SedimentaryCharacteristicsandEvolutionofAsriBasinIndonesiainEarlyTertiary

Sedimentary Characteristics and Evolution of Asri Basin,Indonesia, in Early Tertiary2006PetroleumScienceV_01.3No.3SedimentaryCharacteristicsandEvolutionofAsriBasin,Indonesia,inEarlyTertiaryZhongDakang,ZhuXiaominandZhangQin(SchoolofResourcesandInformation,ChinaUniversityofPetroleum,Beijing102249,China)Abstract:TheAsribasinisatypica1halgrabenbasin.Theeastsideofthebasinisasteepslopecontrolledbysyn.riftingandthewestsideisawidespreadgentleslope.IntheearlyTertiary.itwasfilledwithterrigenousclasticsedimentscomposedoftheBanuwatiandTalangAkarformationsfromboaomtotop.The1atterisfurtherdividedintotheZeldamember(1owerpart)andtheGitamember(upperpart).Thepreviousstudiessuggestedthattheearlytertiarysedimentsarealluvial,fluvia1andswampdeposits.Inthispaper,basedonthecorelithology,wel1logsandseismicdata,thesedimentsshouldbealluvia1.fluvia1and1acustrinesystems.The1acustriResystemincludessubaqeousfan.fandeltaanddelta.shore.shallow1ake.deep1akeandturbiditefandeposition.Alluvia1fan,subaqeousfanandfandeltasedimentsweredepositedintheearlystageand1ocatedonthesteepslopeadjacent.Thedeltaicsedimentarysystemwasusuallydistributedonthegentleslopeofthebasin.IntheearlyTertiary,thebasinevolutioncouldbedividedintofourstages:initia1subsidence(matchingBanuwatiformation).rapidsubsidence(matchinglowZeldamember).steadysubsidenceorfluctuation(matchingmiddleZelda)anduplifting(matchingupperZelda).Atthefirststage,thealluvialfan,floodplain,braidedstreamsedimentsweredepositedfirstwiththickbrownconglomerateandpebblesandstones,andthensubaqeousfansedimentswereinterbeddedwiththethick1acustriRemudstones.Atthesecondstage.shoreshallow1akeanddeeplakeandturbiditefansedimentsweredeposited.withthinfinesandstonesandsiltstonesinterbeddedwiththickmudstones.Atthethirdstage.thickfandeltaanddeltasandstonesweredeposited.At1astcamefluvia1meandering,anastomosedandswampsediments.Sedimentsupplywasmainlyfromthewestandtheeast,partlyfromthenorth.Keywords:Sedimentarycharacteristics,evolution,Asribasin1.IntrOducti0nTheAsribasinisovalinshape.coveringabout2500kmfromnorthtosouth.Themaximumthicknessatthedepocenterisaroundl8,000feet(5,500m).ItisadjacenttotheHerabasininthewestandtotheSundabasininthesouthwestfFig.11.NowitisundertheJavasea.Thebasementconsistsofgranites,granodiorites,fnarbles,schistsandquartzites(TonkinandHimawan,l999).IntheearlyTertiary.theeasternsideofthebasinwasasteepslopecontrolledbysyntiffing,andthewesternsideawidespreadgentleslope(Howells.1997andColeandCrittenden.1997).Soitwasatypicalhalf-grabenbasinwithterrigenousclasticsediments.Duringthelatetertiary.terrigenousmarinetransitionalandmarinesedimentsweredeposited.ThelowertertiarystratigraphyincludestheBanuwatjandTalangAkarformationsfrombottomtotop.ThelatterwasfurtherdividedintotheZeldamember(1owerpart)andtheGitamember(upperpart)(Fig-2,.TheZeldamembercouldbedividedintothelower.middleanduppersections.Fromsequencestratigraphy.thelowerTertiarycouldbedividedinto6thirdordersequences(Aldrich,eta1.,l995).Amongthem.theBanuwatiformationcorrespondstosequenceI:thelowerpartofthelowerZeldatosequenceII.theupperpartofthelowerZeldatosequenceIII.themiddleZeldatosequenceIV,theupperZeldatosequenceVandtheGitatosequenceVI.Thepreviousstudiessuggestedthattheearlytertiarysedimentswereattributedtoalluvia1.fluvialandswampdepositionfYoung,l995andGardner,eta1.,2003).Recently,amoredetailedsedimentationanalysiswasmadebasedonanintegratedstudyofcorelithology,welllogsshapeandseismicreflectioncharacteristics.ThescopeofthisPaDerincludesthesedimentaryfaciesclassification,sedimentaryfaciescharacteristicsofeachfacies.1ateraldistributionoffaciesandsedimentaryevolutionofbasinintheearlyTertiary.2.Sedimentary.faciesclassificationTotally,aboutl3kindsofsedimentaryfaciesintheearlyTertiaryareinterpretedintheAsribasinbasedoncoreimage,welllogshapecombination,seismicreflectioncharacteristicsandlithology(Tablel,.Thealluvialdepositionincludesthealluvialfanandfloodplainfacies(subfacies).Thesetwofacies(subfacies)weremainlydevelopedattheearlystageofbasinevolutionanddistributedintheBanuwatiformationintheeasternAsribasin.2PetroleumScience2006Fig.1LocationofAsribasins,keywellsand2DseiemiclinesTable1SedimentaryfaciescategoryinAsribasinVerticalLateralFaciesandsubfaeiesdistributi0ndistributionAlluvia1fanBanuwatiEasternAsriFloodplainBanuwatiCentralAnastomosedGitaWidelyinbasinMeanderingGitaWidelyinbasinBraidedBanuwafiCenWalbasinSwampGiraWidelyinbasinMide-upperDeltaWesterngentleslopeZeldaMiddle-upperFandeltaEasternsteepslopeZeldaSubaqueousfanBanuwatiEasternsteepslopeAlongtheedgeofShore1akeZeldabasinNearCenwalpartofShallow1akeZeldabasinSemi-deepanddeeplakeLow.ZeldaCentralpartofbasinMiddupperCenwaldeeppartof1,urbiditefanZeldabasinThefluvialandswampsediments,whichweremainlydistributedintheGitamember,arefoundwidelyinthebasin.ThefluvialsedimentarysystemCanbedividedintothreecategories;braided,meanderingandanatomosedstreams.Thedeltasedimentarysystem.includingnormaldeltaandfandeltasediments,aredistributedinthemiddleupperZeldaandsometimesinthelowerZeldainthelatestageofbasinevolution.Thenormaldeltasedimentsaremostlylocatedinthewesterngentleslopeofthebasin,withfandeltasintheeasternsteepslopeadiacenttothesynriftinginthebasin.ThedeltasedimentsaredistinguishedfromthefandeltasedimentsinthattheforlTlermainlyconsistofsandstonesandweredevelopedonthegentleslopeofthebasinatthelatestageofbasinevolution.whilethelaterconsistofconglomeraticsandstone.andweredevelopedonthesteepslopeofthebasinattheearlystageofbasinevolution.Thesubaqueousfansedimentswerealsodevelopedonthesteepslopeofthebasinattheearlystageofbasinevolution.ThevconsistofconglomeratesandsandyconglomeratesandaredistributedinthelowerZeldaandcameearlierthanthefandelta.Lacustrinesedimentaryfacies,whichweredevelopedintheZeldamember,includedtheshorelakealongtheedgeofthebasin.theshallowlakenearthecenalpartofthebasin,andthesemideepanddeeplakeinthecentralpartofthebasin.Lacustrinesedimentsaredominatedbymudstoneinterbeddedwithsomediscretefmesandstoneandsiltstone.InthesemideepordeepJake.turbiditefansweredevelopedfromtheslumpingofunstablesedimentsinthedeltafrontorfandeltafront.Vlol_3NO.3SedimentaryCharacteristicsandEvolutionofAsriBasin,Indonesia,inEarlyTertiaryAsriBasinStratigraphyFig.2StratigraphyofAsribasin(AfterCNOOC,2003)3.Sedimentarycharacteristics3.1AlluvialfaI1faciesBecauseofthelimitationofthecoringjob,thealluvialfanwasnotcored.Fromthewelllogs,thealluvialfaniscomposedofconglomerateandpebblesandstones.Ontheseismicsection,wedgeshapereflectioncharacteristicscanbeobserved(Fig.3)andthealluviaIfanconsistsofasetofseismiceventswith4PetroleumScience2OO6averagelowaltitudeandmoderatepoorcontinuity,showingthewedgeshapeconfiguration.Itislocatedat,thebottomoftheBanuwatiformation.ItiSdistributedneartheeastboundaryfaultsofthebasin.Fig.3Interpretedalluvialfanwithwedgeconfigurationonseismicsection3.2FloodplainfaciesFromthecombinationofseismicdataandsedimentaryfacies,thisfaciesWaSdevelopedinfrontofthealluvialfan.Thewelllogsshowthatthethickmudstonesareinterbeddedwithsomediscretethinsandstones.Spontaneouspotential,gammarayandresistivitylogsareofobvioustoothshapecharacteristics.WlellHariet-2alsoindicatedthiskindoffaciesintheBanuwatiformation(Fig.4).llHarie12GrtDepthIhOORt氏(J9595()Md,IOf,3(J0(,ceBascic,eFaciesft翳_:=一豳席Braidedi一stream1220l瞄:.圈剪二耄一l,.=-.;=-1240d豳ll口I蟹目触_一1露lI11藤Fig.4InterpretedfloodplainandbraidedstreamOilfloodplain3.3Braidedstreamfacies0nthefloodplainsomebraidedstreamsweredeveloped.Thiskindoffacieswasinterpretedfromtheseismicdataandwelllogs.Fillingreflectioncouldbeobservedontheseismicsection.ThewelllogOfHariet2from12.10012.400flintervalrevealsaconglomeraticcoarsesandstonetosandstonesuccession.whichdisplaysanincompletebinarycombinationofbraidedstreams(Fig.4,.Theseismicdatashowchannelfillingcharacteristics.Thebraidedstreamfaciesoriginatedfromthealluvialfananddevelopedoverthefloodplain.3.4MeanderingstreamfaciesItconsistsofgreytowhitefinesandstoneswithobliqueandripplebeddings.Vertically.itshowsobviousupcoarseninglithologicalsequence,inwhichthelowerpartiSfinesandstone.changingupwardtosiltstone,shalysiltstoneandsiltymudstonegradually.ThisiSobservednotonlyoncorelithologicsuccession.butalsoonwelllogs(Figs.5and61.Thereexistsatypicalbellshapeonwelllogcurves.ItrevealstheV_01.3No.3兰!tlcteristicsandEvolutionofAsriBasin,Indonesia,inEarlyTertiary5channelmigrationtoleveeoverbank.MeanderingfacieswaswidelydevelopedintheGitamemberintheAsribasin.3.5AnastomosedstreamfaciesIntheGitamemberoftheAsribasin,theanastomosedstreamsystemiswidelydistributed.Itconsistsoffinegrainsandstone.Itstypicalsedimentarycharacteristicsismainlyfixedchanne1.Thissituationcanbefoundinmanywells.Forexample.inwellDarleneIasetofthick,finesandstonefillingwasdevelopedinanastomosedstreamchannelsonthetopoftheGitamember.Thefinesandstoneismassive.Thetopfinesedimentsaremissing.Thiskindofdepositiondisappearsquicklylaterally,whichindicatedfixedchanneldepositionandthatchannelmigrationwaslimited.Itdisplaysaboxshapeonspontaneouspotentialandgammaraylogs.WellDarlene一0Fig.5RepresentativecoresofmeanderingsedimentationfromwellHariet1(middleZelda)SpGrDepthL5SystemSeqStrataLith,)l)v.11.6()3531)Md/lOf鬻*:=三:_妻莹49().量50()一鬣鬟i精麓*嘲鏊i叠15l0,三520一Fig.6WelllogscharacteristicsofmeanderingsedimentationfromwellDarlene13.6SwampfaciesTheswampfaciesconsistofcoa1.carbonaceousmudstoneanddarkmudstonefromthewelllogs.Thesecoalbeds,whichcanbecorrelatedoverthewholebasin,canbeeasilyinterpretedonwelllogsandseismicdata.Onwelllogs,theydisplaylowgammaraysignal,highresistivityandhighspontaneouspotentia1.Sometimes,thecoalbedsaremisinterpretedassandsonwelllogs.Theyshowclearplanereflectiononseismicdata.SwampdepositionismainlydistributedinthemiddleoftheGitamember.3.7SubaqueousfanfaciesThesubaqueousfanfaciesconsistofconglomeraticsandstone,sandstoneandripplefinesandstonefromthewelllogsinAsribasin.Asubaqueousfancanbedividedintoinnerfan.midfanandouterfansections.Theinnerfanconsistsofthickmassiveconglomerateandsandyconglomerates.WeUlogsshowalowamplitudetoothshape(Fig.7lower).Themidfanconsistsofalternativesandstonesandmudstones(Fig.7middle)andtheouterfanconsistsofthickmudstoneinterbeddedwiththinsandstones(Fig.7upper).Asubaqueousfanshowsanormalcycle/rhythmupwards.Itshowswedgereflectionontheseismicsection(Fig.8,Thesesedimentarycharacteristicsshowarapidsubaqueousdeposition.Vertically,thesubaqueousfanisinterbeddedwithhugethickmudstoneofsemi-deeplake(Fig.71.thesubaqueousfanwasmainlydevelopedonthesteepslopeofthebasinfromBanuwatiformationtolowerZeldamember,sometimestomiddleZelda.6PetroleumScience2006elJI)atlelle一()SpSp(jr(JIDepthlI_【)StrataEim(1(1,Faclc,slcmSca101CeIt5一t(Jt(ItI52I)3S3S二()MdI()f(1I5孝r_l,-一.SubaqueousIIlotlterfan9R()车_=葺0I)IJ专F三=-L.7_,ddll0旃0曩一.ciiAsr.0SpSp(jlrDepth1.ithotovRIStratan【13Mid;t()r52FacICSSystemSeqLlcnce50t10It)5C3(65UL-Zelda鼍970罩宝<=一cSubaquesTST=【】-.,mid.tim多至f99(7eIlAsri一0Spl1GI(jrDepthLlthotogyRtStrata503(JMdtOf150FacICSSystemSequcnce6(7l20I20603o50L.Zelda妻Fl020圈,SubaqueousHlinncrfanf024一兰l)22:=l726lf2NFig.7Welllogsofinnerfan,midfanandouterfanofsubaqueousfanFig.8Progradationalseismicfacies(S)andwedgeshapeseismicfacies(w)intheAsfiBasinVl01_3No.3SedimentaryCharacteristicsandEvolutionofAsriBasin,Indonesia,inEarlyTertiary73.8DeltafaciesFromthecoreofwellHariet1.thedeltafaciesshowsmediumtofinesandstoneswithparallelbedding,obliquebedding,crossbeddingandripplebedding(Fig.91.Itcanbedividedintodeltaplain.deltafrontandprodelta.Thedeltafrontincludessubaqeouschannelandmouthbardeposits,whichusuallydisplayreverselithologicalsuccessionvertically(Fig.9)andcanbefoundonSPandGR(Spontaneouspotentialandgammaraylogs)curvesofsomekeywells(Fig.10).IntheAsribasindeltasedimentswereusuallydistributedonthegentleslopeofthebasinfrommiddletoupperZelda.Onthedeltaplainsedimentsweredeposited.channelandinterchannelTheforlTlerwerefilledwithmediumthicksandstones.whichshowedanormalrhythmsimilartothefluviallithologicalsuccessiongeneratedbychannelmigration(Fig.1l1.Intheinterchannelareathinmudstonesorsiltymudstoneswimparallelbeddingweredeposited,whilethedarkmudstonesofshallowtosemideeplacustrineweredistributedwidelyintheprodeltaarea.VcllHarieI一()Fig.9RepresentativecoresofdeltafromwellHariet1interpretedasdeltafront,nSpLIrLJrDepthSuataFaciesS,stemNequence8()l25I2580l50l500Md,1(1一三92(1III930DeltaLZetda二940Front=l950Fig.10Welllogsofadeltafront(interpretedasamouthbar)WellHariet2SpspGrGrDepthRtStrataI_ithojOgVSystembequence42(49249242C2()l2(12()2fMd/10f2081O兰=I-n毛薹82(=MZclda:>一;83(一jFig.11Welllogsofadeltaplain(interpretedaschannelfilling)Asingledeltasedimentationcomprisesacompletereversecycleconsistingofprodeltaanddeltafrontatthelowerpart,andanormalcycleofdeltaplainfilledwithchannelsedimentationintheupperpart.Multipledeltasaremadeupofseveralsuchcompletecycles.ThiscanbeseenfromthewellsinthemiddletoupperZelda,forexample,wellsHariet2andDarlene1.3.9FandeltafaciesThefandeltafacieSiSsimilartothedeltafacies.ItiSalsomadeupoffandeltaplain,fandeltafrontandprofandelta8PetroleumScience2006sediments.Vertically,itdisplaysoneormovecompletecyclesfromreversecycletonormalcycle(Figs.12and13).ButitCanbedistinguishedfromthedeltasedimentationbythefollowingaspects:Firstly.fandeltaiSusuallyJocatedonthesteeperslopeofthebasin.whichiSsimilartothesubaqeousfan.Secondly.itiSusuallycoarserthanthedeltaWcIIAsri一0sedimentsandrichinpebbles.Itwasdepositedwimconglomeraticsandstones.Thirdly,fandeltaswereoftendevelopedlaterthansubaqueousfanandoverlaidsubaqueousfansedimentintheverticalfaciessequence.ThisfaciesrelationshipCanbeseenfromthewelllogsofwelIDarlene1andAsri1.SI)Sp(11(11I)cpthRllrill.1L_ll1()l<,Faciesstem,heqtlellCelffIfJIfJ406564f)MdIf29-臣,0一i嚣纛一68();LFalldeltaIST鞭女辫Pam軎%毒.:=主=l_圈:考,喜690)M一eldaj量Er一暑.Fig.12Welllogsofafandeltaplain(interpretedaschannelfilling)We11Asri一lII)LJrLJI)cpthRlStrataLitholog)1I1FaciesS3,stcm5eClUnceff7【l()j65b530Md/l01一.t-二,650FandeltaS丁ltonl一-,.壬手_=号:=卜车?謇辜660.,._LZelda.二=l;670.=1每.一0J=JE.f:-二;Fig.13Welllogsofafandeltafront(interpretedasamouthbar)Fig.14Representative?