PA97 - OR- 10 INDONESIAN PETROLEUM ASSOCIATIOP Proceedingsof the PetroleumSystems of SE Asia and Australasia Conference, May1997 THE ROLE OF MUDVOLCANOESIN PETROLEUM SYSTEMS: EXAMPLESFROM TIMOR, THE SOUTH CASPIAN ANDTHE CARIBBEAN Paul Warn* La Ode Ichmm** ABSTRACT World-wide, mud volcanoes have been documented in areasofoverpressure whereexplosiveexpansionof trapped methane has occurred during argillokinesis. In an area with high sedimentation rate, such as the Gulf of Mexico or the South Caspian, there may be no time forfine-grainedsedimenttode-waterbeforebeing covered by impermeable material. Inan accretionary wedge setting, such as Timor, this process is complicated by overthrusting of off-scraped material which increases the overburden pressure and providesmanymoreavenuesforthemigrationof fluids throughthesystem. Insome cases, suchasis seen in the Caribbean, the fluidscan escape directly to the seabottom through high permeabilitybeds due tothesubductionofcoarser-grainedmaterial.When thishappenstheremaybenodiapirism.Inother cases, such as in Venezuela,the fore-arc may be the siteofrapid,laterally-derived,sedimentationand fluids fromtheoverthrustedrocksmay be forced to escapethroughseveralkilometersofrecentdeltaic sediments. Since these fluids may include petroleum, thishasobviousexplorationpotential.Often,the source rock for the mud diapirism is the same source forthepetroleumsystem.Theoverpressurewhich gives rise to the mud volcanismalso causes increased expulsionefficiencyand hydrocarbonssampled from mud volcanoes tend to be immature. If thereare nosuitable reservoir rocks, there maybe no commercial accumulations. However, many giant fieldsareassociatedwithmudvolcanoes,suchas those in Azerbaijan. *Unocal - U. S. A.**Unocal Indonesia Co. INTRODUCTION World-wide, mud volcanoes have been documented in areasofoverpressurewhereexplosiveexpansionof trappedmethanehasoccurredduringargillokinesis. The role of mud volcanoes in petroleumsystems is. a contentiousone, given that they are oftenassociated with petroleumseeps but that the oiland gasat the surfaceisoftenfoundtobeoflowmaturity.For reasonswhicharediscussedin thispaper,fheyare oftenassociatedwiththefrontalthrustsof accretionary margins in tectonically active areas. Mud volcanoes are a result of overpressure. According toBrown(1990),theformationofdiatremesis normallyassociatedwithmuddiapirism.Belowa certaindepth(2.5-3 km)strain hardeninggivesa characteristicscalyfabrictothemud,allowingthe migrationoffluidsincludingoilandgas-saturated brine (above this level strain hardening gives way to strainsoftening). At around1.5 -1 kmdepth, rapid expansion of dissolved gas as it comes out of solution quickly causes the formation of migration pathways to thesurface,typicallyalonglinesofweakness associatedwithlocalizedextensionaltectonicsover the top of the underlying diapir (Figure1). Overpressureisgenerallyassociatedwithrapidly depositedsedimentswhichhavenothadtimeto dewaterbeforebeingcoveredwithimpermeable material.Pressurescanbefurtherincreasedby tectonicstressi.e. thelateralmigrationoffluids by seismicpumpingsuchascanoccurinanactive margin.Thiscontrastswiththeformationof mud islandson a passivemarginsuch as theMississippi 955 IPA, 2006 - Proceedings of an International Conference on Petroleum Systems ofSE Asia and Australasia, 1997Disc ContentsContentsSearch956deltaoftheGulfofMexicofromrelativelyshallow-buriedmud-richlayers.Intheactivemargincase(Figure2),thefluidsmayescapedirectlytothesurface(orseabottom)ifthereare sufficientlypermeablebeds, in which case there isnodiapirism.However,mudvolcanoesaretypicallyseenalong the frontalthrustof the accretionary wedgewheretheyarecontinuallyformedbeforebeingoverwhelmedbythemigratingfrontof thewedge.IntheCaribbean,aselsewhere,agravitylowisassociatedwiththeaccretionaryprism.SouthofBarbados, the frontalthrust is within the sedimentfanoftheOrinocoRiver.HigherpermeabilitybedsassociatedwiththisallowthemigrationoffluidsdirectlyintotheAtlanticandproduceelevatedtemperatureswithinthewatercolumn.Tothenorth,on thedistal peripheryof thefan, the permeabilityofthesubductedmaterial is muchlower. Mudvolcanoescanbeobservedhere,justinfrontofthefrontalthrust.ThelocationofmudvolcanoesinN.E. VenezuelaisalsoalongthefrontalthrustcausedbytheobliquecollisionwiththeSouthAmericanplate.Aseismicline(Figure3) throughtheareashows theunderlyingdiapirintheCarapitaFormation.Fluidshavebeenseismicallypumpedaheadofthethrustzoneintoazone ofweaknesswhere youngersedimentsare beingsqueezedoverandundertheadvancingthrustfront.Thefluidsthenescapeupwardsthroughyoungersediments.Themudmatrixinasalsacanconsistofclay-sizedparticles of all formationsthat exist vertically fromthemotherdiapirtothesurface(includingolderformationsfromthehangingwallsofreversefaults)suspendedinabrinethatisa mixtureofformation(s)andmeteoricwaters.Byitsnature,therefore,mudfrommud volcanoesis notoriouslydifficultto age andreliance isoften(mis-)placedon theassociatedejecta.Bothmudandhydrocarbons,however,havebeensampledatmudvolcanoesalongthistrendandpalynological and biomarker evidence (DGSI/EPOCA,1995)suggestacommonsourceintheMioceneCarapita Formation. Maturity is described as immaturetolowmature.By contrast, intheMaturinsub-basinthe mainsourcerockforreservoiredoilisthematureUpperCretaceousQuerecalFormation;theCarapitaisaregionalseal.ContinualtectonicunderplatingofthethrustbelthasoccurredduetoobliquecollisionbetweentheCaribbeanandSouthAmericanplatessince the Miocene. Even where theCarapita is locallymatureinthedeeperforelandbasin,andhascontributedsomehydrocarbonstothepetroleumsystem,ittendstobemoregas-pronethantheQuerecal.Themudvolcanoseepscanthereforebeseen as anomalous in that the high pressures generatedduetotectonicstresshaveforcedtheexpulsionofyoungerimmaturehydrocarbonsatthesurfacealongwiththeirsourcerock.Another active margin isseen in Figure 4. In this casethecontinentofAustraliaismovingnorthward,subductingundertheouterBandaArc.Therearemanymudvolcanoesinthisareaalongthesouthern(frontal)thrust (Figure5). The phenomenonof escapetectonics (of the islandof Sumba) has complicatedthetectonicpicture,causingthebackthrustillustrated(Figure6). Note the muddiapirsformedaheadof theSavuThrust.Unfortunately,theresolution "oftheseismicdataisinsufficienttotellwhethertheseareaccompaniedbymudvolcanoes.ThelithotectonicunitspresentinTimorcanbedividedintoautochthonous,allochthonous,andpara-autochthonousunits. The main mud volcano fieldsareassociated with the Bobonaro Complex which consistsofa matrixofextrudedscalyclaysderivedfromtheKeknenoSeries of theGondwanaSequence(Harris etal.,inpress).Thisispartofthepara-autochthon,ofAustralianmarginorigin.ALandsat photograph(Figure7)shows the islandofRoti,thesoutherntipofTimorandasmallislandcalled Pulau Kambing. This island, as well as much ofthesurfaceofRoti, isa mudvolcano(seeFigure8).InsouthernTimor,theBobonaroconsistsmainlyoffine-grainedargillaceousmaterialandformslarge(1 kmacross) flat-lying mudvolcanoes(Figure9,10)with numeroussmall gryphonsandsalsas. Bubbles ofmethanecontinuallyrisetothesurfaceofthe salsasalongwithablackishscumwhichwassampledbyUNOCALforgcochcmicalanalysis(Figure11). ThematerialwasfoundtohavehighTOCbutlowmaturity.Nobiomarkeranalysiswasattempted.However,analysisofoilsampledat thePualacaseepwasfoundtobe(locally)matureandwasdatedas957beingof Upper Triassic-MiddleJurassic age (the sameastheBobonaroComplex).In northern Timor, the Bobonarocontains larger clastsof material that have beenbrought to thesurface.Thisallowsittoformmoresteeplysidedmudvolcanoes.Blackishscumwasalsopresenthere,thoughinsmaller quantities, and was sampled for analysisalongwiththe mud.Theconclusionreachedwasthat,asinVenezuela,theoverpressurewhichhadproducedthemud volcanoes hadallowed immature oil and biogenicgastorise tothesurfacealongwiththesourcerock.The phenomenaof muddiapirismand mudvolcanoesmaybeseenastheruleratherthantheexceptionalongtheAustralianmargin.MuddiapirismisalsoreportedaheadofthefrontalthrustinNewGuinea.Thelargestnudvolcanoprovinceintheworldisfoundin the circum-Caspian region. Mud volcanoes inAzerbaijanare located in clustersalongcompressionalfoldscaused by the continual northwards movement oftheArabianPromontory,whichhasalsocausedtheupliftoftheCaucasusandElburzmountainrangesandstrike-slipmotionalongtheTalyshfault.Themudmatrixofthevolcanoes(Figure12)reportedlyvarieswithlocation(thoughthisiscurrently the subjectof an integratedstudy by Unocal)butispredominantlysourcedfromtheMaykopFormation(Oligocene-Mioceneage)asistheassociatedoilandgaseruptedatthemudvolcanoes.IncommonwithboththeAustralianmarginandVenezuela,theoilstendtobeoflowmaturity.Thisshouldnotbesurprisingas(a)overpressureismorewidespreadinareasofrapiddepositionwhichhavenot yet reached thermal equilibrium(b) most of the oiland gas tends to beeruptedwith theclay matrix of itssource rockinsuspensionand(c) mudvolcanoestendto be ephemeraleventsinGeologicTime. Thus, if theoilsweresufficientlymaturetheymighthavebeenexpectedto migrate out of thesource rock prior to theformationofmudvolcanoes.Almostall oil fieldsin Azerbaijanare associatedwithmudvolcanoesthoughthecausalitymayonlybeduetotheirsharedlocationsinthefoldedanticlinalfeaturesthatoccur verticallyovershale diapirs, ratherthananyactiveroleinprovidingmigrationpathwaystoreservoirs(diatremes,asnoted,areindividuallyshort-livedphenomena).OilsampledbyUNOCALfromthe Cheleken peninsula in Turkmenistanis olderthanthattrappedinreservoirsaroundtheflanks.Certainly,evenmajoreruptionsofmudvolcanoeshavenolongterminfluenceover productionratesoftheoilfields.HugemonetarysumsarecurrentlybeinginvestedinoffshorefacilitiesinanoffshoreareaofAzerbaijanwhich is prone to mudvolcanismwithaccompanyingsoilinstabilityproblems.Asa result,studiesofmudvolcanismin thisarea have focusedon theprobabilityofnewmudvolcanoformationnearplatforms,pipelines, etc.CONCLUSIONSMudvolcanoesareaworldwidephenomenonalongaccretionarymarginsandtheirassociatedejectacanbekeytounderstandingthepetroleumsystemsassociatedwiththesebasins.Thehydrocarbonsthataresampledatmudvolcanoestendtobeimmature,having been expelled fromsource rocks"early" due torapidlocalreleaseofoverpressureinthediatreme.REFERENCESBreen,N.A.,Silver,E.A.,Hussong,DM. ,1986,StructuralStylesofanaccretionarywedgesouthoftheislandofSumba,Indonesia,revealedbySeaMARCIIsidescansonar,GeologicalSocietyAmericaBulletin, v97, pp 125 0-1261.Brown,K.M.,Thenatureandhydrogeologicsignificanceofmuddiapirsanddiatremesforaccretionarysystems,JournalofGeophysicalResearch, v95, pp8969-8982.DGSI/EPOCA,1995,TheoilandgasseepsofN.E.Venezuela(DGSI/EPOCApublication).EPOCA,1995,SeismicatlasofVenezuela(EPOCApublication).Harris, R.A., Sawyer, R.K., Shaw, H., Audley-Charles,M.G.,OriginoftheBobonaroComplexofTimor,Indonesia,in press.Langseth, M.G., &Moore, J.C.,1990, Introductionto958 specialsectionontheroleoffluidsinsedimentReed, D.L., Meyer, A.W., Silver, E.A., Prasetyo,H., accretion, defomlation, diagenesis, and metamorphism1987,Contouritesedimentationinanintraoceanic in subduction zones, J ournal of Geophysical Research,forearc system: eastern Sunda arc, Indonesia, Marine ~ 9 5 , ~~8 7 3 7-874 1.Geology, v76, pp223-241. FIGUREI - Idealizedcross-sectionthroughshalediapir(nottoscale),showingdiatremeformationasa resultoffluidescape. FIGURE 2 - Accretionan. prism shon-ing ajcnues of fluid escape due to seismic pumping 96 1 a 0 w 0 12r a, 3 I m Merged Free AirBouguer Gravity [showing areas of interest 1 ]17"-135"E, 5"-1"S, +260mgal] - FIGURE 4 - Gravity map of N.W. Australian active margin, showing areas of interest discussed in the text. . ...: :: 'y.. . ._ . :... .. . . 8 L 9 E t 364 non-volcanic arc Seismic sequences A-E are within a lkm thick post-early-Pliocene to recent fine-grained contourite mound of reworked calcareous ooze Mud Diapirsa 25km I- SouthNorth 1 .o 2.03.0 4.0 -2.0 -3.0 4.0 Savu Thrust (after Reed et al.) 6-Mud volcanoesalongSavu Thrust, Indonesia. 965 I 966 ai 3 cd :: 0 0 0 P-i3 3 967 ru 0 970 41 0