new evolution of an intra-slope apron, offshore niger … · 2019. 7. 7. · recent sea-floor and...

EVOLUTION OF AN INTRA-SLOPE APRON, OFFSHORE NIGER DELTA SLOPE 181

EVOLUTION OF AN INTRA-SLOPE APRON, OFFSHORE NIGER DELTA SLOPE:IMPACT OF STEP GEOMETRY ON APRON ARCHITECTURE

MARK D. BARTONShell International E and P, 3737 Bellaire Blvd., Houston, Texas 77025, U.S.A.

e-mail: [email protected]

ABSTRACT: A high-resolution 3-D seismic dataset from the offshore Niger delta slope was utilized to study the stratigraphic architectureand evolution of a near-seafloor intraslope apron that overlies an abrupt break in slope. Elements that constitute the apron are from oldestto youngest: (1) a package of prograding lobes, (2) a complex of laterally offset stacked channels, and (3) a sinuous deeply incised bypasschannel. Apron evolution reflects the adjustment and response of sediment gravity flows to an evolving slope gradient. Lobes are depositedas flows enter the basin and encounter an abrupt decrease in slope, decelerate, and lose confinement. As the step is healed, flows remainconfined and form channels. Eventually, the apron becomes a site of erosion and bypass as down-dip basins become linked by a commongraded profile. A comparison with published examples of slope aprons suggests that the geometry of the step may impact the architectureof the apron. Aprons formed above mild breaks in slopes should be thinner, more channelized, and potentially more dissected then apronsformed above severe breaks in slope.

KEY WORDS: slope, step, fan, apron, submarine lobe, submarine channel, channel–levee, architecture, seismic stratigraphy, shallowanalogue, seismic geomorphology

Application of the Principles of Seismic Geomorphology to Continental-Slope and Base-of-Slope Systems:Case Studies from Seafloor and Near-Seafloor AnaloguesSEPM Special Publication No. 99, Copyright © 2012SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-304-3, p. 181–197.

INTRODUCTION

Intraslope basins are sites where sand-prone deposits accu-mulate; they represent a class of economically important hydro-carbon plays around the world (Prather, 2003). Recent sea-floorand near-sea-floor studies of submarine slope fans and apronshave improved our understanding of their gross morphology,evolution, and controls on deposition (Beaubouef and Friedmann,2000; Fonnesu, 2003; Prather and Pirmez, 2003; Adeogba et al.2003). However, the internal architecture and depositional ele-ments that comprise stepped slope aprons is not well docu-mented. The objective of this study is to use a high-resolutionthree-dimensional (3-D) seismic data set to characterize the inter-nal architecture of a near-sea-floor apron that occupies a shallow,stepped basin located along the western Niger delta slope.Deepwater outcrop analogues from similar depositional settingsare integrated with the seismic-based interpretations to providea picture of the geometry, connectivity, and facies architecture ofthese deposits that is below the resolution of the seismic. Thisinformation can be used to better characterize the internal archi-tecture of less well-imaged reservoirs that may have formed incomparable depositional settings.

The Niger Delta, located along the western margin of Africa,forms a symmetrical protrusion into the Gulf of Guinea thatcovers area of about 210,000 km2 and reaches a maximum thick-ness of about 12 km (Damuth, 1994). It consists of a regressivesequence of Tertiary clastics that prograded over a passive-continental-margin sequence of mainly Cretaceous sediments.(Doust and Omatsaola, 1990). The submarine slope starts 50–80km offshore of the coastline at a water depth of about 200 m.Sediment delivered to the submarine slope are sourced from thedelta as well as embayments that flank the margins of the delta(Burke, 1972).

Due to rapid sedimentation, the Niger Delta is the site of activeloading, growth faulting, and shale remobilization at depth (Allen,1965; Damuth, 1994; Pirmez et al., 2000; Steffens et al., 2003). As

a result, the slope consists of alternating bathymetric highs andlows that (1) affect the path of sediment gravity flows passingthrough the slope, and (2) act as sediment traps (Prather 2000,2003). Prather (2003) further classified the Niger Delta slope as an“above-grade, slope that exhibits subtle changes in depositionalgradient resulting in low-relief stepped or terraced topography”.

The study focused on a shallow intraslope basin located 250km northwest of the present Niger Delta (Fig. 1). The area coversa 25 km by 15 km region (375 km2) positioned about 80 kmdownslope from the shelf break in a mid- to lower-slope setting.Water depth increases from approximately 2200 m in the east toover 2800 m in the west.

Data and Methods

The interval studied was within the upper 350 ms of strataand extended from the sea floor down to a base horizon thatextended across the study area. Additional horizons subdividethe interval into major packages. Due to the complexity of theevents, each horizon was mapped along every in-line (25 mspacing) for the length of the survey. Amplitude and isochoremaps were extracted for each horizon and the intervals betweenthem. Though there are no well data to calibrate the seismicresponse, high amplitudes in the image (red/yellow) are inter-preted to result from high-impedance material, i.e., sands, andzero-crossings in the runsum volumes are interpreted to corre-spond roughly to contacts between sandstone and mudstoneunits. The seismic volume has a frequency near 65 Hz and an in-line and cross-line spacing of 25 m x 37.5 m. Estimated verticalresolution is about 8 m. Outcrop analogues presented includefollowing: (1) Colleen Canyon, Brushy Canyon Formation, westTexas; (2) Willow Mountain, Bell Canyon Formation, WestTexas; (3) Popo Channel, Brushy Canyon Formation, west Texas,(4) Plane Crash Canyon, Brushy Canyon Formation, west Texas,and (5) the Condor West Channel, Cerro Toro Formation, south-ern Chile.

Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/4259915/9781565763043_ch09.pdfby gueston 07 July 2019

MARK D. BARTON182

Slope Morphology

The base horizon is the reflector that defines the base of thesystem studied. It represents a relatively continuous event onwhich subsequent events onlap or downlap. The horizon dis-plays a complex physiography composed of high-relief, discon-tinuous, mud-cored ridges that trend parallel to the dip in slopeand low-relief steps that develop orthogonal to the ridges (Fig. 2).Amplitude maps above and below the horizon indicate that theridges form corridors that funnel sediment into distinct channelfairways that shift location through time (Fig. 3). Below the basehorizon, corridor three is occupied by a series of sinuous channelsystems while corridors one, two, and four appear inactive.Above the base horizon, corridors one, two, and four are occu-pied by sinuous channel systems whereas corridor three appearsto be inactive. In corridor four a fan-like apron deposit is presentbeneath the channel systems. This deposit was the primary focusof this study (Fig. 4). An abrupt break in slope separates corridorfour into a pair of steps (referred to as steps one and two), definedas the relatively-flat lying portions of the slope, separated by ashallow ramp, defined as the relatively steep portion of the slope(Fig. 5). The lowermost step (step two) covers an area that is about

18 km in length (in a down-dip direction) and 12 km in width (ina direction parallel to the regional slope). It displays an averagegradient of around 1.0˚. The corresponding ramp displays anaverage gradient of about 4.0˚ and a lateral extent of 3–4 km. Stepsone and two are linked by a narrow incised channel that is around400 m in width and up to 30 ms in depth. The fill above the basehorizon consists of two main elements: (1) an older, relatively thin(less the 90 ms), funnel-shaped sediment wedge, referred to as aslope apron, and (2) a younger, thicker interval (up to 350 ms)composed of a divergent network of channel–levee complexes.

Apron Architecture and Evolution

The slope apron originates from the incised-channel systemlocated along the ramp between steps one and two narrow (Fig.6). It is about 10 km in width and 16 km in length. Maximumthickness of the apron is about 90 ms and occurs near the entrypoint at the transition from ramp to step. The apron spreads outand thins down dip. Internally, it is composed of thicks andthins that display a divergent pattern. In addition, portions ofthe apron, including the primary exit point for flows leavingstep 2, have been erosionally replaced by younger channel–

Study AreaStudy Area

50 km

Map courtesy of Shell Deepwater Services

AvonCanyon

MahinCanyon

Niger Delta

FIG. 1.—Map of the Niger Delta region, West Africa. The study area is located 250 km to the northwest of the Niger Delta.



Base Horizon:appx . 250 msbelow sea floor

Contour interval is 50 ms

Corridor 2Corridor 2

Corridor 1

Corridor 1

Corridor 3

Corridor 3

Corridor 4Corridor 4

DetailedStudyArea

MinimumAmplitude Map:100 ms belowbase horizon

MinimumAmplitude Map:100 ms abovebase horizon

Outline ofdetailedstudyarea

Corridor 3ChannelSystem



A B

FIG. 2.—Base horizon structure map (ms below sea level) of the study area. Depth varies from 2700 ms (red) to 3600 ms (blue) andincreases from northeast to the southwest. Horizon displays a complex physiography composed of high-relief ridges that trendparallel to the slope and low-relief steps that develop orthogonal to the mud-cored ridges. Ridges form sediment corridors orfairways that are numbered 1–4.

FIG. 3.— A) Amplitude map 100 ms below base horizon. Corridor three is occupied by a sinuous channel system that partially spillsover into corridor two. Other corridors lack bright amplitudes and appear inactive at this time. B) Amplitude map 100 ms abovebase horizon. Corridors one, two, and four are occupied by sinuous channel systems; corridor three appears to be inactive.

levee systems. The northern portion of the apron is significantlythicker (by about 20 ms) then the southern portion. In crosssection the apron displays a wedge geometry that overall thinsdown dip (Fig. 7). Large portions of the apron have beencompletely removed by erosion from overlying channel–leveesystems. Reflector amplitude and continuity are variable buttend to increase in a down-dip direction within the apron. Instrike view, reflectors in the apron converge to the south and onlap to the north (Fig. 8). Their amplitude and continuity appeargreater in the southern half of the apron, whereas the basehorizon appears more irregular and erosional in the northernhalf of the apron. Minimum-amplitude maps for the base andtop apron horizon are shown in Figure 9. The dimmest ampli-tudes occur near the sediment entry point and down the axis ofthe system, whereas the brightest amplitudes are distributedalong the flanks of the system.

Based on reflector patterns, amplitudes, and bounding sur-faces the apron is subdivided into three distinct packages. Eachpackage represents a discrete phase of deposition, which fromoldest to youngest are referred to as (1) a lobe-dominatedpackage, (2) a channel-dominated package, and (3) a bypass-channel package (Fig. 10). The lobe-dominated package domi-nates the southern and eastern portions of the apron and con-sists of moderate- to high-amplitude reflectors that displaymoderate to high continuity. Its basal surface appears conform-able to slightly erosional. The channel-dominated package isrestricted to the northern part of the apron. The basal surfaceappears erosional. Reflector amplitude is more variable and less


MARK D. BARTON184

FIG. 4.—Amplitude extraction from the base apronhorizon. A fanlike apron is present in corridorfour beneath the distributive channel systemshown in Figure 3B.

FIG. 5.—Base horizon map (ms below sea level)from corridor four in the surrounding the areaoccupied by the fanlike apron shown in Figure4. The corridor consists of a pair of broad stepsseparated by a relatively narrow ramp. Step 2ranges in depth (below sea level) from 3300 msin the east (boundary marked by red dashedline) to 3600 ms in the west. It is confined onthree sides by anticlinal structures to the southand east and by a channel–levee system to thenorth. Step 2 is linked to step 1 by a narrowincised channel, 400 m wide, that flows fromeast to west.

FIG. 6.—Apron isochron map. The point of maxi-mum thickness is about 90 ms (red is thick, blueis thin) and occurs at the proximal portion ofthe apron near the break in slope. The apron iscomposed of a series of thicks (red) and thins(blues) that diverge and thin basinward. Areasof zero thickness are shaded purple and resultfrom erosion of overlying channel–levee sys-tems.

N

5000 m Fan-likeapron depositwithin corridor

four

MinimumAmplitudeMap: appx.250 msbelowseafloor

Outline ofdetailedstudyarea

Step 2Step 2

Step 1Step 1

Contour interval is 50 ms

Exit Point

RampRamp

Entry point and incised channel

N

4000 m

Maximum Thickness 90 ms

Region of theapron erosionallyremoved

Path of overlyingchannel-leveecomplexes

X

Y

N

4000 m

A

B



FIG. 7.—A) East-to-west dip cross section across apron, illustrating apron geometry and regional slope. The section is about 20 kmin length. Location is shown in Figure 6 (see section A–B). B) Apron deposit, shaded purple. The ramp between step 1 and 2 isthe result of an anticlinal structure and displays a gradient of about 100 ms per km. The base apron horizon across step 2 displaysa gradient of about 15 ms per km, and the top apron horizon displays a gradient of about 20 ms per km.

FIG. 8.—A) North-to-south strike section across the fan apron. Cross section is about 12 km in length. Location is shown in Figure 6(see section X–Y’). B) The apron shaded green (lobe-dominated), magenta (bypass channel), and yellow (channel-dominated) isabout 10 km in width. It is confined to the south by an anticlinal structure. The northern edge is bounded by a channel–levee systemthat trends from east to west.

Flat (10 ms/km)

Ramp

(55

ms/

km)

5000 mA

B

Lobe-dominated: High Amp / High Cont.Channel-dominated: High Amp / Low Cont.Bypass Channel: Low Amp / Low Cont.

2500 mA

B


MARK D. BARTON186

N

4000 m

FIG. 9.—Minimum-amplitude map extracted from the base apron horizon. Regions of the apron completely eroded by channel–leveesystems indicated by white fill. The brightest amplitudes are interpreted to correspond to relatively sand-rich areas, and thedarkest amplitudes (blue and green) to relatively sand-poor areas. Note sinuous amplitude dim that extends down central portionof the apron.

continuous than in the lobe-dominated package. The bypass-channel system is incised into the channel-dominated packageto the north and the lobe-dominated package to the south.Reflectors are relatively dim in comparison to the other pack-ages. By volume, the lobe-dominated package makes up about60 percent of the apron, the channel-dominated package 25percent, and the bypass channel 15 percent. In a strike sense thepackages are laterally offset, with little overlap existing be-tween adjacent packages.

Lobe-Dominated Package—

The lobe-dominated package shows a progressive thinningand spreading from east to west. Reflectors display divergentribbonlike patterns, interpreted as distributary channels, thatpass down dip into broad, fan-shaped amplitudes, interpretedas lobes (Fig. 11). There are ten individual lobe elements. Indi-vidual lobe elements are 1–4 km in width and 2–6 km in length.The elements prograde or step basinward, with younger lobeelements often appearing to incise into older lobe elements,especially near the proximal portion of the apron. Each lobeelement is associated with a distributive channel system thatbranches off from a larger, long-reach channel interpreted as a

feeder channel (Fig. 11). In cross section the channels display acuplike geometry near the up-dip portion of the apron and alens-shaped geometry near the down-dip portion (Fig. 11). Theshort-reach, radially arranged channels are around 100–200 min width. The larger, long-reach channels are 400–500 m inwidth. Channels often display dimmer amplitudes then associ-ated lobe elements, suggesting that they may not be as sand-rich. The larger, long-reach channels are occasionally flanked bybroad, apron-shaped elements interpreted as a lateral (Figs. 12,13). Individual splays range from 1 to 2 km in width and from2 to 3 km in length.

Channel-Dominated Package—

The channel-dominated package displays moderate- to high-amplitude reflectors with low continuity (Fig. 14). The package isrestricted to the northern half of the apron and displays a widthof 2 to 3 km and a thickness that varies from 60 to 80 ms. Withinthe package, reflector terminations map out as ribbonlike ele-ments that overall trend from west to east (Fig. 15). Several of theelements display arcuate geometries. The elements are inter-preted as crosscutting channel elements that form a complex ofhighly amalgamated channels. The reflector terminations result



N

4000 m

Lobe-dominated PackageChannel-dominated PackageBypass Channel System

FIG. 10.—Map of primary stratigraphic elements constituting the apron. The apron consists of three laterally stacked packages listed,in ascending stratigraphic order, a lobe-dominated package (green), a channel-dominated package (yellow), and a bypass-channel system (magenta). Amplitudes display bright divergent patterns in the lobe-dominated package, bright arcuate patternsthen the channel-dominated package, and sinuous dims in the bypass-channel systems. Areas where the base horizon has beenerosionally modified by overlying channel–levee systems are shaded gray. Areas where the apron is thin (less then 10 ms), andmud-rich (based on extremely dim amplitudes) are shaded white.

from the onlap of channel fills onto channel margins, and thetruncation of older channel by a younger channel. Individualchannel elements range in width from 500 to 750 m. The brightestamplitudes occur near the base of the channels and may representcoarse-grained lag deposits. Dim amplitudes occur near theedges of the channels and may represent fine-grained channel-margin or overbank deposits.

Bypass-Channel Package—

The bypass channel is a deeply incised, throughgoing channelsystem that dissects the apron into a northern channel-domi-nated package and a southern lobe-dominated package (Figs. 16,17). The package is up to 2 km in width and 90 ms in depth. Itconsists of three parts: (1) a narrow, sinuous, low-amplitude,throughgoing channel element, (2) a series of linear- to pod-shaped high-amplitude reflectors located at the inside bends ofthe sinuous channel, and (3) a region of chaotic, low-amplitudereflectors that flank the margins of the sinuous channel. Thesinuous channel element is 300 to 400 m in width. Depth increasesin a down-dip direction from about 40 ms at the entry point toabout 80 ms at the exit point. About two kilometers down dip

from the entry point the depth of the channel abruptly increasesfrom about 45 to 65 ms. The abrupt increase in incision is inter-preted as an up-dip-migrating knickpoint. The knickpoint repre-sents a drop in base level and readjustment of the equilibriumprofile through erosion and the base of the channel (Adeogba,2003). Up dip of the knickpoint, the channel is relatively straight,while down dip of the knickpoint it becomes progressively moresinuous. The pattern suggests that the up-dip portion of thechannel was deepening by way of knickpoint migration anderosion, while the down-dip portion of the channel had achievedbase level and was widening by lateral channel migration. Thehigh-amplitude pods, present at the base of the bypass-channelpackage, are deposits of the laterally migrating channel. In crosssection, high-angle truncations can be mapped that follow thedirection of channel migration (Fig. 17B). The deposits are be-lieved to consist of very coarse-grained material, reworked fromprevious sediments, by flows that for the most part passedthough the system. Chaotic, low-amplitude reflectors that flankthe channel are interpreted as fine-grained slump and inner-leveedeposits. The depth of incision and the presence of a throughgoingchannel is evidence of sediment bypass to the next sub-basinlower on the slope (Adeogba, 2003).


MARK D. BARTON188

BDC BDC BDC

FC

FCBDC

FC

BDCFC

BDC

FIG. 11.—The lobe-dominated package is composed of 10 lobe elements (green fill with boundaries indicated by dark lines) and acorresponding set of branching, distributary-like, channels (yellow fill). Channels are easily traceable in cross section and displaycuplike to lenslike geometries. The main feeder channel (FC) and various branching distributary channels (BDC) are indicatedon the cross-sectional views with arrows.

Channel–Levee Architecture and Evolution

The interval between the sea floor and the top apron horizonconsists of a distributive network of channel–levee complexes(Figs. 18, 19). The channel–levee complexes are up to 300 ms inthickness (measured from base of channel to levee crest) and 2–4 km in width (measured between levee crests). Individual chan-nel elements are 200–500 m in width and highly sinuous. Cross-cutting relationships indicate that the channel–levee complexesare not contemporaneous but rather separate events related toabrupt lateral shifts in the position of the channel through timedue to avulsion. Three main channel–levee complexes, num-bered 1 through 3 in ascending stratigraphic order, are identified.The position of the channels appears to be controlled by topogra-phy, with channels converging across the ramp and divergingacross the step. Avulsion points are located near breaks in slope.The first avulsion point occurs near the western edge of step 1 andresults in channel system (CLC-1) being diverted to the north ofthe apron bypass channel. The second and third avulsion pointsoccur at the ramp-to-step transition above step 2 and result inchannel systems CLC-2 and CLC-3 being diverted to the south.The avulsions also appear to occur after a period of channelaggradation. Within channel–levee elements 1 and 2, the eleva-tion of the final active channel fill is about 50 ms below the leveecrest and nearly 250 ms above the erosional base of the system

(Fig. 15B). Each of the channel systems (CLC-1, CLC-2, and CLC-3) incise through the underlying apron. HARP-like deposits,originally defined as high-amplitude reflection packets (Pirmezet. al., 1997), are associated with channel–levee complex 3. Thedeposits display funnel-shaped geometry, up to 100 ms in thick-ness and 8 km in width. They are confined by an anticlinalstructure to the south and by topography created by channel–levee complex 2 to the north. The internal architecture of thedeposits was not investigated, but the base looks erosional andthe internal character appears channelized.

DISCUSSION

The slope apron at OPL315 evolved in three distinct phasesthat correspond to a lobe-dominated package, a channel-domi-nated package, and bypass-channel system. Each phase is inter-preted to reflect the adjustment and response of sediment gravityflows to an evolving slope gradient (Fig. 20). Deposition beganafter subsidence and the formation of a shallow, stepped basin. Alobe-dominated slope apron is formed as originally confinedsediment gravity flows encounter an abrupt decrease in slope,decelerate, lose confinement, and deposit their load. As the slopebreak is healed, and a local graded profile achieved, apronaggradation ceases. Incoming sediment gravity flows remainconfined, eroding and replacing portions of the apron by chan-



Figure 12:

3000 m

Location of 2d seismic line (L = left, R = right)

Area of splayerosionallyremoved

A

B

L

R

L R

Splay (red)Splay channel (red)

Feeder channel(brown)

A

B

3000 m

L R

FIG. 13.—A) Event is interpreted as a splay lateral to the main feeder channel. Feeder channel is shaded brown, flanking crevasse splayand channel is shaded red, and underlying lobe channel is shaded yellow. Crevasse channel extends from margin of feederchannel and dissects splay. B) Seismic cross section interpreted.

FIG. 12.—A) Minimum amplitude within upper portion of lobe-dominated package adjacent to feeder channel. B) Seismic crosssection, location shown in Part A.


MARK D. BARTON190

nels. Over time, down-dip basins become linked by a regional orcommon graded profile and the step becomes a site of erosion andbypass. Flows consolidate into a single throughgoing bypasschannel that shows evidence of significant incision and enlarge-ment. The model is similar to previously described models forslope aprons (Beaubouef and Friedmann, 2000; Prather, 2003;Adeogba, 2003). The primary difference is the development of alocal graded profile that results in significant portions of theapron being channelized.

A comparison with published examples of slope aprons,such as OPL 211 (Prather et al., 2007) shows significant differ-ences in geometry and architecture (Fig. 21). Differences in-clude: (1) the geometry of the ramp and step, (2) the stackingpattern of depositional units, (3) internal architectures, and (4)the degree of dissection. At OPL 211 the ramp is steeper (80ms/km vs. 55 ms/km), the step flatter (0 ms/km vs. 10 ms/km), and the apron thicker (120 ms vs. 80 ms), than at OPL 315.At OPL 211 the apron is made up of vertically stacked, progra-

FIG. 14.—A) Minimum amplitude of channel-dominated package. B) Seismic cross section, location shown in Part A.

2400 m

1000 m

N

Location of 2D seismic line

X

Y

X Y

A

B



dational units. At OPL 315, the units erode and stack in a lateralfashion. Internally, depositional units at OPL 211 consist ofdistributive channel–lobe complexes. At OPL 315, initial depo-sitional units consist of distributive channel–lobe complexes

while later units consist of channel complexes. The apron atOPL 315 is completely dissected by the bypass channel, whereasdissection of the OPL 211 apron is focused near the entry andexit points.

1000 m

N

Location of 2Dseismic line

X

Y

X Y

A

B

FIG. 15.—A) The channel-dominated package is up to 60 ms in thickness and several kilometers in width. Reflectors display numerouscrosscutting relationships interpreted as channels (black dashed lines define channel boundaries). Channels display low-sinuosity, arcuate, map patterns that vary in width from 400 to 750 m. B) Interpreted seismic cross section. To the left of the crosssection, the channel complex is eroded and replaced by the bypass channel (purple fill). To the right of the section, the channelcomplex is eroded and replaced by a younger channel–levee complex (brown fill).


MARK D. BARTON192

Differences in element geometries and stacking patternsbetween the two systems may result from variations in thesteepness of the slope and corresponding step (Fig. 22). Sedi-ment gravity flows encountering a sharp break in slope, such asOPL 211, which displays a steep slope passing into a flat step,

are likely to transfer a larger component of the flow energylaterally than sediment gravity flows encountering a mild orshallow break in slope, such as OPL 315, which displays ashallow slope passing into a slightly dipping step. In the OPL211 case, the greater lateral transfer of energy at the slope break

2000 m

N

Location of 2D seismic line

X

Y

X Y

A

B

FIG. 16.—A) Minimum amplitude of channel-dominated package. B) Seismic cross section; location shown in Part A.



High amp reflectorsLow amp reflectorsChaotic, low amp reflectors

Knickpoint

X

Y

X Y

A

B

FIG. 17.—A) The bypass channel is characterized by sinuous amplitude dim that crosscuts the entire apron. The bypass-channelcomplex is up to 2000 m in width and 70 ms in thickness. The final channel fill is 250 to 350 m in width. The channel is deeperand more sinuous down-dip of an apparent knickpoint (see arrow). The knickpoint is suggested by an abrupt downstream (tothe west) increase in channel depth of 25 to 30 ms. Dim amplitudes indicate that much of the channel is filled with relativelymud-rich deposits. Amplitude patterns down dip of the knickpoint shows a meandering channel pattern with the developmentof point-bar-like deposits. B) Interpreted seismic cross section. The bypass channel (shaded pink and gray fill) deeply incisesthrough the previous apron deposits (shaded white). Wedge-shaped, low-amplitude reflectors flank the channel and areinterpreted as levees (brown fill). High-angle truncations (indicated by blue dashed lines) indicate that channel migration wasfrom left to right relative to the figure and punctuated by several episodes of cut and fill (see strike cross section, increments20 ms).


MARK D. BARTON194

causes the flow to lose confinement, channels become unstable,and lobes are deposited in an aggradational to laterally stackedfashion across the break in slope. By contrast, in the OPL 315case, the relatively shallow break in slope allows flow energy tobe transferred basinward. As a result, the flow remains con-fined, channels maintain stability, and lobes are deposited in aprogradational fashion across the step.

SUMMARY

Two types of systems are seen across the step slope in thestudy area: a slope apron, and a distributive network of chan-nel–levee complexes. Deposition of the slope apron began aftersubsidence and the formation of a shallow stepped basin. Theapron consists of three distinct phases of sedimentation: a lobe-

4000 m

N

X Y

Location of 2dseismic line

X

YA

B

FIG. 18.—A) Minimum-amplitude map of the interval between the sea floor and the top apron horizon. B) North-to-south cross sectionacross the study area. B) Seismic cross section; location shown in Part A.



dominated package, a channel-dominated package, and a by-pass channel. A lobe-dominated slope apron is formed as origi-nally confined sediment gravity flows encounter an abruptdecrease in slope, decelerate, lose confinement, and deposittheir load. As the slope break is healed, and a local gradedprofile achieved, apron aggradation ceases. Incoming sedimentgravity flows remain confined, eroding and replacing portions

of the apron by channels. Over time, down-dip basins becomelinked by a regional or common graded profile and the stepbecomes a site of erosion and bypass. Flows consolidate into asingle throughgoing bypass channel that shows evidence ofsignificant incision and enlargement. Differences in apron ar-chitecture may reflect difference in the geometry of the step.Aprons formed above mild breaks in slopes should be thinner,

Channel-Levee Complex 1Channel-Levee Complex 2Channel-Levee Complex 3

‘HARP’ like reflectors

4000 m

N

X Y

B

A

FIG. 19.—A) Interpretation of interval shown in Figure 18A, showing three main channel–levee complexes numbered 1 through 3 inascending stratigraphic order. The channel–levee complexes display a distributive or divergent pattern across the step, withavulsion points occurring near the transition from ramp to step. The channel complex 3 consists of a broad, apron-shaped depositincised by a sinuous channel system. B) In cross section, the channel–levee systems are numbered and color-coded in the samefashion as on the base map. The underlying apron deposit is colored purple. The channel–levee systems are up to 300 ms inthickness (measured from base of channel to levee crest) and several kilometers in width.


MARK D. BARTON196

FIG. 20.—Model describing the evolution of theintraslope apron at OPL 315. A) Local sub-sidence results in the development of astepped slope profile. B) A lobe-dominatedslope wedge forms as originally confinedsediment gravity flows encounter an abruptdecrease in slope, decelerate, lose confine-ment, and deposit their load. C) The slopebreak is healed, a local graded profile isachieved, and apron aggradation ceases. In-coming sediment gravity flows remain con-fined, eroding and replacing portions of theapron by channels. D) Down-dip basins be-come linked by a regional or common gradedprofile, and the step becomes a site of inci-sion and bypass.

FIG. 21.—Slope aprons from OPL 211 and OPL315 (this study) are compared in terms of stepgeometry, stacking pattern of depositionalunits, and internal architecture. At OPL 211the ramp is steeper (80 ms/km versus 55 ms/km), the step flatter (0 ms/km versus 10 ms/km), and the apron thicker (120 ms versus 80ms), than at OPL 315. At OPL 211 the apron ismade up of vertically stacked, progradationalunits. At OPL 315, the units erode and stack ina lateral fashion. Internally, depositional unitsat OPL 211 consist of distributive channel–lobe complexes. At OPL 315, initial deposi-tional units consist of distributive channel–lobe complexes, whereas later units consist ofchannel complexes. The apron at OPL 315 iscompletely dissected by the bypass channel,whereas dissection of the OPL 211 apron isfocused near the entry and exit points.

FIG. 22.—Diagram illustrating the impact of rampgeometry on flow character. Sediment gravityflows that pass over a steep ramp or a sharp breakin slope (such as OPL 211) transfer a larger pro-portion of the flow energy laterally than flowsthat pass over a shallow ramp or a mild break inslope. The sudden transfer of energy laterallycauses the flow to rapidly lose confinement anddeposit lobes. Lobes continue to stack laterallyand vertically as the lateral transfer of energyprevents channel formation and progradationacross the step. In contrast, flows that encounter amild break in slope (such as OPL 315) transfer thegreater part of the flow energy basinward. Chan-nels remain stable, with flows gradually losingconfinement. As a result, lobes are deposited inprogradational fashion across the step.

Flow encounters a sharp break in slope

Flow energy translated laterally

Channels are unstable, lobes are deposited

System aggrades

Flow encounters a mid break in slope

Flow energy translated forward

Channels are stable

System progradesSystem progrades

Shallow Ramp (this study)

Steep Ramp (OL 211)



more channelized, and potentially more dissected than apronsformed above severe breaks in slope.

ACKNOWLEDGMENTS

I thank Petroleum Geo-Services for allowing seismic datautilized in this study to be published. The paper benefited greatlyfrom discussions with Alesandro Cantelli, Carlos Pirmez, BradPrather, and Ciaran O’Byrne. Support for this project provided byShell Exploration and Production, including managers SteveTennant, Mark Hempton, Steve Naruk, and Marc Alberts.

REFERENCES

ADEOGBA, A.A., 2003, Depositional controls and geometries of deep-waterdeposits as imaged by near-surface 3-D seismic data, Niger Delta,Nigeria: M.S. Dissertation, Stanford University, Stanford, California,82 p.

ALLEN, J.R.L., 1965, Late Quaternary Niger Delta, and adjacent areas—sedimentary environments and lithofacies: American Association ofPetroleum Geologists, Bulletin, v. 49, p. 547–800

BEAUBOUEF, R.T., AND FRIEDMANN, S.J., 2000, High resolution seismic/sequence stratigraphic framework for the evolution of Pleistoceneintraslope basins, western Gulf of Mexico, in Weimer, P., Slatt, R.M.,Coleman, J., Rosen, N.C., Nelson, H., Bouma, A.H., Styzen, M.J., andLawrence, D.T., eds., Deep-Water Reservoirs of the World: Gulf CoastSection SEPM Foundation, 20th Annual Bob F. Perkins ResearchConference, p. 40–60.

BEAUBOUEF, R.T., ABREU, V., AND VAN WAGONER, J.C., 2003, Basin 4 of theBrazos–Trinity slope system, western Gulf of Mexico: the terminalportion of a late Pleistocene lowstand systems tract, in Roberts, H.H.,Rosen, N.C., Fillon, R.H., and Anderson, J.B., eds., Gulf Coast SectionSEPM Foundation, 23rd Annual Bob F. Perkins Research Conference,p. 182–203.

BURKE, K., 1972, Longshore drift, submarine canyons, and submarine fansin the development of Niger Delta: American Association of Petro-leum Geologists, Bulletin, v. 56, p. 1975–1983.

DAMUTH, J.E., 1994, Neogene gravity tectonics and depositional processeson the deep Niger Delta Continental Margin: Marine and PetroleumGeology, v. 11, p. 320–346.

DOUST, H., AND OMATSAOLA, E.M., 1990, Niger Delta, in Edwards, J.D., andSantagrossi, P.A., eds., Divergent/Passive Margin Basins: AmericanAssociation of Petroleum Geologists, Memoir 48, p. 201–238.

FONNESU, F., 2003, 3-D seismic images of a low-sinuosity slope channel andrelated depositional lobe (west Africa deep offshore): Marine andPetroleum Geology, v. 20, p. 615–629.

PIRMEZ, C., HISCOTT, R.N., AND KRONEN, J.K., JR., 1997, Sandy turbiditesuccessions at the base of channel–levee systems of the Amazon Fanrevealed by FMS logs and cores: unraveling the facies architecture oflarge submarine fans, in Flood, R.D., Piper, D.J.W., Klaus, A., andPeterson, L.C., eds., Proceedings of the Ocean Drilling Program,Scientific Results, v. 155: College Station, TX, p. 7–33.

PRATHER, B.E., 2003, Controls on reservoir distribution, architecture andstratigraphic trapping in slope settings. Marine and Petroleum Geol-ogy 20, p. 529–545.

PRATHER, B.E., AND PIRMEZ, C., 2003, Evolution of a shallow ponded basin,Niger Delta slope: 2003 American Association of Petroleum Geolo-gists, Annual Meeting, Expanded Abstracts, v. 12, p. 140–141.

STEFFENS, G.S., BIEGERT, E.K., SUMNER, H.S., AND BIRD, D., 2003, Quantitativebathymetric analyses of selected deepwater siliciclastic margins:receiving basin configurations for deepwater fan systems: Marineand Petroleum Geology, v. 20, p. 547–561.


new evolution of an intra-slope apron, offshore niger … · 2019. 7. 7. · recent sea-floor and...

Documents