stratigraphic response to evolving geomorphology in a submarine apron perched on the upper niger...

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Special Publication No. 99

Application of the Principles Seismic Geomorphology to Continental Slope and

Base-of-slope Systems: Case Studies from Seafloor and Near-Seafloor Analogues

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SUBMARINE APRON, NIGER DELTA SLOPE 145

STRATIGRAPHIC RESPONSE TO EVOLVING GEOMORPHOLOGYIN A SUBMARINE APRON PERCHED ON THE UPPER NIGER DELTA SLOPE

BRADFORD E. PRATHERShell Upstream Americas 200 North Dairy Ashford Road, Rm EPC-A2126, Houston, Texas 77079, U.S.A.

e-mail: [email protected] PIRMEZ

Shell Research & Development, 3737 Bellaire Blvd., Houston, Texas 77001-0481, U.S.A.present address: Shell Nigeria Exploration and Production Company, Lagos, Nigeria

e-mail: [email protected]ÁN SYLVESTER

Shell Research & Development, 3737 Bellaire Blvd., Houston, Texas 77001-0481, U.S.A.e-mail: [email protected]

AND

DANIEL S. PRATHERUniversity of Kansas, School of Engineering, 2120 Learned Hall, 1530 W. 15th Street, Lawrence Kansas 66045, U.S.A.

e-mail: [email protected]

ABSTRACT: This submarine apron is an analog for the stratigraphic architecture of shallow ponded basins common to stepped, above-gradeslopes, where late-stage bypass valleys and channels did not form. Deposition of this apron began within shallow ponded accommodation.Sediment gravity flows entering the basin pass through a leveed channel that incises underlying slope muds. Flows spread, becomingdepositional once reaching lower-gradient area within ponded accommodation. Incisions at the distal end of the basin suggest that gravityflows downcut the basin sill as they bypass the basin during filling of ponded accommodation. A channelized apron downlaps the pondeddeposits, healing the stepped topographic profile formed after ponded accommodation fills. Collapsing flows exiting the entry-pointchannel create plunge-pool scours in the proximal part of the apron. Sediment gravity flows exiting the plunge-pool scour accelerate overthe steeper face of the apron, eroding bypass channels as healing progresses. Avulsion takes place as the height of the lower apron unitbuilds, forcing flows to bypass and erode the southwestern flank of the lower apron. Avulsion leads to deposition of an upper apron unit.Throughout deposition of the aprons, flows leave the basin through a gather zone at the exit point of the basin, forming a tributary scourpattern. Acceleration of these flows as they top the basin sill forms a deeply incised submarine valley. Erosion of the sill progresses byheadward-migrating knickpoints that truncate apron deposits.

KEY WORDS: submarine apron, stepped slope profile, perched apron, Niger delta slope, above-grade slope, plunge pool, apron gradients

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. 145–161.

INTRODUCTION

Study of near-seafloor analogues is important because theyprovide higher resolution of basin-fill stratigraphic architecturethan conventional deep-imaged seismic data. Shallow analogshave a variety of uses throughout the life cycle of typical deepwaterplays, including: (1) identification of drilling hazards, (2) model-ing of slope depositional processes, (3) architecture calibration ofseismic facies, and (4) reservoir modeling. Detailed mapping ofshallow, well-imaged seismic sequences improves our under-standing of models of deepwater depositional processes. Pat-terns of deposition controlled by slope gradient, entry points, andaccommodation are identified most confidently in the near-seafloor setting. Where they reoccur through multiple deposi-tional episodes, near-seafloor features make particularly usefulanalogs for deeper sequences. Variable acoustic rock propertiesin most near-seafloor settings, however, require coring for reli-able lithologic calibration.

Although typical seismic from near-seafloor analogs have lessresolution than outcrops, they provide three-dimensional infor-mation typically lacking from outcrops. Near-seafloor seismic is

capable of resolving surfaces related to episodes of starvation,bypass, and/or erosion that are related to both reservoir bedlength and connectivity. Outcrops show us that units bounded bythese surfaces can be too thin to be mapped confidently withconventional seismic. Dimensional data such as channel width,thickness, sinuosity, meander-belt width, and areal extent ofslumps from images of shallow analogs can be applied to reser-voir models where appropriate.

The objective of this study is to demonstrate how geomor-phology of the slope, such as gradient, entry-point position, andaccommodation, controls patterns of deposition within a shallowponded basin located across a “step” on the upper slope offshoreNigeria (Fig. 1). Together with work by Pirmez et al. (2000),Fonnesu (2003), Deptuck et al. (2003); Deptuck et al. (2007),Deptuck et al. (this volume), and Adeogba et al. (2005), this studyextends our knowledge about the evolution of stepped slopes,and the deposition of submarine aprons perched on the slope thatare unaltered by formation of bypass channels and submarinevalleys. We focus this study on the details that characterize thestratigraphic evolution of the intraslope basin located in thesoutheastern portion of an area designated as Oil Mining License

BRADFORD E. PRATHER, CARLOS PIRMEZ, ZOLTAN SYLVESTER, AND DANIEL S. PRATHER146

(OML) 134 (Fig. 1) where conventional three-dimensional seis-mic data that cover an area of 225 km2 (15 km x 15 km) can beintegrated with giant piston cores and a sub-bottom profilingsurvey (Fig. 2).

DATA SOURCES AND INTERPRETATION

Horizon interpretations of the seismic data were done bothmanually and with auto-trackers from a three-dimensional seismicvolume binned with a spacing of 25 m x 12.5 m; this provides fordetailed spatial imaging of fine-scale erosional and depositionalfeatures. In the case of the OML 134 three-dimensional survey, the80 Hz frequency content (at -10 dB) provides a vertical resolutionof 6 m (1/4 wavelength, assuming a velocity of 1500–1750 m/s).This is about twice the vertical resolution expected from conven-tional 30 Hz seismic data typical at reservoir levels (Fig. 3). Thevolume was processed to approximate a zero-phase wavelet.Besides a conventional reflection-coefficient three-dimensionalvolume, a seismic attribute that combines semblance and ampli-tude, trace shape of the seabed reflector, and -90° phase-rotatedvolumes were used for mapping purposes. Higher-frequencycontent of near-seabed seismic allows for detailed imaging ofmorphology and smaller-scale geologic features than seismic fromconventional exploration depths (Prather 2000; Steffens et al. 2004).

Twenty-three 4-inch diameter piston cores penetrated theuppermost part of the upper apron (Fig. 2). Core recoveriesranged from 1.68 to 18.06 m—recovery depends partially onthe composition of sediment at the seabed. Each of the corespenetrated about 1–2 m of hemipelagic drape, and all but four,fan cores 3, 17, 19, and 22, recovered sand below the drape.Muddier sediment tends to have better recoveries than sandysediment. Some of the shortest recoveries may indicate thepresence of sand, but this cannot always be confirmed. Bothfan core 1 and 17 provide lithologic information about theslope drape on the flank of the basin.

ACCOMMODATION AND SLOPE MORPHOLOGY

The study area in OML 134 is located on the upper NigerDelta slope west of the present location of the Niger River in1100–1400 meters of water (Fig. 1). The Niger Delta covers anarea of about 75,000 km2 and extends for more than 300 km fromits apex to its shoreline (Whiteman 1982; Doust and Omatsola1990). The Niger Delta continental slope extends westwards foranother 150 km from the present-day shelf–slope break. Thedelta and the slope together comprise a sedimentary wedge thatcovers 140,000 km2 and reaches about 12 km in thickness (Evamyet al. 1978; Whiteman 1982; Doust and Omatsola 1990).

’

FIG. 1.—Perspective view of Niger Delta slope, showing typical shale-based above-grade slope with curvilinear shale-cored ridgesand adjacent plunging syncline lows. Zoomed seafloor image comes from the survey area indicated in lower left inset. Study areais outlined by dashed line with OML 134 block. Line of section shown in Fig. 5 follows the X channel.


The large-scale wedge or shelf-margin prism grew by re-peated transits of the Niger Delta across its shelf. Mulder andSyvitski (1995) characterize the modern Niger River as a “clean”river, with a water discharge of 6140 m3/s and a sediment load of1270 kg/s, consisting of grain sizes ranging up to and includinggravels. At times when the delta reached the shelf edge, majorphases of shelf-margin progradation occurred and deepwaterslope and basin-floor deposits accumulated.

Fault-bounded sedimentary depocenters and intervening shale-cored anticlines characterize the modern Niger Delta slope andouter shelf (Cohen and McClay 1996). Lateral shale withdrawalfrom beneath the advancing deltaic load, combined with compres-sional uplift and folding of slope strata, drive the tectonics of thearea and thus the morphology of the slope (Cohen and McClay1996). The regional gradient of the present-day Niger Delta sloperanges from 1.0 to 1.2°, but it locally steepens to gradients of 1.7° to2.2°. Local gradients throughout the margin can be considerablysteeper than the regional average. The lowest gradients occur inbetween deeply rooted shale-cored anticlines that have a curvilin-ear trend paralleling the coastline (Steffens et al. 2003).

The morphology of the western Niger Delta slope is a primeexample of a stepped above-grade slope (Prather, 2003). Above-grade slopes in the classification scheme of Prather (2003) refer toa downslope seafloor profile that is elevated above the level of atheoretical concave-upward smoothed graded profile. Steppedslopes are a class of above-grade slope that exhibits subtle changesin depositional gradient resulting in low-relief stepped or terracedtopography. “Steps” are local areas of decreased gradient that lackthe three-dimensional closure characteristic of ponds. The part ofthe continental slope that connects steps is called the “ramp” (Fig.

4). Healed-slope accommodation dominates stepped slope pro-files. Healed-slope accommodation is the space across a step andramp on the slope below a three-dimensional convex hull fit to therugose seafloor topography (Prather, 2003; Steffens et al., 2003).Healed-slope accommodation is created by local subsidence and isthe space left after filling of ponded accommodation. Althoughstepped above-grade slopes typically lack the well-developedponded accommodation such as characterizes the ponded above-grade slope of the Gulf of Mexico, both ponded accommodationand slope accommodation can be present (Prather, 2003).

Steffens et al. (2003) conclude that ponded accommodation israre to nonexistent across the modern Niger Delta middle andupper slope, where healed-slope accommodation dominates andthe overall morphology is that of a stepped above-grade slope.Anastomosing dip-oriented drainage corridors extend for 100km westward across the upper and middle parts of this steppedslope (Steffens et al., 2003). This is particularly evident in thelower slope, where drainage pathways become more strike-oriented as higher-relief toe thrusts increasingly influence thedirection of sediment transport. The little ponded accommoda-tion found on the Niger delta slope occurs in the lower slope(Steffens et al., 2003). Where the steps connect through the toethrusts, tortuous corridors result in which sediment gravity flowspass (Smith, 2004). In this way deposits in these corridors recorda complex tectonosedimentary history, with numerous activestructures and throughgoing submarine valleys and channelsthat maintained a stepped above-grade slope profile (cf. Prather2003) throughout the Neogene (Heiniö and Davies 2007).

The study area consists of a series of perched submarineaprons. Submarine aprons in our scheme include fan aprons(O’Byrne et al., 1999), ponded-basin deposits, and healed-slopedeposits (Prather et al., 1998; Prather et al., this volume). Perchedsubmarine aprons include perched slope fills (Beaubouef andFriedmann, 2000), healed-slope aprons (Booth et al., 2002), andtransient fans (Adeogba et al., 2005). Submarine aprons areassociated with convergent-baselapping seismic facies (cf. Pratheret al., 1998). Slope aprons consist of outer levees associated with

FIG. 2.— Index map showing locations of seismic, cores, and crosssections projected on a seabed contour map.

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FIG. 3.—Comparison of conventional seismic frequency at normalexploration and production depths to seismic frequency nearthe seafloor in OML 134.


bypass canyons and channels, channel story elements, mass-transport deposits, and turbidite-lobe story elements (Barton,this volume; Hay, this volume; Deptuck et al., this volume;Prather et al., this volume). Direct age control for the interval ofinterest is not available, but jump correlations to interpretationsfrom Cohen and McClay (1996) suggest that the dsp horizon (Fig.5) is approximately of Pliocene–Pleistocene age and the baseapron reflector is of Pleistocene age.

These apron systems have been the subject of several studiesin recent years (Iunio et al., 1998; Friedmann et al., 1999; Pirmezet al., 2000; Prather and Pirmez, 2003). The aprons occupyhealed-slope accommodation created across steps on the slopeprofile as shale-cored structural features rose during slopeevolution (Fig. 5). The steps link across ramps at present viasubmarine valleys and channels (Fig. 1; Pirmez et al., 2000).

Pirmez et al. (2000) believe that the distinct rim and onlappinggeometry in the subsurface below the steps suggest they wereclosed intraslope basins in the past (Fig. 5). The most downdipperched submarine apron in the OML 134 area passes updipinto lower-gradient unconfined slope deposits that bury earlierperched submarine aprons (Fig. 5). Apparently, recent rates ofsedimentation exceeded local rates of subsidence, resulting in apresent-day bathymetry characterized by only subtle breaks inslope above the basin rims (Pirmez et al., 2000). Earlier deposi-tional and erosional events also affected the local topographyand influence subsequent deposition (Friedmann et al., 1999).Depositional processes and morphology, causing both pondingof sediment and diversion of flows, are most strongly influ-enced by counter-regional structures associated with shale-cored anticlines at depth.

The near-seafloor submarine apron that is the focus of thisstudy occupies a topographic low formed between localizeduplifts of mobile shale within the underlying slope (Fig. 5).Evidence of movement of underlying shale is expressed in thenear-seafloor geology as a mud volcano, ridges, and linked faults(Fig. 1). An intraslope basin forms in the hanging wall of thenorthwest-striking fault across the crest of the buried shale ridge.A series of down-to-the-north faults forms the north flank of theshale ridge. An east-striking shale-cored ridge bounds theintraslope basin to the south, as does the regional Niger Deltaslope to the east. A structural saddle with steeply plungingsouthwestern flanks forms where a domal structure and the east-striking shale-cored ridge meet (Fig. 6A). Intersection of the east-striking shale ridge and the north-striking continental slopeforms another structural saddle in the southeastern part of thestudy area. The combination of these structural elements forms adoubly plunging syncline with a shallow westward-plungingsyncline extension below the position of the seafloor submarineapron that is the subject of this study.

Submarine aprons and unconfined slope deposits occupy thetwo synclines at depths below the near-seafloor apron. Channel-scale and valley-scale scours evident on the dsp horizon (see Fig.5) and the isochore map (Fig. 6B) suggest deposition of theseearlier submarine aprons from submarine gravity flows enteringthe slope from the southeast (Fig. 6A). Thickened stratigraphyinto the synclines (Fig. 6B) suggests that deposition of these unitslikely healed-over some of the topography created within thesyncline. Either some amount of accommodation remainedunderfilled and/or additional uplift of surrounding shale-coredstructural elements created the accommodation occupied by theseabed apron. Underfilled accommodation in the syncline com-bined with continued subsidence has resulted in formation of anearly circular topographic low with ~ 25 m of ponded relief (Fig.7A).

CONFIGURATION OF THE ENTRY CHANNEL

Three channels cross the seafloor in the study area; Pirmez etal. (2000) designate them X, Y, and Y’, respectively (Fig. 1). Thesechannels link the upper slope to a shelf margin sourced fromrelatively small, updip incised coastal river systems unlike thelarge incised valleys associated with the Opuama or Afamvalleys, located on eastern and western Niger deltas respec-tively. The X channel originates below a shelf-margin delta andincised valley in the northeast corner of the OML 134 three-dimensional seismic survey (Pirmez et al., 2000). Here the Xchannel cuts older chaotic, transparent deposits derived fromadjacent shelf-margin deltas deposited within the uppermostintraslope basin on the shelf (Pirmez et al., 2000). The X channelvaries in width, depth of incision, character of fill, and sinuosity

FIG. 4.—Morphological parameters of the Niger Delta slope Xchannel (modified from Pirmez et al., 2000). Measurementsare averaged over 2 km. The zones of increased gradient andchannel depth correspond to ramps between steps.

FIG. 5.—Regional cross section of study area (line of section isshown in Fig. 1).


depending on local gradient, degree of confinement, and char-acter of the substrate. Where the X channel crosses faults andshale diapirs, local gradient and the depositional patterns change.It disappears below the entry point of the intraslope basin,located in the southwestern corner of the OML 134 three-dimensional survey at about the 62 km mark (Fig. 4; Pirmez etal., 2000). The channel re-emerges downslope of a knickpoint atthe distal end of the intraslope basin, where it joins with a largesubmarine valley (the Y channel of Pirmez et al., 2000) that cutsthrough several shale-cored ridges as it trends westward, down-slope of the study area, eventually feeding a large submarineapron located at the present base of slope (Fig. 1).

At least six smaller channels or slope gullies flank the Xchannel (Fig. 8). Together with the X channel they form a distribu-tary-channel pattern emanating from a point along the X channelseveral kilometers updip of the apex of the submarine apron.Hanging U-shaped incisions associated with these gullies alongthe walls of the X channel suggest that the gullies existed prior toformation of the X channel but were abandoned as the X channelcut down through the slope to its present depth. Levees flank theX channel in its lower reaches, draping the slope gullies andextending downdip onto the top of the submarine apron (Fig. 8).The submarine apron deposits backfill each of these channels,indicating that except for the X channel the slope gullies predatethe latest phase of deposition on the apron. Extension of the leveesonto the upper surface of the apron indicates that the levees arecoeval with the latest phases of apron deposition.

The slope gullies and the X channel incise the top of thehemipelagics where they enter the intraslope basin, modifyingthe morphology of the topographic surface at the base of thesubmarine apron (Fig. 9). These channels are deflected aroundtopography as they cut down to the bottom of the basin, suggest-ing that the rugose topography near the entry point existed whilethe basin was unfilled. The X channel extends farthest downdip,

ending at the top of ponded accommodation (Fig. 9). The otherchannels end at various positions on the basin entry slope as theyfed younger parts of the apron (Fig. 9).

A broad downdip-oriented tributary drainage pattern is evi-dent near the exit point of the basin (Fig. 10). Since this area isburied by submarine apron deposits and is updip of knickpointerosion created during later bypass of the basin, it evidentlydeveloped early in the history of the basin fill, probably by erosiveflows bypassing the area of ponded accommodation. The upperreaches of these tributary scours end at the downdip limit ofponded accommodation.

APRON ARCHITECTURE

The OML 134 submarine apron represents the last and far-thest downdip of the intraslope basin fill sequences along the Xchannel in OML 134 (Fig. 4). The apron occupies the intraslopebasin located in the southwest corner of the three-dimensionalsurvey below where the X channel disappears into the apron andupdip of where it reemerges from the apron before linking withthe Y channel (Fig. 1). The submarine apron in OML 134 consistsof at least three units — a thin low-relief ponded apron (?) and twoperched aprons which downlap the underlying unit and pro-grade across the intraslope basin floor (Fig. 11). The blue horizonseparates the perched apron into lower and upper units (Fig. 11).Discontinuous, highly reflective seismic facies characterizes thelower apron, whereas more continuous highly reflective seismicfacies characterizes the upper apron (Fig. 11, 12). This is also thedeepest seismic event that connects the basin entry point directlyto the basin exit point (Fig. 11).

The submarine apron that occupies the OML 134 intraslopebasin is generally circular in planform (Fig. 13B), reflecting theinfilling of nearly circular ponded and healed-slope accommoda-tion (Fig. 13A, C, E). Although we are not able to isolate and

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FIG. 6.—A) Perspective view of sub-apron dsp horizon (see Fig. 11 for detailed location of this mapping horizon). B) Isochore map(color-fill) of the interval between the base of the apron and the dsp horizon (time structure map shown in gray contours fromFig. 6A). Note that the thickest parts of the aprons in this interval are offset relative to the structural lows. Contours are in two-way travel time.


FIG. 7.— Measurement of A) depth and slope gradients of the B) upper apron, C) lower apron, and D) base surfaces along the centralapron transect. Red triangles indicate locations of slope breaks. Line of section is shown in Fig. 2.

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independently map a seismic event associated with a pondedsubmarine apron, the presence of demonstrable ponded accom-modation of about ~ 25 m (Fig. 11, 13A) and a correspondingsingle baselapping seismic event that onlaps the intraslope floorbelow the spill point (Fig. 11) suggests the existence of a low-reliefponded apron (sensu Prather et al., this volume).

Healed-slope accommodation makes up the remaining ~ 150m of sediment thickness (maximum) in the basin. Maximumthickness of the basin fill is offset from the deepest point of thebasin towards the entry point. The basin fill thins to the southeasttowards the basin exit point. Thinning results in part from down-lap towards the basin rim and in part from erosional truncationbeneath a knickpoint at the seafloor that cuts progressivelydownward into the large submarine valley that represents thedowndip extension of the X channel (Fig. 11). The exit point of thebasin corresponds to the structural saddle located in the south-western corner of the study area (compare Fig. 6 and Fig. 13A).Such erosional truncation of the downdip portion of the basin fillis a typical characteristic of perched aprons.

An isochore thickening of strata symmetrically distributedaround the X channel (Fig. 14B) and “gull-wing” cross sectionssuggest the presence of levees near the basin entry point. Threecurvilinear thins that run parallel to the levees suggest that thelevees are incised by small slope gullies. The gullies overtop thelevees at a break in slope a few kilometers updip of the apronentry point where the channel crosses a fault (Fig. 8).

The seafloor, which is the top of upper apron in OML 134, hasa prominent knickpoint that connects downdip to a submarinevalley (Fig. 13E). This submarine valley merges with a larger

valley to the east (termed the Y channel by Pirmez et al (2000) thatcuts across the entire slope, connecting the shelf–slope break to awell-defined submarine apron at the toe of slope (Fig. 1). Slopeprofiles, both down the feeder channel axis and through theaprons at several levels, show gradually flattening gradients (Fig.7D). The gradient change between the feeder channel and thebaselap surfaces takes place within ponded accommodation andrepresents a break in slope that is perched well above the ultimatetoe of slope outboard of the deep-water fold belt of the Niger deltaslope. Over time, as the OML 134 step healed by deposition of theapron, this perched break in slope progressively steps back intothe feeder channel (Fig. 7B, C). The downdip limit of the perchedapron corresponds to the downlap edge of the apron on top of anyponded apron deposits. Length of the slope across the stepincreases from 5 km to 12 km during progradation of the apron.These line lengths are always less than the distance across thestep, except for the last phase of deposition, when the apronprograded to the basin exit point.

Lower Apron

The surface of the lower apron has an asymmetrical radial-fanshape with its apex slightly off of center to the north relative to thebasin entry point (Fig. 13C). The gradient across the proximal partof the lower apron is lower than the gradient across the sameregion in the upper apron (Fig. 7). Thinning, multiple knickpoints,and irregular topography suggest that the asymmetry of theapron is related to a broad area of erosion on its southern flank.The depocenter for the lower apron is also offset updip of the

FIG. 8.—Perspective view of the entry channel showing the locations of splays and associated channels or gullies that incise leveeslateral to the entry-point channel.


FIG. 9.—Close-up view of the entry-point configuration below the submarine apron, which has been stripped away for viewing thehemipelagics at the bottom of the basin. Note that the X channel ends at the top of ponded accommodation whereas the downdiptermini of slope gullies, indicated by red triangles, occur at higher levels.

FIG. 10.—Perspective into the intraslope-basin exit point towards the southwest. This perspective shows the top of the conformablesuccession that underlies the basin fill (interpreted as unconfined slope deposits). It shows linear erosional patterns converginginto the basin exit point and the incision from the late-stage submarine valley that emanates from the basin exit point and cutsinto the mapped horizon. The black band marks the top of ponded accommodation.


underlying ponded accommodation, suggesting that the bulk ofthe deposition did not occur within the pond (Fig. 14A). Stratalslices through a “seismic texture” volume shows that the lowerapron consists of complexes of small distributary channels andlobes (Fig. 15A). Channels emanating from the basin entry pointfan out laterally across the prograding apron toward the basinexit point and are confined to the thickest part of the lower apron(Fig. 15A).

Thickening apron deposits, immediately downdip of the ba-sin entry point, reflect the location of multiple erosional features(Fig. 11). Only the shallowest of these erosional features can bemapped outside of the proximal part of the lower apron, becausedeeper erosional cuts are only partially preserved. The latest ofthese erosional features modifies the top of the lower apronsurface where the entry channel merges with the proximal apron,forming a dip-elongated scour (Fig. 16). Channels exit the scourarea and shallow as they extend down the apron, before they linkup with a knickpoint that leads to the basin exit point. The scouris possibly analogous to the “plunge pools” reported from thecontinental slope off California and from outcrops of the Gresd’Annot, SE France (Lee et al., 2004).

Amalgams of flute-shaped scours each approximately 500 macross coalesce across on the southeastern flank of the lowerapron, forming a scour field. Seismic reflectors are truncated

below the surface, suggesting that the scour field forms anerosional surface produced from the amalgamation of thesesmaller-scale flutes. Several of the most prominent scours con-nect updip through channels to the basin entry point. Gradientsacross the eroded apron flank are steeper than gradients per-pendicular to the main apron axes of either the lower or theupper aprons (Fig. 7). The position of the scour field suggeststhat it formed as flows entering the apex area of the lower apronwere diverted to the southwest out of the plunge-pool scourlocated at the apex of the lower apron. Bonnel et al. (2005)interpret scour fields on the Rhone Fan as either relics of buriedchannel topography or products of flow stripping from a bendin the channel, combined with a break-in-slope feature on thesurface of the Rhone Fan. In a similar way the lower-apron scourfield appears to be either a precursor to, or coeval with, avulsionacross the top of the lower apron. In either case this led to a shiftin the locus of upper apron deposition, off the main lower-aprondepositional axis.

Upper Apron

The surface of the upper apron has a symmetrical, radial fanshape with its apex centered downdip of the of the entry-pointchannel (Fig. 13E). However, there is a lateral (eastwards) shiftin thickening in the upper apron compared to the lower apron(Fig. 13F). The upper apron depocenter corresponds to thelocation of the broad, eroded area at the top of the lower apron.Isochore and trace shape patterns suggest the presence of dis-tributary lobes within the upper apron. The lobes emanate fromthe basin entry point, switch laterally in a compensating fashionas they prograde downslope across the top of the lower apron,and then switch to the southeast (Fig. 15B). Sediment waves,levees, and scour flutes are also present at the seafloor andrepresent depositional environments in the uppermost part ofthe upper apron (Fig. 9). This surficial expression suggests thatthe levee and the sediment waves probably formed towards theend of upper-apron deposition, possibly coeval with depositionof the southernmost lobes (Fig. 15B). The scour is ~ 200 m wideand connects to the entry point through a broad (1.8 km wide),shallow trough on the top of the apron just inside the outer levee(Fig. 13E).

The sediment waves are deflected around and over the low-relief levee, modifying their shape just downdip of the entrypoint (Fig. 8). These bedforms appear to have steep lee sides,with a total amplitude of approximately 1.5 m or less and anaverage wavelength of ~ 62 m across a slope ranging from 1.0°to 0.5° (Fig. 17). The sediment waves disappear approximately10 km downdip of the entry-point channel. Comparison withmeasurements of bedforms compiled by Wynn et al. (2002)suggests that they may be sandy. Their relatively high acousticimpedance, and location extending from the channel floor intothe proximal apron apex just downdip of the basin entry point,further suggests that they are sandy. Some simple calculations,assuming these features to represent antidunes and using themethodology of Prave and Duke (1990), suggests a flow thick-ness of about one-twelfth of the wavelength, or about 5 m,assuming a densimetric Froude number of 1. This thicknessshould correspond to the height of the velocity maximum of theturbidity currents, which occurs at approximately one-quarterof the total flow height. So total flow thickness would be of theorder of 20 m, well within the observed relief of the modernfeeder channel updip (Fig. 4). Furthermore, assuming a typicalflow density of 1050 kg/m3 (about 3% concentration by vol-ume), we estimate a flow velocity of the order of 3 m/s, whichis sufficient to carry granules and pebbles in the bedload.

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Fig. 11.—Stratigraphic architecture of the OML 134 submarineapron (modified from Prather 2003). Line of section is shown inFig. 2.

FIG. 12.—Strike view of OML 134 apron showing mapping hori-zons, stratigraphic units, and seismic facies. Line of section isshown in Fig. 2.


FIG. 13.—A) Perspective rendering of the base of the submarine apron; note the closed contour that marks the top of shallow (~ 30m) ponded accommodation. B) Total isochore of the combined lower and upper aprons shows circular plan form, leveed channelfeeding the apron and exit-point valley. C) Perspective view of lower apron. D) Isochore map of the lower apron. E) Perspectiveview of upper apron. F) Upper apron isochore. Depth and thickness units are two-way travel time (ms).

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FIG. 14— A) Time isochore map of the lower apron (color-fill contours) and time structure map of basin bottom surface (graycontours). B) Time isochore map of the upper apron (color-fill contours) and time structure map of the lower apron top (graycontours).

FIG. 15.—A) Distributary channel and lobe complexes within the lower apron. Colors represent a seismic attribute computed froma combination of amplitude and semblance. Horizon slice is extracted a few milliseconds below the blue horizon (see Fig. 12 formapping horizons); gray contours are from the time isochore map of the same interval (Fig. 14A). B) Trace-shape map of seabedreflector showing compensating lobes, levees, and exit-point submarine valley. Note the close correspondence of the trace shapeand the thickness of the upper apron (gray contour representation of Fig. 14B).

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Although seabed cores in the proximal apron were short andrecovered no sand, cores downdip of the sediment waves showthat sands in the granule and pebble size range are present inthis area (Fig. 18).

The sediment waves described here are similar to cyclic stepsfound in bedrock rivers (e.g., Wohl, 2000) and created in flumeexperiments with cohesive and erodible beds (Sawai 1977; Koyamaand Ikeda 1998). Cyclic steps occur in net-erosional (Parker andIzumi, 2000) or net-depositional (Sun and Parker 2005) flows.Fildani et al. (2006) suggested that the steps in the distributarychannel on the Monterey Fan are analogous to the former, whereasfields of sediment waves found throughout the fan are analogousto the latter. Lamb et al. (2008) hypothesize that steps in a the lineardepression in a distributary channel on the Eel River submarinecanyon are cyclic-step bedforms created by turbidity currents.

DISCUSSION

Our preferred interpretation for the stratigraphic evolutionof this shallow-ponded intraslope basin is built on the assump-tion that there are ponded deposits onlapping the intraslopebasin floor in OML 134 and that these are overlain by a subma-rine apron that downlaps the top of ponded deposits (Fig. 11).In this scenario deposition began following subsidence of theintraslope basin and creation of ponded accommodation. Thepresence of older apron units above the dsp horizon suggeststhat subsidence occurred relatively early and the basin was atleast partially filled. Thickening of unconfined slope units abovethis older apron unit suggests that any residual topography thatremained after deposition of the older aprons was filled, possi-bly forming a local graded to stepped-slope profile. Reforma-tion of ponded accommodation at the base of the younger apron

FIG. 16.—Perspective-view detail of the top of the lower apron showing entry-point scours, scour field, and channels exiting thescoured area.

Fig. 17.—A) Seismic profile and B) topography measurementsacross the field of sediment waves at the apron entry point (seeFig. 8 for location).


unit probably occurred during deposition of the draping hemi-pelagic (?) unit that forms the base of the shallow intraslopebasin (Fig. 5).

An alternative interpretation for the stratigraphic evolution ofthis shallow intraslope basin is built on the assumption that asingle ponded apron does not cover the entire intraslope-basinfloor, but rather there are multiple ponded aprons, resolved byseismic as the toesets of the prograding clinoforms, that charac-terize the apron (Fig. 19). Under this scenario deposition beginsas an apron progrades into ponded accommodation of ~ 30 m

FIG. 18.—Selected cores from the proximal area of the upper apron showing lithologic columns (refer to Fig. 2 for core locations).

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depth. The apron clinoform builds angle until bypass begins onits proximal part. Denser parts of sediment gravity flows bypassthe upper part of the clinoform and pond in front of the apron andbehind the downdip basin sill. Progradation continues until theapron grades to the basin sill. The rest of the depositional historyis the same as in the preferred scenario. We expect that thisscenario is less likely, inasmuch as we observed that the gradientof clinoforms within the apron remains constant (Fig. 7). If thetoes of the clinoforms were healed over during progradation, weexpect that the intra-apron clinoforms would shallow upwards.


PONDING AND EARLY-STAGE BYPASS PHASE

Introduction of sediment gravity flows into the ponded slopebasin occurred through the leveed slope channels that inciseunderlying slope mud at the proximal end of the basin. Thesediment gravity flows became depositional and spread uponentering ponded accommodation. If there was any ponded ac-commodation, this phase of deposition would produce a low-relief ponded apron in the basin. If the alternative scenario wasthe case and no ponded accommodation existed in the basin, thendeposition of a perched submarine apron would occur. In eithercase incisions between the rim of the basin and the basin exit point(Fig. 10) suggest an episode of early sediment bypass as sedimentgravity flows downcut the basin sill after available accommoda-tion was filled (Fig. 20A). Early bypass produced truncation at theexit point similar to that seen in the shallow ponded Brazos–Trinity Basin II (Prather et al., this volume).

Healing Phase

Filling of the pond was followed by progradation of twoapron sequences. The aprons downlap the ponded deposits,healing some of the topography of the step (Fig. 20B, C). Sedi-

FIG. 19.—Alternative interpretation for the stratigraphic architec-ture of the OML 134 perched submarine apron (modified fromPrather, 2003).

FIG. 20.—Stratigraphic evolution of the OML 134 intraslope basin.

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ment gravity flows entered a newly created step after filling ofponded accommodation. The flows formed a broad apron withcontinuous to discontinuous seismic facies associated with aproximal distributary-channel complex and a distal tributarynetwork of channels and lobes that converges toward the exitpoint of the basin. Lateral deposition away from the channels atthe head of the apron is partly responsible for the overall up-dipshift in depocenter during deposition. Convergence of the flowsforced by increased lateral confinement formed a gather zone oftributary erosional features observed at the basin exit point(Figs. 20B, C).

As healing progressed, the bypassing portion of gravity flowsaccelerated over the steep lee face of the basin sill, developing adeeply incised submarine valley (Fig. 20). Erosion of the sillcontinued by headward-migrating knickpoints in the gatherzone, truncating the distal parts of the apron (Fig. 20). At OML 134these knickpoints do not migrate across the apron, so the subma-rine valley does not connect directly to the basin entry point asobserved in many other aprons (O’Byrne et al., 2004; Barton, thisvolume; Bohn et al., this volume; Deptuck et al., this volume;Prather et al., this volume). The reason for this incomplete con-nection in OML 134 is not fully understood, but it is likely relatedto early apron abandonment, and shutdown of turbidity currentinput into the system.

Transition upward from the channelized aprons with theirdiscontinuous seismic facies to more continuous seismic faciesassociated with compensating sandy (?) apron lobes took placeacross the blue horizon (Fig. 20D). The blue horizon marks the firsttime in the basin-fill history that a depositional surface connects theentry point to the exit point without the intervening step created atthe run-out edge of the prograding apron (Fig. 7). Depositionalprofiles of the aprons evolve through time, from an initial shallowpond of 25 m depth to a progressively lengthening depositionalsurface on top of the overlying perched aprons (Fig. 7). Lengthen-ing occurs by coeval progradation of the distal edge and backstep-ping of a break of slope into the basin entry point. Measurement ofgradients on the slope updip of the intraslope basin (i.e., a ramp)and across the top of the perched apron shows no significantchanges throughout the fill history of the basin (Fig. 7).

Late-Stage Bypass and/or Abandonment Phase

The change from a perched apron characterized by aggrada-tional distributary channels and lobes with multiple plunge-poolscours at the entry point, to an apron characterized by laterallyshifting distributary lobes, levees, and sediment waves, suggestsa change in the character of sediment gravity flows entering thebasin. A channel to lobe transition in the lower apron within thearea of the step, (Fig. 15) and an upper unit consisting of smallsandy (?) lobes also within the area of the step (Fig. 8), demon-strates that flows entering the step throughout apron depositionwere small enough to have sand runout distances less than theslope length across the step. Decrease in the size of lobes and achange from sandy (?) lobes to a muddy lobe in the upper part ofthe apron further suggests a decrease in the size of sedimentgravity flows entering the basin and possible abandonment of thefeeder channel. If the sediment flows are decreasing, then theabsence of a throughgoing bypass channel or submarine valleysuggests that the lobes and levees are parts of an abandonmentfacies assemblage.

The association of a well-developed knickpoint with laterallyshifting distributary lobes, levees, and sediment waves in theupper apron suggests that this assemblage of seismic facies maybe indicative of bypass conditions and could serve as a model forthe stratigraphic architecture of perched aprons either prior to

knickpoint migration and incision associated with formation of abypass channel and/or valley (Barton, this volume; Deptuck etal., this volume; Prather et al., this volume) or for aprons wherea bypass channel and/or valley did not form due to abandon-ment. Preservation of the assemblage of bypass features is rare inmost perched aprons because perched aprons are generally char-acterized by the presence of a bypass channel or a valley. Aban-donment may result from either eustatic sea-level rise, avulsionof the X channel, or possible capture of the updip drainage basinby larger river systems incising the shelf, such as the Opuama orAfam (Doust and Omatsola, 1990), shutting down sedimentsupply to the smaller incised coastal systems on the exposedshelf. Alternatively, the change in facies across the blue horizonindicates limited accommodation across a basin now in a state ofdepositional equilibrium. Once the slope reaches equilibriumeverywhere in the basin, thick apron deposits cannot be accom-modated, resulting in thinner individual lobe elements and morefrequent lateral shifting.

CONCLUSIONS

Sixty-nine percent of producing deepwater reservoirs occurin slope aprons (Prather et al., 2009), of which perched apronsmake up a significant proportion (28%; Fig. 21). The OML 134system is therefore a useful analog for a significant subset ofproducing deep-water reservoirs. If the fundamental control onreservoir architecture and distribution across varied slope pro-files is the interaction between local gradient change and grainsize of sediment gravity flows, regardless of absolute positionalong the slope, then the architecture of the OML 134 perchedapron should also be analogous to submarine aprons elsewhereregardless of absolute slope position and to as much as 79% ofproducing reservoirs from deep-water globally (Fig. 21).

The intraslope basin described from OML 134 is an analog forthe stratigraphic evolution of shallow ponded basins common tostepped, above-grade slopes, where late-stage bypass valleys andchannels did not form. This case study suggests that compartmen-talization of reservoirs by late-stage-erosion channels and/or val-leys should not be expected for analogous apron types, and reser-voir architectures should be largely preserved in their originaldepositional form. Depositional history of the OML 134 apron issimilar to the fill histories of other perched aprons on ponded orstepped slope profiles consisting of (1) ponded-apron depositionfollowed by (2) early bypass, and creation of a knickpoint front atthe exit-point gather zone, (3) apron deposition with prograding

FIG. 21.—Frequency distribution of types of slope reservoirs(from a proprietary Shell database; Prather et al., 2009).


distributary channel–lobe complexes, (4) late-stage sediment by-pass through a single knickpoint connected to a submarine valley,and/or (5) abandonment. The upper apron provides a uniqueexample of a thin abandonment or bypass facies assemblage ofsmall lobes without the leveed, bypass channel or valley com-monly observed on the tops of other perched aprons such asobserved in Basin II of the Brazos–Trinity system (Winker, 1996;Beaubouef and Friedmann, 2000; Prather et al., this volume) andBenin Major (Deptuck et al., this volume).

At conventional exploration and production depths and seis-mic frequencies the entire intraslope basin fill would be consid-ered part of a convergent base-lapping seismic facies (sensuPrather et al., 1998). Convergent base-lapping seismic facies areoften assumed to be indicative of depositional ponding and thepresence of areally extensive “sheet” sand reservoirs. The OML134 example suggests that this expectation is not always wellfounded. In the case of OML 134 the thickest reservoirs, up toabout 100 meters thick in this case, probably occur close to theintraslope basin entry and are located neither in the center of thebasin nor at the basin exit point or backstop position. Expectedreservoir architecture in the perched apron is dominated bydistributive channel systems, and there is little evidence of “sheet”sands. OML 134 also provides an example of vertical facieschange from distributary channel complexes to a “bypass faciesassemblage” consisting of sandy distributary lobes, sedimentwaves, and levees that cap the perched apron as depositionalequilibrium is reached (after the apron progrades to the basin exitpoint). OML 134 can be used to develop reservoir-architecturescenarios used for construction of static models for a class ofhydrocarbon-bearing perched-apron reservoirs that show littleevidence of knickpoint migration and incision.

The exit point of the OML 134 apron may also offer someuseful information for understanding discontinuous and com-partmentalized reservoirs such as in the Macaroni field (Booth etal., 2003), located at the exit point of the Auger intraslope basin inthe Gulf of Mexico. The OML 134 analog shows that sands at exit-point position have the potential of occupying isolated channelsassociated with early bypass of the step followed by furtherstratigraphic isolation due to erosion from the overlying knickpoint(Fig. 10).

ACKNOWLEDGMENTS

We gratefully acknowledge Shell International E&P for per-mission to publish this paper. We also acknowledge the helpfulcontributions from our Shell colleagues Marcel de Jong, RichardBirch, David Steele, Herman Lauferts, Gary S. Steffens, CiaranO’Byrne, Matthijs van der Molen, and Alesandro Cantelli.Maurizio Orlando guided the release of seismic data from ENI,and reviews by Ron Steel and Joe Cartwright provided construc-tive critiques of the paper.

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