Overview of Deepwater-Reservoir Elements

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

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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.


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


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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


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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


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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


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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


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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


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


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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


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


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


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


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.

2.5 mi0

0

Tw

o W

ay T

rave

l Tim

e (se

c)

0.5

66 Ma

14.35 Ma

W E

99 Ma 1.0

15.3 Ma

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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.

(b)


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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.

60

WESTEAST

40

80

FE

ET

20

0

60

40

FE

ET

20

80004000 FEET0

01

234

5

67

89

1011 AXIS

AXIS

AXIS

AXIS

AXIS

AXIS

AXIS

AXIS

AXIS

AXIS

AXIS

mud clast conglomerate

amalgamated,

massive sandstoneamalgamated,

low concentration turbidites

non-amalgamated,

low concentration

turbidites

laminated

mudstones

Examples of Producing Reservoirs in this Book

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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.


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/

Bruhn, C. H. L., J. A. T. Gomes, C. Del Lucchese, and P. R. S. Johann, 2003, Campos Basin: reservoir characteriza-tion and management—Historical overview and future challenges: OTC Proceedings Contribution No.15220, 12 p.

Bruce, R., G. Bowers, and R. Borel, 2000, Well planning for shallow water flows and overpressures-the Kestrelwell: OTC Proceedings Contribution No. 13104.

Chapin, M. A., P. Davies, J. L. Gibson and H. S. Pettingill, 1994, Reservoir architecture of turbidite sheet sand-stones in laterally extensive outcrops, Ross formation, western Ireland, in P. Weimer, A. H. Bouma and B. F.Perkins, eds., Submarine fans and turbidite systems: Gulf Coast Section SEPM Foundation 15th. AnnualResearch Conference, p. 53-68.

Eaton, L. F., 1999, Drilling through deepwater shallow-water flow zones at Ursa: SPE/IADC Drilling Conference,Amsterdam, March 9-11.

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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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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Documents

Overview of Deepwater-Reservoir Elements