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