Imaging_Channels_In_Nile_Delta

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Predicting sand distribution andreservoir presence is a major explo-ration risk in the Nile Delta offshoreEgypt. However, integrating state-of-the-art technologies including 3-Dseismic reflection surveys, seismicattributes, amplitude extractions, andcoherency techniques can reduce thisrisk through recognition and moreaccurate mapping of submarine val-leys and channel complexes. In addi-tion, identification of sediment inputpoints and pathways may result inmore accurate interpretations of sanddistribution patterns.

Seismic profiles from the area ofinterest indicate that Upper Miocene(Messinian) evaporites created a

series of bright reflectors that areoverlain by a Lower Pliocene trans-gressive sequence which, based onwell control and seismic character,appears relatively sand-rich. Theshale-prone Middle Pliocene sectionis relatively dim, with weak and dis-continuous reflections and someobvious erosional surfaces. TheUpper Pliocene and Pleistocene sec-tions are complex, with abundanterosional and slump surfaces. Theyare seismically brighter, and well con-trol indicates that they are sandy.

Within the Lower Pliocene inter-val, strike-oriented seismic profilessuggest that channels and valleys arepresent. Seismic character of the val-

ley/channel fills is complex, with mul-tiple incised surfaces, aggradationalfill, and evidence of lateral accretion onsome sections. Coherency horizonslices show the Upper Miocene-LowerPliocene channel systems are confinedto a single valley that is broad, rela-tively deep, and generally sinuous.Higher in the stratigraphic section, sin-gle valleys divide into multiple chan-nel courses which are narrower,shallower, and less sinuous. Thesechanges are interpreted to reflect thetransition from more proximal to moredistal submarine channel facies as NileDelta system tracts backstepped dur-ing the Late Miocene-Early Pliocenetransgression.

580 T HE LEADING EDGE JUNE 2000 JUNE 2000 T HE LEADING EDGE 0000

Imaging submarine channels in thewestern Nile Delta and interpreting theirpaleohydraulic characteristics from 3-D seismic

WILLIAM A. WESCOTT, Consultant, Houston, Texas, U.S.

PAUL J. BOUCHER, BP Amoco Egypt Gas Business Unit, Cairo, Egypt

Figure 1. Nile Delta Neogene chronostratigraphic chart. Key horizons are Pleistocene NDT-10, Pliocene NDT-15, NDT-20, NDT-30, and Late Miocene NDT-40.

CORNER

CoordinatedbyLindaR.Sternbach


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These submarine, delta-frontchannel complexes appear in planview to be very similar to fluvialchannels. Assuming the channel pat-terns are the result of similar physi-cal processes in both depositionalsettings, submarine channels should

be subject to the same type of paleo-hydraulic analyses that have beenapplied to ancient fluvial deposits.

In this article, we look at the mor-phological characteristics of threesubmarine valley/channel com-plexes (Rosetta, Abu Madi, and Kafrel Sheik channels) and attempt tointerpret the processes that formedthem within a sequence stratigraphiccontext and use that analysis to infersand/shale ratios of the sedimentsthat were transported by these chan-nel complexes.

Stratigraphy and depositional set-ting. In 1996, Amoco Egypt OilCompany initiated a project to betterunderstand the Cenozoic stratigra-phy of the Nile Delta. The projectintegrated paleontologic, sedimento-logic, and seismic data to more pre-cisely define the stratigraphicframework for the last 25 millionyears. The initial step was to deci-pher the chronostratigraphy of thedelta using paleontological data to

build a provincial composite stan-dard using graphic correlation tech-niques.

Graphic correlation can deriveprecise, consistent, and accurate bio-

stratigraphic correlations. Theprocess involves crossplotting twostratigraphic sections of similar ageon x-y coordinates and drawing a lineof correlation (LOC) based on fossildatums, or crossplotting the observedranges of a single stratigraphic sec-tion against a database of compositedfossil ranges scaled in dimensionlesschronostratigraphic units. This data-

base is referred to as a compositestandard. A composite standard isconstructed by combining fossilranges from several stratigraphic sec-tions. As more and more fossil groups

are added, the composite standardbecomes more robust and accurate.Nonfossil chronostratigraphic data,such as radiometric dates, can beadded to further refine and calibratethe composite standard. Aworldwidecomposite standard can be developedusing fossil ranges from many dif-ferent widely spaced sedimentary

basins, or provincial standards canbe constructed for individual basinswhere some fossil ranges may have

been restricted because of local envi-

ronmental conditions.On graphic correlation x-y plots,

sloping lines represent periods of timein which there were continuous depo-sition and preservation of accumulatedsediments. Horizontal slopes or ter-races represent intervals of erosion,

nondeposition, or very slow deposi-tion. Plots from a series of wells and/oroutcrops within a basin can be corre-lated to each other and related to geo-logic time. Relative time (compositestandard units) can be converted toabsolute time in mega-annums (MA).When plotted against time in MA, theduration of missing time across uncon-formities can be calculated andmapped, and decompaction factorscan be applied to periods of rockaccumulation to calculate sedimenta-

tion rates. These calculations giveinterpreters more accurate pictures ofdiachroneity of sedimentation and ero-sion rates within a basin and can then

be used to develop more accuratebasin models.

Once the chronostratigraphy was

established, the paleontological datawere looked at in detail, particularlythe abundance and diversity ofspecies and bathymetry indicators,to determine if graphic correlationterraces were caused by faults,unconformities, environmental shifts,or flooding surfaces (condensedintervals). This was integrated withother stratigraphic data to constructa chronostratigraphic chart (Figure1). The positions of the biostrati-graphic terraces were then matched


Figure 3. Coherency horizon slice of Rosetta channel complex 128 ms belowinterpreted NDT 30 surface. Marked profiles are arbitrary lines orientedalong the axes of the channels extracted from 3-D volume in Figure 6.

Figure 2. Arbitrary strike line from western Nile Delta showing interpretedgraphic correlation terraces and sequence stratigraphy.


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with lithostratigraphic eventsrecorded on well logs and then trans-ferred onto interpreted seismic lines(Figure 2) to further develop asequence stratigraphic model for the

basin.This article primarily focuses on

the stratigraphic interval betweenNDT-40 to just above NDT-30(Figures 2 and 3, NDT = Nile Delta

Terrace). This interval was depositedduring the latest Miocene and theEarly Pliocene and mainly comprisesthe Rosetta anhydrite and the sandsof the Abu Madi formation. Thesesediments were deposited during thetransition from lowstand to high-stand conditions. The Rosetta anhy-drite was deposited after a period oferosion that produced the Intra-Messinian unconformity (NDT-40).These evaporites were deposited inlows formed during the downcuttingevent and may represent depositionduring several high-frequency pulsesof sea-level rise and fall during latestMessinian time.

The Abu Madi section is a sand-rich interval that represents sedimen-tation during the latest Miocene. Thesesediments are a major exploration tar-get in the Nile Delta. On the central,more proximal portion of the delta,Abu Madi sands were deposited inlarge canyons that were eroded con-temporaneously with deposition ofthe Rosetta anhydrite. In the more dis-tal, delta front environments theywere deposited over a more continu-

ous, less deeply eroded anhydrite sec-tion during a major transgression. Wewill look more closely at some of thedepositional and erosional features inthe transition zone from proximal dis-continuous anhydrite to the more dis-tal continuous anhydrite section.

The latest Miocene transgressionculminates in the maximum flood-ing event NDT-30. This flooding sur-face defines the base of the Pliocene(Figures 3 and 4). The Kafr el Sheikformation (NDS-30, NDS = Nile DeltaSection) subtly downlaps onto theNDT-30 surface. The Kafr el Sheikformation is composed primarily ofshale with interbedded, isolatedsands deposited within an EarlyPliocene highstand system tract.

Identification/characterization ofsubmarine channel complexes.Submarine channels and channelcomplexes were identified using aproprietary coherency algorithm.Horizon-slice images were extractedfrom a 3-D volume of coherency datathat was flattened on the interpreted

NDT-30 surface. We identified threechannel complexes within the area ofinterest on the western Nile Delta:(from old to young) Rosetta, AbuMadi, and Kafr el Sheik.

All are clearly identifiable on thecoherency data. Coherency images ofthe channel complexes were used tocalculate meander length and sinuos-ity. Arbitrary vertical seismic lineswere extracted from the 3-D seismicvolume perpendicular to the axis of thechannels, and the channel bases wereinterpreted on these lines. Then, arbi-trary lines oriented along the axes ofthe channels were extracted from the3-D volume. These axial lines weretied to the cross-lines and channel

bases were interpreted on them aswell. Because of the uncertain nature

of the seismic responses at the actualbase of the channels (due to a lack ofrock data to calibrate the seismicresponse), it was not possible to usethis pick to calculate width/depthratios.

Calculation of paleoslopes hingedon the ability to tie the interpretedchannel base to a horizontal datum.We used the base of the Rosetta anhy-drite as our datum for the three chan-nel complexes. This horizon waschosen because it is a relatively con-tinuous and unambiguous seismicevent across the area of the study.Interpreted bases of the individualchannel complexes were then sub-tracted from the base of the Rosettaanhydrite. After evaluating the stack-ing velocities in the area, the calcu-


Figure 4. Arbitrary seismic lines perpendicular to Rosetta channel complex(base of the channel complex = yellow) showing the channel complex cut-ting down into the Rosetta anhydrite (bright reflectors).


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lated times between the two eventswere converted to vertical distances

by applying a constant velocity of 1.2m/ms of two-way traveltime. Valleylength was measured as a straight linefrom the updip portion of the chan-nel complex on the coherency imageto the downdip end.

Rosetta. The image of the Rosettachannel complex (Figure 3) is acoherency horizon slice extracted 128ms below the interpreted NDT-30 sur-face. On seismic profiles (Figures 4and 5) it is apparent that this channelcomplex incises the underlyingRosetta anhydrite. This channel com-plex is characterized by well-definedchannel edges, with a relatively lowsinuosity. The minimum averageslope calculation for this channel com-plex is 4.4 m/km (0.044%).

Abu Madi. The coherency horizon slice

that best images the Abu Madi chan-nel complex (Figure 6) is 72 ms belowthe interpreted NDT-30 surface. Thischannel complex occurs stratigraph-ically above the Rosetta anhydrite. Inthis system, seismic profiles (Figures7 and 8) suggest that the channel com-plex is confined within fan/overbankdeposits. This system is characterized

by a relatively moderate sinuosity.The minimum average slope calcula-tion is 6.3 m/km (0.063%).

Kafr el Sheik. The Kafr el Sheik chan-nel complex is best imaged by acoherency horizon slice (Figure 9) 80ms above the interpreted NDT-30horizon. Seismic profiles (Figures 10and 11) show that this channel com-plex has well-defined edges. This sys-tem is characterized by moderatesinuosity. The minimum averageslope calculation is 3.5 m/km(0.035%).

Limits and potential problems withpaleohydraulic analysis. In manyareas of the world, submarine chan-nels, channel complexes, and valleyscan be clearly imaged using 3-D seis-mic attributes. In plan view, thesepaleogeomorphic submarine formsare very similar to features created bysubaerial fluvial processes. Althoughfluvial and submarine density flowprocesses have obvious differences,some of the hydraulic processes aresimilar, particularly the basal and lat-eral boundary conditions when theflow is confined within canyons orchannel levee complexes. One majorhydraulic difference affecting theprocess that forms these features isthe density contrast at the interfacewith the upper bounding surface ofthe flow. In the fluvial setting, thisdensity contrast between water andair is sharp and occurs at one atmo-sphere of pressure. In the submarine


Figure 5. Arbitrary seismic line (top) down the centerof Rosetta channel complex. Base of the channel isblue. Lower image is flattened on the base of theRosetta anhydrite.

Figure 6. Coherency horizon slice, showing the AbuMadi channel complex 72 ms below interpreted NDT-30 surface. Marked profiles are arbitrary lines orientedalong the axes of the channels extracted from 3-D vol-ume in Figure 9.

Figure 7. Arbitrary seismic lines perpendicular tothe Abu Madi channel complex (base of the channelcomplex = yellow).


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environment, the upper boundary istransitional between sediment-ladenwater and less dense seawater andoccurs at pressures greater than oneatmosphere.

However, because the resultantfeatures are similar in both environ-ments, theoretically one might assumethat they should be amenable to thesame type of paleohydraulic analysesthat have been applied to ancient flu-vial systems. If indeed this approachis correct, then it can be a significanttool for predicting reservoir charac-teristics, specifically by evaluatingparameters that are related to sand/shale ratios. These parameters includesinuosity, meander wavelength, slope,and width/depth ratios. Wide, shal-low, straight channels are more proneto transport sand. Narrow, deep, sin-

uous channels transport finer-grainedsediments.

In this study, some complicationshave arisen because certain assump-tions must be made in order to calcu-late slopes from seismic data. Weflattened the seismic on a horizon

beneath the channel complexes.However, during the times the chan-nels were formed, the horizon we flat-tened, the Rosetta anhydrite, was mostlikely not flat but had a basinwardslope. Therefore, the slopes calculatedare probably lower than the actual gra-dients, which is why we refer to themas minimum average slopes. Also, wewere unable to calculate width/depthratios for the channel complexes

because the seismic character variesfrom trace to trace along the length ofthe complex, making it difficult to con-

sistently measure depth. On sometraces, the top of the channel complexis defined by peaks, and in other casestroughs; thus, there is no consistencyin the measurements. Additionally, thetrue base of the complex could not beaccurately mapped for the same rea-son. Consequently, we were forced tomap on a consistent but arbitrary baseof channel-complex horizon. In areaswith well control penetrating chan-nel/valley complexes, seismic can becalibrated to real rock data and theseproblems may be somewhat amelio-rated.

Discussion and conclusions. TheRosetta channel complex formed dur-ing the earliest phase of the latestMiocene-earliest Pliocene transgres-sion in the eastern Mediterranean Sea.


Figure 8. Arbitrary seismic line (top) down the cen-ter of the Abu Madi channel complex. Base of thechannel is green. Lower image is flattened on thebase of the Rosetta anhydrite.

Figure 9. Coherency horizon slice, showing the Kafrel Sheik channel complex, 80 ms above interpretedNDT-30 horizon.

Figure 10. Arbitrary seismic lines extracted perpendic-ular to the Kafr el Sheik channel complex (base of thechannel complex is yellow). See Figure 9 for the loca-tion of cross-lines.


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During this time, submarine channelsincised Messinian-age lowstandevaporites forming relatively steep-walled canyons. The incised channelcomplexes served as sediment bypassconduits transporting sedimentacross the shelf into deeper-water set-tings. The resistant walls inhibitedmeandering of the channels confinedwithin the canyon resulting in the lowsinuosity of this channel complex.

This submarine valley filled with sed-iment as the transgression continued.

The Abu Madi channel complexformed as the transgression contin-ued and the sediment supply sys-tems were no longer confined by theresistant evaporite canyon walls.Eventually, the channel avulsed andestablished a steeper slope (Figure12). In response to the increased val-ley slope, the sinuosity of the chan-nels also increased. The increasedsinuosity may also reflect a decrease

in the amount of coarse-grained sed-iment transported by the channelsand a relative increase in the amountof mud. Adecrease in the sand/mudratio is also consistent with deposi-tion in progressively more distalenvironments during the transgres-sion.

The Kafr el Sheik channel com-plex was deposited above the NDT-30 maximum flooding surface in a

highstand systems tract. This chan-nel system formed in the most distalposition of the three studied com-plexes. The low valley slope requiredthe channels to straighten and adapta less sinuous pattern in order toeffectively transport the sedimentsupplied to it. Based on regional cor-relations, this channel complex trans-ported less sand and more mud thanthe channel complexes that formedduring the rising limb of the trans-gression.

Overall changes in channel pat-terns, from early transgressionthrough subsequent highstand,reflect a progressive deepening ofenvironments and landward shiftingof the sediment source, resulting ina decrease in sand deposition and anincrease in mud. These processesresulted in a fining and thinningupward stratal architecture whichcan be seen in the North Alexandria

F-2 well in the center of the studyarea.

Submarine channels and valleyscan be successfully recognized using3-D seismic and coherency techniques.Tying channel characteristics to wellcontrol within a sequence stratigraphicframework can aid the prediction ofreservoir facies within undrilled explo-ration areas. However, not enough ispresently known about the geomor-phic processes within submarine chan-nels to accurately predict sand/shale


Figure 11. Arbitrary seismic line (top) extracted down the center of the Kafr el Sheik channel complex. Base of thechannel is yellow. Lower image is flattened on the base of the Rosetta anhydrite. See Figure 9 for location of thearbitrary line.


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ratios based only on the geometries ofchannel complexes as imaged by seis-mic attributes.

Suggestions for further reading. Moreinformation on graphic correlation andcomposite standard methods can befound in Graphic Correlation (SEPMSpecial Publication 53). Details on fluvialchannel response during eustatic sea-level changes can be found in articles byWescott (AAPG Bulletin, 1993) andSchumm (Journal of Geology, 1993). LE

Acknowledgments: This work, originally pre-sented at AAPGs 1998 convention, wasauthorized by Amoco Corporation and RepsolExploration Egypt S.A. Paleontological andbiostratigraphic support was supplied byWilliam N. Krebs. Editorial comments byLinda Sternbach were greatly appreciated.

Corresponding author: [email protected]

Bill Wescott received his undergraduate degreein geology fromFranklin and Mar-shall College. Afterserving in the U.S.

Army, he contin-ued his geologiceducation, earninga masters degree

from Southern Illi-nois University atCarbondale and aPhD from Color-ado State Univer-sity. In 1979, he

moved to Houston and worked on a series ofdomestic exploration projects for AmocoProduction. In 1984, he transferred to

Amocos international new ventures groupand spent the next 15 years engaged in a vari-ety of worldwide exploration activities, pre-

dominantly in Africa and the Middle East.From 1988 to 1990, he was the explorationsupervisor for Amoco Kenya PetroleumCompany. He is presently a consulting geolo-

gist specializing in sedimentology and sequencestratigraphy.

Paul J. Boucher received a bachelors degree(1990) in geology from Salem State College anda masters in geophysics (1994) from Texas

A&M University. In 1994, he joined Amoco inHouston and began working on Amocos hold-ings in Egypt. Currently he is an interpreta-tion geophysicist in the BP Amoco Egypt GasBusiness Unit, Nile Delta Exploration Group.

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Figure 12. Composite coherency image showing the Rosetta channel complex (orange) incised into Rosetta anhydrite.The image of the Abu Madi channel complex 56 ms above the Rosetta channel complex image is superimposed onthe upper right. The Abu Madi channel complex is yellow. Light yellow outlines levee/overbank deposits associatedwith this channel system. The composite image shows the evolution of the Rosetta channel complex into the AbuMadi system as the incised valley filled with sediment and the channel avulsed its banks, forming the unconfinedAbu Madi system.

Imaging_Channels_In_Nile_Delta

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