MarineandPetroleumGeology34(2012)119e133

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MarineandPetroleumGeology

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Rockphysics-basedseismictraceanalysisofunconsolidatedsedimentscontaininggashydrateandfreegasinGreenCanyon955,NorthernGulfofMexico

ZijianZhanga,b,*,2,DanielR.McConnella,1,De-HuaHanb,2

ab

AOAGeophysicsInc.,6100Hillcroft7thFloor,Houston,TX77081,USA

DepartmentofEarthandAtmosphericSciences,UniversityofHouston,312Science&ResearchBldg1,Houston,TX77204-5007,USA

articleinfo

Articlehistory:

Received24May2011Receivedinrevisedform10November2011

Accepted11November2011

Availableonline25November2011Keywords:GashydrateJIPLegIIGC955

SeismicamplitudeRockphysicsTraceanalysis

Spectraldecomposition

abstract

Thegashydratepetroleumsystematthe2009GulfofMexicoGasHydrateJointIndustryProjectLegII(JIPLegII)GreenCanyon955(GC955)siteshowsacomplexseismicamplitudeandwaveformresponseofhighlynegativeandpositiveamplitudeswithcontinuousanddiscontinuouscharacterwithininferredgas-hydrate-andgas-bearingsandreservoirs.Logging-while-drilling(LWD)dataobtainedduringJIPLegIIandconventional3-Dseismicdataallowedfortheidenti?cationofthickhighlyconcentratedhydratelayersbyintegratingrockphysicsmodeling,amplitudeandthinlayeranalysis,andspectraldecompo-sition.RockphysicsmodelingwithconstraintsfromthreeJIPLWDholesallowedfortheanalysisofvariationsinacousticamplitudecharacteristicsasaproductofhydratesaturation,gassaturation,andreservoirthickness.Usingthewelllog-derivedacousticmodels,thickhighlyconcentratedgashydratewithandwithoutunderlyingfreegasaccumulationshavebeenidenti?ed.Theseresultssuggestthatthickhighlyconcentratedgas-hydrate-bearingsandunits(withthicknessesgreaterthanhalfoftheseismictuningthicknessandgashydratesaturationsgreaterthan50%)underlainbygascanbediffer-entiatedfromsandscontainingonlygas,butthingas-hydrate-bearingsandunitswithlowgashydrateconcentrations(withthicknesseslessthanhalfoftheseismictuningthicknessandgashydratesatu-rationslessthan50%)aredif?culttoidentifyfrompost-stackseismicamplitudedataalone.WithinGC955,wehaveidenti?edsixzoneswithseismicamplitudeanomaliesinterpretedasbeingcausedbygashydratedepositswithvariablelateralextent,thicknessandsaturation,andinsomecasesoverlyingfree-gas-bearingintervals.Syntheticseismicimagesproducedfromwell-log-andmodel-derivedvelocityanddensitydistributionsmimicsimilarre?ectioncharacteristicsinthecorresponding?eldseismicdata.

ó2011ElsevierLtd.Allrightsreserved.

1.Introduction

Gashydrateisanice-likecrystallinesolidconsistingofgasmoleculesenclosedbycagesofwatermolecules(Sloan,1998).Althoughgashydrateisestimatedtobethelargestglobalreservoirofhydrocarbons(Kvenvolden,1993),mostnaturalhydrateaccu-mulationsarecharacterizedbyrelativelylowconcentrations(lowenergydensity)andaredisseminatedinthesedimentsasgrainsandnodulesofvarioussizes.Ontheotherhand,highlyconcen-tratedthickgashydrate-bearingsandlayersinpolarregionsandindeepwatercontinentalslopeenvironmentsmaybeanexploitableenergysource(Boswell,2009).

*Correspondingauthor.AOAGeophysicsInc.,6100Hillcroft7thFloor,Houston,TX77081,USA.Tel.:t17135322624;fax:t17135326572.

E-mailaddresses:zhang@aoageophysics.com(Z.Zhang),dan_mcconnell@aoageophysics.com(D.R.McConnell),dhan@uh.edu(D.-H.Han).1

Tel.:t17135322624.2

Tel.:t1713743

3982.0264-8172/$eseefrontmatteró2011ElsevierLtd.Allrightsreserved.doi:10.1016/j.marpetgeo.2011.11.004

Abottomsimulatingre?ector(BSR)isawidelydescribedseismicindicatorofgashydrate,butitmayfailtodelineategashydratedepositsandeveninthecasewhereitdoesindicatethepresenceofhydrateitfailstoyieldanyinformationontherelativeconcentrationofthehydrate.However,inmostcasesBSRsobservedinpost-stackP-waveseismicdataarebelievedtoindicatethepresenceofgashydrateinthedeepwatermarineenvironment(Shipleyetal.,1979;KvenvoldenandBarnard,1983;Andreassenetal.,1990;HyndmanandSpence,1992).Becausethestabilityofgashydrateiscontrolledmainlybytemperature,pressure,porewatersalinityandgaschemistry(Sloan,1998),theBSRisusuallycoincidentwiththephasechangebetweengashydrateandfreegas.Asanindicatorofmostlythephaseboundarybetweengashydrateandfreegasitisnotparticularlypredictiveofgashydratedistribution(XuandRuppel,1999).

ThepropertiesandcausesofBSRshavebeenthesubjectofmuchwork.StudiessuggestthattheBSRiscausedbyasharpdropinP-wavevelocityintheunderlyinggaszone(Singhetal.,1993;MacKayetal.,1994;ZhangandMcMechan,2006).Numerous

120Z.Zhangetal./MarineandPetroleumGeology34(2012)119e133

expeditionsofODP(OceanDrillingProgram)andIODP(IntegratedOceanDrillingProgram)haveshownthatthepresenceofaBSRdoesnotcorrelatetohighconcentrationsofgashydratejustabovetheBSR(e.g.ODP164,LuandMcMechan,2002;IODP204,Tréhuetal.,2004;IODP311,Riedeletal.,2006).

PriortotheJIPLegIIexpedition,seismicdatawasusedtopredictthedistributionandsaturationofgashydratedepositspriortoacquisitionofdrillingdatainonlytwooccasions,bothwithencouragingresults(Daietal.,2008;Leeetal.,2011).Thepro-spectingmethodsusedtoselectsitesfortheJIPIIdrillingexpe-ditionwereasuccessfulcombinationoftraditionalgeological-geophysicalprospectingincludingfull-waveforminversionfol-lowedbygashydratesaturationmodeling(Boswelletal.,2012a,b;Shelanderetal.,2012;Leeetal.,2009,2011;Inksetal.,2009).ThetraditionalthroughgoingBSRsarenotassociatedwithgashydratedepositsateithertheWR313orGC955sites(Boswelletal.,2012a,b;Sheddetal.,2012)yetthicksigni?cantgashydratedepositswerediscoveredattheselocations.

Althoughthepresenceofgashydrate-bearingsanddepositscanbepredictedbyconventionalseismicprospectingmethods,amoredetailedunderstandingisrequiredtoaccountforthegreatercomplexityandheterogeneityofmostgashydratesystems.Thehighqualityexploration3-DseismicandloggingdataacquiredbytheJIPLegIIthroughhighlysaturatedgashydratedepositsallowfortheinvestigationandunderstandingoftheseismiccharacter-isticsofgashydratedeposits.InthisreportwefollowapproachesusedbyLeeetal.(2009)toshowthatcombiningrockphysicsmodelingandthinlayeranalysisisapromisingapproachforthedelineationandcharacterizationofgashydratedepositswithseismicdata.Inparticular,thinlayerseismicanalysismaybeparticularlyrelevanttounderstandingseismicresponseofgashydrateandgashydrateoccurrencesoverfreegasnearthebaseofthegashydratestabilityzone(BGHSZ).Thinlayeranalysishasbeenwidelyappliedtoseismicdatafromoilandgasdepositsinclasticreservoirswithsomedegreeofsuccess.Widess(1973)showsthatathinlayerwiththicknessgreaterthanone-eighthoftheseismicwavelengthcanbedetectedwithseismicamplitudeanalysis.Partykaetal.(1999),Partyka(2005),MarfurtandKirlin(2001)alsousespectraldecompositiontopredictthethicknessofimagedsedimentarylayers.WhileChopraetal.(2006)andPuryearandCastagna(2008)usespectralinversionforlayerthicknessdeter-mination.AdriansyahandMcMechan(2002)detectthinreservoirsfromattributeanalysis,acousticimpedanceinversion,andfull-wave?eldmodeling.However,theapplicationofthinlayeranalysistogashydratedepositsisarelativelynewdevelopment(Leeetal.,2009).

Thephysicalpropertiesofsedimentscontaininghydrateintheporespacedependonthehydratemorphologyandconcentration.Arockphysicsmodelforgashydrateinsandlayersisagoodtooltodescribethephysicalpropertiesofhydrate-bearingsediments.Itnotonlydescribesphysicallymeaningfulrelationshipsbetweenseismicparametersandsedimentaryproperties,butitcanalsobecalibratedwithloggingdata.Becauseofthis,rockphysicsapproacheshavebeenusedtosuccessfullydescribethephysicalnatureofgashydratedeposits(Helgerudetal.,1999;Jakobsenetal.,2000;LeeandCollett,2001;Lee,2002;Leeetal.,2009)andmapgashydratedistributionwhencombinedwithseismicdataanalysis(Daietal.,2008;Boswelletal.,2009a).

Thisstudyhastwoobjectives.The?rstistoinvestigatethezero-offsetseismiccompressionalwaveamplituderesponsetovariablegashydratesaturation,freegassaturation,andlayerthicknessesinsandreservoirsusingarockphysicsmodeldevel-opedinthisstudy,alongwithseismicandLWDdataacquiredatGreenCanyon955(GC955)intheGulfofMexico.Thesecondobjectiveistoseparatethickandthingashydratedepositsin

seismicdatausingspectraldecompositioncalibratedbyLWDdataobtainedduringJIPLegII.TherockphysicsmodelsarelocallycalibratedbyLWDdataandsomaynotbedirectlyappliedtootherareaswheregeologicalconditionsandelasticpropertiesofthesediments(e.g.sandandclaycompressional-andshear-wavevelocities)aresigni?cantlydifferent.However,theprocedurespresentedherearelikelysuitableformostothermarineanddeepwatermarineclasticsettings.2.Geologicsetting