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5-149
5 Overview of Deepwater-Reservoir Elements
Introduction
As we stated in Chapter 1, several reservoir elements have now been recognized by var-ious workers and are used routinely in industry: channel-fill, levee (thin beds), sheets(amalgamated and layered), and mass-transport deposits (slides). We describe each of theseelements in a systematic manner in Chapter 6 through Chapter 9. A series of three unusualdeepwater elements (remobilized sands, chalk turbidites, and carbonate debris aprons) aredescribed in Chapter 10. Pitfalls in the interpretation of different elements are briefly summa-rized in Chapter 11.
The discussion of each reservoir element is organized by scales of observation. We firstdescribe regional aspects of each element using data sets at the exploration scale (seismic: surfaceand shallow subsurface; buried elements at exploration and development scale). We then describemore development-scale data sets: outcrops, cores, conventional, and borehole image logs.
The purpose of this chapter is to give an overview to the following five chapters. Wewill: (1) describe the elements and try to equate different terminologies that have been used bydifferent workers (a non-trivial issue); (2) discuss which data sets we use to describe deepwa-ter elements and their resolution; (3) discuss how deepwater systems vary in grain size andsediment-delivery systems; (4) describe the hierarchy of deepwater deposits and how these dif-ferent elements stack stratigraphically through time; (5) discuss shallow analog studies andtheir importance, and (6) address how production from various elements varies between differ-ent basins, and within the same basin, in systems of differing age.
Elements and Nomenclature Issues
The study of deepwater systems evolved from several separate disciplines that eventuallymerged (outcrop geology, marine geology and geophysics, oceanography, subsurface geologyand geophysics). As a consequence, different terminology was used to describe these featuresbased on different data sets (Chapter 1; Figure 1-8). In addition, different disciplines also usedtheir own terms. Below, we define how we used these terms and equated them in this book.
Sequence stratigraphic terms
Sequence stratigraphic terminology was developed to describe all of those sedimentsthat were deposited within certain positions of a relative cycle of sea level (Chapter 3). Archi-tectural terms are used within a sea-level context.
Within the lowstand-systems tract, three elements are recognized (Figure 3-4): basin-floor fans, slope fans, and prograding complex. The downdip portion of basin-floor fans areequivalent to sheet sands (lobes) (Chapter 8); the updip portion are equivalent to amalgamatedchannels (Chapter 6). These are usually the highest net:gross of the deepwater system.
The slope fan is a general term used for lower net:gross systems (Brown et al., 2005) andincludes several elements: channel-fill (Chapter 6), levee-overbank and their equivalent thinbeds and crevasses (Chapter 7), extensive slides, debris flows and mass-transport deposits(Chapter 9). The prograding complex consists of prograding shallow-marine deposits (deltas,shorelines and related deposits), slope, and deep-marine muds. In some conditions, localizedturbidites can develop, called “shingled turbidites.” These features have been characterized ashigh frequency basin-floor fans, composed largely of sheet deposits with minor channels
Overview of Deepwater-Reservoir Elements
5-150
(Mitchum et al., 1993). Shingled turbidites tend to be muddier and more poorly sorted thantrue sheets because they are associated with more muddy parts of the system.
This classification was largely based on seismic stratigraphic appearance, and strati-graphic position within a depositional sequence. The terminology was developed prior to theusage of 3-D seismic data; many of the stratigraphic boundaries between the systems are morediffuse and not as rigorously defined as is indicated by this classification.
Process/accommodation terms
The terminology for fill-and-spill literature also mixes architectural elements and timingof sedimentation (Chapter 3: Figure 3-13 to Figure 3-16; Prather et al., 1998). The pondingand fill facies (A facies) are equivalent to sheet sands; bypass facies (B facies) is equivalent tochannel-levee systems, and drape facies (D facies) is equivalent to condensed sections. Theterms were developed to explain the timing of sedimentation and relate it to changes in accom-modation within intraslope settings.
Architectural terms
The usage of two sets of terms for this book warrant further discussion: the use of sheetsands versus depositional lobes, and the use of thin beds for deepwater reservoirs.
We use the terms that have been used for generally similar deposits, depositional lobesand sheets. Depositional lobes were originally defined by Mutti and Ricci Lucchi (1972) andlater modified by Mutti and Normark (1987, 1991). The term was originally an outcrop-basedterm, with interpreted 3-D geometries. In the collaborative work of Mutti and Normark (1987,1991), the term was applied to modern-fan studies for those sediments deposited beyond theterminus of a channel. Sheet sands is an architectural term used by Chapin et al (1994) andMahaffie (1994) to describe the geometry of sand beds in both outcrop and the subsurface.
Importantly, the extensive use of 3-D seismic has clearly imaged lobate-like bodies atthe terminus of channels in confined and unconfined basins (Figure 8-1, Figure 8-6, Figure 8-8, Figure 8-10, and Figure 8-14). The term now is applied to those sedimentary bodies in deep-water that have a lobate shape. Chapin et al. (1994) suggested that amalgamated sheet sandsare equivalent to proximal lobes and layered sheet sands are equivalent to medial and distallobes.
We use the terms synonymously throughout the book, with the architectural term sheetsmore commonly. Richards et al. (1998) distinguish between the two terms in their classifica-tion (Figure 1-3), noting that lobes are more common in sand-rich systems and sheets morecommon in mud-rich systems. We do not use the terms in that fashion.
The term thin beds also has two general uses. In general, thin beds refers to interbeddedto interlaminated sandstones and shale, both of which are up to a few cm thick. Thin beds canoccur in many depositional settings, ranging from continental to deepwater, and reflect varia-tions in the energy of the environment. In deepwater settings, thin beds can occur in levee-overbank, channel margin, late channel fill, and in distal layered sheets (distal lobe). For thisbook, we use the term thin beds exclusively for those that occur as reservoir in levee–overbanksettings (Chapter 7).
Sediment Grain Sizes and Delivery Systems
Several key papers in the 1990’s defined the essential controls on deepwater systems.From these publications grew a four-fold classification of fan systems based on grain sizes anddelivery systems: gravel-rich, sand-rich, mixed sand-mud, and mud-rich (Reading and Rich-ards 1994; Richards et al., 1998) (Figure 5-1). Three different end-member, sediment-deliverysystems were identified: single point-fed source fans, multiple point source submarine ramp,
Hierarchy and Scales of Heterogeneity Within Architectural Elements
5-151
Figure 5-1. Principal architectural elements of deepwater clastic systems. After Reading andRichards (1994). Reprinted with permission of AAPG.
and line-sourced submarine slope aprons. The salient characteristics for these four systems aresummarized in Table 5-1 and Figures 5-2 to 5-5.
Chapter 6 through Chapter 9 are subdivided by major architectural elements. However,within any element, there can be a wide range of grain sizes and variations in delivery systemsas the element evolves.
Hierarchy and Scales of Heterogeneity Within Architectural Elements
The concept of scales of heterogeneity is one of the more difficult to define and grasp.Although there is a clear hierarchy of scales in deepwater depositional systems, as there is inmany naturally ordered systems, it is not uncommon to use the improper terminology whendescribing a feature. For example, the following deposits might all be defined as becoming‘finer-grained upward’: (a) an individual Bouma Ta bed 5 cm (2 in) thick; (b) a 10 m (33 ft)thick channel-fill; and (c) a 100 m (330 ft) thick lowstand-systems tract of a depositionalsequence. Because of the same descriptor for these deposits of differing scale, a similar set ofprocesses may be erroneously inferred.
Examples that are discussed in separate chapters include (1) different lithologies andfacies, and associated processes (Chapter 4); (2) hierachy of channel-fill deposits (Chapter 6);different stacking patterns related to the time-frequency of stratigraphic cyclicity (Chapter 3);inner and outer levees, master channels and internal channel-fill in leveed-channel deposits(Chapter 6 and Chapter 7); hierarchy of sheet sands and sandstones (Chapter 8); and small- tolarge-scale slides and mass-transport deposits (Chapter 9). Also, a hierarchy of shales affectsreservoir performance; for example, a shale layer lining a concave-upward scour surface willhave much less influence on vertical flow of reservoir fluids than will a shale layer that is later-ally continuous across an entire reservoir.
It is also important to remember that different tools and techniques measure or imagereservoir heterogeneities at different scales (Figure 5-6), for example: microscopic (pore andgrain scale), mesoscopic (vertical sequence in core or outcrop), macroscopic (interwell) andmegascopic (field-wide) scales (Krause et al., 1993). The filtering of seismic reflection data to
GRAVEL-RICHSYSTEMS
SAND-RICHSYSTEMS
MUD/SAND-RICHSYSTEMS
MUD-RICHSYSTEMS
SYSTEM TYPE WEDGES CHANNELS LOBES SHEETSCHAOTICMOUNDS
SLUMPS & SLIDES
SLUMPS & SLIDES
CHANNELIZED-LOBES
DEPOSITIONAL LOBES
DEPOSITIONAL LOBES
CHANNEL-LEVEE
CHANNEL-LEVEE
BRAIDED
CHUTES
PRINCIPAL ARCHITECTURAL ELEMENTS
Overview of Deepwater-Reservoir Elements
5-152
Table 5-1. Reservoir characteristics of contrasting types of deepwater siliciclastic systems (after Reading and Richards, 1994).
Feeder System Type
Point Source Submarine Fans Multiple Source Submarine Ramps Linear Source Slope Aprons
Dominant Grain Size category
Mud MFMud/Sand
MSFSand SF Gravel GF Mud MR
Mud/Sand MSR
Sand SR Gravel GR Mud MAMud/Sand
MSASand SA
Gravel GA
Principal architectural elements
Proximal area Channel-levees
Channel-levees Channels Wedges Channel-
leveesChannel-levees Channels Wedges Slumps,
slidesSlumps, slides Channels Wedges
Distal area Distal Sheets Lobes
Channel-ized lobes
Distal sheets
Distal sheets Lobes
Channel-ized lobes
Distal sheets
Offset Lobes Slumps
Debris flows
Seismic architecture
Channel-levees, Dis-tal parallel reflections
Channel-levees and mounds
Construc-tional, low and relief mounds
Wedges
Channel-levees, Dis-tal parallel reflections
Mounds
Construc-tional, low and relief mounds
Wedges Chaotic mounds
Chaotic mounds Mounds Wedges
Sand Percentage
<30% sand>30 - <70% sand > 70% sand
Variable 5-50% (>50% gravel)
<30% sand>30 - <70% sand > 70% sand
Variable 5-50% (>50% gravel)
Highly Vari-able 0-20%
Highly vari-able 0-20% Highly variable
Highly variable
Sandbody Geometry
Large/len-ticular chan-nels with multiple, variable scale sand\ silt and mud fills. High degree of heterogene-ity. Distal fan domi-nated by thin sand, silt and mud sheets.
Lenticular channels dominated by sand or mud fill. Down-dip lobes formed of interbed-ded and alternating sand, silts and muds.
Broad sheet-like to low relief lobate sand-body geom-etries dominated internally by channel-ized sand-stone units.
Irregular intercon-nected grav-els. Proximal areas domi-nated by conglomer-ates and breccias. Sands domi-nant within medial to distal parts of system.
Moderate size sand-bodies within over-all large channel form. Sands commonly isolated in both down-dip and up-dip direc-tions.
Offset stacked. Lenticular channel sandbodies bounded by levee fines passing down-dip into offset stacked lobate sand-bodies formed of sandstones and mud-stones.
Broad sheet-like to low relief lobate sand-body geom-etries dominated internally by channel-ized sand-stone units.
Irregular intercon-nected gravels. Proximal areas domi-nated by conglomer-ates and breccias. Sands dom-inant within medial to distal parts of system
Limited sand devel-opment. Commonly continued to slide scars and slump generated lows.
Laterally extensive separated by silts and muds
Lobate sand-bodies domi-nated by intercon-nected chan-nelized units.
Laterally exten-sive dis-tally limited
Turbidite facies A, B, E, F C, D B, C A, B, F A, B, E, F C, D B, C A, B, E, F F D, F D, E A, B, F
Reservoir heterogeneity
HighHigh-Moderate Low High High
High-Moderate Low High High
High-Mod-erate Low High
Sandbody Communication
Vertical Poor Moderate Good Good Poor-Mod-erate
Moderate Good Good Poor Moderate Very Good Good
Lateral Poor Poor Good Poor Moderate Moderate Good Moderate Poor Moderate Very Good Very Good
Table continued on next page
Hierarchy and Scales of Heterogeneity Within Architectural Elements
5-153
Common reservoir trap type
Strati-graphic
Strati-graphic Structural Structural Strati-
graphicStrati-graphic Structural Structural Strati-
graphic Stratigraphic Structural Structural
Play Concepts
1) Strati-graphic trapping within termi-nal lobes 2) Strati-graphic trapping of channel levee and splay deposits
1) Strati-graphic trapping of channel + lobe depos-its 2) Trapping of sands in canyons from slope failure + flow strip-ping
1) Up-dip pinchout of interfan channel sandstones 2) Strati-graphic trapping of mid-fan channel-ized lobes
1) Struc-tural trap-ping of fan conglomer-ates adja-cent to footwall scarps 2) Struc-tural trap-ping of medial/dis-tal sand-stones
1) Strati-graphic trapping within termi-nal lobes 2) Strati-graphic trapping of channel levee/ splay sands
1) Com-bined struc-tural! strati-graphic trap in proximal ramp chan-nels 2) Updip pinchout of channel and/or ramp lobe sands
Combined structural and strati-graphic traps of ramp sand-stones
1) Struc-tural trap-ping of fan conglomer-ates + sand-stone against to footwall scarps 2) Struc-tural clo-sure of media/dis-tal sand-stones
Strati-graphic trapping of detached slumped clastics derived from active or relict up-dip deltaic systems
Stratigraphic trapping of detached slumped clastics derived from active or relict up-dip deltaic sys-tems
Combined structural and stratigraphic traps involv-ing slope apron turbid-ite sand-stones
Struc-tural clo-sure of fan con-glomer-ate and sand-stones against to footwall scarps
Key exploration risks
Reservoir and trap presence, extent and identifica-tion
Reservoir presence, extent,& identifica-tion. seal integrity
Trap requires structural component; Seal integ-rity
Reservoir quality and seal integ-rity
Reservoir quality, trap Presence, extent and identifica-tion
Reservoir definition. delineation and quality; seal integ-rity
Trap requires structural component; Seal integ-rity
Reservoir quality and seal integ-rity
Reservoir and trap presence, extent and identifica-tion; Seal integrity
Reservoir and trap presence, extent and identifica-tion; Seal integrity
Trap requires structural component; Seal integrity
Reser-voir qual-ity and seal integrity
Importance and position on relative sea level cycle
Important lowstands
Potentially important HST, LST
Not impor-tant
Not impor-tant
Important LST Potentially
Not impor-tant Impor-tant HST
Important LST, HST LST & TST
Highly Important HST
Low impor-tance HST & LSW/TST
Low impor-tance LST/LSW
Low impor-tance LST
Table 5-1. (Cont.) Reservoir characteristics of contrasting types of deepwater siliciclastic systems (after Reading and Richards, 1994).
Feeder System Type
Point Source Submarine Fans Multiple Source Submarine Ramps Linear Source Slope Aprons
Overview of Deepwater-Reservoir Elements
5-154
Figure 5-2. Schematic block diagram and wireline logs showing the distribution of a gravel-rich fan system:(a) apron, (b) submarine fan, and (c) submarine ramp. Modified from Reading and Richards (1994) and Rich-ards et al. (1998). Reprinted with permission of AAPG.
AVALANCHING
INERTIA FLOW
TURBIDITY FLOW
200-
1,000m
CHUTES
BASIN
PLA
IN
LINE
SOURCED
TALUS CONES
LINE SOURCED
COALESCING
ALLUVIAL
FANS
TALUS FRINGE
BASIN PLAIN
PROXIMAL
TALUS APRON
SURFA CE PLUME
1-2km
a. Slope Apron
b. Submarine Fan
c. Submarine Ramp
TALUS
AVALANCHING
INERTIA FLOW
TURBIDITY FLOW
ALLUVIAL
FAN SUBAERIAL
DELTA
SAND + GRAVEL
SHEETS + BOULDERSHUMMOCKS, LOBES
AND SPLAYS
CHUTES
BASIN
PLA
IN
200-
500m
SURFACE PLUME
BASIN PLAIN
MEDIAL RAMP
1-5km
2-5km
FAN-DELTA
MOUNTAIN FED
ALLUVIAL FANS AND
FAN DELTAS
AVALANCHING
INERTIA FLOW
TURBIDITY FLOW
GULLIES AND
CHANNELS
BASIN PLAIN
TALUS
NARROW
ZONE
LITTORAL
SURFACE PLUME
PROXIMAL
FAN/RAMP
MEDIAL RAMP
BASIN PLAIN
HUMMOCKY
LOBES AND
SPLAYS 200-
1,000m
TALUS
SLOPES
COALESCING
LOBES
Hierarchy and Scales of Heterogeneity Within Architectural Elements
5-155
Figure 5-3. Schematic block diagram and wireline logs showing the distribution of a sand-rich fan system:(a) apron, (b) submarine fan, and (c) submarine ramp. Modified from Reading and Richards (1994) and Rich-ards et al. (1998). Reprinted with permission of AAPG.
COASTAL
BRAIDPLAINBRAID
PLAINSANDY
SHELF
APRON
TURBIDITES
AND SANDY
DEBRIS
FLOWS
POTENTIAL FOR THE
DEVELOPMENT OF
SLUMPS AND SLIDES WITH
LOCALIZED SANDS
CHANNELIZED
GULLIES
SLOPE APRON
SANDS
BASIN PLAIN
BASIN
PLA
IN
BRAIDPLAIN SHELFSANDS
COALESCING
TURBIDITE
SANDS
250-
2,000m
a. Slope Apron
b. Submarine Fan
c. Submarine Ramp
1-10km
SHELF/DELTA
SLUMP
COASTALPLAIN SHELF
SLOPEAPRON
CANYON
SLUMP
SCAR
BASIN PLAIN
BASIN
PLA
IN
BASIN PLAIN
OUTER FAN
MID-FAN CHANNELISED\
AND UNCHANNELISED SANDS
INNER FAN
SLUMP
SCAR
LONGSHORE
DRIFT
BARRIER
SANDY COASTAL
PLAIN
CANYON FED BY ACTIVE
NEARSHORE LITTORAL DRIFT
OR RELICT SHELF SANDS
SLUMPS
500-
2,000m
INNER FAN
CHANNELS
MID-FAN
CHANNELISED
LOBES
10-50km
HINTERLAND
SHELF
SLOPE
RAMP
COASTAL
PLAIN
PROXIMALRAMP
MEDIALRAMP
DISTAL RAMP
BASIN
PLAIN
BASIN PLAIN
MEDIAL RAMP
PROXIMAL TO
MEDIAL RAMP
CHANNELS
SHELF/DELTA
MULTIPLE RIVER/SANDY DELTA
AND COASTAL SYSTEMS
250-
2,000m
10-50km
Overview of Deepwater-Reservoir Elements
5-156
Figure 5-4. Schematic block diagram and wireline logs showing the distribution of a mixed mud-sand fan sys-tem: (a) slope apron, (b) submarine fan, and (c) submarine ramp. Modified from Reading and Richards (1994)and Richards et al. (1998). Reprinted with permission of AAPG.
SLUMP
SLUMP
WID
E MU
D/S
AND
SHELF
COASTALPLAIN
SHELFSLO
PEAPR
ON
SLUMP
SCARS500-
4,000m
SLUMP
SLOPE APRON
LONGSHORE
DRIFT
MEDIUM GRADIENT
COASTAL PLAIN, DELTAS,
CHENIERS, BARRIED ISLANDS
BASIN PLAIN
MIXED SAND AND MUD
SLUMPS AND SLIDES WITH
LOCALIZED SANDS
10-100km
a. Slope Apron
SHELF/DELTA
SLUMP
COASTALPLAINAND DELTA
SHELF
SLOPEAPRON
OUTER
PLAIN
DEPOSITIONAL
LOBES
MID-FAN
INNER
FAN
SLUMP
SCAR
SLUMP
SCAR
CANYON
CHENIERS, BARRIERS
INNER TO
MID-FAN
CHANNELS
50-
3,000m
DEPOSITIONAL
LOBES IN THE
SUBSURFACE
PROXIMAL TO
MEDIAL RAMP
CHANNELS
SHELF/DELTA
b. Submarine Fan
SLUMP
SLUMP
SLUMPSLUMP
SCARS250-
2,000m
BASIN PLAIN
DISTALRAMP
MEDIALRAMP
PROXIMALRAMP
SHELF
COASTALPLAIN
MIXED-LOAD DELTA
OR SHORELINE
SYSTEM
PROXIMAL RAMP
CHANNELS
10-50km
SHELF
SLOPE
RAMP
LOBES
c. Submarine Ramp
Hierarchy and Scales of Heterogeneity Within Architectural Elements
5-157
Figure 5-5. Schematic block diagram showing the distribution of a mud-rich fan system: (a) apron, (b) subma-rine fan, and (c) submarine ramp. Modified from Reading and Richards (1994) and Richards et al. (1998).Reprinted with permission of AAPG.
COSTALPLAIN SHELF
SLOPEAPRON
ARID
COASTAL
PLAIN
LOW GRADIENT
COASTAL PLAIN
LOW
GR
ADIE
NT S
TARVEO S
HELF
BIOG
ENIC
PR
OD
UC
TION
SLUMP
SLUMP
CO
NTO
UR C
URREN
TS
TURBIDITY
CURRENTS
SLUMP
SCAR
SLIDES IN THE
SUBSURFACE
BASIN PLAIN
MUD-RICH
SLUMPS AND SLIDES WITH
LOCALISED SANDS
1,000-
5,000m
10-100km
a. Slope Apron
ABANDONED
CHANNEL LEVEE
SYSTEM IN
SUBSURFACE
SHELF
SLOPEAPRON
COASTALPLAIN AND
DELTA
LAKE
MAJOR
MUD-RICH
RIVER DELTA
MARSHLAGOON
SLIDE
SCARS
BASIN
PLAIN
MID-
FAN
INNER
FAN
SLUMPS
VALLEY
CANYON
HIGH
SINUOSITY
CHANNELS
AND LEVEESTERMINAL
LOBES
SHELF/DELTA
INNER FAN
CHANNELS
MID FAN
CHANNELS
BASIN PLAIN
OUTER FAN
1,000-
5,000m
10-100km
b. Submarine Fan
SHELF/DELTA
COASTALPLAINSHELF SLOPEAPRON
PROXIMAL
RAMP
CHANNELS
LOW MUDDY
COASTAL PLAIN
DELTAS, CHENIERS
HETEROGENOUS
DEPOSITIONAL
LOBE SANDS AND SILTS
CHANNEL-LEVEES
IN THE SUBSURFACE
SLUMP
SLUMP
BASIN
PLA
IN
LATERAL
FEEDING
1,000-
5,000m
DISTSTAL RAMP
BASIN PLAIN
MEDIAL RAMP
CHANNELS
c. Submarine Ramp
50-250km
Overview of Deepwater-Reservoir Elements
5-158
Figure 5-6. Log-plot showing the relative scales of observation and resolution of different datasets used in studying deepwater settings. SSS=side scan sonar. Figure modified from Minken(2004, personal communication).
a more limited bandwidth of frequencies (spectral decomposition) allows one to image fea-tures of varying thicknesses. Finally, stratigraphic modeling for reservoir simulation(Chapter 14) is scale-dependent because computing limitations often require the scaling up oraveraging of the smaller scale properties to accommodate a limited or pre-determined numberof simulation cells.
In Chapter 6 through Chapter 10, we begin our discussion of each element at the explo-ration (regional) scale, then characterize it at the development (reservoir) scale, combining thevarious techniques and data sets listed above.
Integration of Data Sets
One of the main themes of Chapter 6 through Chapter 10 is the need for geoscientistsworking in teams to integrate multiple data sets that have different scales of resolution. Toemphasize this challenge, we have constructed a series of figures that compare: the areal extentof outcrops (Figure 5-7) with seismic data from two intraslope minibasins (Figure 5-8); wire-line logs from the minibasin (Figure 5-9) with outcrop photographs (Figure 5-10) showingstratal architecture; and, high-resolution seismic data (Figure 5-11) with outcrop information(Figure 5-12).
Shallow Geophysics
Scales of Resolution
Well Tests
Shallow Seismic
Outcrop Studies
Regional Analysis
Core Facies and Petrophysical Analysis
Conventional Well Logs
Mineralogic Analysis
Investi
gati
ve T
ech
niq
ue
10 10 10 10 10-6 -3 0 3 6
Meters
Borehole Image Logs
Seismic Reflection
SSS
Integration of Data Sets
5-159
Figure 5-7. Map view outline of three deepwater outcrops commonly used by companies for res-ervoir and stratigraphic analog studies: (a) Brushy Canyon, Permian, western Texas, USA(Beaubouef et al., 1999), (b) Tanqua-Karoo, Permian, South Africa (Morris et al., 2000) and(c) Ross Formation, Upper Carboniferous (lower Pennsylvanian) western Ireland (Martinsen etal., 2000), (d) outline of the Mensa and Thunder Horse minibasins in northern Gulf of Mexico(adapted from Lapinski, 2003; van den Berg, 2004). Location of shallow, allochthonous salt isshown in pink. Locations for Figure 5-8 and wells are shown. Note that the areal distribution ofthese outcrops is about the same scale as 1-3 mini-basins.
Three of the better visited deepwater outcrops in the world are shown in Figure 5-7a-c:the Permian Tanqua-Karoo strata of South Africa, the Permian Brushy Canyon Formation ofwestern Texas, USA, and the Upper Carboniferous (lower Pennsylvanian) Ross Formation ofwestern Ireland. The outline of the entire outcrop is shown in Figure 5-7a-c. Plotted at thesame scale are adjacent intraslope basins in the northern deep Gulf of Mexico, the Mensa andThunder Horse basins (Figure 5-7d).
Several key things can be observed. First, even the best exposures of outcrops are onlythe size of one or two intraslope minibasins, typically the scale that geoscientists work at inexploration. Second, the orientation of the outcrops is similar to those orientations of randomvertical profiles through a 3-D seismic data set. In Figure 5-8, the ground profile of the BrushyCanyon outcrop is superposed on a seismic profile across the reservoir level (14.3-13.05 Ma)in the Thunder Horse minibasin and field. The seismic profile illustrates that the outcrop isabout three to four seismic wavelets, and is slightly less thick than the reservoir intervals atThunder Horse Field. Two wireline logs from the field are shown in Figure 5-9. An outcrop
3 miles
(a) (b)
(c)
(d)
3 miles3 miles
3mi0
MC 773MC 773MC 773
MC 73MC 73131MC 731
MC 687MC 687MC 687
MC 776 MC 778MC 778MC 778MC 778
MC 822
ThunderhorseMini-Basin
MensaMini-Basin
Fig. 5-9
Overview of Deepwater-Reservoir Elements
5-160
Figure 5-8. Two sections, plotted at the same vertical and horizontal scales: illustrate that the total thickness ofthe Brushy Canyon Formation is about the same as one depositional sequence in the Thunder Horse Field.(a) North-northwest to south-southeast cross section across the Delaware Mountains, west Texas. Base surfaceis the approximate base of the Brushy Canyon Formation. Upper surface shows present surface of erosionacross the Brushy Canyon Formation. Dashed line shows the approximate restored top of the Brushy CanyonFormation. BB=Brushy Bench; BM=Brushy Mesa; UCH=Upper Cmanhat; CC= Colleen Canyon; CC1=Cor-doniz Canyon. See Figure 1-1 of Beaubouef et al. (1999) for locations of topographic features. (b) Flattenedseismic profile on the 14.35 Ma horizon from the Thunder Horse minibasin, northern deep Gulf of Mexico. SeeFigure 5-7d for location of profile, and Figure 5-9 for two nearby wells that penetrate the main reservoir inter-val. Modified from Lapinski (2003). Reprinted with permission of Todd Lapinski.
photograph of the Brushy Canyon shows the strata are similar in thickness to the producinginterval at Thunder Horse (Figure 5-10). Thus, both the log and outcrops are quite comparablein scale.
These displays illustrate the kinds of details that can not be recognized on regional seis-mic profiles, and why these two additional data sets begin to help address the gap in scalebetween seismic and wireline logs. Only the thicker stratigraphic features can be seen at bothscales in the seismic.
The recent trend of interpreting shallow subsurface 3-D seismic hazard surveys hasclearly demonstrated the potential for further bridging this scale gap. The shallower 3-D seis-mic data can give far better resolution than deeper seismic data (120 vs. 40 Hz), providing theopportunity for more detailed analog studies, especially when integrated with outcrops(Figures 5-11, 5-12). In Chapter 6 through Chapter 10, we show 3-D seismic images from theshallow sedimentary section, which are crucial for illustrating key features of each reservoirelement.
Steffens et al. (2004) reviewed the significance of using shallow analog studies in deep-water systems, citing three primary applications: (1) understanding depositional processes,(2) building architectural models for deeper buried images, and (3) for addressing shallow haz-ards problems.
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Figure 5-9. Two wireline logs from the main reservoir in the Thunder Horse Field: (a) MC 778#1, (b) MC822#1. Location of wells are shown in Figures 5-7d and 5-8. Total thickness of the 15.3 Ma-14.35 Ma sequenceis about 650 m thick (2200 feet); total sand thickness is about 250 m thick (800 feet). Compare these wells withthe thickness and lateral continuity of channel-fill strata in the Brushy Canyon Formation (Figure 5-10b).Wireline logs are from Lapinski (2003). Reprinted with permission of Todd Lapinski.
1. As we review in Chapter 1 and Chapter 4, the integration and routine usage of 3-D seis-mic data were essential to causing geoscientists to re-evaluate many of their assumptionsabout sedimentary processes in deepwater. Key images that are observed on shallow 3-Dseismic data include: wide ranges of channel evolution from straight to sinuous in plan-form (Chapter 7 and Chapter 8), facies variabilities and distribution of mass-transportdeposits (MTDs) (Chapter 9), and fill- and spill processes between intraslope basins(Chapter 3).
2. Shallow 3-D seismic data allow the opportunity to image recurring depositional ele-ments and their stacking patterns (channels and their fill, levee-overbank, sheets, andMTDs), with the ultimate goal of constructing accurate 3-D architectural models. Aspectrum of morphologies of different elements can be imaged, which can then be usedfor analogs to deeper, buried units, and for outcrop analog studies (Figures 5-11, 5-12).Important dimensional data can be compiled (e.g., channel widths and morphologies,thickness of different elements). These data can also be used to study the multitude ofwell-imaged stratigraphic trap geometries at the edge of intraslope basins. Abundant cal-ibration is needed for these studies to have high ultimate impact on exploration anddevelopment.
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Another important aspect of shallow 3-D studies is that many companies use these tocalibrate their deeper seismic data for exploration. Because the physical properties of thesediments of each deepwater basin differ, interpretation of the near-surface section canbe an important technique for understanding the seismic response of different sediments.An important step in exploration in any new basin is to understand the seismic reflectionresponse of different sediments in the basin.
3. The use of 3-D seismic data imaging of the shallow section has become increasinglyimportant in drilling hazards assessment and seafloor features. In the early 1980’s, 2-Dseismic surveys were used to study shallow gas problems in the North Sea that affectedbottom-founded rigs. More recently, 3-D data are now used to evaluate the regionalstructural and stratigraphic trends to help in well-site assessment. For example, threespecific problems have been described: (a) shallow flow problems, (b) drilling throughMTDs, and (c) possible pipeline ruptures over the relief created by shallow MTDs.
(a) While drilling in the shallow subsurface interval, many companies have penetratedshallow sand bodies (primarily late Pleistocene slope channels) that are overpres-sured in relation to the underlying and overlying stratigraphic section (Bruce et al.,2000; Ostermeier et al., 2000). Drilling has been done using one mud weight; whenthe overpressured sands are penetrated, fluid leaks into the hole causing many unex-pected problems. The delays in drilling and problems associated with these sands cancause significant increases in drilling costs. A good example of this is the development of the Ursa field in northern deep Gulf ofMexico (Eaton, 1999). A subsea production template was placed on the seafloor.While drilling the template for development wells, a shallow, overpressured sand waspenetrated, ultimately causing buckling of casing of the wells that had already beendrilled and cemented. The ultimate cost to replace the subsea template by the fourcompanies operating the field was about $100 million. Clearly, recognition of thesepotential shallow flow features is essential to reducing well costs.
(b) A second drilling hazard is the occurrence of shallow mass-transport deposits(Chapter 9). MTDs commonly are overcompacted in the shallow subsurface (< 100m; 330 feet), so that jetting or pile driving operations through them can significantlydecrease penetration rates (Shipp et al., 2004).
(c) Latest Pleistocene MTDs are common in many deepwater settings where they occurin the upper tens of meters of sediments. They are commonly covered with a thindrape of Holocene sediments. For proper design of subsea infrastructure (pipelines,production manifolds), it is important that we understand the distribution of shallowMTDs. When pipes are laid on the seafloor, differential compaction can occur, wherethe MTDs are more stable and the sediments consisting of hemipelagic drape com-pacts more (Kaluza et al., 2004). With this compaction, pipes can rupture. Thus,understanding their distribution is critical to avoid these kinds of engineering prob-lems.
Several other issues in shallow hazard studies are of major concern. For surficial prob-lems, the general issue of sea-floor stability is of critical concern for production facilities. Oneissue is the proximity of rigs to sea-floor expulsion features or active faults. In the subsurface,a key issue is the prediction of lithologies; for example, when drilling and setting casingpoints, there is a real need to avoid setting a casing point in a sand body, which can give rise toa bad cement job. Other concerns are the avoidance of drilling (a) across faults, (b) throughshallow gas pockets, and (c) through any sands (channel- fill or proximal levee). Finally, thereis a growing need to understand the distribution of hydrates in near surface sediments and theeffects of long-term development facilities overlying hydrates, and their possible responses.
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Figure 5-10(a) Regional photograph looking northwest of the Brushy Canyon Formation in the Guadalupe and Delaware Mountains, west Texas, USA.Photograph includes the northern portion of the area shown in Figure 5-7a.
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Figure 5-10(b) Outcrop photograph of the Brushy Canyon showing three high-frequency sequences. Basal sequence consist primarily of sheet depositswith some channels, and the amount of channelization increases in the upper two sequences. Note the scale of the deposits is similar to those shown in thewireline logs in Figure 5-9. After Beauboeuf et al. (1999). Reprinted with permission of the AAPG.
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Figure 5-11. High-frequency seismic images (250 Hz) of the near-surface deepwater sediments (a, b) that are similar in scale to outcrops (Figure 5-12).(a) Seismic horizon slice taken 20 ms below seafloor in one intraslope basin, late Quaternary, northern deep Gulf of Mexico. Two distinct upfan channelbelts (A, B) to the right (north) change downfan to channel mouth lobes. Also present are basin margins, mud volcano, and a “slump.” Location ofFigure 5-11b is shown. (b) Seismic profile across the distal portion of (a). Note that the lobes A and B have slightly mounded appearance amongst the lat-erally continuous sheet-like reflections that lapout against the side of the basin. The deposits are up to 50 ms in twtt. See Figure 5-11a for location of pro-file. After Beauboeuf et al. (2003). Reprinted with permission of the Gulf Coast Section SEPM Foundation. See Chapter 8 for further discussion of thefigure.
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Figure 5-12. Outcrop photograph of sheet sandstones, Grootvontein section, Permian Skoorsteenberg Formation, South Africa. Underlying correlationpanel with measured sections show lithofacies and degree of amalgamation within the sheets. Photograph shows a portion of the outcrops described inthe correlation panel. Photograph shows sheet sands at the same scale as the seismic profile in Figure 5-11b. Amalgamated massive sandstones are mostsimilar to the channel mouth lobes shown in Figure 5-11a, b. After Sullivan et al. (2000). Reprinted with permission of Gulf Coast Section SEPM Foun-dation. See Chapter 8 for further discussion of the figure.
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Reservoir Elements and ProductionDeepwater reservoir systems produce from different architectural elements. The percent
of production from these architectural elements varies greatly from basin to basin. For exam-ple, Lawrence and Bosmin-Smits (2000) estimated that 60% of the production in the northerndeep Gulf of Mexico is from sheet sands, about 25% from channel-fill deposits, and 15% arefrom thin beds in levees. In contrast, Pacht et al. (1992) characterize the production in a smallarea in current shallow water depths. They noted that about 15% of the production is fromsheet sands (basin-floor fans), 43% is from levee-thin beds and channel-fill sediments, 30%from deltaic–related strata, and 7% is undeterminable. The differences in statistics betweenthese two studies are due to different portions of a basin being studied.
Furthermore, in offshore Angola, primary production is from amalgamated channel-fillreservoirs and some sheets sands. In the western Nile, reservoirs occur primarily in amalgam-ated channel-fill deposits and in thin levee beds. Channel-fill deposits are the reservoirs inoffshore Mauritania (Vear, 2005) and Kutei (Fowler et al., 2004; Saller et al., 2004). In off-shore Brazil, reservoirs are primarily in channel-fill and sheets (lobes) deposited in a variety ofsettings (Bruhn, 1998; 2001; Bruhn et al. 2003).
In addition, in some basins, different age deposits produce from different deepwater ele-ments. For example, in the North Sea, there is a distinct evolution in the architecture andnet:gross in the producing systems (Hurst et al., 2005). The Upper Jurassic systems are gravel-rich, the Lower Cretaceous systems are sand-rich, Paleocene into Oligocene reflect sand-richchanging upward into more mud-rich systems. Reservoir architecture changes from areallywidespread, amalgamated channels (upper Paleocene) to relatively narrow incised channels(upper Eocene). Abundant remobilized sands occur in the upper section of the North Sea(Chapter 10). The upward change in grain size reflect two tectonic events of northwesternEurope: Late Jurassic rifting, and the Paleocene uplift of the Scottish Highlands.
Examples of Producing Reservoirs in this Book
In each of the following five chapters, we include a series of systematic summaries ofrepresentative deepwater fields. These examples were selected based on a variety of factors,primarily those with thorough documentation, and ones that are representative of a variety ofchallenges that companies may experience in development of certain reservoir elements. Mostof the examples that we cite in Chapter 6 through Chapter 10 occur in > 500m of water today.
Interestingly, some of the older, onshore petroleum-producing basins in the world arefrom deepwater reservoirs. Examples include many of the California reservoirs (Midway-Sun-set, Ventura Avenue, Elk Hills, Santa Fe Springs, Wilmington) discovered between 1894 and1930. However, we did not include many of these “classic” deepwater fields, such as those inCalifornia, in our discussions because of a lack of critical information for complete analyses.
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References
Beaubouef, R. T., C. Rossen, F. B. Zelt, M. D. Sullivan, D. C. Mohrig, and D. C. Jennette, 1999, Deep-water sand-stones, Brushy Canyon Formation, West Texas: Field Guide, AAPG Hedberg Field Research Conference.
Beaubouef, R. T., V. Abreu, and J. C. Van Wagoner, 2003, Basin 4 of the Brazos-Trinity slope system, western Gulfof Mexico: the terminal portion of a late Pleistocene lowstand systems tract, in H. H. Roberts, N. C. Rosen,R. H. Fillon, and J. B. Anderson, eds., Gulf Coast Section-SEPM Foundation 23rd Annual Bob F. PerkinsResearch Conference, p. 182-203.
Browne, G.H., P.R. King, K. E. Higgs, and R.M. Slatt, 2005, Grain-size characteristics for distinguishing basinfloor fan and slope fan depositional settings: outcrop and subsurface examples from the late Miocene MountMessenger Formation, New Zealand: New Zealand Journal of Geology and Geophysics, v. 48, p. 213-227.
Bruhn, C. H. L., 1998, Deep-water reservoirs from the eastern Brazilian rift and passive margin basin: AAPG Inter-national Conference and Exhibition, Rio de Janeiro, AAPG Short Course Notes.
Bruhn, C. H. L., 2001, Contrasting types of Oligocene/Miocene, giant turbidite reservoirs from deep water Camposbasin, Brazil: AAPG Distinguished Lecture Notes: www.aapg.org/
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Fowler, J. N., E. Guritno, P. Sherwood, M. J. Smoth, S. Algar, I. Busono, G. Goffey, and A. Strong, 2004, Deposi-tional architectures of Recent deepwater deposits in the Kutei Basin, East Kalimantan, in R. J .Davies, J. A.Cartwright, S. A. Stewart, M. Lappin, and J. R. Underhill, eds., 3D seismic technology: Geological Society(London) Memoir 29, p. 25-33.
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Lapinski, T. G., 2003, 3-D stratigraphic and structural evolution of the Thunder Horse mini-basin, Mississippi Can-yon, northern deep Gulf of Mexico: unpublished M.S. thesis, University of Colorado, 165 p.
Lawrence, D. T., and D. F. Bosman-Smits, 2000, Exploring deep water technical challenges in the Gulf of Mexico,in P. Weimer, R. M. Slatt, J. L. Coleman, N. Rosen, C. H. Nelson, A. H. Bouma, M. Styzen, and D. T.Lawrence, eds., Global Deepwater Reservoirs: Gulf Coast Section–SEPM Bob F. Perkins 20th AnnualResearch Conference, p. 473–477.
Mahaffie, M. J., 1994, Reservoir classification for turbidite intervals at the Mars discovery, Mississippi Canyon807, Gulf of Mexico, in P. Weimer, A.H. Bouma, and B.F. Perkins, eds., Submarine fans and turbidite sys-tems: Gulf Coast Section-SEPM Foundation 15th Annual Research Conference, p. 233-244.
Martinsen, O. J., T. Lien, and R. G. Walker, 2000, Upper Carboniferous deep water sediments, western Ireland: ana-logues for passive margin turbidite plays, in P. Weimer, R. M. Slatt, J. L. Coleman, N. Rosen, C. H. Nelson,A. H. Bouma, M. Styzen, and D. T. Lawrence, eds., Global Deep-Water Reservoirs: Gulf Coast Section-SEPM Foundation 20th Annual Bob F. Perkins Research Conference, p. 533-555.
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Morris, W. R., M. H. Scheihing, D. V. Wickens, and A. H. Bouma, 2000, Reservoir architecture of deepwater sand-stones: examples from the Skoorsteenberg Formation, Tanqua Karoo Sub-basin, South Africa, in P. Weimer,R. M. Slatt, J. L. Coleman, N. Rosen, C. H. Nelson, A. H. Bouma, M. Styzen, and D. T. Lawrence, eds.,
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Saller, A. H., J. T. Noah, A. P. Ruzuar, and R. Schneider, 2004, Linked lowstand delta to basin-floor deposition, off-shore Indonesia: An analog for deepwater reservoir systems: AAPG Bulletin, v. 88, p. 21–46.
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