TheGC955siteislocatedabout310kmsouthofNewOrleansatwaterdepthsof2000e2200m,onthemiddlecontinentalslopeinthenorthernGulfofMexico(Fig.1).TheGC955siteisonly2000e3000mfromthebaseoftheSigsbeeEscarpmentsoutheastoftheGreenCanyonre-entrant(Fig.1).TheupperPleistocenedepositsinthisareaarethick(>4000m),andconsistofbothrapidlydepositedandslowlydepositedclasticsediments.

DeepwaterenvironmentsintheGulfofMexicoarecharacter-izedmainlybyhemipelagic?ne-grainedsediments.However,theGC955siteiswithinalargesubmarinefan,theMississippiFan,whichwasdepositedinthedeepwateroftheGulfofMexicoduringlatePlioceneandPleistocene(Weimer,1990).Shelf-margindeltasweredevelopedandchannel-leveesystemsweredepositedduringsealevellowstands.Thesechannel-leveesystemsallowforcoarse-grainedsedimentdeliverytotheslopeviasubmarinecanyons.ShallowunconsolidatedsandsarecommonlyencounteredduringdrillingoperationsintheGulfofMexico(Ostermeieretal.,2002).Duringsealevelhighstandshemipelagicclaysdominatemarinesedimentation.ThecoursegrainedchannelcompleximagedinseismicdataatGC955wascon?rmed?rstbyindustrydrilling(McConnell,2000).Thefocusofthestudyisthesandsofachannel-leveesystem,whichareinterbeddedwithinhemipelagicclays(Boswelletal.,2012a,b).

ThepetroleumsystemforgashydrateinGC955includeshighlyfaultedsandreservoirs,gas-rich?uidsand?uidmigrationpathways,andarelativethickzoneinwhichtemperatureandpressureconditionsaresuitablefortheformationofgashydrate(McConnelletal.,2009).Incisedsea?oorchannelsthatextendfromtheSigsbeeescarpmentindicatethatdebrisandturbidite?owshavelikelybeendepositedand/orbypassedGC955intherecentgeologicpast(Fig.1;McConnelletal.,2009).Alargeburiedsand-pronechannel/chutehasbeenindenti?edwithinGC955(Fig.2).Asea?oor?uidexpulsionmoundhasalsobeenidenti?edinGC955,whichisevidenceofanactivepetroleumsystem.Seismicdataanalysisalsosuggeststhatgashydrateistrappingconventionalfreegasaccumulationswithinthechannelsandsandthechannel-leveedepositsinGC955(McConnell,2000;Heggland,2004;Jonesetal.,2008;McConnelletal.,2009)astheupwardmigrationofgasthroughfaultedsand-pronesedimentsishinderedbygashydrateformingatandabovethebaseofgashydratestability(Jonesetal.,2008;McConnelletal.,2009).

3.Seismicamplitudedataanalysisandinterpretations3.1.Preliminaryobservations

TheGC955sitecontainsacomplexfaultsystemoverthecrestofalargesaltstructure(Heggland,2004).Thefaultingisclearlyseeninseismicdata(Fig.2)withsomefaultsreachingthesea?oor.Verticaldisplacementsofthefaultsareestimatedtobeuptow50m.Sediments,asdescribedabove,areinterpretedtobedeepwaterclasticdepositsofclays,silts,andsands.Theseismic

Z.Zhangetal./MarineandPetroleumGeology34(2012)119e133121

Figure1.GC955islocatedinthesoutheastcorneroftheGreenCanyonprotractionarea.Therightpanelshowsthesea?oorshadedreliefmapofthestudyarea(with50mcontourinterval),andthelocationsoftheJIPLegIIwells(GC955-I,GC955-H,andGC955-Q)andconventionalindustrydrilledwells(GC955#1andGC955#2).

databetween2.95sand3.2sischaracterizedbystrongcontinuous

re?ections,whichareinterpretedtobecharacterizedbythin

sand-pronesedimentlayers(HorizonB,markedasBinFigure2)

(McConnelletal.,2009;Guerinetal.,2009).Thedensitypro?les

obtainedfromwellsGC955-HandGC955-Ishowasharpdecrease

indensityatHorizonB,correspondingtoadensitylog-derived

sedimentporosityofabout52%.Withinthesedimentarysectionnearthepredictedbaseofthegashydratestabilityzone(3.1sand3.4s)thereisasigni?cantincreaseinre?ectionamplitudes(Fig.2),whichislikelycausedbythepresenceofgashydrateandfreegaswithinsand-dominatedsediments.However,theamplitudeanomaliesdonotformatradi-tionalBSR,whichisusuallyparallel-sea?oor,andisacontinuoushigh-amplitudere?ectionwithnegativepolarity.Notonlyis

theFigure2.Arbitrarylinethrough3-DamplitudevolumeshowingJIPandindustrywells.Thepro?leshowsstrongre?ectorsarehighlyfaultednearthebaseofgashydratestabilityzone(BGHSZ)between3.1sand3.4s.Sixselectedfeatures(Zone1toZone6)arediscussedinthetextandTable1.TheBGHSZisinterpretedbyJonesetal.(2008)andcrossesthesixzones.SeeFigure1forthesection

location.

122Z.Zhangetal./MarineandPetroleumGeology34(2012)119e133

Table1

Seismiccharacteristicsandestimateddepthsofsixrepresentativegashydrateand/orfreegasaccumulationsinGC955.Interestedzones

Seismicsections

Characteristics

Depthstothetopofamplitudeanomalies(mbsf)

Zone

1

Highpositiveamplitude(HP)overhighnegativeamplitude(HN)atthecrestofthestructure,Moderatepositiveamplitude(MD)onthebaseofthestructure.

Theminimum:426;Themaximum:497.

Zone

2

Alateralphasereversalwithhighpositiveamplitude(HP)ontherightsideandhighnegativeamplitude(HN)ontheleftsideoftheseismicsection.

Theminimum:395;Themaximum:431.

Zone

3

Highpositiveamplitude(HP)overhighnegativeamplitude(HN)discontinuityre?ections.Theminimum:432;Themaximum:458.

Zone

4

Twoseparatehighpositiveamplitudes(HP)re?ections.Theminimum:448;Themaximum:490.

Zone

5

Highpositiveamplitude(HP)overhighnegativeamplitude(HN)continuityre?ection.Theminimum:436;Themaximum:458.

Zone

6

Moderatepositiveamplitude(MP)overhighnegativeamplitude(HN)continuityre?ection.Theminimum:528;Themaximum:534.

traditionalBSRabsent,buttheinferreddistributionofgashydrateaswellasfreegasalongthebaseofgashydratestabilityappearstobeirregular.Wehaveidenti?edaseriesofdistinctacousticrelationshipswiththe3DseismicvolumeinGC955andseparatedtheseacousticanomaliesintozonesnumberedfromZone1toZone6(Fig.2).Thezonesshowavarietyofamplitudesandwaveformpatterns(Table1).3.2.Seismicandwelldata

ThreewellsweredrilledinGC955aspartofasevenwellprogramdesignedtotestforgashydrateinsandsduringJIPLegII.EachofthethreewellsinGC955werethoughttobeprospectiveforgashydrateinsandsbasedonpetroleumsystemand3-Dseismicamplitudeandinversionanalysis(Boswelletal.,2009b;McConnelletal.,2009;Boswelletal.,2012a,b).LWDmeasurementsincludeacompletesuiteofcompressional-waveacoustic,bulkdensity,gammaray,resistivity,andneutronporositylogmeasurements(Guerinetal.,2009;Collettetal.,2012).TheLWDlogdatafromthehydrate-bearing-sandsectionsalongwithmostoftheclay-richsedimentarysectionsareofhighquality(Figs.3and4).Theonlydegradedwell-logdatawasrecordedinthewater-bearingsandsectionsofeachwell,whichcanbeattributedtotheenlarged

Z.Zhangetal./MarineandPetroleumGeology34(2012)119e133123

(washedout)natureoftheboreholeintheseintervals(Figs.3and4).

Figure3isacompositewell-logdisplayofloggingdataforGC955-I,whereathicksandzonewasidenti?edbasedonthechangeingammaraylogresponsebetween360and480mbelowthesea?oor(mbsf).TheLWDlogsatGC955-Ishownoobvioushighresistivitiesoracousticvelocitiesinthegammarayinferredsandunitsuggestingthatgashydratesaturationatthepre-drilltargetisveryloworthat,morelikely,nogashydrateispresent(Collettetal.,2012).IntheGC955-Hwell,however,resistivityincreasesfrom1to50U-mandsonicvelocityincreasesfrom1.6km/sto2.8km/sbetween410and450mbsf,indicatingthepresenceofhighlyconcentratedgashydrate(Fig.4a).Lowgammarayvaluesbetween375and490mbsfindicatethathighlyconcentratedgashydrateoccursinasandsection.Thegashydratesaturationsinthissectionwereestimatedfromresistivitylogsatvaluesexceeding70%(Guerinetal.,2009).LoggingresultsfromwellGC955-QaresimilartothoseatwellGC955-H,indicatingthathighlyconcentratedgashydrateispresentinasandsectionwithintheintervalfrom425to445mbsf(Fig.4b).Gashydratesaturationswereestimatedtobe>50%inthesandsectionsintheGC955-Qwell(Guerinetal.,2009).

Theseismicdataavailableforthisstudyaregoodquality,post-stack,timemigrated3-DdataacquiredbyWesternGecoin1997.ThedatawasreprocessedfortheJIPbySchlumbergerforgashydrateresearchpurposes(Jonesetal.,2008).Thedominantfrequencyisapproximately30Hz,withhigherfrequencies(40e60Hz)intheshallowsection.Inlinesarespaced25mapart;crosslinesarespaced12.5mapart.

3.3.Rockphysicsanalysisandgashydrateidenti?cation

Oneofthemaingoalsofthisstudyistodevelopandvalidateanintegratedinterpretationschemeforthedetectionandcharacter-izationofhighlyconcentratedhydrate-bearingreservoirs.LWDlogshavebeenusedtoidentifyandcharacterizegashydrateinvariousgeologicsettingsbuttheyprovideinformationononlytheareaimmediatelysurroundingeachwell.Seismicdatacandelineate