump

0

0

4000 ft

1000 m

T22S

b

A3 Lenses

15 m

MEMBER 5

40 API 120GAMMA

FT

11400

11450

11500

0° 10° 20°30° 40° AZIMUTHRESTORED DIP

STRUCTURAL DIP

STRUCTURAL DIP

a

b

c

d

e

UPPER

UPPER

UPPER

LOWER

LOWER

LOWER

LOWER

UPPER

FLOW


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an unconformity or channel base) represents progressive flattening of the channel floor as it isfilled with sediment. (2) A relatively constant upward dip magnitude, but a possible variationin azimuth is a typical pattern near the center of the channel. (3) The lack of computed dipswithin an interval indicates that there are massive sandstones without detectable bedding vari-ations. (4) Abrupt changes in dip represent a slumped interval, (5) A fine-scale, upwarddecrease in dip over a thin interval indicates a mudstone drape over a sandy feature.


The total thickness of the reservoir, between the channel floor and the top of the A3sandstone, varies from 15 m (49 ft) to more than 37 m (120 ft) within the field (Figure 6-69).The A3 reservoir sandstone varies in thickness from 3 to 10 m (10 to 33 ft). Because the A3sandstone is composed of a series of offset stacked lenses, lateral continuity is variable (Figure6-71a).


Because the A3 sandstone is composed of a series of offset stacked lenses, vertical con-nectivity is poor between its lenses (Figure 6-71).


Sandstones are typically very fine grained, of variable thickness, and interbedded withthin siltstones and shales. Partial and complete Bouma sequences are present, along with a bio-turbated sandstone unit. The relative order of bed occurrence is —from most abundant to leastabundant—Tae, Tace, Tabce/Tade, and Tabcde. Sandstones are feldspathic sublitharenites withan average detrital composition of Q69F15L11 and 3% clay matrix. Carbonate and quartzcements form 20% of the total rock composition.

Reservoir quality

Range and average porosity and permeability measured on 182 plugs from cores fromthree wells are: 5.5–27.4% (average = 22.1%) and 0.1–100 md (average = 15 md). There is nosystematic relation between grain size, bed thickness, and reservoir quality because of thepatchy distribution of cement and the diagenetic alteration of feldspar to clay.

Drive mechanism

Unknown.

Girassol field, offshore Angola

Key references

Kolla et al., 2001; Beydoun et al., 2002; Navarre et al., 2002.

Location

Girassol field is located offshore Angola, in block 17.


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Significance

Girassol field was a major discovery in deepwater (more than 1300 m [4260 ft] deep)offshore Angola. It is an example of a channel-lobe complex that has been investigatedintensely because of its large size (estimated at 0.75 BBOOIP; see Chapter 2). Although it hasnot been on production for long, we mention Girassol here because of the detailed evaluation ithas been subjected to, only part of which has been published. A conventional 3D seismic sur-vey acquired and processed in 1996 was considered to be of excellent quality; a later 3D high-resolution survey was conducted that nearly doubled the spatial and vertical resolution, thusproviding an even greater level of detail than had existed before. Improved design of appraisaland development wells reduced the total number of wells by 25%, compared with the plandeveloped from the original 3D seismic. The level to which this field can be imaged with state-of-the-art seismic acquisition is the purpose of this summary.

Trap

The field consists of a combined structural and stratigraphic trap. Salt withdrawal cre-ated a large anticline (turtle) structure. The channel-fill reservoir crosses the crest of thestructure.


Following Cretaceous rifting and separation of Africa and South America, large vol-umes of sediment were deposited along the Angolan continental margin as a result of (1) upliftand westward tilting of the African craton, (2) sea-level changes, and (3) increased river drain-age. Sediment supply, coming mainly from the Tertiary Congo River, resulted in aprogressively basinward shift of the margin and the evolution of the Congo Fan. All of therecent offshore West Africa discoveries, including Girassol, Dalia, Hungo, Kuito, and Lan-dana fields, are within the upper Oligocene/lower Miocene Melembo Formation of the CongoFan.


Estimated reserves for the field are 724 MMB and 450 BCFG (see Table 2-1 inChapter 2). The field was brought on line in December 2001, with two wells tied back to aship. By mid-2002, the field had reached 200,000 BOPD from eight wells.


Seismic expression of Girassol field is generally considered to be excellent. The 19963D seismic data set revealed a highly complex, chaotic internal architecture (Figure 6-27). The3D high-resolution survey, acquired in 1999, had a frequency content of about 120 Hz, com-pared with 30–60 Hz in the original survey. From the original survey, channels and lobescould be identified. The more recent survey revealed individual stories within the complex(Figure 6-27). A significant new observation was the existence of shale layers that separateindividual channel sequences. Shale-prone channel fills cutting the field along the depositionalaxis were also identified; they could be lateral fluid-flow barriers. Well planners used thesenew findings to optimize sweep efficiency. Highly deviated wells were steered through severalindividual, isolated reservoir sandstones that were laterally offset (Figure 6-27). This surveyformed the baseline for a 4D seismic survey in late 2002.


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Not available.


Not available.


Not available.

Reservoir quality

Not available.

Drive mechanism

Not available.

Summary: Lessons learned

1. Deepwater channels act as conduits for coarse-grained sediment, connecting the outershelf to the basin floor and bypassing the lower slope.

2. In plan view, the morphology of channels varies from straight to highly sinuous. Gradi-ents, volumes, grain sizes, and frequency of flows all affect the shape and evolution ofthe channel. Environments transition downdip into sheets, through an area known as thechannel-lobe transition zone.

3. Channels have relatively low aspect ratios (10:1 to 300:1) and are considerably longerthan they are wide. Channels vary from erosional to erosional/aggradational to purelyaggradational (channel-levee) types.

4. On seismic-reflection data, channel fills show a variety of geometries, including shin-gled reflections (laterally migrated packages), offset patterns with aggradational fill, andentirely aggradational fill.

5. Lithofacies and grain-size distribution are also highly variable in channel-fill depositsand create many baffles and barriers to pressure and fluid communication.

6. The typical evolution of channels within one depositional sequence is from a base ofcoarse-grained, stacked, amalgamated channels (with higher net:gross values) thatchange upward to leveed-channels with lower net:gross values. This commonly occur-ring stratigraphic (vertical) variation results in two different architectural styles for thereservoirs and may require two different development strategies for optimal production.

7. Although channel fills are internally complex, the complexity is arranged in a hierarchi-cal pattern, which is recognizable at outcrop (large) and multifold seismic scales. It maybe more difficult, but not impossible, to identify the hierarchy in wellbores and cores.

8. Different outcrops and different reservoirs have been discussed in this chapter, to showthe reader the range of potential complexity that may be encountered in a channel-fillreservoir. Excellent connectivity and continuity of reservoir strata may exist in a grav-elly or sandy channel system; in such cases, reservoir performance can be maximizedwith relatively few wells. With increasing shaliness in the strata, complexity and com-partmentalization increase, thereby requiring the placement of more closely spacedwells.

References

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9. Because of the extreme complexity of channel fills, reservoir performance can vary lat-erally within a reservoir. Proper well spacing and orientation are imperative for effec-tively draining hydrocarbons from channel fills. Proper well placement requires aknowledge of the nature of the fill that can only be determined with sufficient data earlyin the life of the field. Early acquisition of 3D seismic data, cores, borehole-image logs,and well-test information is undoubtedly more important in channel-fill reservoirs thanin other types of deepwater reservoirs and is recommended. Such early acquisition mayprevent companies from having to drill additional infill wells or revamp the originaldevelopment strategy.

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