saller2002

Journal of Petroleum Geology, vol.25 (2), April 2002, pp 123-150 123DEPOSITIONAL AND DIAGENETIC HISTORY OF THEKERENDAN CARBONATE PLATFORM, OLIGOCENE,CENTRAL KALIMANTAN, INDONESIAA. H. Saller* and S. Vijaya**The Kerendan carbonateplatform(Oligocene Berai Limestone) covers a subsurfaceareameasuringapproximately11by16kminthewesternmostKuteiBasin,CentralKalimantan. Aggradationof the Kerendan platform occurred during a major Oligocenetransgression,and is contemporaneous with aggradationand backstepping of the Baritoshelf margin which was located approximately 30 km to the south. The Kerendan platformis approximately 1,000 m thick, and comprises three aggrading seismic sequences identifiedbythe downlapofbasinalstrataat theplatformmarginanddownlapoftransgressivestrata within the platform. Carbonatedepositionstarted in the Late Eocene, and endedwhentheupperlimestonesequencedrownedandwascoveredwithshaleintheLateOligocene(approximately28.6Ma).Threedepositionalareascanbedistinguishedinseismic sections: (1) a platform interior (lagoon); (2) a slightly raised platform rim (1-2kmwide);and(3)abasinward-dippingplatformmarginandslope.Themarginof theplatform is identified by inflections on the seismic profiles where the relatively flat platformtop begins to slope basinward.DepositionalmodelsfromoutcropscombinedwithcorefromthreeKerendanwellswereusedtoextrapolatedepositional faciesontotheseismically-definedplatform.Platform-interior(lagoon)faciesconsistlargelyoffossiliferouswackestonesandpackstones, and porosities are generally lower than 5%. The platform rim is characterizedbyinterbeddedbioclasticwackestones,packstones,grainstonesandboundstones,withgrainstonesincreasingtowardtheplatformmargin.Porositypreferentiallyoccursinpackstones, grainstones and boundstones. The platform-rim deposits have greater porosity(5-13%) than the platform interior because the platform rim is more grainstone-rich, andbecause acidic waters compacting out of basinal shales concentrated dissolution near theplatformmargin.Poretypesincludevuggyporosity,microporositywithingrains,andintercrystalline porosity in dolomite in the upper part of the platform rim. Different poretypeshaveresultedinvariablebutlocallyveryhighpermeabilities(greaterthan100mD).Permeabilitiesdecreasewithdepthasvuggyporesdecrease,andmicroporositybecomes dominant.*Unocal Corporation,Sugar Land, Texas 77478, USA.author for correspondence,[email protected]**UnocalIndonesia,Balikpapan,Indonesia.Kerendan carbonate platform, Oligocene, Central Kalimantan 124INTRODUCTIONUpper Eocene and Oligocene carbonate shelves and isolated platforms are importantstratigraphic features in SE Asia (Wilson et al., 1999), but only a few have been describedin thesubsurface(GrtschandMercadier, 1999). Althoughoutcropscanprovidelargesurfaces of exposure, subsurface data have other advantages including (1) clear illustrationof large-scale geometries from seismic data; (2) continuous vertical sections in wells notcomplicatedbypoorexposure;and(3)porosityanddiageneticfeatureswhichare notoverprinted by surface weathering.An Oligocene carbonate platform is present in the westernmost part of the Kutei Basinin Central Kalimantan (Figs. 1 and 2; van de Weerd et al., 1987) and has been characterizedusinga grid of 2D seismic profilesandwells with cores. The platform is composedofcarbonatesassignedto the OligoceneBerai Limestone.The purposeof thispaperis todescribe the geometry, facies, diagenesis and porosity evolution of this Oligocene carbonateplatform.EXPLORATION HISTORYOilproductionineasternBorneo(EastKalimantan)startedinthelate1800s.TheTanjung field (Fig. 1) was discovered in the Barito Basin in 1939. More than 600 MM brlof oil were originally in place here, and more than 100 MM brl of oil have been producedfrom Paleoceneand/orEocenealluvialsandstonesofthe TanjungFormation(Kusumaand Darin, 1989). In the early 1970s, Unocal signed a Production Sharing Contract (PSC)for the Teweh Block which included the northernmost Barito Basin and westernmost KuteiBasin (Fig. 1). Seismic data were acquired and several wells were drilled in this block inthe 1970s. The Kerendan #1 well (Fig. 2) was drilled in 1982 and tested significant volumesof gas from Oligocene platform carbonates (van de Weerd et al., 1987). A more extensivegrid of seismic data was acquired between 1985 and 1987. Field geological programmesalso studied outcrops to help determine the stratigraphic and structural evolution of Tertiarystrata in the area. Several more wells were drilled in the Teweh Block in 1988 and 1989,including Kerendan #2 and Kerendan #3 (Fig. 2), both of which penetrated the platformcarbonates and recovered more than 100 m of core. Kerendan #2 was drilled in a platforminterior location, and Kerendan #3 was drilled on the platform rim and margin.GEOLOGICAL SETTINGThe structural and tectonic evolution of the Kutei Basin has been discussed by manyauthorsincludingHamilton(1979),vandeWeerdetal.(1987),Mossetal.(1997),Chambers and Daley (1997), and Cloke et al. (1999). Moss et al. (1997) suggested thatthe basins basement includes fragmented Upper Cretaceous microcontinental, ophiolitic,and accretionary prism material intruded by Cretaceous plutons. Rifting occurred in thisregion in the late Paleocene and early to mid Eocene (van de Weerd et al., 1987; Moss etal.,1997).TheUpperEoceneandOligoceneapparentlyrepresentasagphasewithwidespreadcarbonatedeposition(vandeWeerdandArmin,1992).Collisionandthesubductionof blocksbeneathNW Borneoresultedin upliftof the Crocker-RajangandCentral Kalimantan Mountains in the Late Oligocene and Early Miocene (Sandal, 1996;Moss et al., 1997).Large volumes of clastics were shed off these mountainsduringtheLate Oligocene and Early Miocene, and those sediments can be seen on seismic sectionsto prograde from NW to ESE in Central Kalimantan (Saller et al., 1993). Eocene throughLower Miocene strata in Central Kalimantan were foldedandthrustfaulted duringtheMiddle to Late Miocene.125 A.H.Saller and S.VijayaFig. 1. Tertiary sedimentary basins in Borneo,with the location of Unocal and partners 1990Teweh Block and the Kerendan platform in Central Kalimantan.Kerendan carbonate platform, Oligocene, Central Kalimantan 126Fig. 2. Oligocene paleogeography in the Teweh Block area. The Kerendan platform is located~30 km north of the margin of the Barito shelf (after Saller et al., 1993).Fig. 3. Schematic cross section of Upper Eocene through Lower Miocene stratigraphy across theTeweh Block. Upper Oligocene and Lower Miocene deltaics prograded from theWNW to the ESE (after Saller et al., 1993).127 A.H.Saller and S.VijayaThe general Tertiary palaeogeography of eastern Borneo was discussed by van de WeerdandArmin(1992)andWilsonandMoss(1999).Wilsonetal.(1999)describedotheroutcrops of Eocene, Oligocene and Miocene carbonates in NE Borneo and Sulawesi, andput together a series of depositional and stratigraphic models. Van de Weerd et al. (1987)discussedthe depositionalsettingof the westernmostKutei Basinincludingthe TewehBlockinCentralKalimantan.Salleretal.(1992,1993)describedUpperEoceneandOligocene shelfal strata from outcrops and the subsurface of the adjacent northern BaritoBasin.The Kerendan platform is located approximately 30 km north of the Barito shelf marginwhich was aggrading and backstepping during the Oligocene (Figs. 2 and 3). The descriptionofoutcropsectionsandtheanalysisofseismic datafrom theBaritoshelf hasallowedsequencestratigraphicanddepositionalmodelstobedevelopedfor UpperEoceneandOligocene carbonates and clastics (Saller et al., 1993). Four major Oligocene sequenceswere identified on the Barito shelf (Fig. 3), and each sequence (200-500m thick) has beendelineated in outcrops and on seismic lines. The sequences are: (1) Upper Eocene to LowerOligocene (34.0-38 Ma), (2) middle Oligocene (29.7-32.0 Ma), (3) lower Upper Oligocene(28.2-29.7 Ma), and (4) Upper Oligocene (N3; >24-28.2 Ma) (Saller et al., 1992, 1993).Ages are based largely on strontium isotope ratios and the strontium isotope ratio versustime curve of Miller et al. (1988). All numerical ages in this paper are relative to the Haqet al. (1988) time scale.SEISMICINTERPRETATION OF KERENDANThe Kerendan platform has been studied on a grid of seismic sections (Figs. 4 and 5).Theseismicdatashowthat theplatformisapproximately1,000m thick,andcontainsthree aggrading seismic sequences with boundaries distinguished by downlapping basinalFig. 4. Map of the Kerendan platform showing the location of seismic lines and depositionalareas. Part of line KT85-10 is shown in Fig. 5.Kerendan carbonate platform, Oligocene, Central Kalimantan 128Fig. 5. Seismic line KT85-10 (above) uninterpreted; (below) interpreted with sequences and depositional areas.Two-way travel-time (in seconds) is shown at the right.129 A.H.Saller and S.VijayaFig. 6. Structural cross section between Kerendan wells 1, 2 and 3. Numbers are ages in Ma from strontium isotope analysesand the Miller et al. (1988) curve.Kerendan carbonate platform, Oligocene, Central Kalimantan 130clastic reflectors at the platform margins, and mounding and local downlap of transgressivecarbonate strata within the platform (Fig. 5). The platform top covered an area measuringapproximately 16 km long and 11 km across. Maximum depositional relief of the Kerendanplatform was approximately 1000m at the end of platform growth. The depositional dip ofthe platforms slope was low during deposition of the lower platform (upper Eocene), andincreasedto approximately15o duringdepositionofthe uppermostcarbonatesequence(Fig. 5). Apparent relief on the platform and slope may have been increased by compactionofbasinalshales;however;mostshalesoccurabovetheplatform,andhencetheircompaction would not have affected the platform morphology. The entire platform wasstructurally tilted (down to the SSW) during the Middle to Late Miocene.Three seismic facies tracts can be distinguishedin the uppermost carbonate sequenceon the platform: (1) a slightly deeper platform interior (lagoon); (2) an elevated platformrim; and (3) a basinward-dipping platform margin and slope (Figs. 4 and 5). The margin oftheplatformisidentifiedbytheinflectionpointonthedepositionalprofilewheretherelatively flat (depositionally horizontal) platform rim begins to slope basinward. Seismicsections show relief at the top of the platform that suggestthat the outer 1-2 km of theKerendan platform was a topographicallyelevated rim (Figs. 4 and 5). The lagoonwardmargin of the platform rim was identified by the lagoonward dip of the top Berai Limestonereflector and thinning of the upper Berai Limestone from the rim to the interior (Fig. 5).The platform interior is characterized by high-amplitude, parallel and continuous reflectors;whereas the platform rim is characterized by lower amplitude, more discontinuous reflectors.Carbonate platform facies were predicted prior to drilling Kerendan #2 and #3 by usingseismic characteristics and outcrop models from Saller et al. (1992, 1993).Fig. 7. Schematic cross section of the Kerendan platform showing depositional facies.131 A.H.Saller and S.VijayaDEPOSITIONALENVIRONMENTS ANDHISTORYWell penetrations have helped to date the three sequences identified in the seismic dataaswellastoconfirmlithologiespredictedfromtheoutcropmodels.Basedonbiostratigraphic analysis of cuttings, the lowest carbonate sequence is Upper Eocene andincludes interbedded sandstones, shales and limestones (Figs. 6 and 7). The middle sequenceconsistsofplatformcarbonateswhicharemainlyLowerOligocene(Tertiarylargeforaminifera stage TcdofAdams, 1984) based on biostratigraphy of cuttings. The uppercarbonatesequenceismiddletoUpperOligocene(33-28.6Ma),basedonthebiostratigraphy(Tertiary largeforaminifera stageTeof Adams,1984)andstrontiumisotope dates on coralline algae and large foraminifera in cores, using the curves of Milleret al. (1988)(Fig. 6).Thefaciestractsidentifiedintheuppercarbonatesequenceinseismicdatacanbedistinguishedin core. The platform-interior facies cored in Kerendan #2 consists mainlyof fossiliferous wackestones and packstones with large foraminifera (rotalines and manymiliolines including Borelis), coralline algal fragments, molluscs, and some thin, branchingcorals (Plates 1A-C and 2A-D: see pp. 144 - 150). Cores from Kerendan #3 indicate thatthe platform rim was dominated by fossiliferous packstones and grainstones (Plates 1D-F,2E-H and 3A-F). Coralline algal fragments and large rotaline foraminifera are the maingraintypes.Thethirdfaciestract isthebasinward-slopingplatformmarginandslope.Outcropanaloguessuggestthatdepositsinthisareaincludecoral-richwackestones,packstones and boundstones passing downdip into lithoclastic conglomerates interbeddedwith carbonate mudstones and shales (Saller et al., 1992, 1993). Where penetrated by theKerendan #3 well, the edge of the platform margin facies tract is coral-rich. No subaerialexposure surfaces were identified in core.DEPOSITIONALHISTORYAsummaryoftheOligoceneandMiocenedepositionalhistoryinterpretedfortheKerendan platform is shown in Fig. 8. Lower to Middle Eocene terrestrial clastics (sands,shale and coals) were deposited during rift and sag phases of the palaeo-Barito and westernKutei Basins. Upper Eocene carbonates and clastics (Fig. 7) formed the base for the isolatedcarbonate platform deposited during the Early and middle Oligocene. A period of platform-wide deepening after the Early Oligocene (Tcd) platform resulted in deposition of a layerof dark carbonate mudstones across the platform (Figs. 6 and 8A). The uppermost carbonateplatformdevelopeddistinctfaciestracts(platform interior,platformrim, andplatformmargin/ slope) as the platform top approached sea level (Fig. 8B). Deepening of the platformatapproximately28.6Maresultedindepositi onofmoreopen-marine,coral-richwackestones,packstonesandboundstones(some shaley) across the platform (Fig. 8C).ProgradingprodeltashalesfollowedbyshallowerdeltaicstratacoveredtheKerendanplatform duringthelatestOligoceneandEarlyMiocene(Fig.8D).Thedeepermarineprodelta shales supplied the topseal required for the gas reservoir.DIAGENESISDiagenesis has greatly altered the Kerendan platform carbonates. Most porosity in theKerendanplatform is secondary, formed duringdiagenesis(Plates 1D-F; 2G,H; and3).Subaerial exposure and freshwater diagenesis were not identified in cores. Major diageneticprocessesaffectingtheKerendancarbonatesincluded:(1)theconversionofhigh-magnesium calcite to low-magnesium calcite; (2) development of microporosity in high-magnesiumcalcite grains;(3) dissolutionand/or neomorphismof aragonite;(4) calciteKerendan carbonate platform, Oligocene, Central Kalimantan 132Fig. 8. Schematic summary of the depositional history of the Kerendan platform.133 A.H.Saller and S.Vijayacementation;(5) mechanical compaction; and (6) pressure solution. Diagenetic processesthatare locallyimportantincludedolomiteprecipitationandvuggydissolution.Fig.9qualitativelysummarizesthetimingandintensityofdiageneticprocessesbasedonpetrographicdata.At the time of deposition, high-magnesium calcite (> 4 mole % MgCO3) was present incorallinealgalfragments,echinodermfragments,manylargerotalineandmiliolineforaminifera, andsomecarbonatemuds.Generally,the conversionofhigh-magnesiumcalcitetolow-magnesiumcalciteoccurswithnoidentifiablepetrographicchange(Friedman,1964; Land et al., 1967; Gavish and Friedman, 1969). Much of the porosity inthe Kerendan platform consists of microporosity within originally high-magnesium calcitegrains, and this could be related to: (1) dissolution during stabilization of high-magnesiumcalcite to low-magnesium calcite, (2) partial selective dissolution before stabilization, (3)partial selective dissolution during deeper burial after stabilization, or (4) a more pervasiveearly microporosity that was filled during burial in some grains and not others (similar toSaller and Moore, 1989). The preferential occurrence of microporous grains in packstonesandgrainstonessuggeststhatdissolut ionwastheproductoffluidswhichflowedpreferentially through grain-rich rocks; however, dissolution could have occurred duringearly or late burial.Fig. 9.Qualitative summary of diagenesis in the Kerendan platform.Kerendan carbonate platform, Oligocene, Central Kalimantan 134Aragonitewasoriginallypresentincoralfragments,somegreenalgae,andsomemolluscs, but all of the aragonite has been either dissolved or calcitized. Most moulds ofaragonitic grains have been filled with carbonate cement, but a few are open. Dissolutionand/or calcitization could have occurred in freshwater conditions shortly after deposition,or in burial fluids during moderate to deeper burial. Calcitized corals have highly variableisotopic compositions, which do not clearly indicate a particular diagenetic environment(Fig. 10). Lack of recognized subaerial exposure surfaces suggeststhat most diagenesisdid not occur in meteoric water.Calcite cement has occluded substantial amounts of porosity in the Kerendan platformcarbonates, and fills many intergranular, vuggy, intragranular, mouldic and fracture pores(Plates 1D-F; 2E-H and3A-E). Calcite cements are generallyequant,thoughprismaticcements occur in some fractures and early reefal cavities (Plate 2E). Most early cementzones are iron-poor, but some outer zones are iron-rich. Iron-rich cements are more commonwithin, and near to shale-rich intervals, suggesting that the shales were the source of theiron.Most cements appear to postdate aragonite dissolution and compaction. Cementationafter aragonite dissolution is indicated by the loss of boundaries between original aragonitegrains and original porosity (Plate 2F), and also the collapse of internal sediments in coralsafter aragonitedissolution.Somecementsare cutbyfractures andothersfillfracturesindicatingcementationbeforeandafter fracturing(Plate3B).Abundantgrain-to-grainpressure solution (Plate 2G) indicates that there was little intergranular cementation priorto deep burial and the onset of pressure solution, because intergranular cements help limegrainstones and packstones to resist compaction.Samples of calcite cements, other carbonate components, and bulk rock were used forstable carbon and oxygen isotopic analyses (Fig. 10). Modern meteoric waters from riversin Kalimantan (the Rivers Barito, Mahakam and the Benangin, which is a small tributaryof the Barito) have stable oxygenisotopiccompositions(d18O values) of -6.4 to -9.5 (SMOW), and Tertiary meteoric waters probably had similar compositions. Two travertinesanalyzed from modern caves in the Teweh area had d18O values of -8.0 and -10.9 (PDB),whichareconsistentwithprecipitationfrom waterswithd18Ovaluesof-5.8and-8.8(SMOW) at 28oC(usingtheequationof FriedmanandONeil,1977).In contrast,twosubsurfacewatersfromtheKerendanplatformhadd18Ovaluesof+6.2and+8.0(SMOW).WelldatasuggestthattheKerendanplatformiscurrentlyatapproximately120oC. Using a geothermal gradient of approximately 3oC/100 m (estimated by S. Brandand R. Sweeney, 1984,pers. commun.), carbonates of the Kerendan platform may havebeen as hot as 170oC.Calcite cements filling aragonite moulds in the Kerendan platform haved18O values of-3.5 to -6.2 (PDB)(Fig. 10). Calcite cements filling aragonite moulds in the Teweh areaoutcrops haved18O values of -4.6 to -11.1 (PDB). Fracture-filling calcite cements aregenerally isotopically lighter with Kerendan samples havingd18O values of -7.5 to -11.2(PDB), andoutcropsamples havingd18Ovaluesof -8.2to-12.3 (PDB)(Fig.10).Most cements haved13C values of -2 to +2 (PDB). The wide range of stable oxygenisotope compositions indicate that carbonate cements either precipitated over a wide rangeoftemperaturesorinwaterswithlargeisotopicvariationsorboth.However,thewiderange in temperatures and isotopic compositions of waters that could have affected Kerendancarbonates make a unique interpretation of the isotopic data difficult.POROSITY EVOLUTIONIn general, carbonates progressively lose porosity during burial (Schmoker and Halley,1982),andmuchporositylossintheKerendanplatform wasprobablyrelatedtodeepburial.TheerosionofLowerMiocenedeltaicstrataoverlyingtheKerendanplatform135 A.H.Saller and S.VijayaFig. 10. Stable isotopic composition of samples of Oligocene carbonates in the Teweh block.Kerendan carbonate platform, Oligocene, Central Kalimantan 136together with vitrinite reflection data suggest that the Kerendan platform was buried about1,500mdeeper than it is at the present (total depth of approximately 4,300m; S. BrandandR.Sweeney,1984,pers.commun.).Carbonatemudstonesandwackestoneshaveporosities of 60-80% when deposited (Enos and Sawatsky, 1981). Micritic rocks probablyunderwent significant mechanical compaction, although this is difficult to quantify. Twotypesofpressuresolutionare common:grain-to-grain,andstylolitic(Plates 2A,Gand3F). Both lead to reduced porosity directly, and also indirectly by dissolving CaCO3 whichthen precipitates into pores as cement.Late saddle (baroque) dolomite cement is concentrated in the upper part of the BeraiLimestoneintheKerendan#3wellwhereitisassociatedwithvuggydissolutionandsometimespartiallyfillsvugs(Plate2F).Saddledolomiteisgenerallyassociatedwithdeepburialenvironments(>60oC;RadkeandMathis,1980).Dolomitecementsfillingvugs have d18O values of approximately -8.0 (Fig. 10). Using the dolomite/water curvesof Land (1985), the Kerendan dolomite cements could have precipitated at approximately145oC from waters withd18O of +6.2 (SMOW)(similar to present-day formation waters).If precipitated from waters with an isotopic composition similar to modern seawater (d18Oof0),thedolomitecementswouldhaveprecipitatedatapproximately87oC.Bothscenarios are consistent with dolomite precipitation during deep burial (>2,000m).Severalobservationssuggestthatmuchofthedissolutionporosity,especiallythenonfabric-selective dissolution, is related to acidic waters compacting out of adjacent shalesduring moderate to deep burial (200-3,000 m). First, porosity is highest near the marginsoftheKerendanplatformwhereacidicwatersemanatingfromshaleswouldbemostabundant. Second, much of the porosity, especially nonfabric-selective, is petrographicallylate, when maturation of organic material might have generated acidic waters which wereFig. 11. Model for porosity development in the Kerendan platform during burial.137 A.H.Saller and S.Vijayaexpelled from the shales during compaction. Third, saddle dolomite is associated with thedissolution,anditformedduringdeeperburial.Fourth,theisotopiccompositionofKerendan formation waters (+6.2 and +8.0, SMOW) is consistent with waters generatedduring the smectite to illite transition (Land, 1983). Creation of acidic waters by organicreactions during burial was postulated by Surdam et al. (1984) and Crossey et al. (1986).A model for late burial fluids being expelled from adjacent shales and forming porositynear the margins of the platform is shown in Fig. 11.DISTRIBUTION OF POROSITY AND PERMEABILITYThe distribution of porosity and permeability in the Kerendan platform is a function offacies-selective and nonfacies-selective diagenesis. The platform-interior limestones coredin Kerendan #2 have low porosity and permeability -- generally less than 5% and 1 mD,respectively(Fig. 12)(average porosityof 2.9%;arithmetic mean permeabilityof 0.36mD; geometric mean permeability of 0.06 mD).Platform-rimlimestonescoredinKerendan#3hadanaverageporosityof5.4%,arithmeticmeanpermeabilityof4.4mD,andgeometricmeanpermeabilityof0.39mD(Fig. 13). Two main types of porosity were present here: nonfabric-selective and fabric-selective.Nonfabric-selectiveporosity(5-13%)occurs20to45m belowthetopofthecarbonate buildup,while fabric-selective porosity was dominant below that. Nonfabric-selective porosity includes vugs up to 1cm across (Plates 1-3 and Fig. 14). Permeabilitymeasured in full-diameter core samples of vuggy rocks was 1-100mD (Upper samplesin Fig. 13); however, most vugs were filled with drilling mud (Plate 1D,E) which couldnot be removed, and hence true subsurface permeability was much greater. Facies-selectiveporosity(commonly 5-15%) is dominant deeper in the platform, and mainly consistsofFig. 12. Graph of porosity versus permeability for platform-interior limestones cored in well Kerendan #2.Kerendan carbonate platform, Oligocene, Central Kalimantan 138microporosity within grains (mainly coralline algal grains) in packstones and grainstones(Plates1F,2E-H,3:Fig.14).Measuredpermeabilityinsamplesdominatedbyfabric-selective microporosityis generallyless than2mD evenin samples with 15%porosity(Lower samples in Plate 2F and Fig. 13).Outcrop facies models combined with seismic stratigraphic interpretations permittedus to construct a reservoir model for the Kerendan platform with only three well penetrations.The platform is characterized by porous platform-rim facies (penetrated by Kerendan #1and#3)andnonporousplatform-interior facies(penetrated byKerendan#2) similar tothose found at outcrops (Saller, 1992, 1993). A three-month productiontest in platform-rim limestones in Kerendan #1 indicated that the gas reservoir had substantial lateral extent(greater than 1.5 km). Resistivity logs indicate a similar gas/water contact in the Kerendan#1 and #3 wells, which supported a single gas column that is continuous from Kerendan#1 to #3. The gas column probably extends around the platform rim down to the gas/watercontact in the southern part of the platform.DISCUSSIONDepositional facies at Kerendan are very similar to those at nearby outcrops (Saller etal.,1992,1993).IntheKerendanplatformandnearbyoutcrops,platform-marginandshelf-margin limestones are generally wackestones, packstones and boundstones with robustcorals, large foraminifera and coralline algae (Figs. 7 and 15). Platform-rim and shelf-rimstrata are dominated by packstones and grainstoneswith large rotaline foraminifera andcoralline algae (fragments and rhodolites)(Figs. 7 and 15). Lagoonal carbonates are mainlywackestones and packstones with large foraminifera (including many miliolines), molluscs,and thin-branching coral (Figs. 7 and 15).Fig. 13. Graph of porosity versus permeability for platform-rim limestonescored in well Kerendan #3.139 A.H.Saller and S.VijayaFig. 14. Kerendan #3 gamma-ray and density porosity logs, depths 9,300 to 9,650 ft, with information from cores.Kerendan carbonate platform, Oligocene, Central Kalimantan 140The upper Kerendan platform is relatively symmetric with raised rims on both sides.This is similar to the Lower Oligocene carbonate platform currently exposed in the GunungAnga area (Fig. 2; Saller et al., 1993); however, many Oligocene carbonate platforms inSE Asia are asymmetric, such as the Tonasa Formation in Sulawesi (Wilson et al., 2000)and the Malumpaya platform in the SW part of the Philippines (Grtsch and Mercadier,1999). The symmetry of the Kerendan and the Gunung Anga platforms suggests that theywere notsubstantiallyaffectedbysyndepositionaltilting,localizedclasticinfluxesorsignificant windward versus leeward variations in carbonate productionduring the mainphases of deposition.Subaerial exposure and classic sequence boundaries (Van Wagoner et al., 1988) werenot recognized in the Kerendan platform. Rather, the key stratigraphic boundaries withinthe platform are intervals characteristic of more rapid accommodation and deepening similartothosereportedbySalleretal.(1993).Inseismicsections,thoseboundariesarecharacterizedbyirregularandmoundedreflectorsoverlyingdepositionallyhorizontalreflectors.Outsideoftheplatform,reflectorsterminateagainsttheKerendanplatform(Fig. 5). Regionalgeologicinformation indicates that these are deep-marine shales thatrepresent the distal toes of a clastic system; therefore, those reflectors are best viewed asseismic downlaps(Saller et al., 1993).Hence, the margin of the Kerendanplatfom is aseismicdownlapsurfacewhichcouldrepresentsedimentationduringhighstandsorlowstands of sea level, and may include both. Schlager (1989, 1999) noted similar featureswhich are difficult to interpret within the models of Van Wagoner et al. (1988).Biostratigraphy and strontium isotope chronostratigraphy have provided relatively high-precision dating of the Kerendan platform and Central Kalimantan outcrops. The timingof the Kerendan sequences, and especially the end of platform deposition, do not correlateclearly to other stratigraphic events nearby or in other parts of SE Asia or in other parts ofthe World. Theendof carbonatedepositionat Kerendanat 28.6Ma doesnotcorrelateclearly with major step-backs on the nearby Barito shelf (29.7 and 28.2 Ma; Saller et al.,1993). Strontium isotope dating precision of +/- 0.3 Ma (Saller et al., 1993) would allowoverlap of the 28.6 and 28.2 Ma ages; however, those ages are repeated in several samplesFig. 15. General depositional model for Oligocene shelfal carbonates in Central Kalimantan(after Saller et al., 1993).141 A.H.Saller and S.Vijayain each location suggesting the difference is real and not an artifact of analytic precision.Likewise, the end of Kerendan carbonate deposition does not apparently correlate to theend of other carbonate platforms or shelves in southernSulawesi (Wilson et al., 2000),southern Philippines(Grtsch and Mercadier, 1999),or other parts of SE Asia (Wilson,2002). Also, the deepening associated with drowning of the Kerendan platform does notcorrespondwithalarge-scale eustaticsea-levelrise,asperHaqetal.(1988).Lackofsynchroneity in the depositional events suggests that local structural movements were thedominant controls on sequence development. The drowning of the Kerendan platform isthoughtto be relatedto subsidencecausedbyloadingassociatedwith theapproachingdelta complex (Saller et al., 1993). Lack of pronounced subaerial exposure features suggestslow, low-amplitude variations in eustatic sea level during the Oligocene.Carbonate platforms have drowned repeatedly during geologic history; however ,thereasons for this are often difficult to determine precisely (Schlager, 1989, 1999). Clearly,rapid subsidenceand/or eustatic sea-level rise contribute to carbonate platform and reefdrowning (Erlich et al., 1990; Greenlee and Lehmann, 1993; Saller et al., 1993). Howevernutrients and muds associated with approaching deltas can decrease light penetration andrates of carbonate growth, and hence contribute to carbonate platform drowning (Hallockand Glenn, 1986; Wilson and Lokier, in press).The Kerendan platform is covered by deepwater prodelta shales, indicating deepeningimmediatelyaftercarbonatedeposition.Strontiumisotopeagesinuppermostplatformand lowest shaley strata in Kerendan #2 (Fig. 6) suggest that more than 200 m of shallow-marine to deep-marine sediments were deposited in ~100,000 years at the end of Kerendanplatform deposition. That suggests that relative sea level (eustatic sea level + subsidence)rose by more than 200 m in 100,000 years when the Kerendan platform drowned. Shaleysediments associated with an approaching delta were present around the Kerendan platformand may have decreased carbonate production by nutrient poisoning (Hallock and Glenn,1986)ordecreasedlightpenetrationduetothemuddywaters(WilsonandLokier,inpress); however, rapid structural subsidence appears to be the main reason for the drowningof the Kerendan platform.CONCLUSIONS(1) TheKerendanplatformcoversanareaapproximately11by16km,andhasaninferredthicknessofapproximately1000mwithslopeanglesincreasingtoapproximately 15o by the end of deposition.(2) Theplatformdevelopedduringthreeperiodsofcarbonategrowth:LateEocene(Tb; with clastics), Early Oligocene (Tcd) and middle Oligocene (33-28.6 Ma).(3) During deposition of the upper sequence, the platform had an elevated, high-energyrim where grain-rich sediments were preferentially deposited. At the same time, theplatform interior was lower energy, and micrite-rich sediments were deposited. Theplatform was drowned and was covered by prodelta shales during the Late Oligocene(approximately 28.6 Ma).(4) No subaerial exposure surfaces have been identified in cores from the platform.(5) The platform was buried to depths of more than 14,000 ft (4,300 m) which degradedmost depositional porosity.(6) The platform rim has porosities of 5-13%, whereas the platform interior generallyhas less than 5% porosity.(7) Porosity is concentrated along the platform rim (a) because of the more grainstone-richfacies,and(b)becauseacidicwaters,whichapparentlycompactedoutofadjacent basinal shales, concentrated vuggy dissolutionnear the platform margin.Microporosity within grains is dominant in the platform rim at greater depths.Kerendan carbonate platform, Oligocene, Central Kalimantan 142ACKNOWLEDGEMENTSWethankmanypeopleatUnocalandUnocalIndonesiawhohelpedusduringthisstudyincluding:Rich Armin, Andrewvande Weerd,Paul Ware, AlCrawford,GregorDixon,JohnColeman,PatCorbett,andSumar Mahadi.Themanuscriptwas improvedthanks to JPG reviews by Giancarlo Rizzi (Baker Atlas), S. 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White grains are mostly large rotaline foraminifera (#2,9,381 ft).D.Coral boundstone with vugs filled with reddish drilling mud (V) and vug filled with coarse calcite cement (white; C). Coralmorphologies include massive (M) and laminar (arrows). Micritic grey internal sediments (I) are also present (#3, 9,417 ft).E.Coral-foram packstone with vuggy porosity, fractures (black arrows), and matrix porosity (dark gray). Vugs (V) are oftenfilled with reddish drilling mud. Minor kaolinite cement (white arrow) is present (#3, 9,474 ft).F.Coralline algal-foraminifera wackestone/ packstone with stylolites (white arrows) and kaolinite (black arrows) filling stylolite-related and vuggy pores. Porosity is patchy (irregular surfaces) and mainly small moulds and microporosity (#3, 9,513 ft).145 A.H.Saller and S.VijayaKerendan carbonate platform, Oligocene, Central Kalimantan 146Plate2. Thin section photomicrographs from wells Kerendan #2 (A-D) and #3 (E-H). Samples wereimpregnated with blue resin prior to thin sectioning so porosity appears blue. Thin sections werestained with a potassium ferricyanide/Alizarin red S solution, so iron-poor calcite is pink, iron-richcalcite is purple, iron-rich dolomite is light blue, and iron-poor dolomite is unstained.A. Wackestone/ packstone with intensely compacted large rotaline foraminifera (F) separatedby dark pressure-solution contacts (#2, 9,526.3 ft).B. Wackestone with gastropod (G), milioline foraminifera (M), and other small bioclasts (#2,9,714 ft).C. Wackestone/packstone withmilioline foraminifera(M)includingBorelis(B),rotalineforaminifera (R), and coralline algae (A). Some miliolines and coralline algal fragmentshave been partially dissolved causing them to appear dark blue (#2, 10,199 ft).D. Long thin coralline algae (red; a) and echinoderm fragment (e) in a dolomitic micrite matrix(#2, 9,519 ft).E. Coral boundstone with bored internal cavity (edges shown by arrows), partly geopetallyfilledwithpeloidal internal sediments(I). Theremaining cavity was linedwithcloudy,radiaxial cement (R), and then filled with clear calcite cement (L). The bored internal cavitycuts across a coral (C; #3, 9,442.6 ft).F. Coral (C) truncated by vuggy dissolution (arrow). The resulting vug was lined with equantto bladed calcite cement (E), and then filled with blocky calcite cement (T) and iron-richbaroque dolomite (B; light blue; #3, 9,446.7 ft).G. Bioclastic packstone with a microporous large foraminifera (L) that has been compactedinto a stylolite (arrow). Porosity is adjacent to the stylolite in some places (white arrow)indicating that the stylolite was present before dissolution and collapse of the foraminifera(#3, 9,577.5 ft).H. Coral boundstone with fossil fragments and micrite. A fracture in the middle of the photo ispartly filled with equant calcite cement (pink) and is partly open (#3, 9,412.2 ft).147 A.H.Saller and S.VijayaKerendan carbonate platform, Oligocene, Central Kalimantan 148Plate3. Thin section photomicrographs of platform-rim facies from well Kerendan #3.A. Small vug (blue) in a bioclastic packstone/grainstone with large rotaline foraminifera (L).Thevug truncates a stylolite (arrow) suggesting that dissolution occurred after the stylolite formed(9,586.4 ft).B. Bioclastic packstone/ grainstone with partial to complete dissolution of some grains resultingin mouldic porosity (M). An open fracture (F) extends down the middle of the photo. Somecoralline algal fragments are present. Sample has approximately 15% porosity (9,621.8 ft).C. Bioclastic packstone/grainstone with partial dissolution of encrusting rotaline foraminiferafragments (F) resulting in microporosity (blue). Note echinoderm fragment (E) (15% porosity,9,621.8 ft).D. Bioclastic packstone with dolomite (D) and partial dissolution of fossil fragments includinga large fragment of coralline algae (A)(9,458.4 ft).E. Bioclastic packstone with microporosity (blue) in a fragment of coralline algae (A)(9,619.2ft).F. Grainstone with approximately 16% porosity and 1 mD permeability. Many original grainswere fragments of coralline algae that haveundergone partial dissolution resulting inmicroporosity (dark blue). Echinoderm fragments (E) commonly have syntaxial cements(arrow)(10,208.5 ft).149 A.H.Saller and S.Vijaya