Holtar_2000_m73ch29

Holtar, E., and A. W. Forsberg, 2000, Postrift development of the Walvis Basin,Namibia: results from the exploration campaign in Quadrant 1911, in M. R.Mello and B. J. Katz, eds., Petroleum systems of South Atlantic margins: AAPGMemoir 73, p. 429446.

Chapter 29

Postrift Development of the Walvis Basin,Namibia: Results from the ExplorationCampaign in Quadrant 1911

Abstract

The exploration campaign in offshore Namibia gained new momentum in 1992 when new licenseswere awarded in the independent Namibia. Seismic and well data from Quadrant 1911 has brought newinsight to the stratigraphy of the vast Namibian shelf.

A stratigraphic breakdown of the northern Namibian offshore area is proposed. The post-breakupsuccession is divided into seven major stratigraphic units or groups, from W1 (oldest) to W7 (where Wdenotes the Walvis Basin). Each group is described in terms of geometry, with examples from seismicexpression and structural maps, and lithofacies as seen in well data.

The W1 Group consists of volcanic and volcanoclastic rocks, succeeded by the W2 Group shallow-marine carbonates. After a tectonic event causing block faulting, the W3 Group consisting of siltstones andclaystones with limestone stringers was deposited. The W4 Group consists of claystones andsiltstonesandstones, including claystones with very good hydrocarbon source rock potential. InQuadrant 1911, the W4 Group also includes a large volcanic center. The W5 Group consists of mudstoneswith interbeds of sandstones and thin limestones. Following the CretaceousTertiary boundary, the W6Group mudstones, claystones, and marls were deposited, followed by claystones of the W7 Group.

The sedimentary succession of Quadrant 1911 postdates the Neocomian Etendeka plateau basaltsfound in onshore Namibia. After onset of the drift phase in late Hauterivian time, the Walvis Basinsubsided and a marine transgression eventually took place. Shallow-marine platform carbonatesprevailed until an Albian tectonic event resulted in complex block faulting and the formation of severalsubbasins. Subsequent volcanic activity created a series of volcanic centers localized near the Walvis Ridgebathymetric feature. The southern African craton was uplifted, leading to the formation of large-scalewestward-prograding wedges. Later sedimentation largely followed the evolution of a passive continen-tal margin, responding to relative sea level changes and paleoclimate.

INTRODUCTIONIn 1992, Quadrant 1911 was awarded as exploration

license 001 to a group consisting of Norsk Hydro (opera-tor), Saga Petroleum, and Statoil. Since then, a total of8000 km of seismic lines has been acquired by theNH9206 and NH9404 surveys, covering the 11,619-km2quadrant situated in the Walvis Basin, Namibia. Thisbasin was undrilled until the 1911/15-1 well was finishedin early 1994. Prior to this well, the nearest offshore explo-ration wells to the north were the Block 9 wells of theAngolan Kwanza Basin and, to the south, the Kudu wellsof the Orange Basin. In mid-1995, the 1911/10-1 well was

drilled, and a total of four wells have now been drilled inthe Walvis Basin.

The seismic grid varies from 2 2 km on the basinflank areas to 2 4 km in the basin areas and on the east-ern platform. A high-resolution aeromagnetic surveycovers all of Quadrant 1911. In addition, regional lines ofgood quality were used to establish a framework for thearea. These include the ECL-89 and ECL-91 surveys andthe Nopec/Namcor regional survey N2R-93 whichcovers areas to the north, south, and east of the quad-rant. In the latter survey, regional line N2R-93-229,recorded to 14 sec two-way traveltime, has been partic-ularly useful.

429

Erik Holtar

Norsk Hydro Exploration and Production InternationalOslo, Norway

Arne Willy Forsberg

Norsk Hydro Canada Oil & Gas Inc.Calgary, Alberta, Canada

There are several structural elements in Quadrant 1911(Figure 1). To the east lies the relatively stable ElephantPlatform, where the main passive marine shelf devel-oped. The central part of the quadrant is dominated by acomplex rift graben system called the Dolphin Basin. Tothe west are a few highs that have a thinner cover ofpostrift sedimentary rocks; these are collectively calledthe Outer Highs. To the north, the outstanding feature isa large volcanic center named the Phoenix High.

The stratigraphic breakdown is based on seismicmapping of geometrically confined depositional units(Figure 2). Data from wells 1911/15-1 and 1911/10-1 wereused to further describe and interpret these units. Wepropose that the post-breakup succession be divided intoseven major units, suggested here to be ranked as strati-graphic groups, ranging from W1 (oldest) to W7(youngest), where W denotes the Walvis Basin. Thedescribed part of the stratigraphic section broadly corre-sponds to the transitional and thermal sag tectono-strati-graphic sequences of Light et al. (1993).

The transitions between the individual groups repre-sent significant changes in gross depositional patternsreflected in lithologic breaks or major changes in geome-try (thickness distribution). Each of the seven major unitsconsists of one to six subunits that are ranked as forma-tions (Figure 3). Most of these are regarded as classic

transgressive, highstand to lowstand tract sequencecycles related to relative sea level changes. Some units,especially in the Earlymiddle Cretaceous succession, areconfined by lithologic changes that are interpreted to berelated more to local tectonic events.

BASEMENT ROCKSOnshore Namibia, the youngest predrift rocks are the

Etendeka Group, as described by Milner et al. (1995),which are interbedded basalts, quartz latites, and minorlatites. This group overlies the Paleozoic Karoo Sequence,and at its base, the volcanics are overlying and interbed-ded with eolian sandstones of the Etjo Formation. Ages(40Ar/39Ar) from stratigraphically equivalent units in theSouth American Paran Basin (Renne et al., 1992) date thevolcanics at 133 1 Ma (Valanginian), which is supportedby similar studies of the Etendeka Group in Namibia.

Light et al. (1993) proposed a subdivision of theNamibian shelf into five main tectono-stratigraphicsequences: basin and range, synrift I, synrift II, transi-tional, and thermal sag. In the Walvis Basin, the synrift IIsequence is a very thick northward- and westward-thick-ening trough believed to be contemporaneous with theEtendeka volcanics. The transitional sequence represents

430 Holtar and Forsberg

Figure 1Location map showing the structural elements of Quadrant 1911 in Walvis Basin, offshore Namibia, southernAfrica.

the Hauterivianmiddle Aptian interval, and eolian sand-stones and volcanics in wells on the Kudu gas discoveryfar to the south on the Namibian shelf mark the onset ofthermal sag following the end of rifting. The thermal sagsequence comprises the middle AptianHolocene sectionwhich mainly represents a passive margin.

A strong seismic event in the eastern part of Quadrant1911 is interpreted to be the top of Paleozoic or older base-ment. The event horizon dips steeply to the west, anddisappears below 7 sec two-way traveltime before reach-ing the well locations. Eastward, the event approaches thesurface before reaching the coast. The event is intersectedby a few northsouth trending normal faults with throwdown to the east. A reliable correlation from this platformarea into the basin and further onto the outer highs hasnot been established. Generally, the reflectors describewedge-shaped packages that thicken to the southwest.The uppermost of these events has been labeled w100.

W1 GROUPWe have used the w100 reflector to define the base of

the lithocolumn as described in this chapter. In the1911/15-1 well location, the reflector ties in just below the

total depth of the well and represents the shallowestpossible base of the basaltic beds that constitute the W1Group. Vertical seismic profile data from the wellsuggests that there is a marked increase in velocity (i.e.,density) at this depth. The interval between reflectorsw100 and w201 thins toward the northeast. The latterreflector ties into the well at 3947 m at the top of theyoungest basalt. To the southwest, the interval betweenw100 and w201 thickens abruptly and displays a patternof strongly divergent seaward-dipping wedges (Figure4). Regional mapping based on a relatively open seismicgrid shows a decrease in thickness from 3 sec two-waytraveltime in Quadrant 2010 to 0 sec along a NNWSSEtrend that crosses the Elephant Platform and the north-eastern corner of Quadrant 1911. The inferred conti-nentocean boundary is situated just southwest ofQuadrant 1911 (Figure 5).

Interpretation and 2-D modeling of aeromagnetic dataindicate a gradual eastward decrease in thickness overthe area of Quadrant 1911 from >1000 m in the west to afew hundred meters in the east. In the extreme northeast-ern comer of the quadrant, a strong linear magneticanomaly indicates the termination of the entire lavawedge. The w100 and w201 reflectors accordinglyconverge in this area. The thickness and distribution of

Chapter 29Postrift Development of the Walvis Basin, Namibia 431

Figure 2Seismic section across Quadrant 1911 (westeast) in the area of the 1911/15-1 well showing the suggested strati-graphic subdivision into the W1 through W7 Groups.


Figure 3Summary electric log from well 1911/15-1 shown with the stratigraphic column for this part of the Walvis Basinand the suggested stratigraphic division into the W1 through W7 Groups. DTCO is the sonic velocity log.

the basalt flows indicate that their origins are connectedto the breakup of the Atlantic Ocean. The westward-dipping reflectors represent stacked subaerial floodbasalts that flowed to the east from the spreading axis.The South Atlantic volcanic margin is described in moredetail by Gladczenko et al. (1997).

In the 1911/15-1 well, Group W1 consists of a series ofbasaltic lava flows separated by tuffaceous layers andoccasionally very thin beds of siliciclastic strata. The baseof this unit was not reached when the total depth of thewell was set at 4586 m. The top of the >563-m-thickcontinuous flow series is at 4023 m. The electric logs showthat individual lava flows range in thickness from lessthan 1 m to about 20 m (Figure 6). One core (at 43404352m) incorporates the base and top of two thick flows andthe interstitial sequence. On the basis of different litho-logic and textural characteristics, the cored section can bedivided into four intervals:

Interval 1 (43404343.1 m) comprises the lowerpart of one lava flow. Above 4342.25 m, the flowunit is massive with rare amygdules. Below thisdepth, the rock is slightly reddened and amyg-dules are abundant. Also, feldspar crystals changeto a more acicular morphology. This interval prob-ably represents the base of the flow.

Interval 2 (4343.14345.4 m) comprises a compositeflow unit produced by repeated surging of lava.Thin, nonamygdaloidal layers are glassy and prob-ably represent freezing at the very base of eachsurge, all of which are thin enough to be amyg-daloidal throughout.

Interval 3 (4345.44347.25 m) is a unit containingseveral interbeds of siliciclastic sedimentary rocks.The base is marked by a distinctive reddened andsilicified argillaceous siltstone. This is interpretedas a pedogenically modified lacustrine horizon


Figure 4Seismic section through the Outer Highs (westeast) showing seaward-dipping reflectors within the W1 Group.

truncating the underlying lava flow. It is overlainby a thin, reddened amygdaloidal lava flow.Throughout this flow unit are thin, irregular silt-stone layers that are thought to have been washedinto fractures and open hollows within the flow.Thin-section investigations of the siltstones revealthat the mineralogic composition differs fromwhat would be expected if they were derivedsolely from erosion of the surrounding lavas. Theydo contain plagioclase feldspar of volcanic origin,but they also contain common monocrystallinequartz and micas. This indicates a major contribu-tion from a nonvolcanic basement source. Thecommon presence of highly unstable aegerineaugite or hornblende grains also suggests that thesource was very proximal to the site of deposition.

Interval 4 (4347.254352 m) comprises the top of asingle thick lava flow unit. It is characterized by an

upward increase in modal size, size range, andabundance of amygdules. Local reddening andsignificant mineralized fractures mark the top ofthe flow. Flow banding structures become visibleas amygdule abundance decreases downward.

Attempts to date the lavas radiometrically have thusfar not yielded a conclusive age. The occurrence of twolava surges interbedded with the lower part of the over-lying carbonate sequence suggests that there is no signif-icant gap in time between the W1 and W2 Groups.Barremian marine microfossils recorded in cuttings fromthe WI Group are probably caved from the W2-1Formation, the lower unit of the W2 Group (see Figure 3).

In the Kudu gas field, southwest of Luedertiz, conti-nental deposits consisting of a mixture of clastic andvolcanoclastic strata of Barremian and older age areencountered in the reservoir sequence. These depositsinclude the main reservoir in the Kudu 9A-3 well andpossibly also in the 9A-2 well. This reservoir, named thelower gas sand by Wickens and McLachlan (1990),comprises a medium-grained anhydritic sandstone


Figure 5Structural map (in time units) of the top of theW2 Group, Walvis Basin, Namibia. Note the onshoreoutline of the Etendeka Group volcanics. Also note thedashed line marking the eastern limit of basalts from thespreading center. The dashed line on the west marks theonset of the seaward-dipping wedges. Contour interval is500 msec two-way traveltime.

Figure 6Wireline logs of the W1 Group volcanics fromwell 1911/15-1.

interbedded with subaerial basalts and volcanoclastics.The depositional environment of the sandstones is inter-preted as eolian, possibly a coastal dune complex(Wickens and McLachlan, 1990). Based on these observa-tions, it seems reasonable to assign an age not youngerthan Barremian to the lavas.

W2 GROUP

Seismic and Stratigraphic Characterization

The pattern of seaward-dipping reflectors typical ofthe W1 Group terminates at the w201 reflector. The depo-sitional geometry between the w201 and w202 reflectorsis represented by a parallel band of strong reflectors. Thew202 reflector ties in near the top of the W2 Groupcarbonates in wells 1911/15-1 and 1911/10-1.

The main depositional trend of the w201w202 inter-val seems to be parallel to the present-day coastline(Figure 7). To the south of Quadrant 1911, a westwardthinning of the interval is observed. This may reflect adepositional thinning onto an outer paleohigh, or it maybe the effect of more compactable argillaceous facies. Notrace of erosion is observed in this area. On the crests ofthe Outer Highs, significant postdepositional erosion hasreduced the thickness of the w201w202 interval andpartially removed it (see Figure 4). To the northeast, thisinterval also thins, probably by onlap onto a landwardrise. Generally, the w201w202 interval thickens consid-erably in the northwestern corner of Quadrant 1911,where it shows apparent onlap from the west. It is unclearwhether the interval in this area represents the W2 Groupas seen in wells, a basinward equivalent of the W2 Group,or a series of late-stage lava flows possibly belonging tothe W1 Group.

In the 1911/15-1 well, the carbonate deposits of the W2Group rest directly on volcanics at 4023 m. The boundarybetween W1 Group volcanics and W2 Group carbonatesis represented by a distinct increase in logged gamma-rayresponse (see Figure 6) which coincides with a decrease inneutron density. The top of the carbonates is at 3652 m,giving a total thickness of 371 m for the carbonates.

Subdivision of the W2 Group

We suggest a division into two stratigraphic units (orformations, in this case) for the W2 Group. The lower partof the lower W2-1 Formation is comprised of 41 m ofmuddy limestones, marls, and glauconitic claystones.High, spiky gamma-ray readings (up to 100 API) in thispart of the formation are due to uranium enrichment andcould indicate stagnant environments during deposition.Analysis of sidewall cores from this interval does notshow elevated total organic carbon (TOC) contents(

indices (HI) in the range of 100200 mg HC/g TOC (typeIII kerogen). Based on these screening data, the carbon-ates can be regarded only as potential source rocks if thereare significant lateral facies changes.

Well-preserved diagnostic microflora and microfaunaare scarce in the lower interval of the W2 Group.Barremian marine microfossils recorded in cuttings fromthe volcanics of the W1 Group are probably caved fromthe W2-1 Formation. The maximum age of the W2-1Formation is thus most probably Barremian. The mostprobable age range for the W2-2 Formation is lateAptianmiddle Albian. The W2 Group is thus assigned aBarremianmiddle Albian age. This implies that the basalcarbonate deposits are somewhat older than the massivesalt deposits and overlying Pinda Group carbonates andevaporites of the Kwanza Basin in Angola (Abilio, 1986).

The succession of various depositional facies repre-sented in the W2 Group is summarized in Figure 8. Thereturn of lava flows in the lower part of the group indi-cates a reemergence of the area after the initial marinetransgression. The coarser grained zones encountered inthe core are interpreted as middle platform shoaldeposits, while the thicker oolithic zones in the upper-most part of the W2-2 Formation are considered to beplatform margin grainstones. The development from

restricted lagoonal environments to more high-energy(barrier island) environments, as evidenced by theupward increasing proportion of grainstones containingooides, indicates a landward shift of facies during ageneral marine transgression.

W2 Group Reservoir

The main reservoir target in well 1911/15-1, drilledinto the top of rotated prerift blocks, turned out to be shal-low-marine carbonate deposits of AptianAlbian age.Prior to drilling, this interval was regarded as the mostprospective within the license area, mainly because of itshigh number of large structural closures as defined by thew202 reflector. The lateral distribution of the differentcarbonate facies is difficult to predict, especially the distri-bution of its reservoir properties.


Figure 8Schematic stratigraphic column showing depo-sitional transgressive facies development over time andspace in the W2 Group carbonates. Also shown (on theleft) are the wireline logs from well 1911/15-1, WalvisBasin.

Figure 9Plot of porosity and oil saturation versus depthfor the W2 Group carbonates in well 1911/15-1. (RKB is rigkelly bushing, the reference for wireline log depths.)

The cored part of the carbonates revealed zones withvery good reservoir properties. Porosities of 1525% weremeasured within the algalpelletal grainstone and pack-stone facies (Figure 9). A high proportion (7095%) of thisporosity occurs in macropores. These intervals havepermeabilities ranging from 10 to 500 md and seem tocorrelate well with low-velocity intervals distinguishedon the sonic velocity log (see the DTCO log in Figure 3).According to the logs (sonic velocity and gamma), suchhigh-porosity zones are especially found in the upperone-third (about 100 m) of the W2 Group limestone. Thenet to gross ratio in this interval (36503750 m) is esti-mated to about 35%, with an average porosity of 20%.

Enhanced oil saturation (1540%) was recorded insamples from low-porosity zones of the carbonates in thecore (Figure 9). The porosity versus oil saturation plot inFigure 10 shows an inverse relationship between the twoparameters. Such a relationship is generally found indepleted reservoirs. Saturated fraction gas chro-matograms (C15+ GC) from core extracts have a lighterend-biased appearance. They also show low isopre-noid/n-alkane ratios and carbon preference indexes near1.0, indicating a mature source (Tissot and Welte, 1984).

Figure 11 shows the phytane/n-C18 versus pris-tane/n-C17 ratios for the saturated fraction of the coreextracts plotted together with those derived from otherparts of the W2 Group. The difference in isoprenoid/n-alkane ratios between the core samples and the samples

from other parts of the sequence is striking. While ratiosfrom all other samples indicate that the host rocks havenot yet reached the main hydrocarbon-generating zone,the ratios for the core extracts indicate a very maturesource rock. The ratios from the core extracts are in factabout the same as or even lower than ratios found inmany condensates.

The source of these hydrocarbons has not yet beenestablished. However, based on our observations, webelieve that the traces of residual oil encountered in theW2-2 Formation core extracts may represent remnants ofhydrocarbons that have migrated from a mature sourcekitchen, either in fluid phase as a light oil or in a gaseousphase as a condensate.

W3 GROUP


In the 1911/15-1 well, the base of the W3 Group ischaracterized by a significant downward drop in gamma-ray (Figure 8) and transit-time log responses at 3652 m,coinciding with a marked neutron density increase. Thetop is at 3479 m, where the w401 reflector ties into thewell. Its total thickness is 173 m in the well. On structuralhighs, the W3 Group is thin to absent.


Figure 10Plot of residual oil saturation versus porosityfrom retort analysis for the W2 Group carbonates in well1911/15-1.

Figure 11Plot of phytane/n-C18 ratios versus pristane/n-C17 ratios for rock extracts from various formations inwell 1911/15-1.

In contrast to the parallel band of strong reflectors seenbelow, the seismic data exhibit a transparent and diver-gent pattern above reflector w202, which defines the baseof the W3 Group (Figure 2). The divergent pattern showsthat the area experienced a period of differential subsi-dence and tilting of individual fault blocks. The organiza-tion of faults on the structural depth map in Figure 12reveals that a three-pronged graben feature was formedas a consequence of this major tectonic restructuring ofQuadrant 1911. Farther south, the relief dies out, and thesurface becomes a more smooth westward-dipping slope(see Figure 5).

The top of the W3 Group is defined at the prominentw401 reflector, which is very continuous but exhibitssome degree of deterioration to the east. Within the W3Group, the w301 reflector defines the top of the majorsedimentary wedges that thin onto the crest of mostrotated fault blocks, including the drilled 1911/15 struc-ture (Figure 2). Onlap onto the w202 reflector occurs inmost areas. The transparent pattern below the w301reflector may in certain areas be continuous over thehighs and cover several fault blocks. In other areas, thispattern appears to be limited to the structurally lowerpart of the individual blocks.

The interval above the w301 reflector shows ratherstrong, continuous, and generally more widely distrib-

uted parallel reflectors. At its lower boundary, the intervalseems to exhibit some degree of onlap or downlap.Above underlying faults or flexures, the parallel internalpattern may change abruptly and become chaotic andopen with a mounded character. These features terminateupward at the w302 reflector, where the pattern becomessignificantly more concordant over the entire basin area.

Although the thickness of the interval between thew202 and w401 reflectors may reach up to 800 msec two-way traveltime in some of the half-grabens in Quadrant1911, it is still rather thin on a regional scale. In a depocen-ter in Quadrant 2112 to the southeast, the interval reachesthicknesses up to 1200 msec.

Within Quadrant 1911, the W3 Group has significantlyhigher interval velocities derived from seismic stackingvelocities than seen in the wells. The interval velocitiesincrease with increasing thickness of the group, reachingvalues of more than 5000 m/sec. The high interval veloc-ities are clearly not compatible with shales or poroussandstones and instead may suggest that the lithologiesof the W3 Group are more calcareous in the rift basinscompared to what is observed in the well. The transitionfrom the carbonate environments of Group W2 to the sili-ciclastics of Group W3 may be regarded as overall trans-gressive and is apparently related to a tectonic phase thatoccurred in the middle Albian.

Bulk cuttings from the W3 group yielded TOCcontents from 0.3 to 2% with HI values generally in therange of 50250 mg HC/g TOC. The richer samples arefound scattered throughout the group. As the groupthickens dramatically into local graben areas, the organicfacies may improve downflank into more basinalsettings. Vitrinite reflectance values of about 0.5% Ro andhigh pristane/n-C17 and phytane/n-C18 ratios (Figure 11)in the saturated hydrocarbon fraction of the extracts indi-cate low or moderate maturity.

Subdivision of W3 Group

Based on seismic mapping and log response, a three-fold division of the W3 Group is proposed. The W3-1Formation is easily distinguishable on well logs by itshigh, spiky gamma-ray signature (36523637 m) (Figure8) that contrasts with both the underlying carbonates ofthe W2 Group and the overlying claystones of the W3-2Formation. Lithologically, this unit is dominated by glau-conitic siltstones grading upward to claystones with lime-stone stringers. The top of the W3-1 Formation is at 3637m, giving it a thickness of only 15 m. Although this isactually below seismic resolution at the well, by definingthe w301 reflector as the top of the formation, its thicknessincreases to more than 1000 m in graben areas.

The W3-1 Formation was deposited during or justafter the main period of differential subsidence. This unitis of middle Albian age and it is very condensed in thewell. As the W3-1 Formation thickens dramatically intograben areas, the lithologies probably become morevaried. The recorded high gamma log and low sonicvelocity log signatures indicate that the unit may be a


Figure 12Structural depth map for the base of the W3Group, Quadrant 1911, Walvis Basin, Namibia. Contourinterval is 1000 m (below mean sea level).

potential source rock for hydrocarbons, especially down-flank from highs, such as the one drilled by the 1911/15-1 well. Maturation modeling (using the PetroMod 2D soft-ware package) suggest that the thick, downflank depositsof the W3-1 Formation reached maturities well into thecondensategas zone as early as TuronianSantoniantime. Although a direct correlation between these poten-tial petroleum source rocks and the residual hydrocar-bons encountered in one of the cores has not been estab-lished, they may very well have been parts of the samepaleopetroleum system.

On well logs, the W3-2 Formation can be distinguishedfrom the W3-1 Formation by a sharp upward decrease ingamma log response, coinciding with a moderateincrease in neutron density. The W3-2 Formation consistsof 68 m of light gray claystones, occasionally gradingupward to silty claystones. The top is at 3569 m where thew302 reflector ties in. Although the thickness is not asgreat as for some parts of the underlying W3-1Formation, it reaches more than 800 m in the central partsof graben areas. Lithologic facies changes within this unitcan be expected in the chaotic mounded sections whichmay be composed of carbonate buildups or sandydeposits.

The W3-2 Formation was deposited during a mainlypassive, intermediate stage of sedimentary fill intomiddle Albian age grabens. Faulting seems to have beenrestricted to the main basin-bounding faults in the westand east and occurred with little or no tilting of theminor fault blocks. The observed change from siltylithologies in the W3-1 to more argillaceous lithologiesin the W3-2 suggests deposition in deeper and/orcalmer waters. The chaotic mounded sections seen onthe crests of tilted fault blocks are interpreted either asbuildups of biogenic carbonates or as siliciclastic sandridges created by wave and current action. Low-angleaccretionary clinoforms observed on the eastern plat-form suggest that sediment input from the Namibianmainland caused a westward progradation of a coast-line which may have reached into the Quadrant 1911area at the time. The age of the unit is late Albian, possi-bly stretching into the early Cenomanian.

The upper part of the W3 Group is assigned to the W3-3 Formation. In well 1911/15-1, it consists of siltyclaystones grading upward into siltstones. The maincriteria for identifying the transition between units W3-3and W3-2 on well logs are moderate upward decreasinggamma-ray responses coinciding with decreasing sonicvelocity and neutron density responses. The w401 reflec-tor ties into the top of the W3-3 Formation in the well at3479 m (3122 msec). W3-3 thickness is 90 m, but itincreases to close to 400 m in a depocenter southwest ofthe well.

The W3-3 Formation is interpreted to represent a late-stage passive infill into the Albian topography. Thicknessvariations in the basin can mostly be attributed to accom-modation space created by differential compaction ofunderlying W3 Group strata. Its history of depositionspans most of the Cenomanian.

W4 GROUP


The base of the W4 Group corresponds to the w401reflector, which ties in at 3479 m to the 1911/15-1 well.This reflection is attributed to a positive impedancecontrast between the higher velocity silty deposits of theW3-3 Formation and the low-velocity, high gamma-rayshales at the base of the W4-1 Formation. The reflector isvery strong and continuous, only deteriorating to the eastwhere a reduction in the impedance contrast probablyreflects a facies change. A shallower reflector, the w501,seismically defines the top of the W4 Group. This ties inat 3163 m to the well. The total thickness of the W4 groupis thus 316 m.

Age dating based on palynology indicates that thedepositional history of this group started in the latestCenomanian, continued through the Turonian andConiacian, and ended sometime in the late Santonian.This gives the W4 Group a depositional time span of 68m.y. and makes it roughly time equivalent to the clasticCabo Ledo, Itombe, and NGolome Formations in theKwanza Basin of Angola (Abilio, 1986).

The age of the W4-2 Formation ranges fromearlymiddle Turonian to late Santonian. Based on theseismic observations described above, this is the mostprobable time of growth of the Phoenix High volcano andsome of the other bathymetric features that have beenlinked to the Walvis Ridge in this area.

In large parts of the graben area, the internal seismicpattern of the W4 Group changes upward from parallelwith an onlapping lower boundary to oblique reflectorsthat generally dip eastward (Figure 13). The boundingsurface between these two different internal patternscoincides with the base of a second high gamma-rayshale package at 3366 m in the 1911/15-1 well (top of theW2-1 Formation; see Figure 3). Based on these log andseismic features, we have divided the W4 Group intotwo formations.

Although the W4-1 Formation is only 72 msec (113 m)thick at the well location, its sedimentary sequence attainsa thickness of as much as 150 msec in the deeper part ofthe basin. The entire W4 Group thins eastward onto theElephant Platform, where the two formations becomeseismically inseparable. This eastward thinning is incontrast to the drastic thickening of the overlying W5-1Formation observed toward the east.

In the northern part of Quadrant 1911, a volcaniccenter, the Phoenix High, is distinguished by seismic data(Figure 14) and high-resolution magnetic data. By follow-ing the various reflectors that define the external andinternal geometry of this feature to where they convergeand interfinger with the interpreted sedimentary pack-ages, its relationship to the W4 Group becomes evident.The growth of the Phoenix High volcano is thus inter-preted to have been contemporaneous with the deposi-tion of sediments in the W4 Group (probably the W4-2


Formation). Internal seismic patterns also indicate thatthe growth of the volcanic complex may have triggeredsoft sediment rearrangement during deposition of theW4-2 Formation (Figure 13b).

The seismic profile (Figure 14) beneath this volcanicsequence is difficult to interpret. It is probable that themain fault separating the platform to the east from thegraben to the west continues approximately northsouthand that the volcanics now straddle these two tectonicelements. At least two more volcanic centers are identi-fied, one as a northward continuation of the PhoenixHigh and one just to the northwest of Quadrant 1911(Figure 5). Based on mapped reflectors that lap onto theseother volcanic features, they seem to have been contem-poraneous with the Phoenix High, or possibly slightlyyounger. The outlines of the Phoenix High and the othervolcanic features are also detectable on detailed bathy-metric maps of the area and have been related to theWalvis Ridge bathymetric feature by previous authors(Light et al., 1993).

W4 Group Source Rock

In the 1911/15-1 well, the most characteristic logfeatures of the W4 Group are the two distinct highgamma-ray carbonaceous claystone intervals that occurat the base of each subunit. The main lithology of the W4-1 Formation is greenish gray claystone with minorinterbeds of siltstone. The formation totals 113 m in thick-ness, and the lower 22 m is comprised of dark yellow-brown argillaceous and calcareous siltstones with fairlylarge amounts of carbonaceous material. Log signaturesand weak shows on cuttings and sidewall cores gaveearly indications of source rock type deposits. Furtheranalysis proved that, within this interval, the TOCcontent varies from 5% to >10%. Rock-Eval pyrolysis(Figure 15a) indicates very good type II kerogen, with HIvalues reaching 600 mg HC/g TOC. The rest of the W4-1claystones also exhibit source rock quality, with typicalTOC values in the range of 16% and HI values generallyranging from 200 to 400 mg HC/g TOC.


Figure 13(a) Seismic section (westeast) showing the internal seismic pattern of the W4 Group. (b) Slump model for theW4 Group interpreted from the above seismic section.


Figure 14Seismic section across the Phoenix High, Walvis Basin. This is a composite section, running WSW-ENE (on theleft) and NNW-SSE (on the right).

Figure 15Geochemical data from sidewall cores (SWC) and cuttings of the W4 Group, well 1911/15-1. (a) Plot of HI versusTOC. (b) Plot of HI versus Tmax.

The W4-2 Formation is comprised of dusky yellow-brown claystones and siltstones. The latter sometimesgrade into very fine sandstones. Carbonaceous material iscommon throughout, and the TOC content is generally25% with HI values typically at 350500 mg HC/g TOC.As indicated by Tmax data (Figure 15b) and by the weakshows recorded while drilling, these sedimentary rockshave reached maturities only in the very upper part of theoil-generating zone in the location of the 1911/15-1 well.

The high content and quality of the organic matter inthese sediments and the high uranium content in thelower part indicates an initially anoxic paleoenvironmentfollowed by dysoxic conditions. Bottom water circulationwas probably poor or absent.

W5 GROUP


In the 1911/15-1 well, the W5 Group consists of darkgray to olive gray mudstones with interbeds of sand-stones and thin limestones. Thin beds of very fine grainedsandstone are found in the upper part of some of theformations (Figure 3). The base of the group is at 3163 mand the top at 2463 m. This total thickness of 700 mincreases to more than 1200 m on the Elephant Platformto the east. Palynologic age dating indicates that thedepositional history of the W5 Group lasted from lateSantonian to late Maastrichtian time.

The base of the W5 Group marks the end of a tectoni-cally active period that involved volcanism and erosion offootwall blocks. As a response to significant differentialsubsidence between the exposed African continent andthe offshore area, large sedimentary clinoforms began toreach the Quadrant 1911 area at this time and a typicalpassive margin shelf was established. The depositionalgeometry of the W5 Group is hence dominated by largeprograding wedges that accumulated in the northeasternpart of Quadrant 1911, whereas parallel laminated bedsof moderate thicknesses are typical for the western area.

Subdivision of W5 Group

We suggest subdividing the W5 Group into six strati-graphic units. Each of these is interpreted to represent agenetic sequence with a transgressive system tract at thebase followed by a highstand and eventually a lowstandsystem tract. Basin floor fans are occasionally recognizedbasinward of highstand or lowstand clinoforms.

The W5-1 Formation is well developed on theElephant Platform, with thicknesses reaching more than1000 m. On seismic lines, four systems tracts can be recog-nized within this formation (Helland-Hansen, 1995). Theinitial transgressive systems tract is generally thin in thisarea and not readily recognized. As can be seen in Figure16, a major highstand event created a clear shelf edgeclinoform followed by a forced regression wedge and alowstand wedge. The forced regression wedge shows

steep foreset beds that accumulated entirely in front of theprevious shelf. The lowstand wedge has less steep foresetbeds, and its strata partially accumulated on the shelf.During highstand, the shelf developed along a north-westsoutheast trend. Deposition shifted basinwardduring the subsequent lowstand, mainly on the previousslope. On the seismic profile (Figure 16), we have inter-preted high-density submarine fan and turbidite depositsin the Dolphin Basin that are correlated to the highstandpart of the W5-1 Formation. In well 1911/15-1, the 210-m-thick W5-1 Formation is defined between 31632953 m.The age of this formation is late SantonianearlyCampanian.

Along the southern edge of the Phoenix High, a largeerosional channel cuts deeply into the W4 Group. Seismictie to the well indicates that this feature was createdduring the lowstand period of W5-1 deposition and wasfilled during the subsequent transgression, as evidencedby the base of the W5-2 Formation. In well 1911/15-1, thetotal thickness of the W5-2 Formation is 117 m (29532836m); its age is early Campanian. The overlying W5-3Formation is defined at 28362738 m, with a thickness of98 m, and its age is Campanian.

The w504 reflector near the top of the W5-4 Formationdefines the top of a massive mounded fan system in thesoutheastern part of Quadrant 1911. It corresponds to theupper lowstand part of this formation. The fan system,which we would expect to contain coarse clastics, extendsfarther south, but it thins completely out downflankbefore reaching the site of well 1911/15-1. In the well, theW5-4 Formation is defined at 27382652 m. This unit wasdeposited mainly during the late Campanian.

The W5-5 Formation is defined at 26522522 m with athickness of 130 m. It is dated as Maastrichtian. At the topof the formation, there is a sandstone interval(25222572.5 m) consisting of fine- to medium-grained(but occasionally coarse-grained) quartz with kaolinite asa secondary mineral. In the upper 1215 m, it is tightlycemented with silica, but judging from wireline logs, theporosity in the rest of the sandstone varies between 25and 30%. No hydrocarbon shows have been recorded inthese sandstones. They are not readily detectable on seis-mic lines as a geometric feature, but they are assumed tobe present where the seismic character appears discontin-uous. The tightly silica-cemented upper part of the sand-stone interval should give a pronounced reflector, but theevent is not clear and continuous on the seismic. This maybe caused by slumps or channeling at the well location.The sandstones are thought to represent a basin floorsheet sand shed into the basin during the lowstand partof the W5-5 Formation.

In the eastern part of the Elephant Platform, the inter-nal reflectors of the W5-6 Formation dip to the west as thethickness of the unit increases (Figure 17). In map view,the feature is elongated northwestsoutheast along theshelf edge and has a seemingly hard and irregular uppersurface. Combined with observations from the overlyingW6 Group, it is interpreted to be a set of carbonatemounds or reefs. This interval is not penetrated by wellsin offshore Namibia.



Figure 16Seismic section from the west (left) toward Elephant Platform to the east (right) with the sequence stratigraphicnomenclature superimposed on formations of the W5 Group.

Figure 17Seismic section from the west (left) toward Elephant Platform to the east (right) showing the reef-like features inthe upper part of the W5 Group and lower part of the W6 Group.

A marly claystone interval in the W5-6 Formation inwell 1911/15-1 is interpreted to represent a basinalrestricted onlapping wedge associated with the moundedfeatures near the shelf edge. In well 1911/15-1, the W5-6Formation is defined between 25222463 m (59 m thick).The unit yielded Maastrichtian age microfossils.

W6 GROUP


In well 1911/15-1, the base of the W6 Group is at 2463m and the top at 922 m, with a resulting thickness of 1541m. The recorded age of the W6 Group ranges from thelate early Paleocene to middle Miocene. Maastrichtianforaminifera and dinocysts recorded in the basal part ofthe group in the well are believed to reflect Paleoceneerosion and resedimentation of latest Cretaceous stratafrom the shelf. The base of the W6 Group is an noncon-formity because the early Paleocene record is missing.

At the time of deposition of the basal W6 Group, apronounced shelf edge was already established on theElephant Platform. This edge reached out to, but did notcover, the Phoenix High. The main part of the W6 Groupin the 1911/15-1 well is dominated by pelagic mudstones,claystones, and marls. In its lower part however, onedistinct sandstone interval is recognized in the well,unconformably overlying the W5 Group. The presence ofkaolinite as a major secondary mineral in these sandstonesalso indicates substantial weathering of micas andfeldspars onshore.

On the paleoshelf edge, northwestsoutheast trendinglarge reef-like features were established in the lowermostpart of the group, superimposing on the mounded featuresof the uppermost part of the W5 Group (Figure 17). Thesefeatures remain undrilled to date, but can be regarded asleads for future hydrocarbon exploration in the area.

Subdivision of the W6 Group

The W6 Group is subdivided in six formations (Figure3); this division is based on the same principles as for theW5 Group. The sequence stratigraphic relationships,however, are not as clear for the W6 Group.

In well 1911/15-1, the W6-1 Formation is defined at24632353 m, giving a thickness of 110 m. Biostrati-graphic studies indicate a maximum age of late earlyPaleocene for the interval. The formation comprises onemajor coarsening-upward unit underlain by the distalheterolithic part of a basin floor fan.

The basal W6-1 sandstone interval (23972463 m)consists of irregularly interbedded sandstones, siltstones,and claystones. The grain size of the quartz sand variesfrom fine to very coarse. Measured helium porositiesfrom two cores in the sandstones were 2631%, withpermeabilities in the range of 150590 md. The irregulartopography of this sandstone interval makes it readily

identifiable on seismic sections. The sandstones are inter-preted to represent a channelized basin floor fan complexand can be traced updip to a point source east of thesoutheastern corner of Quadrant 1911. Deposition wasrestricted to the eastern slope and the southern DolphinBasin, not reaching the Outer Highs.

In Quadrant 1911, two laterally distinct reef-likefeatures are identified at the base of the W6-1 Formation.Both are characterized by a gentle talus slope to the eastand a very steep slope to the east (Figure 17). Interpret-ation of seismic lines along strike strongly suggests thatthe southern feature predates the northern. If thesefeatures are reefs or large bioherms, their presence indi-cates a period of stable depositional conditions and sedi-ment starvation in basinal areas in earliest Paleocenetime.

In the W6-2 Formation, the interpreted carbonatefeatures shift updip relative to the W6-1 Formation(Figure 17). These map out as long, continuous barrierreefs and are the youngest of such features observed inthis area. The main part of the W6-2 Formation in well1911/15-1 is interpreted to represent the basinal restrictedonlapping wedge associated with the reefal features. Theformation is defined from 2353 to 2197 m (156 m thick)and is late Paleocene in age.

In well 1911/15-1, the W6-3 Formation is defined at21972025 m. The 172-m-thick formation is also assigneda late Paleocene age. The lower half of the formationrepresents a pronounced transgression, presumed tohave drowned the shelf edge reef structures. During thefollowing highstand, major sedimentary slumps appar-ently moved downslope from the Phoenix High andabout 30 km along the shelf to the south.

Before the deposition of the W6-4 Formation, the maindepocenter shifted northward. In the northern part ofQuadrant 1911, the thick highstand and lowstand tractsof this formation prograded across the top of the PhoenixHigh, thus moving the shelf edge significantly westward.In well 1911/15-1, the 422-m-thick (20251603 m) marls,siltstones, and claystones of the W6-4 Formation areEoceneearliest Oligocene in age. In the upper part of theW6-4 Formation is a large undrilled basin floor fanlocated centrally in the Dolphin Basin.

In well 1911/15-1, the W6-5 Formation is defined from1603 to 1173 m. The 430-m-thick claystones of this forma-tion were deposited in early Oligocenemiddle Miocenetime. The northward shift in deposition along the shelfseems to have continued, and the W6-5 Formation has itsmain depocenter on the Phoenix High itself. Severalchannels appear at the base of the formation, runningnormal to and down the shelf slope.

We define the W6-6 Formation from 1173 to 922 m,giving a thickness of 251 m. This unit is inferred to havebeen deposited during the middle Miocene, but only thelower part of the interval is represented by cuttings fromwells in Quadrant 1911. The deposition is characterizedby an increased thickness basinward of the previous shelfedge, bringing the shelf edge westward. The thickestdeposits are to the immediate west of the Phoenix High.


W7 GROUPIn well 1911/15-1, the W7 Group is defined from 922 to

522 m, for a total thickness of 400 m. The top of this unitis the present sea floor. The group is interpreted to consistprimarily of claystones and is believed to be middleMioceneHolocene in age.

The transition between the W7 Group and the W6Group is thought to be related to a dramatic eustatic sealevel drop that occurred in middlelate Miocene time.Based on evidence of a paleocoastline on the present-daysea floor, we estimate that sea level then was about 150 mlower than it is today. The reflector w701 ties into the baseof the W7 Group in the 1911/15-1 well, above the levelwhere well cuttings were obtained. The reflector can betraced westward to the site of the DSDP 362 well (Figure5), where it ties to this well at about 1776 m (below meansea level), corresponding to the middlelate Miocenetransition according to Bolli et al. (1978).

Near the base of the W7 Group, the shelf totallycovered the Phoenix High. Reflector w701 displays elon-gated channels normal to and down the slope, as well asvery large northsouth trending channels on the OuterHighs. The seismic data shows no lithologic contrastbetween the channel fill and the surrounding strataconsisting of claystones. The channels are interpreted tobe of a nonerosional character. They may have served asfeeder channels running down the shelf slope, accumu-lating sediments at approximately the same rate as thesurroundings. Individual channels are seen to be migrat-ing northward. Features like these seemed to have beencommon along a large part the west African offshore areain MiocenePliocene time. Their sequence stratigraphicsignificance in offshore Gabon has been discussed indetail by Rasmussen (1994).

Toward the top of the W7 Group, deposition shiftedfarther basinward. The shelf edge was established aboutwhere it is today (Figure 1).

CONCLUSIONS

Exploration activity in offshore Namibia in the north-ern Walvis Basin has revealed important informationabout the sedimentary succession that postdates theNeocomian Etendeka plateau basalts found in onshoreNantibia. The proposed stratigraphic subdivision dividesthe post-breakup succession into seven major lithologi-cally and geometrically confined depositional groupsW1 to W7.

After the onset of the drift phase in late Hauteriviantime, the Walvis Basin received subaerial basalt flowsfrom a spreading center to the west. Following subsi-dence as the spreading center moved farther westward, amarine transgression eventually took place as the embry-onic South Atlantic Ocean advanced northward. Shallowmarine platform carbonates then prevailed in the area ofQuadrant 1911 from the latter Barremian to middleAlbian time. In Albian time, a local tectonic event resulted

in complex block faulting and the formation of severalsubbasins and highs. This marked the termination of thecarbonate platform and the transition into a clastic-domi-nated depositional system. In CenomanianTuroniantime, a series of volcanic centers localized near the WalvisRidge bathymetric feature emerged. This tectonic episodewas followed by the formation of large-scale westward-prograding wedges sourced from the elevated southernAfrican craton. Later sedimentation largely followed theevolution of a passive continental margin, responding torelative sea level changes and paleoclimate. The basalTertiary unconformity marks a period of very limiteddeposition, possibly with the formation of reefal struc-tures on the shelf.

The two wildcat wells drilled in Quadrant 1911 foundgood porous reservoir carbonates of Early Cretaceous agein well-defined structural closures. Compositional analy-sis of residual hydrocarbons, found in relatively highconcentrations in less porous zones, indicate that theseare not indigenous and hence point to a paleopetroleumsystem. The source rock for these hydrocarbons has notbeen identified. Most likely it is to be found within thestrata that were deposited in local grabens created as thecarbonate platform broke up in middle Albian time. Thetime of breaching (failure of seal) of this reservoir is notknown, and play types in younger potential reservoirsupdip have not been tested. A rich source rock containingmarine type II kerogen was deposited in the lateCenomanianSantonian succession, contemporaneouswith the growth of a large volcanic center in the northernpart of Quadrant 1911. Within this part of the WalvisBasin, this source rock has barely reached the top of theoil window maturity where it was penetrated.

AcknowledgmentsWe would like to thank our partners inlicense 001, Statoil and Saga Petroleum, for giving us thepermission to publish these data. The statements in this chapterrepresent the opinions of the authors and not necessarily theconclusions of the whole license group. We would also like tothank our colleagues in Norsk Hydro, Jan Robert Eide and ArneRasmussen, for constructive cooperation and fruitful discus-sions. Bob Martin has had a good hand with the figures.

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