Sand Stones Reservoir Facies Edit

8/2/2019 Sand Stones Reservoir Facies Edit

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MAJOR SANDSTONE RESERVOIR FACIES

The key parameters exhibited by important sandstone reservoir facies and some of theassociated facies are listed in the following pages. Also, relevant diagnostic evidence in the form of cores, logs, and seismic is summarized for cases where it isdeemed particularly relevant.


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Alluvial Fans

Generally, the style of deposition on alluvial fans prevents them from acting as goodreservoirs. To date, there are only a few, clear-cut examples of fields producing fromterrestrial fan facies. They are often, however, extremely important to recognize and

delineate in the subsurface because of their indication of both tectonic setting andsource area composition. Given this, and the fact that fans commonly grade into fandelta and alluvial plain environments (whose sediments have far greater potential tobe good reservoirs), alluvial fans can serve as associated facies of crucial significance.

Summary of Facies Characteristics

Lithology

fanglomerate (some very large fragments)

channel sand, conglomerate

thin shale layers

rapid vertical and lateral changes

commonly red beds

Sedimentary Structures

crude to unbedded (fanglomerate)

imbricated and oriented pebbles

crossbedding in channels (various scales)

crude horizontal stratification

current lineations

Paleontology

rare vertebrate bones, plant debris

more common spores, pollen, often oxidized

Geometry

fan-shaped in plan view

wedge-shaped in radial profile

convex-upward in transverse profile


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Associated Facies

fault-generated mountain fronts

mountain stream valley

alluvial plains (braided river)

playas and eolian facies

Diagnostic Evidence

Cores

The general coarseness, poor sorting, clast angularity, and immaturity of alluvial fansediments are the conspicuous features that dominate most core samples of the upperfan. Finer-grained, cross-stratified or flat-bedded channel sandstones can also beprevalent, particularly from lower-fan sediments. Figure 1 (Devonian alluvial fan

successions in the Hornelen Basin, Norway) shows the internal details of three alluvialfan coarsening-upward sequences from the Devonian of Norway.

Figure 1
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Logs

Figure 2 (Idealized gamma ray and dipmeter logs for an alluvial fan sequence, showingboth fanglomerate and channel development.

Figure 2

Note three major patterns: lowest green dips represent shale breaks and correspond tospikes on gamma ray curve; random "bag o' nails" dips in fanglomerate; and dipclusters that show an upward-increasing blue pattern in channel sands ) shows anidealized gamma-ray/dipmeter profile through a fan. The log is characterized by amonotonous gamma ray curve generated by fanglomerate and coarse braided channel

sand. Several shale layers are indicated simultaneously by spikes of high radioactivityon the gamma ray log and low dips (green motif) on the dipmeter log. These shalesseparate three channels, whose tadpole patterns show a clustering of high-angle dipscaused by crossbedding.


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Seismic

Vertical seismic sections through alluvial fan complexes typically show discontinuousinternal reflectors. ( Figure 3 , Seismic section and interpretation through probable

alluvial fan developed over structurally deformed basement.

Figure 3

Note relatively poor internal seismic character of the deposit.) This should beexpected, given the great lateral and vertical variation in lithology.


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Braided Streams

Many present-day alluvial fans pass laterally into the alluvial plain of a braided river,that is, one characterized by an interlacing veinlike network of low-sinuosity channels

with constantly shifting midchannel bars ( Figure 1 , Block diagram model of a braidedstream system in a semi-arid environment).

Figure 1

Streams and rivers tend to braid when three main factors conspire: (1) high (thoughpossibly seasonal) discharge, (2) relatively steep slopes and (3) large amounts of

coarse bedded sediment.


Lithology

highly variable


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up to 90% coarse, pebbly sandstone


soft sediment deformation

ripples

planar bedding

tabular and trough crossbedding

crude bedding, oriented pebbles

Paleontology

some plant and animal debris, highly oxidized

rootlet horizons

burrows

Geometry

sheetlike, may cover thousands of square miles and be hundreds offeet thick

Associated Facies

proximal: alluvial fan

distal: meandering stream alluvium, sabkha, eolian dunes, playa

(desert lake), possible transition to marine delta

Diagnostic Evidence

Cores

Core samples taken from a braided alluvial section can reveal either a homogeneoussection of coarse, crossbedded and gravelly sandstones or a diverse range of grainsizes and sedimentary structures. Again, well-preserved individual sequences beginwith a sharp erosional base that marks the channel floor, possibly overlain by anupwardly fining progression of grain sizes and sedimentary structures. ( Figure 2Idealized "outcrop" showing succession of grain sizes and sedimentary structures in a

single channel sequence of braided alluvial system.)


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Figure 2


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Logs

Figure 3(Log of braided alluvial sequence showing characteristic monotonous log

response.

Figure 3

Note that the gamma log is neither as clean as that for eolian deposits, not as shaly asmeander channel flood plane alluvial sequences. Azimuth frequency plots reflect lineartrend of this river type ) displays the idealized log response of a braided streamdeposit. Some crude fining-upward portions of the curve can be discerned, but grain-size variation is most often too small to produce a convincing bell-shaped channelprofile, and blocky profiles usually result.

In terms of its dipmeter signature, this facies mainly shows the multiple stacking ofchannels. Within each channel, azimuths and dip amounts are clustered into separablegroupings. Channel switching is characteristic but azimuth changes usually remainwithin a 90 arc. The probable long dimension of the sand body as a whole can oftenbe found by bisecting the arc when it is plotted on an azimuth frequency diagram.
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Seismic

Due to relative lithologic homogeneity, braided stream deposits do not often showinternal reflections. Shales are too thin and localized to generate any significant

responses.Figure 4 (Possible braided stream/alluvial fan deposit in seismic designatedby dashed line) is a probable example of the overall lenslike geometry and poor

internal seismic character of such a deposit.

Figure 4


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Meandering Stream Channels

With greater distance from the sediment source area, a meandering river becomestypical (Figure 1), idealized block diagram showing meandering river system overregion of low slope and continual subsidence).

Figure 1

Alluvial flood plains cut by a single meander channel occur in regions characterized by

relatively low gradients, higher suspended load component, fine- to medium-grainedsediment, and more continuous (nonseasonal) discharge. Sand bodies are created aspoint bar sands resulting from channel migration.


Lithology


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overall, approximately 1:1 sand/shale ratio

point bar: flood-plain siltstone/shale, medium to fine sand, well-sortedand channel pebble lag

abandoned channel: oxbow lake siltstone/shale channel lag

flood-plain shale, coal


scour and fill

surface exposure features; mud cracks, raindrop impressions

ripples

planar bedding

trough, tabular crossbedding oriented pebbles, current lineations

Paleontology

potentially diverse: vertebrates, plant remains, nonmarine mollusks,gastropod shells, spores, pollen, burrows, footprints

Geometry

point bars: stacked to relatively isolated lenticular sand bodies

channels: continuous and discontinuous "shoestrings," sometimes

encased in less permeable sands/silts or flood-plain shales

Associated Facies

most common: deltaic, shoreline/marine shelf, lakes, braided streams


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Diagnostic Evidence

Cores

Core sampling of point bar sands should show the overall fining-upward sequence ofsedimentary types and structures illustrated inFigure 2(Idealized "outcrop" showingupward succession of grain size and sedimentary structures in preserved point bar).

Figure 2

Such sequences are often truncated by overlying channels and the entire suite maynot be seen.


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Logs

Figure 3(Well log showing two upward-fining point-bar sand bodies).

Figure 3

Note the characteristic bell-shaped curve for channels. There is little, however, todistinguish these filled channels as alluvial.) presents several logs that show the

variations and relationships in meandering stream, alluvial floodplain sediments. Twopoint-bar sequences are in evidence.

Both are surrounded by overbank flood-plain shales. Note how the gamma ray curveshows the abrupt change from shale to sand at the base of each channel, as well asthe fining-upward, bell-shaped curve as point-bar sand grades into flood-plain shale atthe top of each channel sequence.


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The dipmeter log for such a section will be a bit complex, but will show three maindepositional surfaces ( Figure 4 , Idealized dip log showing both the filled-in red motif(left) and the upward-increasing blue motif, which indicates individual crossbed sets.

Figure 4

Note that the blue pattern (right) depends upon a narrow dip correlation interval(usually less than 10 ft.), so that both toeset and foreset dips can be recorded by thelogging tool): structural dip (green motif), major accretion slopes (red motif), andcrossbedding (blue motif).


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Seismic

In the subsurface, channels generally create abrupt changes in lithology. Their seismic"visibility" should, therefore, be pronounced. At the same time, where the sharperosional base and sides of the average channel make for good velocity contrast, theupper part of the average channel grades into flood-plain deposits, and thus will notgenerate high-quality reflections. As a result, the typical lens shape of most channelsshould be only relatively clear on high resolution seismic lines, as shown inFigure 5 .

Figure 5

(Seismic expression of a river-cut channel. Note the abrupt termination of flat-lyingreflections against the channel flanks and the change in seismic character between


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these reflections and those within the channel. Note also the steeper slope of the rightflank of the channel, possibly indicating that this was the cut bank.)

Eolian Dunes

The bed forms into which sand settles when transported by wind are mainlyasymmetric ripples and dunes whose overall geometry is much like that of theirsubaqueous counterparts. Most dunes preserved in the sedimentary record appear tobe the transverse type ( Figure 1 , Cross section of barchan or transverse dune

showing the various bedforms and slipface surface).

Figure 1

The dynamics of eolian and aqueous movement are basically similar: they both involvegranular solids being moved by and within "fluids." This is probably the main reasonwhy the eolian environment is particularly difficult to distinguish in the subsurface.


Lithology

clean, well-sorted quartz sandstones (orthoquartzite)

scattered, local interdune shale, evaporite, or lag lenses

layers of heavy mineral concentrations


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pure carbonate sand, much more rare


primarily large- to giant-scale crossbedding with high angle foresets(20-35)

surface exposure features (rain drops, rootlets, tracks and trails, etc.)in interdune lithologies

Paleontology

rare vertebrate remains

oxidized spores, pollen

Geometry

usually sheetlike, upper surface often planed by transgressive seas

Associated Facies potentially variable: alluvial fans, braided streams, sabkha, playa ininterior arid basins; barrier island, lagoonal and shallow shelf facies incoastal settings, often complexly interbedded with water-laid deposits


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Diagnostic Evidence

Cores

Samples of eolianite sections are commonly composed almost entirely of clean, well-sorted quartz sandstone (often called orthoquartzite or quartzarenite) ( Figure 2 ,Idealized vertical sequence of eolian dune and interdune sediments).

Figure 2

Detailed sedimentological analysis has not proven its unqualified worth in strictlydistinguishing dunes from some transitional marine facies. Since eolian sand is often


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reworked from older deposits, such study may reveal mostly "inherited" features. Thicksets of monotonously consistent crossbedding are the prominent sedimentarystructures found in most cores. Traces of oxidized "impurities" between sand grains whether ferric iron, spores, or heavy minerals can be significantly diagnostic.

Logs

Figure 3(Log motifs for eolian sands.

Figure 3

Note well-developed blue pattern of upward-increasing dips along the toeset-foresettransition in individual dune units ) shows a suite of logs typical of the eolianRotliegendes (Permian) group, a productive reservoir in the North Sea. Despite anoverall blocky appearance, the gamma ray curve can be divided into approximately


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50-ft increments, all bordered by narrow spikes of higher radioactivity. Thus, thegeneral profile can be more accurately described as "saw-toothed." Each of the smallkicks (which are more obvious on the density log curve) is asymmetric, with a gentleupward decrease in gamma ray API units. They are caused by the finergrained, mica-containing layers of each new dune that abruptly truncate the foresets of theunderlying dune unit. The 50-ft interval is also strongly evident on the dipmeter. Each

increment begins at the base with low-angle dips (toeset beds), which then increaseupward until reaching a maximum of about 25 to 35 (foreset beds). This maximum isthe most conspicuous part of the dipmeter log and indicates both the large size andconsistent orientation of the crossbedding. Dip azimuths are very constant, directlyindicating the downwind direction. This, in turn, reveals the local elongation of thesand body transverse (perpendicular) to wind direction.

Seismic

In general, subsurface dune deposits are not detectable as such by existing seismicmethods. Sheetlike geometry, association with unconformities, and absence of good

internal reflectors are, as mentioned, also typical of the overall response generated bybraided stream sediments which may over- or underlie eolianites and thus furthermask them. Seismic data, therefore, are perhaps most useful in delineating thedepositional limits, rather than the actual lithology, of a potential dune reservoir.


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Lacustrine Deposits

Unlike the previous environments we have looked at, lakes usually do not define a

single facies, but a collection, and might better be considered to represent a faciesgroup ( Figure 1 , Block diagram illustrating the major facies and subfacies of LakeUnita, northeastern Utah, as it is interpreted to have looked in the Eocene.

Figure 1


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Alluvial, marginal-lacustrine, and open-lacustrine depositional environments existedsimultaneously). Lakes that have occupied intracratonic basins can, to some extent, beconsidered as small inland seas in terms of their major facies. They may be borderedby coastal alluvial plains with swamps, lagoons, and barrier islands (tidal flats are

notably absent). They may also be the site for deltas, which form at major rivermouths, and from which turbidity (subaqueous gravity slide) currents transportsediment into the basin center, creating subaqueous fan deposits. This means thatlithology is often completely undiagnostic for this environment.

The far more subdued water turbulence of the lacustrine environment waves,longshore and subsurface currents as well as its different geochemistry, sometimeseffects significant, partially diagnostic differences from marine counterparts. Lacustrinesediments, for example, are often much more finely bedded (laminated) and containbetter preserved plant debris than those in most marine settings (lagoons being amajor exception). Certainly, paleontology is the most prognostic indicator, butexperience dictates that reworking and redeposition of nonmarine fossils in marinefacies is common.

In general, the consistent indication of aqueous deposition and nonmarine fossils, aswell as the "negative evidence" offered by the lack of marine biota, together indicatethe probability that the facies under consideration is lacustrine. More broadly, tectonicsettings can also afford a strong clue. Small continental basins, as well as rift graben-type basins associated with continental breakup, are strong candidates for having atone time or another played host to large lakes.


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Deltas

The delta environment contains diverse settings for sandstone deposition ( Figure 1 ,Sand deposits of a delta system).

Figure 1


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In the upper delta plain, point-bar or braided-stream channel sands may be deposited.When streams contain high sediment bedload or when marine processes dominate(high-energy deltas), these alluvial channel sand deposits may extend over the entiredelta plain to the shoreline. In river-dominated deltas of low marine energy, alluvial

channel deposits of the upper delta plain give way, through stream bifurcation, to anetwork of essentially straight distributary channel deposits on the lower delta plain.Surrounding these channels are fine-grained bay-fill sediments, often containingcoarsening-upward sandy sequences deposited by crevasse subdeltas.

The subaqueous delta contains distributary front bar sands that may be reworked intobarrier islands by marine processes in abandoned portions of low-energy, river-dominated deltas. In high-energy deltas, winnowing of fine-grained material by waves,currents, and tides creates a variety of sand deposits along the shoreline, in the formof barrier islands, tidal channels, and tidal sand sheets.

The characteristics and diagnostic evidence of braided stream and point bar sandsdeposited in the delta environment are essentially the same. Crevasse subdelta sands

generally form minor petroleum reservoirs. We shall touch on the two major deltaicsandstone facies: distributary channel sands and distributary mouth bar sands.

Facies Characteristics of Distributary Channel Sands

Lithology

fine- to medium-grained sandstone, moderate- to well-sorted

fining-upward grain-size profiles


contorted bedding

ripple formations

planar bedding

trough, tabular crossbedding

scour base

Paleontology burrows

organic plant debris

faunal remains usually absent

Geometry


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linear, straight to sinuous

10 in to 30 in thick

1 km to 5 km wide

Associated Facies

fluvial meander point bar or braided stream

interdistributary bay, crevasse subdelta

distributary mouth bar

Diagnostic Evidence for Distributary Channel Sands

Cores and cutting samples should show a suite of lithologies and structures similar tothat shown inFigure 2 (Idealized lithogenetic sequence of vertically stacked point barsfrom upper delta plain area).


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Figure 2

An upward-fining sequence of medium- to fine-grained, moderately to well-sortedsandstone is typical. Sedimentary structures vary from large-scale cross strata in lower

portions of units to interbedded ripple cross laminations and planar lamination in upperparts. Fragments of plant and coaly material are common.

Logs

SP/gamma ray curves typically display blocky to upward-fining "bell" shapes withabrupt bases. Curves are often jagged, reflecting shale laminations within the sand,and dip-meters in distributary channel sands tend to display red "slope" patterns ofincreasing dip with depth ( Figure 3 , Sp and dipmeter logs of a distributary channelsand reservoir, offshore Louisiana, with a schematic cross section showing location oflogs within the channel.

Figure 3

Note red pattern dip azimuths point toward channel axis). These shapes reflect


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deposition on lateral accretion surfaces and dip azimuths usually point toward thechannel axis and, thus, are normal to channel strike.

Facies Characteristics of Distributary Mouth Bar Sands

Lithology

in proximal bar: clean, well-sorted coarse- to medium-grainedsandstone

in distal bar: coarsening-upward sequence of fine sand, silt, and clay


in proximal bar: small-scale cross laminae and current ripples

in distal bar: small-scale cross laminae, small scour and fill, andgraded sand units

Paleontology

abundant microfossils in prodelta clays at base of sequence withminor bioturbation

microfossils and bioturbations decrease upward

small burrows and shell remains in distal bar

laminations of organic debris in upper sand body (proximal bar)

Geometry elongate in seaward direction with high river influence; arcuate tocuspate-shaped, with increased wave and marine current action

up to 130 in thick and 10 in wide

Associated Facies

prodelta marine shale

delta plain and interdistributary bay silts and clays

distributary channel sands

crevasse subdelta silts


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Diagnostic Evidence for Distributary Mouth Bar Sands

Cores should typically show lithologies and sedimentary structures illustrated inFigure4 (Lithologic column of distributary mouth bar deposit ) i.e., distal shales and siltscoarsening upward to coarse-to-medium, well-sorted sand in upper bar.
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Figure 4

Sedimentary structures are primarily ripple laminations in fine- to medium-grainedsandstones.

Logs

The electric log in Figure 5 (Gamma-ray/SP and dipmeter log of distributary mouth barsequences, subsurface Gulf of Mexico.


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Figure 5

Note blue current dip motifs pointing in direction of current flow ) shows the SP/gammaray curve of distributary mouth bars displays an overall funnel-shaped, coarsening-upward profile. An abrupt break is usually seen at the top of the curves, reflecting thesharp change from clean, well-sorted sand of the uppermost bar to a capping by fine-grained sediments.

On dipmeters, distributary mouth bars are often characterized by patterns of upward-increasing dips (blue patterns) ( Figure 5 ). This pattern reflects deposition byprogressively stronger currents as a bar is built up into shallower water. Dip azimuthsgenerally point in the direction of current flow (seaward) , but variations may beconsiderable.

Seismic

Oblique progradation is the type of reflection configuration typically associated withfluvial delta systems. Sediment input in this environment is high compared to sea level


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rise/basin subsidence, resulting in significantly more lateral progradation than verticalaggradation. The oblique configuration is distinguished by reflections that terminate bytoplap at or near the upper surface, and by downlap at the base ( Figure 6 , Oblique-

progradational seismic reflection pattern typical of deltaic systems).

Figure 6

An actual map view of an ancient deltaic channel or bar sand may be revealed by ahorizontal slice through a block of 3-D seismic data. Figure 7 (Horizontal slice throughblock of 3-d seismic data from Gulf of Mexico, showing lenticular-shaped distributarychannel sand.


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Figure 7

Superimposed structural contours show brightest (darkest) portion of channel, wheregas is indicated, is structurally high) is such a horizontal section from the Gulf ofMexico displaying variations in reflection amplitude along a structurally interpretedhorizon (Brown 1985). We can clearly see a bifurcating distributary channel delineatedby a zone of high amplitudes (darkest tone) cutting from northeast to southwest across

the section.


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Fan Deltas

Fan deltas are alluvial fans that prograde out into a standing body of water from an

adjacent highland (Holmes 1965). As such, they generally develop on the flanks ofbasins next to fault-bounded, elevated source areas (Figure 1 , Typical fan-delta

tectonic setting on flank of rift valley).

Figure 1

When fan deltas form adjacent to contemporaneous faults, thick wedges of coarse-grained deposits accumulate.

Fan deltas have only recently been recognized as important oil and gas reservoirs(Ethridge and Wescott 1984). Rapid facies changes and association with tectonicallyactive basin margins create favorable stratigraphic and structural trapping conditions.Furthermore, potential reservoir beds are often in close juxtaposition with marinehydrocarbon source rocks.


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The cross sections in Figure 2 (Idealized vertical sequence),

Figure 2


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Figure 3 (paleogeographic reconstruction),

Figure 3

andFigure 4 (cross section of shelf-type fan deltas based on data from U.S.
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Figure 4

midcontinent Pennsylvanian-Permian granite wash studies. Based on studies byMcGowen, 1970) illustrate the distribution of facies within a shelf-type fan delta. Thistype of fan delta forms on the broad shelves that typically border intracratonic andplate-divergent basins and often develops extensive progradational sequences.

We see that the proximal and medial parts of the fan, collectively called the fan plain,occupy the exposed portion of the fan-delta system. The distal fan and prodeltaenvironments constitute the subaqueous portion of the fan system.


Lithology

fan plain: poorly sorted, coarse-grained, sands and gravels; oftenhighly arkosic

distal fan: well-sorted, coarsening-upward sequences of sand andgravel, grading offshore into prodelta shales and possible marinelimestones


crude to unbedded (fanglomerate) in proximal fan

large-scale, tabular, and trough crossbedding in braided channels inmedial fan with occasional horizontal stratification

parallel-laminated to massive delta-front sands

Paleontology

rare vertebrate bones, plant debris in fan plain


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shell fragments in delta-front sands

microfossils (marine or fresh water) in prodelta shales

Geometry

overall fan-shaped in plan view

wedge-shaped in radial profile

convex-upward in transverse profile

subaqueous distal facies elongate in seaward direction in fluvial-dominated fans; arcuate to cuspate-shaped, with increased wave andmarine current action

Associated Facies

fault-generated mountain fronts, proximally

marine or lacustrine shales and limestones, distally

Diagnostic Evidence

Cores and cuttings should show a high ratio of coarse- to fine-grained sediment, often

highly arkosic, and an overall coarsening-upward succession in vertical sequence( Figure 5 and Figure 6 ,


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Figure 5

Hypothetical vertical sequence in shelf-type fan delta based on studies of fans and fandeltas along the southern Alaska coast).


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Figure 6


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Logs

Figure 7 (SP and resistivity log of the fan-delta Ivishak formation,

Figure 7

main reservoir in the Prudhoe Bay field, Alaska ) a log of the reservoir Ivishakformation, Prudhoe Bay field, Alaska, illustrates the overall coarsening-upward Spprofile of a fan delta sequence. The base of the sequence consists of very fine-grainedsandstones and mudstones of the offshore that coarsen upward into fine-grained, well-sorted sandstones deposited in a beach-bar shoreline complex. Overlying the shorelinesands are coarse-grained sandstones deposited in braided streams of the distal fanplain that are capped by conglomerates of the proximal alluvial fan. In this example, a

sequence of braided stream channel sand ("upper sandstone sequence") from afollowing cycle overlies the proximal conglomerate facies.


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Seismic

In a study of seismic reflection patterns from offshore Brazilian basins, Brown andFisher (1977) presented patterns characteristic of fan delta/shelf facies. They found the

reflection patterns developed in response to proximal, medial fan facies to be poorlydefined and parallel-layered to reflection free ( Figure 8 , Seismic facies patternscharacteristic of fan delta/shelf reflections, generalized from offshore Brazil seismicsection).

Figure 8

Reflection continuity is very poor to absent, and the external geometry of the reflectionunits is wedge-shaped, thickening toward the source area or toward boundingbasement faults. The distal fan and prodelta facies contain some poorly defined,inclined to horizontal, slightly divergent, layered reflectors increasing in numberbasinward and grading into well-developed shelf reflections.


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Coastal Barrier Islands

Coastal barrier sand bodies are generally narrow, wave-built, sandy islands or

peninsulas that form parallel to shore ( Figure 1 ,

Figure 1


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Figure 2 and Figure 3 ,

Figure 2

Idealized block diagram and cross sections showing principal environments and faciesof a regressive barrier island system).


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Figure 3

As topographic features, they can be perennially emergent or exposed only duringperiods of low tide. They most frequently occur as a linear trend of individual islandsseparated by tidal inlet channels. The main sand body of a barrier island is createdalmost entirely by relatively high-energy, shallow marine processes. In many instances,subaerial reworking by onshore winds leads to the formation of a capping dune field.

The offshore-to-beach profile ofFigure 4 (Typical fan-delta tectonic setting on flank ofrift valley) shows the progression of specific depositional zones.


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Figure 4

The offshore or shelf zone grades landward into the lower and then upper shore facezones, which form the seaward portion of the barrier island. Above the mean low waterlevel is the beach/ dune zone.


Lithology

lower shoreface: fine- to medium-grained sand

upper shoreface: medium- to coarse-grained, well-sorted sand

beach: medium- to coarse-grained, well-sorted sand, occasionally withconglomerate

Sedimentary Structures lower shoreface: small-scale cross lamination and parallelstratification, often hummocky; abundant bioturbation

upper shoreface: high-angle trough cross stratification, planar tabularbedding

beach: low-angle, planar stratification, dipping seaward; possible high-angle cross stratification dipping landward


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Paleontology macrofossils (bivalves and gastropods) and shell fragments

trace fossils: straight burrows of low-level suspension feeders insubaqueous barrier

rootlet horizons (uppermost beach)

Geometry thickness: 10 in (low-energy coasts) to 30 in (high-energy coasts)

elongate: 20 kin to 100 km in length on microtidal (0-2 in) coasts

stunted ("drumstick"-shaped) 3 km to 20 km in length on mesotidal (2-4 m) coasts)

barriers generally absent on macrotidal (>4 in) coasts

isolated, shoestring bodies when formed by rapid transgressing seas

overlapping series of bars when formed by regressing seas

Associated Facies marine shelf shales

lagoonal silts and shales

tidal channel, tidal delta/inlet and washover fan sands

Diagnostic Evidence

Cores

As illustrated by Figure 5 (Theoretical vertical sequence of a barrier island systembased on studies of modern deposits on Oregon coast),


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Figure 5

cores and cuttings should reflect the following basic characteristics of a regressivebarrier island sand sequence:

a progressive and fairly regular upward increase in grain size fromsilt/clay to coarse sand and possibly conglomerate, with maximum grainsize usually occurring in the upper shoreface.

a simultaneous upward improvement in sorting, from fair to good inthe lower shoreface, to excellent within the upper shore face and beach.

a general upward increase in both the abundance and scale of crossstratification, indicative of higher energy levels.

a general upward decrease in the disturbance of primary stratificationdue to bioturbation.

Logs

SP and gamma ray logs through barrier island sands commonly display the smoothfunnel shape that reflects a regular upward increase in grain size, sorting, andpermeability. Greater amounts of fine-grained material depress and round off this


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curve, while barrier island sands that are almost entirely free of clay and silt generatea blockier profile. Figure 6 (Generalized electric log patterns across a barrier island

system,

Figure 6

showing changes in log shape depending on location and relative richness of sandversus shale) shows how log curves ideally vary according to changes in the amount offine and coarse material and to location within the barrier system.

As shown in Figure 7 (Gamma ray log and dipmeter motifs for barrier island sandbodies), dipmeter patterns for barrier bar sands usually display an upward-increasingblue motif reflecting the concave profile of the seaward depositional slope.


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Figure 7

Dips within the barrier sand body may, when plotted on a rose diagram, reveal abimodal pattern. The lower angle dips, which define the main blue motif, representseaward-inclined beds formed by wave swash, while higher dips with oppositeazimuths reflect landward-dipping foresets, presumably from ridge and runneldeposition.

Seismic

A typically seismic response should generate a high amplitude reflection from thesharp upper contact between the coarser beach/dune or upper shore face sands andthe overlying marine or lagoonal shales. A sharp but diminishing reflection is generatedfrom the sides of the sand body, caused by the downward-fining in grain size and a

weaker response marking the transition to the fine-grained base of the sand body.

In a profile showing three pulses of barrier island regression (part a ofFigure 8 ,Seismic profile showing three pulses of barrier-bar regression), note the high amplitudereflection caused by the contrast between the upper barrier sands and overlying

lagoon/marsh material.


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Figure 8

The general depositional slope and the direction of progradation are to the right, asmodeled in the accompanying cross section (part b of Figure 8 , Block diagramshowing how transgressive-regressive sand bodies are composed of a stair-stepmultitude of individual bars).


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Continental Shelf SandsShelf sands form as linear ridges usually oriented oblique to the shoreline, or assheetlike deposits. They occur between the lower shoreface and shelf edge ofcontinental shelves ( Figure 1 , Occurrence of sand deposits on the continental shelf)

and in broad, relatively shallow epicontinental seas, such as the North Sea.

Figure 1

Tidal- and storm-generated currents have been shown to be the two most significantagents responsible for shelf sand deposition.



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Lithology

fine- to coarse-grained sand, moderately sorted, possible pebbleconglomerate at top of unit

generally coarsening-upward grain size profile

minor to abundant glauconite

occasional shale laminations and shale clasts


predominately moderate angle trough and planar crossbedding

some planar laminated bedding

ripple stratification in lower units, often hummocky

bioturbated in lower units

possible scour at base of some high-energy deposits

Paleontology

marine shelf foraminiferal assemblages in associated finegrainedrocks

macrofossil shell "hash" at scour base of some high-energy ridges

Cruziana and Zoophycus ichnofacies

Geometry

commonly series of parallel ridges, asymmetrical in cross section, upto 50 km long, 3 km wide, and 40 in thick

less commonly sheetlike, up to 20,000 sq kin in area and up to 12 inthick

Associated Facies

surrounded by marine shelf shales

possible lower-shoreface fine sands and silts laterally shoreward ofsome shelf ridges

Diagnostic Evidence

Cores


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The upward-coarsening lithofacies sequence, illustrated inFigure 2 (Idealized lithologicsequence of the Viking formation, Joffre-Joarcam area, Canada ), from the productiveCretaceous Viking formation of Alberta, Canada, is characteristic of many shelf sand

ridge deposits.

Figure 2

The basal facies consists of a burrowed, silty gray shale. This facies is overlaintransitionally by ripple-bedded sandstone intercalated with silty shale and containingabundant burrowing. Interchangeable with this ripple-bedded sandstone facies is abioturbated, shaly, fine-grained sandstone.

Next in vertical sequence is a trough crossbedded, fine- to very coarse-grained, well- to

moderately sorted sandstone. Shale clasts are common and the sandstone containsabundant glauconite. Generally this facies has a sharp lower contact and a gradationalupper contact. A pebble conglomerate occasionally forms the top of the sequence.

Overlying the sequence may be another interval of bioturbated or rippled sandstoneand shale, which, in turn, is overlain by crossbedded sandstone.

Logs


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SP/gamma ray log profiles may show a variety of shapes: funnel (coarsening-upward),blocky, serrated, and more rarely, bell-shaped (fining-upward). The type of profiledepends on the amount and occurrence of dispersed clay and clay intervals, which, inturn, are dependent on the nature of shelf near-bottom currents.

Therefore, log shapes of shelf sands tend to reflect flow regimes. In general, a funnel-

shaped, coarsening-upward profile (the most common of shelf sand log profiles)suggests a storm/wave-dominated shelf. A blunt-base, blunt-top signature is morecharacteristic of tidal-current sand bodies (Selley 1976).

Figure 3 (Gamma-ray neutron-density log of the Cretaceous Shannon formation, oilreservoir of Hartzog Draw field,

Figure 3

Wyoming, showing coarsening-upward grain-size profile and corresponding upwardincrease in porosity typical of a shelf sand ) from the Shannon sandstone of Wyomingillustrates the coarsening-upward, funnel-shaped profile common for storm-emplacedshelf sands.Figure 4 (Log of the Cretaceous sub-Clarksville.


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Figure 4

a shelf sandstone in Iola field, Texas, with plots of texture and composition, showing afining-upward sequence probably resulting from rapid deposition by waning stormcurrents) shows a log from the Cretaceous sub-Clarksville sandstone of Texas. Herehowever, the log and grain-size plot show a fining-upward sequence characteristic ofrapid deposition by waning current flows, probably from geostrophic storm currents.


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Figure 5 (Gamma-ray log of shelf tidal current sand body from the North Sea,

Figure 5

showing characteristic blocky shape with blunt base and top associated with manytidal sands ) is a gamma-ray log from an undisclosed North Sea location where thesand body was postulated to originate on a tide-dominated shelf. The log profile hasthe characteristic blocky shape with blunt base and top associated with many tidalsands.


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Seismic

Shelf sands usually coarsen upward from a shale base to a coarse sand orconglomeratic top that is abruptly overlain by marine shale. Laterally these bodies arefringed with tight silt that grades into marine shale. The seismic model in Figure 6(Seismic model of a thin shelf sandstone, Cardium formation, Alberta, Canada ) reflects

these overall lithologic changes by showing a strong event at the upper sharp contactand a lower-amplitude event at the gradational base.

Figure 6

As the reservoir becomes transitional into silt, updip to the right, there is a gradualdecrease in amplitude and the exact boundary between porous reservoir rock and tight

silt is difficult to determine seismically.


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Deep Sea Sands

Deep sea research of present-day ocean bottoms, along with petroleum exploration inancient basins, has shown that a particular type of deep water sedimentary facies ischaracterized by thick sequences of laterally extensive interbedded sands and shales.

These deposits have been variously called deep sea sands, deep water fans, turbidites,submarine fans, and turbidite fans. Although fan-type deposits make up the bulk of thesediment, feeder channel sands and slump deposits can be important subfacies (Figure 1 , Model of a submarine fan). And, though turbidite currents are believed to bethe primary depositional process, processes like debris flow and grain flow can besignificant in the proximal fan area.

Figure 1


Lithology

pebbly conglomerate and massive sand in channels


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upward-fining, vertically graded turbidites that constitute overallcoarsening-upward sequences of sand, silt, and shale in mid- to lower-fan and inter-channel areas


scoured erosion surfaces

dish structures and pillars in channel sands

laminated sands, cross-laminated sands, and laminated, oftenconvoluted, silts and fine sands in turbidites

Paleontology

macrofossils (in situ) rare

micro fossils common in finer-grained sediments

Geometry fans are mound shaped, concave downward in strike profile

fans are lenslike, concave upward in dip profile

individual channels long and narrow or coalesced into sheets

Associated Facies

marine pelagic shales

slope shales


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Diagnostic Evidence

Cores

A diverse range of sediment from boulder beds to fine silt and clay is characteristic ofdeep sea fans.Figure 2

Figure 2


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and Figure 3 (Hypothetical stratigraphic sequence of a prograding submarine fan: C.T.,classical turbidite; M.S., massive sandstone; P.S., pebbly sandstone; D.F., debris flow;S.L., slumps; C.G.L., conglomerate.

Figure 3

Arrows show thickening-upward and thinning-upward sequences) shows the overallstratigraphic sequence typically developed by a prograding fan. The lower portion ofthe sequence consists of often incomplete turbidite sequences (CT). The upper portionof the sequence is dominated by cut-and-fill channel sediments composed of massivesand (MS), pebbly sand (PS), and conglomerate (CGL). The main feeder channel in theupper fan may be filled with debris flow (DF) sediment characterized by massive,

poorly sorted sand with clasts of coarse gravel.


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Figure 4 (Idealized turbidite sequence showing Bouma subdivisions Ta throught Te withhemipelagic subdivisions for the Te unit) shows the ideal lithologies and sedimentarystructures anticipated in cores of the turbidite units.

Figure 4

With increased distance away from the source the coarser, lower units of the sequencebecome missing from nondeposition. Thus, in the lower fan, only the upper, finegrainedportions of the sequence are deposited. Glauconite and carbonaceous detritus areoften found mixed together if sediment is derived from both marine and deltaicsources.


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Logs

Figure 5 (Blocky to fining-upward gamma-ray and dipmeter motif of submarine feederchannels) shows the typical blocky or fining-upward SP/gamma ray profiles of feederchannels of the upper fan with random, scattered dips displayed by the dipmeter.

Figure 5


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Figure 6 (Gamma-ray profiles of proximal fan sands showing fining-upward channel

sands,

Figure 6

and dipmeter showing red "slope" motifs dipping into the center of the channels)shows the thinner fining-upward SP/gamma ray profiles developed in channels of theproximal (mid) fan area. Dipmeters may display red "slope" motifs dipping into thecenter of the channels in a direction perpendicular to channel axes. Thin blue currentpatterns are often absent because crossbedding is usually not well developed in deepsea sands.


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Figure 7 (Gamma log profiles of distal fan sands showing coarsening-upward

progradational sequences,

Figure 7

and dipmeter showing blue dip patterns pointing in the direction of fan progradation)shows the upward-coarsening SP/gamma ray profiles of the distal (lower) fanprogradational sequences. These larger sequences in turn are made up of individualupward-fining turbidite units. Combined with the presence of marine pelagic shaleintervals, the resulting SP/gamma ray profile of a distal fan displays a "nervous" back-and-forth character. Blue dip patterns that may be evident point in the direction of fanprogradation.

Seismic

Perhaps the most direct seismic indicator of submarine fans is a mound-shaped seismic

sequence with an internal hummocky or chaotic reflection pattern.

Figure 8(Seismic section across the Frigg field, a giant gas field in the North Sea.


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Figure 8

The pronounced mound with hummocky reflections from 1.8 to 2.0 sec is a submarinefan, and the "flat spot" around 2.0 sec is a gas-liquid contact ) is a seismic sectionacross the Frigg field, a giant gas accumulation in the North Sea, which produces froma submarine fan. Note the pronounced mound with hummocky reflections from 1.8 to2.0 seconds, centered under shotpoint 150. The high amplitude reflection at 2.0seconds is a "flat spot" representing a seismic reflection off the gas-liquid contact.

The presence of canyons or troughs on a seismic section may indicate the presence ofa submarine fan located basinward of these features. Submarine fans also may bepresent beneath or basinward of features displaying clinoform (sigmoid or oblique)patterns.

Case Study: Marchand "C" Sandstone, Anadarko Basin,Oklahoma

A typical environmental study of a productive sandstone was described by Baker(1979).

The Pennsylvanian (Upper Carboniferous) Marchand "C" Sandstone of the AnadarkoBasin, Oklahoma produces oil from over 90 wells in the East Binger field ( Figure 1 ,Location map of East Binger field, Anadarko Basin, Caddo Co., Oklahoma).


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Figure 1

Regional structure consists of a monoclinal surface with local structural noses andsaddles; hydrocarbon entrapment is purely stratigraphic with porous, permeable sandsgrading laterally into siltstones and shales.

The first step in the study was to determine the external geometry of the sand body byconstructing an isopach map of the gross clean sandstone ( Figure 2 , Isopach of grosssand, Marchand "C").


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Figure 2

The sand body was shown to be an elongate, northwest-southeast trending sandribbon extending over 8 mi in length and 1 to 1.5 mi in width. Sand thickness rangedup to 80 ft, and the sand body was asymmetrical in shape with a steep southwestflank.

A northeast-southwest stratigraphic cross section was then constructed in a directionnormal to the long axis of the field. With streaks of "hot-shale" and local marker bedsproviding correlations, the section showed the sandstone to thicken at the expense ofthe underlying shale, a classic indication of a channel deposit ( Figure 3 , Stratigraphiccross section, C-C', across East Binger field, showing sandstone thickening at expenseof underlying shale).


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Figure 3

However, based on regional isopach mapping ( Figure 4 ) the long axis of the sandbody apparently parallels depositional strike. Channel deposits usually extendapproximately normal to depositional strike. Thus a channel interpretation seemedunlikely.


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Figure 4

Thin sections and hand samples of the sand revealed no woody or carbonaceousdetritus, thus indicating a turbulent environment (beach or shoal). In contrast,glauconite, fecal pellets, and crinoid fossils were present in the sand, indicating amarine origin. Combining this information with the blocky gamma ray profiles andapplying it to Selley's model a sub-tidal sand ridge was suggested. ( Figure 5 , Fourcharacteristic gamma log motifs.


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Figure 5

From left to right: thinly interbedded sand and shale; an upward- coarsening profilewith an abrupt upper sand-shale contact; a uniform sand with abrupt upper and lowercontacts; and, furthest right, an upward-fining sand-shale sequence with an abruptbase. None of these motifs is environmentally diagnostic on its own. Coupled with dataon their glauconite and carbonaceous detritus content, however, they define the originof many sand bodies.)

However, the stratigraphic cross section showed the sand body to be emplaced intothe underlying shale; thus it was unlikely to have been formed as a "ridge."

A core study revealed underlying burrowed, fossiliferous marine shale scoured into bya basal lag deposit, ripple and megaripple crossbeds, herringbone cross-stratification,shale lamina, and slumping of sediments - all very similar to sequences seen inpresent-day subtidal sand ribbons that are formed by tide-generated bottom currentsin the North Sea.

It was finally concluded that the Marchand "C" Sandstone was indeed probably formedas a subtidal sand ribbon but infilled a scoured depression similar to the depressions

present on many modern tide-dominated shelves

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Sand Stones Reservoir Facies Edit