IPA99-G-138

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATIONTwenty Seventh Annual Convention & Exhibition, October 1999

3D SEISMIC VISUALIZATION OF SHELF RIDGES: CASE STUDY FROM OFFSHORENORTHWEST JAVA

Henry W. Posamentier*

ABSTRACT

Detailed stratigraphic evaluation of 3D seismic datafrom the Miocene section of the offshore northwestJava shelf reveals the extensive presence of preservedshelf ridges. These features are deposited as longlinear bodies ranging from 0.3 to 2.0 km wide, over20 km long, and up to 17 m high. Upon closerinspection, these features appear to be asymmetric,characteristically sharp-edged and thicker on one side,gradually thinning with an irregular edge on the otherside. Possible sand waves, smaller in scale, areobserved superimposed upon these ridges andoriented oblique to the long axes of the ridges. Shelfridge deposits tend to be sand prone and are observedto overlie ravinement surfaces. The ridges appear tobe oriented parallel to paleo trunk valleys associatedwith broad structural embayments. In addition toshelf ridges, shelf ribbons, possibly less than 5 mthick and less than 100 m wide, are also imaged.

Sand ridges are common on modern shelves, butrarely observed in the subsurface or in outcrop. Thefeatures observed here represent rare unequivocalexamples of preserved ancient shelf ridges. Theseridges are thought to have formed as a result oferosion and subsequent reworking of sand-prone deltaplain and/or coastal plain deposits by shelf tidalcurrents that became active immediately aftershoreline transgression. They appear to havemigrated across the ancient sea floor and represent asignificant component of the transgressive systemstract.

These transgressive systems tract deposits havesignificant exploration potential insofar as they are

___________________________________________* ARCO Indonesia

commonly sand prone and they tend to be encased inshelf mudstone seal facies. Depending upon thedegree to which sand is present in inter-ridgelocations, these linear sand bodies can comprisepotential stratigraphic traps.

INTRODUCTION

Shelf sediments have been the focus of much analysisrecently, especially in light of the development ofsequence stratigraphic concepts (Plint et al. 1987;Plint, 1988; Posamentier et al., 1988; Swift andThorne, 1991; Posamentier et al., 1992; Posamentierand Chamberlain, 1993; Snedden et al., 1994;Snedden and Dalrymple, 1999). Ample evidenceexists for the presence of sand deposits on modernshelves (Swift and Field, 1981; Stride et al., 1982;Snedden et al., 1994; Snedden and Dalrymple, 1999);however unequivocal documented examples ofancient shelf ridges have been conspicuous by theirabsence in the recent literature. Whereas sandstonesisolated in mid- to outer-shelf settings have beenobserved and documented, they have most commonlybeen interpreted to have formed not as shelf ridgesbut rather as incised valley or lowstand shorefacedeposits (e.g., Plint et al., 1987; Posamentier andChamberlain, 1993). These interpretations reflect theimpact of the sequence stratigraphic approach duringthe past decade. Consistent with this approach, suchshelf sands were thought to have been transportedonto and across the shelf during low stands of sealevel when much of the shelf might have beensubaerially exposed. This mechanism afforded a moreelegant mechanism for transportation of relativelycoarse-grained sediment across a shelf than havingsediment transported across a submerged shelf eitherby traction or suspension, sourced from the shorelinearea (Scheihing and Gaynor, 1991).

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A hybrid mechanism for developing and subsequentlypreserving shelf sands involves the reworking oflowstand deposits (shoreface, deltaic, channel fill,etc.) by shelf currents after transgression hasoccurred. This process is well known in shelf areasinfluenced by high current energy such as thesouthern North Sea (Stride et al., 1982), butnonetheless has not been widely embraced as capableof producing depositional elements that have anypreservation potential.

This study identifies widespread ancient shelf sandbodies in the late Miocene of offshore northwest Java.These features were first recognized on seismic dataand later calibrated with borehole data. Two areasoffshore northwest Java where 3D seismic coveragewas available, were the focus of this study (Fig. 1).The principal focus area was the area around theFXE-1 well covered by 167 km2 of 3D seismic data.The secondary focus area was the area around the E-Field, covered by 110 km2 of 3D seismic data. Theseismic data were sampled at 2 msec intervals and thepeak frequencies observed here are in the range of 30-35 Hz at a depth of approximately 800 m.

REGIONAL SETTING

This paper focuses on the upper part of the MainMember of the Upper Cibulakan Formation, whichcomprises a shallow marine siliciclastic succession ofmiddle to upper-Miocene age (Arpandi et al., 1975;Ponto et al., 1987; Reksalegora, 1993; Reksalegora etal., 1996). An overall north-northeast to south-southwest direction of progradation has beenobserved (Purantoro et al., 1994; Reksalegora et al.,1996). Approximately 60 km to the south-southeastof this area the section thickens into theCipunegara/E15 trough (Noble et al., 1997) andsandstones are less common. It has been estimatedthat sandstone reservoirs within this stratigraphic unitcontain approximately 75% of all the hydrocarbonsdiscovered to date in ARCO Indonesia’s offshorenorthwest Java PSC (Atkinson et al., 1993) within asection up to 700 m thick. Over 700 MMBO and 0.7TCF have been produced from this unit to date.

The study area lies within the southwest-northeasttrending Tertiary Offshore Northwest Java Basin. Theunderlying structural fabric is that of a half-graben,pull-apart extensional basin. Associated faultsremained active during upper Cibulakan time,although structural activity was subdued at this timerelative to the early Miocene and Oligocene.

APPROACH

The initial evaluation of the stratigraphic evolution ofthe Miocene section was accomplished throughdetailed analysis of 3D seismic horizon slices andinterval attributes. Horizon slices were generated at 2msec intervals using CGG Petrosystem’s Stratimagicseismic interpretation software. Horizon slicing wasaccomplished using multiple reference horizons, eachhaving been chosen for its reflection continuity.Successive reference horizons generally were spacedat 100 msec intervals. Each horizon slice representedan amplitude extraction from a level at a fixedinterval below the reference horizon. Where featureswere observed that warranted closer inspection, avariety of additional attributes were generated andanalyzed.

Well logs were tied to the seismic slices for thepurpose of calibrating the seismic images. Well logtendencies (e.g., fining-upward and coarsening-upward trends) relating to features observed onseismic data were noted. Cores were examined toevaluate the sedimentology of the observed features.Primary as well as secondary sedimentary structureswere described, key surfaces were identified, andgrain size and mineralogy were recorded.

SEISMIC OBSERVATIONS

At a number of levels in each of the two focus areas,long linear features were observed on horizon slices(Fig. 2). These linear features are expressed as bandsof alternating light and dark amplitudes, in manyinstances extending across the data set. Theorientation of these bands is quite consistent, rangingfrom north 20-30 degrees east. In some instances thebands narrow and even appear to pinch out and canappear less as bands than as sharp-edged patches (Fig.3). The linear bands commonly range in width fromunder 0.5 km up to 4 km, although in some instancesthey can be significantly narrower than 0.5 km (Fig.4). Commonly the bands appear well defined on oneside and poorly defined on the other.

Figure 5 illustrates a close-up of the band imaged inFigure 2. This feature was examined in greater detailfor the purpose of more accurately describing theinternal and external architecture of this type ofdepositional element. Figure 6 illustrates threeseismic sections across the seismic band imaged inFigures 2 and 5. Figure 6A illustrates the interval

across which various attributes were derived. Theseseismic sections illustrate cross-sectional views acrossthe band shown in Figure 5. In each instance, thecross-sectional view illustrates an apparent reflectionphase reversal at the base of the feature at thenorthwestern edge of the seismic band. Likewisethere is a consistency of expression marking thecross-sectional view of the southeastern edge of theseismic band. On that side, the edge of the seismicband is characterized on some sections by anonlapping reflection (Figs. 6B and 6C).

Several attributes for the interval bracketing theseismic feature illustrated in Figures 5 and 6 weregenerated for the purpose of further evaluating thestratigraphic body. Based on the interpreted top andbase of this feature the resulting thickness map (intime) illustrates the asymmetric aspect of this depositwith the maximum thickness preferentially located onthe northwestern side (Fig. 7). Note also the linearnorthwestern edge of this feature. The dip map of theupper bounding horizon (Fig. 8) illustrates the sharplinear edge as well as the high gradient of thenorthwestern edge of this feature. Other attributes(Figs. 9 and 10) also illustrate the linear aspect of thisfeature as well as the sharp delineation on thenorthwestern side contrasted with the more poorlydefined edge on the southeastern side.

Two seismic attributes provide potential insight as tothe internal stratigraphic architecture of this deposit(Figs. 11 and 12). Figure 11 illustrates a seismicfacies map across the linear seismic anomaly. Thismap illustrates the result of grouping of seismic tracesegments into “families” for the interval shown onFigure 6A, using CGG Petrosystems Stratimagic’sneural network approach. The analysis results in amap illustrating areas with similar seismic responseand therefore similar rock properties. A arcuate-shaped area of uniform seismic response is observedoriented oblique to the long axis of the linearanomaly. A similar arcuate-shaped amplitudeanomaly is observed in Figure 12. On this attributemap, two and possibly three parallel features areobserved similar to that observed in Figure 11.

The seismic anomaly shown in Figure 3B is fromsame formation (i.e., Upper Cibulakan) but at E Field,located approximately 40 km to the northwest. Thisfeature, described in Posamentier et al. (1998), issimilarly sharp-edged on one side, and gradational onthe other. In contrast to the feature shown in Figure 5,

this feature does not thicken sufficiently to allowresolution of top and bottom. Rather, this feature isexpressed in cross-section view as an increasedamplitude (Fig. 13). Several parallel amplitudeanomalies are observed at this level, separated by a 3-5 km gap (Fig. 14).

BOREHOLE OBSERVATIONS

The feature shown in Figure 5 has not been penetratedby drilling. Consequently there is no ground truthcalibration available there. However, the featureshown in Figures 3 and 14 has been penetrated byseveral wells. Both wells (Fig. 15) and cores (Fig.16) illustrate a sand-rich section sharply overlying amud-rich section. Note that the log signaturesassociated with this feature range from coarseningupward to blocky to fining upward. The coarseningupward trend seems to be associated with the sharp-edged side of the patch shown on the western side ofFigure 3 (Fig. 15A). Thinning characterizes the westto east trend across this seismic anomaly (Fig. 15A).

Conventional cores through this feature are availablefrom the EZC-2 well at E-Field (Fig. 16). Thedeposits are intensely burrowed so that no primarysedimentary structures are preserved. The grain sizeranges from upper fine to medium. These sands areabruptly separated from the underlying muddy sectionby a well-defined surface characterized by aGlossifungites ichnofacies, with Thalassanoidestraces common.

GEOLOGIC INTERPRETATION

Both the seismic and borehole observations point toan interpretation of shelf ridges for the band and patchanomalies observed on Figures 2 and 3 respectively.These features appear to have migrated across theshelf, likely during a time of shoreline transgression.Figure 17 illustrates the stages that characterize theformation of these deposits. The presence of upperfine to medium sand within these interpreted featuressuggests that it would be unlikely that these sedimentswere brought out as plumes through the water column(Scheihing and Gaynor, 1991). Rather, theserelatively coarse sediments likely were transportedonto and across the shelf within fluvial or distributarychannels across alluvial and/or coastal plains, en routeto deltas and shorelines beyond. These deposits thenwere eroded during the subsequent transgression andre-deposited on the shelf as palimpsest deposits. Both

ridges and ribbons seem to have formed at that time(Figs. 2, 3, and 4). Based on the near total absence ofsedimentary structures indicative of wave action inthis area (Posamentier et al., 1998), coupled with thepresence of tidal indicators (Posamentier et al., 1998),a tidal current origin for these features is suggested.

The surface at the base of the sand deposits observedin EZC-2 (Fig. 16) separates muddy shelf depositsbelow from shelf ridge deposits above. The presenceof a Glossifungites ichnofacies suggests erosion of apartially indurated substrate (MacEachern et al.,1992). The currents that eroded the substrate alsoacted as winnowing agents resulting in deposition oflag deposits across this erosion surface. The lagnature of such deposits is illustrated in Figure 18.The sand characterized by the bell-shaped gamma-raylog signature at 3574-3590 ft (1089-1094 m)can beseparated into two genetic units near the peak of thegamma ray curve. As shown by the UV-lightilluminated core, the permeability is markedly betterabove this surface than below. Once again, thisbounding surface is characterized by a Glossifungitesichnofacies. This surface is interpreted to constitute atransgressive surface separating underlying regressivedeposits from overlying shelf-redepositedtransgressive lag deposits. The higher permeability ofthe overlying sediment is indicative of its cleaner (i.e.,less muddy) character.

A shelf ridge origin for these deposits is indicated by1) the striking linearity of these deposits as observedon the seismic data (Figs. 2 and 3), 2) the presence ofa sharp erosional contact marked by a Glossifungitesichnofacies at the base of these deposits, 3) thepresence of muddy shelf sediments immediatelyunderlying as well as overlying these deposits, 4) thepresence commonly of a sharp edge on one side ofthese features, and 5) the asymmetry of these features,generally being thicker along the more sharplydefined edge.

The direction of shelf ridge migration can be inferredto be in the direction of the sharp edge of thesedeposits (Figs. 7, 8, 9, and 10). The sharp edge isbelieved to represent the leading edge, whereas thegradational side is believed to represent the trailingedge. The presence of the best developed coarsening-upward log pattern (well EE-1 in Fig. 15A) near theleading edge of the shelf ridge shown in Figure 3,supports the sense of migration and progradation inthat direction.

The detailed architecture of the ridge is shown by twointerval attributes (Figs. 11 and 12). The anomaliessuperimposed on the shelf ridge could represent sandwaves superimposed upon and migrating down thelong axis of the ridge (Fig. 19). Note the apparent enechelon aspect of these features (Fig. 12). Theorientation of these superimposed features suggests asubordinate vector toward the sharp edge of the ridge,with a dominant vector down the long axis of theridge.

Modern Analog

The shelf ridges described by Liu et al. (1998) serveas a modern analog for the deposits described here.Liu et al. (1998) described a field of shelf ridges thatformed during the late Pleistocene to early Holocenetransgression within the greater Pleistocene Yang-Tzeembayment in the East China Sea shelf area (Fig. 20).These ridges were formed in a tide-dominated settingthat characterized the Yang-Tze embayment mouthduring transgression. With each successive landwardstep of the embayment mouth, a new cluster of ridgesformed (Fig. 20). Eventually, the result of thislandward stepping of depositional environments was afield of shelf ridges (the oldest located at the outershelf and the youngest located on the inner shelf) c.200 km wide from one side of the paleo embaymentto the other, and from the outer to the inner shelf (Fig.20).

Individual ridges described by Liu range from 2-5 kmwide, up to 30-40 km long, and up to 25 m high. Atransverse seismic profile across one such ridge isshown in Figure 21. Note the asymmetry as well asthe sharp leading edge. Note also the progradationalarchitecture within this ridge. Although the samelevel of internal detail shown in Figure 21 is notavailable for the Miocene Java sea example, the senseof progradation within the East China Sea example,indicated by the oblique tangential clinoform seismicreflections, is consistent with the sense ofprogradation indicated by the log signature of theMiocene Java Sea example (well EE-1 in Fig. 15A).The similar origin of the two features is furtherunderscored by the similarity of the dimensions,shape, and form.

The striking similarities between the East China Seashelf ridges and the Miocene deposits of offshorenorthwest Java described here suggest a shelf ridgeorigin for the latter features. Taking this comparison

further, we would suggest that the Miocene linearshelf-ridges are oriented parallel to paleo-embaymentlong axes (i.e., north-northwest to south-southeast).This interpretation is consistent with the regionalorientation of land and sea, as well as the orientationof the long axes of structurally influencedembayments (Reksalegora, 1993; Reksalegora et al.,1996).

EXPLORATION SIGNIFICANCE

The findings of this study suggest that shelf ridgescan constitute a significant transgressive systems tractplay type. These deposits are long (>20 km) andnarrow (from a few tens of meters up to 2 km wide),and can be up to 15 m thick. They are also strikinglylinear in plan view. Based on borehole penetrationsthey appear to be sand prone, containing up 80% netsand. This play type is known to produce at severalfields within the offshore northwest Java PSC.

The extent to which shelf ridge deposits canstratigraphically trap hydrocarbons is unclear at thepresent time. It is not clear whether these depositscan be characterized as fractal equivalents of muchsmaller starved ripple sedimentary bedforms. If theycan be so characterized, then the absence of sandbetween ridges will be associated with discretepinchout of sands away from the ridges. Such apinchout, or potential stratigraphic trap, will be bestdefined at the leading rather than the trailing edge ofthese features.

From a field development perspective, the recognitionof these features may provide valuable insight toreservoir compartmentalization and the identificationof possible sweet spots (Figs. 11 and 12). This wouldallow optimization of primary and secondary recoverystrategies.

Where shelf ridges are observed to be depositeddirectly on shelf muds and separated from those mudsby a sharp surface representing a significant time gap,the surface upon which those deposits are lying couldconstitute a sequence boundary. A mode of origin asshown in Figure 17 suggests that in some instancesthe surface upon which the sand ridges lie couldconstitute a zone of sedimentary bypass betweeninner-shelf highstand deposits and outer-shelflowstand deposits. Thus, in those instances thepresence of thin transgressive lag, or thicker shelfridges, would point to the outer shelf as a possiblelocation of lowstand shoreface or deltaic deposits.

SUMMARY AND CONCLUSIONS

Shelf ridges can potentially constitute a significanttransgressive play type. They have a distinctivelylinear form and are common throughout the MioceneUpper Cibulakan Formation of offshore northwestJava. Such deposits are known to produce fromseveral sections in numerous fields within theOffshore Northwest Java (ONWJ) PSC. Shelf ridgesobserved here commonly range from 0.5-2.0 kmwide, >20 km long, and up to 17 m thick. They areobserved to contain up to 80% net sand. Because ofthe winnowing process associated with theirdeposition, reservoir quality is excellent incomparison with deltaic and shoreface sands in thearea.

The shelf ridges observed and described in this studyrepresent one of the first unequivocal examples of astratigraphic feature common on modern sea floorsbut conspicuous in their absence from the geologicliterature with regard to ancient deposits. Thissuggests that shelf ridges do have the potential forpreservation into the rock record and opens the doorfor the possible reinterpretation of similar depositselsewhere.

ACKNOWLEDGMENTS

The author gratefully acknowledges the managementof PERTAMINA and Atlantic Richfield Indonesia,Inc., for permission to publish this paper. Thanks aredue to Jim Howes, Caroline Whorlow, Terry Daw,Sugeng, Sena R., and Wayne Suyenaga forstimulating and constructive discussions. This paperbenefited from a review by Hasan Sidi. However,ultimate responsibility for interpretations containedherein rests solely with the author.

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