Milkov-2005-Gas hydrate systems at Hydrate Rid

Milkov-2005-Gas hydrate systems at Hydrate Rid

GeochimicaetCosmochimicaActa,Vol.69,No.4,pp.1007–1026,2005

Copyright?2005ElsevierLtd

PrintedintheUSA.Allrightsreserved

0016-7037/05$30.00?.00

doi:10.1016/j.gca.2004.08.021

GashydratesystemsatHydrateRidgeoffshoreOregoninferredfrommolecularand

isotopicpropertiesofhydrate-boundandvoidgases

ALEXEIV.MILKOV,1,*GEORGEE.CLAYPOOL,2YOUNG-JOOLEE,3andROGERSASSEN4

BPAmerica,ExplorationandProductionTechnologyGroup,Houston,TX77079,USA

2

8910West2ndAvenue,Lakewood,CO80226,USA

3

KoreaInstituteofGeoscienceandMineralResources,Daejeon305-350,Korea

4

GeochemicalandEnvironmentalResearchGroup,TexasA&MUniversity,CollegeStation,TX77045,USA

(ReceivedDecember15,2003;acceptedinrevisedformAugust30,2004)

1

Abstract—Wereportanddiscussmolecularandisotopicpropertiesofhydrate-boundgasesfrom55samplesandvoidgasesfrom494samplescollectedduringOceanDrillingProgram(ODP)Leg204atHydrateRidgeoffshoreOregon.Gashydratesappeartocrystallizeinsedimentsfromtwoend-membergassources(deepallochthonousandinsitu)asmixturesofdifferentproportions.Inanareaofhighgas?uxattheSouthernSummitoftheridge(Sites1248–1250),shallow(0–40mbelowthesea?oor[mbsf])gashydratesarecomposedofmainlyallochthonousmixedmicrobialandthermogenicmethaneandasmallportionofthermogenicC2?gases,whichmigratedverticallyandlaterallyfromasdeepas2-to2.5-kmdepths.Incontrast,deep(50–105mbsf)gashydratesattheSouthernSummit(Sites1248and1250)andonthe?anksoftheridge(Sites1244–1247)crystallizemainlyfrommicrobialmethaneandethanegenerateddominantlyinsitu.Asmallcontributionofallochthonousgasmayalsobepresentatsiteswheregeologicandtectonicsettingsfavorfocusedverticalgasmigrationfromgreaterdepth(e.g.,Sites1244and1245).Non-hydrocarbongasessuchasCO2andH2Sarenotabundantinsampledhydrates.Thenewgasgeochemicaldataareinconsistentwithearliermodelssuggestingthatsea?oorgashydratesatHydrateRidgeformedfromgasderivedfromdecompositionofdeeperandoldergashydrates.GashydrateformationattheSouthernSummitisexplainedbyamodelinwhichgasmigratedfromdeepsediments,andperhapswastrappedbyagashydratesealatthebaseofthegashydratestabilityzone(GHSZ).FreegasmigratedintotheGHSZwhentheoverpressureingascolumnexceededsealingcapacityofoverlayingsediments,andprecipitatedasgashydratemainlywithinshallowsediments.Themushroom-like3Dshapeofgashydrateaccumulationatthesummitispossiblyde?nedbythegasdiffusionaureolesurroundingthemainmigrationconduit,thedecreaseofgassolubilityinshallowsediment,andrefocusingofgasbycarbonateandgashydratesealsnearthesea?oortothecrestofthelocalanticlinestructure.Copyright?2005ElsevierLtd

1.INTRODUCTION

Naturalmarinegashydratescontainalargeamountofmeth-ane(Milkov,2004c)andareconsideredasanimportantcom-ponentoftheglobalcarboncycle(Dickens,2001)andapo-tentialenergysource(MilkovandSassen,2002,2003).GeophysicalevidencesuchasBottomSimulatingRe?ectors(BSRs)andstrongsea?ooracousticre?ectivitiesarecommonlyusedtoinferthedistributionofgashydratesoncontinentalmargins(KvenvoldenandLorenson,2001;Sageretal.,2003;Johnsonetal.,2003).However,gashydratesamplesarees-sentialtoground-truthgeophysicalobservation,understandtheoriginofthismineral,visualizethemigrationofhydrate-form-ing?uids,andconstrainmechanismsofgashydrateformationnearthesea?oorandindeepsediments.Thelackofextensiveandcomprehensivedirectmeasurementsofmolecularandiso-topicpropertiesofhydrate-boundgasessometimesresultincontroversialviewsontheoriginofgasingashydratesystemssuchastheBlakeRidge(Fehnetal.,2000;DavieandBuffett,2003).

Gashydratesampleshavebeenrecoveredfrom?23loca-tionsworldwide(KvenvoldenandLorenson,2001),and?50individualgashydratesiteshavebeendiscoveredinsomeof

*Authortowhomcorrespondence(alexei.milkov@bp.com).

should

be

addressed.

1007

thoselocationssuchastheGulfofMexico(MilkovandSassen,2001).Mostgashydratesampleshavebeenobtainedviapistoncoring(GinsburgandSoloviev,1998),submersibleoperations(Sassenetal.,1998,2001a,2001b),grabbinganddragging(Suessetal.,1999).Thesesamplingtechniquesarerelativelyinexpensiveandmayprovidedetailedinformationonthearealdistributionofgashydratesoversmallisolatedsea?oorfeaturessuchastheH?konMosbymudvolcanointheNorwegianSea(Milkovetal.,2004a).However,pistoncoringfromvesselsandsubmersiblesisrestrictedtoshallowdepthsof3to10mbelowthesea?oor(mbsf)anddoesnotprovideinsightintothedistributionandoriginofgashydratesatgreatersedimentdepths.

Mostinformationonthedistribution,concentration,compo-sitionandoriginofgashydratesindeepmarinesedimentscomesfromDeepSeaDrillingProject(DSDP)andOceanDrillingProgram(ODP)Legs.Gashydratesampleswerere-coveredinLegs66offshoresouthernMexico(ShipleyandDidyk,1981),67offshoreGuatemala(HarrisonandCuriale,1982),76atBlakeRidgeoffshoreGeorgia(KvenvoldenandBarnard,1983),84offshoreCentralAmerica(KvenvoldenandMcDonald,1985),96intheOrcaBasinoftheGulfofMexico(P?aumetal.,1986),112offshorePeru(KvenvoldenandKastner,1990),127and131offshoreJapan(ShipboardScien-ti?cParty,1990,1991),146offshoreOregon(Hovlandetal.,1995),164atBlakeRidge(Paulletal.,2000),170offshore

1008A.V.Milkovetal.

Costa-Rica(Lückgeetal.,2002),and201offshorePeru(D’Hondtetal.,2003).Atotalof46gashydratesampleswerecollectedandanalyzedontheseLegs.Kvenvolden(1995)reviewedthegeochemistryofhydrate-boundgasesfromLegs66to146andconcludedthatindeepmarinesediments:(1)methane(C1)isthemaincomponentingasmixtures(?99%)andgashydratecommonlyhasstructureI;and(2)gashydrateoccurrencesformfrommicrobialC1generatednearbyormi-gratedonlyshortdistances.Incontrast,sea?oorgashydratessampledbypistoncoringandothershallow-penetrationtech-niquesoftencontainC2?hydrocarbongasesofthermogenicoriginthatmigratedlongdistancesfromdeeplyburiedsedi-mentsandpetroleumreservoirs(Kvenvolden,1995).

ODPLeg204addressedthedistributionandoriginofgashydrateandfreegasatHydrateRidgeoffshoreOregon(Milkovetal.,2003,2004b,2004c;Tréhuetal.,2003,2004).MoregashydratesampleswererecoveredandanalyzedonLeg204(55samplesfrom0–105mbsf)thanduringallpreviousDSDP-ODPLegs.Inthispaper,wepresentanddiscussthemolecularandisotopicpropertiesofhydrate-boundandvoidgases.Thesedataareusedto:(1)infertheoriginofhydrate-forminggases;(2)identifygashydratesystemsandrelatethemto?uid?owregimesandmigrationpathwaysthatprevailinthestudiedarea;and(3)developanintegratedmodelofgashydratecrystallizationattheSouthernSummitofHydrateRidge.

2.GEOLOGICALSETTING

HydrateRidgeisa25-km-longand15-km-wideaccretionaryridgeontheOregoncontinentalmargincomposedofsiltyandsandyturbidites(Fig.1).ComplextectonicandhydrogeologicprocessesoccurastheJuandeFucaplatesubductsbeneaththeNorthAmericanplate(MacKay,1995).HydrateRidgeischar-acterizedbytwosummitsseparatedbyasaddle(Fig.1).ThewaterdepthattheNorthernSummitisshallower(?600m)thanattheSouthernSummit(?800m).Bothsummitsarepopulatedbygasvents,authigeniccarbonaterock,outcropsofmassivegashydrate,andachemosyntheticcommunity(Suessetal.,1999;Sahlingetal.,2002;Torresetal.,2002).Waterandgasrapidlymigratefromthesubsurfacetothesea?ooratbothsummits(Tryonetal.,2002).Thesummitsaresurroundedbyareaswithsigni?cantlylessgas?uxtothesea?oor(Tréhuetal.,2002).Foldedandupliftedsediments(Johnsonetal.,2003)andastrongregionalBSRconsistentwithgashydrateandfreegasoccurbelowandbetweenthesummitsasseenonhigh-resolution3Dseismicsurveys(Tréhuetal.,2002,seetheinterpretationofselectedseismicpro?lesinFig.2).

TheshallowsedimentsofHydrateRidgehavebeenstudiedduringnumerousresearchcruises(Suessetal.,1999;Tryonetal.,2002;Sahlingetal.,2002)andsubmersibledives(Torresetal.,2002).The?rstinsightintothedeepsedimentswaspro-videdbyODPLeg146drilledtoinvestigate?uidmigrationpathwaysandthenatureofgashydratesat?veSites(888–892)ontheCascadiaMargin(Westbrooketal.,1994).Site892wasdrilledontheNorthernSummit(Fig.1).NineSiteswerecoredandeightSiteswereloggedduringODPLeg204attheSouth-ernSummitandsurroundingareastostudythedistributionandconcentrationofgashydratewithinthegashydratestabilityzone(GHSZ)andfreegasbothwithinandbelowtheGHSZ(Milkovetal.,2003,2004b,2004c;Tréhuetal.,2003,2004).

Fig.1.Bathymetry(afterSuessetal.,1999)andstructures(afterJohnsonetal.,2003)ofHydrateRidge.ThelocationsofODPSites892and1244to1252areindicated.LinesA-A=,B-B=,andC-C=correspondtotheapproximatepositionofcross-sectionsinFigure2.Starsindicatemudvolcanoes(mv)nearthenorthernsummitandcarbonatepinnacleattheSouthernSummit.TheinsetillustratestheregionaltectonicsettingofHydrateRidge,whichislocatedwithinCascadiaaccretionarycomplexformedasaresultofsubductionofJuandeFucaplatebeneaththeNorthAmericanplate.

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