ContentslistsavailableatScienceDirectMarineandPetroleumGeologyjournalhomepage:www.elsevier.com/locate/marpetgeoOldmarineseismicandnewsatelliteradardata:PetroleumexplorationofnorthwestLabradorSea,CanadaChristopherD.Jauera,*,PaulBudkewitschbabBedfordInstituteofOceanography,GeologicalSurveyofCanada(Atlantic),Box1006,Dartmouth,NovaScotiaB2Y4A2,CanadaCanadaCentreforRemoteSensing,Ottawa,Ontario,CanadaarticleinfoArticlehistory:Received3December2009Receivedinrevisedform4February2010Accepted5March2010Availableonline24March2010Keywords:PetroleumSeepsSaglekBasinRadarsatSARdataOilslicksSeismicinterpretationSeafloorstructuresBasaltColdwatercoralsabstractThispaperpresentssomenewconceptsinthepetroleumsystemsofthenorthernLabradorandsouthernBaffinIslandoffshoreregionofeasternCanada.ThefocusofthisworkistheregionoftheHekjaO-71gasdiscoveryof1979byAquitaine,oneofonlyfivewellsdrilledbetween1976and1980withinanareacoveringsome166,000squarekilometerswithintheSaglekBasin.Thisstudyemergedfromabroadscalere-examinationofthepetroleumpotentialofthisarea“fromthecrustup”usingregionalreflection,refractionandpotentialfielddata.AnopportunitytouseSAR(syntheticapertureradar)datafromtheRADARSAT-1Earthobservationsatellitewastakentoincor-poratealternativedatasourcestosupportthisexploratoryreviewandresourceassessment.Examinationofthefinalmapofinterpretedslick-likefeaturesontheseasurface,revealedaclosecorrelationofsomeoftheseoccurrencestoseveralbathymetricfeatureswhichhaveunderlyingseismicsignaturessimilartopreviouslyidentifiedgashydrate“pipes”orchimneyanomaliesasseenindatafromoffshoreNovaScotiaandintheIrishSea.Globally,manyactivemarinehydrocarbonseepsappearontheseaflooras“pockmarks”;inthiscasenoseafloordepressionswereseenonconventionalmultichannel2-Dseismicdataassociatedwithactiveseepage.Instead,verydistinctmound-likestructuresareseenassociatedwithseepsattwolocationseastofHudsonStrait.ThepresenceofsignificantcoldwatercoralincloseproximitytotheactiveseepagesitesmayshowalinkbetweenseafloorpetroleumseepageandcoralreefdevelopmentsimilartothatobservedoffshoreNorway.Closeexaminationofthenearsurfaceseismiccharacterledtotheinterpretationofwhatwasoriginallyseenasseismicnoiseinvintagedata,aslikelybeingfluidescapechimneysorgaspipes,alongtheeasternedgeoftheHekjastructuralcomplex,wherefavourablehydrocarbontrappingconditionsarepresent.Noactiveseepswereseenthere;thismaybeduetootherfactors,suchasthenatureofthefluidsthatareescapingorthepossibilityofselfsealingactivitybybiologicalagentssuchasbacterialmattingwithassociatedcarbonatehard-groundformation.OneseepanomalywasnotedrelativelyclosetothemajorbasalticeruptivecomplexneartheGjoaG-37well.Thismaybeduetothepresenceofvolcaniclastic-hostedhydrocarbonssimilarinstyletothoserecentlydiscoveredattheRosebankfieldinPaleocenevolcanicsintheFaroe-ShetlandBasin.Theoccurrenceofanactiveseepinavolcanically-dominatedterrainraisesthepotentialforaradicalshiftinacceptedtypesofpetroleumleadsandwheretheymayoccur.Thepresenceofapparentoilseepageversusthepasthistoryoffindingonlygas-proneaccumulationsisastrongindicatorthatasecond,oil-pronepetroleumsystemexistsintheSaglekBasin.Thesignsofactivepetroleumsystemsasshownbyoilseepanomalieswithconfirmingseismicevidenceofdiscreteseafloorstructuresincloseproximitymakesacompellingargumentforre-examiningtheseareaswithaneyetowardsstratigraphicinsteadofstructuralplays.CrownCopyrightÓ2010PublishedbyElsevierLtd.Allrightsreserved.1.IntroductionThisstudywaspartofaregionalpetroleumresourceassessmentbytheGeologicalSurveyofCanadatoreviewexistingdiscoveriesandexplorenewconceptsinthepetroleumsystemsontheoffshorenorthernLabradorShelf,DavisStraitandBaffinBayregions(Fig.1).*Correspondingauthor.E-mailaddress:chris.jauer@nrcan.gc.ca(C.D.Jauer).0264-8172/$eseefrontmatterCrownCopyrightÓ2010PublishedbyElsevierLtd.Allrightsreserved.doi:10.1016/j.marpetgeo.2010.03.0031380C.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e1394
Fig.1.GeographicallocationoftheSaglekBasinwithrespecttoeasternCanada,thedetailedmapshowsthewelllocationswithinthestudyarea.Themajorstructuralfeaturesareshownontheregionalgeologicalmap.
Comparedtomanyotherhydrocarbonbearingbasins,littledetailedgeoscienceinformationispubliclyavailableforthisregionotherthanregionalreconnaissancegeophysicalgridsofpotentialfielddataandseveralexploratoryseismicsurveys.AregionalsynthesiswascompiledbyBelletal.(1989)fortheLabradorSeabutthisworkcoveredonlyasmallportionoftheSaglekBasinwhichisthefocusofthiseffort.Thedrillinghistoryforthestudyregionisverysparseasthereexistonlyfoursignificantwellsdrilledbetween1976and1980foranareaofsome166,000kilometres2(Fig.1).TheHekjaO-71gasdiscoveryof1979byAquitaineintheSaglekBasinprovidessomeimportantdetailsregardingthegeologyofthisbasin.Thiswellshowedacalculateddrillstemtestabsoluteopenflowof504,323m3/dwith17m3/dof54APIcondensatefromthelowerC.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e13941381
EoceneGudridsandstone(Kloseetal.,1982).Howeverastepoutwell,RaleghN-18,drilledin1980onanadjacentstructureonly26kmawaywasdry.TheseismicstructuralmapoftheHekjagasdiscoveryshowsgoodfourwaystructuralclosure.Ourseismicmappingofthesurroundingfieldareainthisstudyshowsthatthestep-outwellwasplacedinastructurallylowlocationofamuchlargeradjacentfeature.TheRaleghN-18wellappearstolieinastructuralvalleyadjacentamuchlarger,asyetuntestedstructuralhigh.Theeasternmostwell,GjoaG-37,wasdrilledin1979onaprominentbathymetrichigh190kmfurthereastthatappearsstructurallycontrolledbytheregionalUngavaFaultzoneandfoundanunderlyingeruptivecomplexofthickPaleocenebasaltflowsintermixedwithshalelayersandarelativelythinoverlyingsequenceofPaleogenethroughNeogeneclasticswithnohydro-carbonshows.Tothesouth,theCabotG-91wellwasabandonedatadrillingdepthof290m;itisshownonlyasareferencepoint.Thesouth-ernmostwelladdressedbythisstudywasRutH-11,drilledin1981.IttestedagentlebasementstructureformedoveralowerEocenebasalt.TherewerepotentialsourcerockintervalsintheoverlyingMarklandshalesbuthydrocarbonswereabsentinallreservoirzones.2.GeologicalsettingTheSaglekBasinisthenorthernmostofthetwomajoroffshorebasinssituatedalongtheriftedmarginofLabrador.TheseareextensionalbasinswhoseoriginsdatefromthebreakupofPangeaandseafloorspreadingeventsbetweentheGreenlandandNorthAmericanplates.SaglekBasinisboundedtothesouthbytheOkakArchfromtherelativelybetterunderstoodHopedaleBasin.Botharehydrocarbonprovinces,buttherehasbeennoproductiontodatefromeitheroftheselargelygas-pronebasins(Enachescu,2006a,b).ThewesternlimitsoftheSaglekBasincompriseasystemofbasinboundingfaultsthatcloselyoffsetthecontinentalProtero-zoiccoastline.TheeastwardlimitcanbeplacedattheextinctspreadingridgeaxialriftanditsassociatedbasalticeruptivecomplexthatdatesfromthelatestEoceneriftingoftheNorthAtlantic.Thisvolcanicallydominatedterrainextendsforseveralhundredkilometrestothesouth-westasshowninthegeneralisedgeolog-icalmapinFig.1.TothenorththisbasinisoffsetfromtheLadyFranklynBasinbytheUngavaFaultzone,atransformfaultthatterminatesatthewesternendofthespreadingridgeaxis.ThetectonicsettingoftheSaglekBasinissomewhatatypicalwithregardstopetroleumhabitat.TheMesozoicthroughrecentsectionofpredominantlyclasticsedimentaryfilloverliescontinental,transitionalandoceaniccrustrespectively,fromeasttowest.Thismixofcrustaltypeshasbeencorroboratedbyarecent630kmregionalwide-angleseismictransectwhichwasabletodistinguishthesecrustaltypesbytheiracousticvelocities(Funcketal.,2007).TheearliestsynriftrocksknowninthebasinaretheAlexisFormationvolcanicsfoundtothesouthofthestudyareaalongtheLabradorShelfwhichhavebeendatedintherangeof122to139Ma(earlyCretaceous)thatrepresentthefirstphasesoftheintra-cratonicriftingofLabrador-Greenland(Umpleby,1979).TheserockswereoverlainbytheMarklandFormationduringtheperiodofthermalsubsidencethatfollowedduringtheLateCretaceous;thisisamarineshalewithlocallyvaryingfacies,alongtheshorewardbasinmargintherearetwosandstoneunits,theFreydissandstonefoundtothesouthofthestudyareawhichdatesfromtheCampanianandthelowerGudridmemberwithinthestudyareawhichisDanianinage(Umpleby,1979;Belletal.,1989).DuringtheearlyPaleocenetherewasaresurgenceofeast-westspreadingintheLabradorsearegionthatresultedinamajorvolcanicepisode.AstheMarklandwasstillbeingdepositedduringthisperiodtherearevariousareaswhereintercalatedsectionsofbasaltandshaleorothermarinesequencesarefound;thiswasseenattheGjoaG-37testwherefourseparatebasalticflowsareseparatedbymarineshalesoverasectionofover1300mthickness,thewellbottomingwithinMarklandForma-tionshales.TheregionallywidespreadtopoftheEocenebasaltcanbeusedasaseismicmarkertodefinethelimitsofthisbasin.Seismicimagingbelowthebasaltsishighlycompromisedbutthisseismicsurfaceshowscharacteristicsea-warddippingreflectionsinthedeepersectionleadingofffromthewesternsideofthebasin.Thisseismicmarker,whenconsideredasasurfaceisdiachronousasthebasaltsatHekjaaredatedas48.7Æ1.8MacomparedtothebasaltsatGjoawhichare59.5Æ1Ma(Williamsonetal.,2003);althoughthebasalttopisquiterecognisableonbothlogsandseismicdata,itisnotpossibletodistinguishbetweentheearlierandlater-stageflowswiththeseismicdatausedforthisstudy.TheMarklandFormationissucceededbytheCartwrightFormation,anotherpredominantlyshaleunit,thelowerportionhoststhemiddleGudridsandstoneswhichdistallygradedowndipintotheupperMarklandshales(Balkwilletal.,1990).ItshouldbenotedthattheinformalstratigraphicdescriptionsherehavethelowerGudridintheMarklandandthemiddleandupperGudridmembersintheCartwright;thisisoneofseverallithostratigraphicchallengesyettoberesolved.EarlyworkbyUmpleby(1979)viewedthisentiresectionasoneunit,buttherearesufficientwelllogandpalynologicalsampledifferencesseeninthedrilledwellstoregardthestratigraphyasbeingmorecomplicatedandneedingbetterdefinition.AsimplifieddiagramoftheregionalstratigraphyisshowninFig.2.ThePaleogenethroughNeogenesedimentaryfillabovethebasaltunitsisessentiallyclasticwithonlyveryminoramountsofcarbonaterock.TheseclasticswerelargelysourcedfromtheancientBellRiverdrainagesystem(Fig.2b),whichdwarfsthebetterknownmajorfluvialsystemssuchastheMississippiRiversystemoftheGulfofMexico.Thepaleo-MackenzieRiver,inthewesternpartoftheNorthwestTerritories,wasthemajornortherntributaryoftheBellRiversystemwhichdrainedtheWesternCanadianCordilleratotheLabradorSeaviaHudsonBay.Evidenceforthispaleo-drainagesourcehasbeenfoundinthemineralogyandpalynologyofterrestrialsedimentsindrillcoresfromtheLabradorSea(Duk-RodkinandHughes,1994).IntheSaglekBasinahighvolumeofsedimentinfluxhasexistedfromDaniantopresent,asisshownbythedevelopmentofextensivestackedchannelsystemsprogradingouttotheeastandsouth-eastintodeeperpartsofthebasin.Theassociateddeltafrontsthatarepartofthisdepositionalsystemhosttheoneprovenreservoirunit,themiddleGudridsandstonesofthelatePaleocenetoearlyEoceneCartwrightFormation.Thishigh-latitudeterrestrialinputintothedepositionalenvi-ronmentfromsuchahugeareacontainsresinite,asasourcerockforthebasin’spetroleumsystem(Kloseetal.,1982).Thisplant-derivedmaterial,oftenassociatedwithcoalformation,isaknownsourceforoilinseveralCenozoicbasins(Snowdonetal.,2004).Thebestexampleofasimilar,highlatitudeareawitharesinitesourceforPaleogene-NeogeneoilsistheBeaufort-Mackenziebasin(Henryetal.,2006).Thepetrologicaldescriptionofthe76mgrossreservoirsectionfoundatHekjaO-71showsDanianagecoarse-grainedsandstoneswithcrossbedding,interbeddedwithshalesinadeltaicsetting.1382C.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e1394
Fig.2.a)SimplifiedstratigraphicdiagramoftheSaglekBasinfromsouthwesttonortheastshowingthemajorgeologicalelements.b)InterpretedPaleoriversystemsthatwerethedominantsourcesofsedimentsupplyintothestudyarea.
Thenetpayof44mhasporosityof16%with10mDpermeabilityand28%watersaturation.Althoughthesearegoodparametersforapetroleumreservoir,thenatureofa“Mackenzietype”petroleumdepositisonewhereinthesourcematerialisrandomlydistributedthroughoutthereservoirsands.Whilethisdistributionmayseemproblematicthenatureoftheclasticrockspromotesa“wicking”effectforhydrocarbonsthatenhancestheirstorageinthereservoir(Fowler,pers.comm.2009).ThesourcerockoftheGudridconsistsofcoalysiliciclasticmaterialwithavitrinitereflectancevalueof0.55%whichrepresentsonlyborderlinematurityforoilgeneration.Asthisintervalproducedgasandcondensate,theactualsourceofthehydrocarbonsisnotclearbecausethevitrinitereflectanceandwholerockevalu-ationofpetroleummaturityshowstherocksarejustenteringtheoilwindowatdepthsof3000m(Fowleretal.,2005).AslighterhydrocarbonstendtomigrateupwardsitwouldappearhighlyC.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e13941383
probablethattheactualsourcerocksforthisdepositliedeeperinthesection,mostlikelyinlowerCartwrightorMarklandshales.ThemidEocenethroughOligoceneKenamuFormationoverliestheCartwrightFormationandisapredominantlymarineshale,withtheonewell-developedchannelsandunit,theLeifMember,neartheEoceneeOligoceneboundary,whichappearstobecoin-cidentwiththemajorbasin-wideBaffinBayUnconformity.TheoverlyingNeogeneMokamiFormationisasimilarmarineshaledominatedsectionwithafewsandsequences.Thesucceed-ingSaglekFormationconsistsofsandytosiltymudstonesthatbecomesseawardsmoreshaleprone;anotherbasinwideuncon-formityseparatestheupperandlowerunits.Welldevelopedpro-gradingfore-setswithinterspersedincisedchannelcomplexesarequiteprominentontheseismicdatainthisinterval,possiblybestdevelopedtotheeastofHudsonStrait.ThetopupperSaglekhasnotbeenwelldefinedasitistypicallybehindthesurfacecasingandconductorpipeofallthewellsdrilledheretodate.ThicknessofthefullSagleksectionrangesfrom1173matRuttothesouthto1419mseentothenortheastatGjoaG-37.Thetop-mostdepositsarePlio-Pleistoceneglacialclasticsandrangeuptoseveralhundredmetresinthickness.Theycompriseamuddymixstrewnwithnumerouspebbles,cobblesandothererratics.Thispresentsasasurficialboulderpavementthatcanposeaseveretechnicalchallengetodrillingoperationsbymakingthesettingofthesurfacecasingverydifficult.Duetothepaucityofwellcontrolhere,therearesomeincon-sistencieswiththebiostratigraphicagedatesassignedtothelith-ostratigraphy-basedformations.Thebasalbasaltunitagesarealsoproblematic,withagesrangingfrom63to55Ma,theyreflectseveralstagesoftheopeningoftheLabradorSea.3.GeophysicalinterpretationandhydrocarbonexplorationThe“fromthecrustup”approachtointegratealltheregionalgeophysicaldataforhasyieldedsignificantinsightsintothetectonicevolutionofthenorthernLabradorSeaandDavisStraitregion.Regionalgravity,aeromagneticsandseismicrefractionstudiesgaveafairlybroadunderstandingoftheprerift,synriftandpostriftgeodynamicevolutionoftheSaglekBasin.Usingsome15,000line-kilometresof2-Dseismicreflectiondatatiedtothefewindustrywellspresenthasconfirmedsomeofthestructuralfeaturesinter-pretedbyotherdata,anexample,theaxisoftheextinctspreadingridgecomplexwasimagedbyseismicreflectionmappingofthebasaltsandfoundtooverliethegravitymapofthatstructure(Oakey,2005).Acomparisonofthemappingofthismajortectonicfeaturebygravity,magneticandreflectionseismicimagingisshown(Fig.3)anditsrelationtotheHekjastructuralcomplexisobvious.Theseismicdatausedinthestudyspanned30years,from1972to2002,withprocessingrangingfrombasicstackswithsimpledeconvolutionfilteringtodatathatincludedprestacktimemigration.Themajorityofthismixofdatawasacquiredpriorto1982,muchofthisdatasufferedfrommultiplesandthenaviga-tionalinaccuraciesinherentinpre-GPSdata.Insomecasesthebest“groundtruth”ofthenavigationdatawasbycomparisonwithbathymetry.Additionally,whilemuchofthedatawasreasonablyinterpretable,thewidespreadfloodbasaltflowsmadeanacousticbarrier,belowwhichinterpretationswerenotpossible.TheHekjadiscovery(Fig.4),whichhasanapparentstructuralrelationshipwiththeregionaltectonicsasseenontheregionalseismicmapoftheEocenebasalt,wasimagedwitharelativelyloosegridofseismiclinesaveragingaboutonelineevery5kmoveranareaoflessthan500km2.Theavailableseismicgridovertheentireareaconsideredherehasamuchlowerlinedensity,makingforagoodregionalreconnaissanceofthebasinbutprobablymissinganyadditionalstructuresthatarelessthan25kilometresinlength.ThetranspressivemovementsrelatedtotheactivityoftheEocenespreadingridgehavecreatedthepost-depositionaltrapstructureatHekjaandthebroadregionwherethisstructuralinfluenceispresentcanbeinferredfromseismicmapping.ThereareadditionalprospectiveareasalongthisstructuralfairwaythattrendssouthwestwardsthroughHekja,andtothenortheastthatrequireadditionalseismicdataforfurtherstudy.Aroundthediscoveryarea,theseismicdatacoveragewasaboutonelineevery5kmwithlinesgenerallyhavingagroupintervalof50m.Withalocallysomewhatdenserseismiccoverage,thestep-outlocationatRaleghwasmappablewithreasonableconfidence(Fig.5).Thiswellwasdry,eventhoughallthecriteriaforasuccessfulprospectwerepresent,i.e.,structure,reservoirandsealaswellasproximitytoasimilarappearingdiscovery.Althoughtherewasexcellentreservoir,theGudridsandstoneherehad70mgrosspaywith22%porositybutwasbarrenofhydrocarbons.ThestructuraltopfortheGudridwasat3473metresdepthatRalegh,comparedto3200metresdepthatHekja;asignificantmishapinthechoiceofthislocationhadoccurred.TheareaofclosureclaimedfortheHekjadiscoverywas44km2,givingrecoverablereserveestimatesofabout56.7billionm3orw2Tcf(Kloseetal.,1982).Theareagrantedhereforthesignificantdiscoverylicence(SDL)was11.1km2,bycomparison,theclosedstructureimmediatelywestofRaleghhasanareaofclosureapproaching100km2anditappearstobehydrocarbonchargedbasedontheseismicallyobservedventstructures.Atthispoint,basedontheavailabledata,thelimitsofaconventionalprospectassessmentwouldappeartohavebeenlargelyreached.ForSaglekbasinasandstonedepositionalfairwayhasbeenestablished,themechanismforformingstructuraltrapformationisknown,thedistributionofsourceandsealrocksroughlyestablished(Kloseetal.,1982;Wielensetal.,2009).Newhighqualityseismicandespecially3-Ddatashouldbeabletobetterresolvegeologicalstructureandamplitudeanomaliesthatwouldindicatefluidcontacts.AnactualseismicamplitudedimspotcanbeseenatHekjaO-71onthemostrecentdata,buteventhisevidencewillnotbeentirelyconclusive.Ideally,thenextpossiblepieceofevidencetolowertheexploratoryriskwouldbesomeformofremotegeophysicalpro-spectingthatmightmoredirectlyindicatehydrocarbons.Thisiscurrentlybeingdoneinmanynewerexplorationareasbyrevisedelectromagnetictechnologiessuchastherecentlydevelopedcontrolledsourceseabedloggingtechniques.Unfortunately,suchnewtechniquesarerelativelyexpensiveandstillyieldambiguousresultsunlesscollectedinconcertwithhighqualityseismicreflectiondata,soforthisstudyweelectedtousethemuchcheapermethodofsatelliteremotesensing,usingRADARSAT-1technologytoindicatethepresenceofoil-seepagesontheseasurface.DatacollectionovertheareaofinterestconsistedofacquiringseveralpassesofRADARSAT-1datacollectedduringthemonthsofSeptembertoNovemberin2003and2004,coveringanareaofover250,000km2.4.Detectionofoilseepsfromsatelliteremotesensing:radarmethodsforoilslickmappingWiththeavailabilityofcivilianspaceborneSARsystems,imagingoftheseasurfacehasrevealednumerousexamplesofvarioustypesofseasurfaceslicksandseveralexperimentshavebeencarriedouttobettercharacterisethesefeatures(e.g.Bernetal.,1993;Gadeetal.,1996).Astheangleofincidenceoftheradarbeamfromorbitisobliquetotheseasurfacespecularreflectionfromcalmwaterisdirectedawayfromthesensor.There1384C.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e1394
Fig.3.Comparisonofregionalscalegeophysics:gravity,magneticsandreflectionseismic.TheBouguergravityanomalymapatlefttophascorrectedforbathymetryandhighpassfilteredtoenhanceresolution.Comparedefinitionofthespreadingridgefeaturetotheaeromagneticmapattopright.Thethreewellsshownbythesymbolsarefromwesttoeast,Hekja,RaleghandGjoa.Forrelativedimensionsrefertothe3-DrenderingoftheseismicdepthconversionofthetopEocenebasaltsurfaceatbottom(notecoloursshownareallrelative).
needstobeafairlyhighseastatepresent,i.e.thecombinationofwaveheight,periodandcharacteroftheseasurface,toenableconditionsforsomeradarreflectivitytobedirectedbacktothesensor.OnemechanismrecognisedthatenhancesabackscatterresponseiscausedbyBraggscattering.ThedataexaminedinthisstudyismainlyfromtheRADARSAT-1satellite,collectedin2003.RADARSAT-1dataisaC-bandradarthathasawavelengthof5.6cmandscansthesurfaceatanglesof10to59degreesfromverticaltoprovidedatainavarietyofbeammodesandresolution(Luscombeetal.,1993).C.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e13941385
Fig.4.TheHekjaO-71discoveryimagedonrecent2-Ddata.EvenmoderndatahasproblemspenetratingtheEocenebasaltlayer(red)whichwasusedastheregionalseismichorizonformappingtheextentoftheSaglekBasinshowsasthedepthconvertedsurfacebelow.ThebasalticzoneatHekjaisatleast950mthick.TheGudridreservoiristheoverlyingyellowhorizon.ThelowermapshowstheuseofthetopofbasaltseismicmarkerasaproxyfordefiningthestructurallayoutoftheSaglekBasin.
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Fig.5.InterpretedhydrocarbonventfeaturesneartheHekjadiscovery:onepockmarktypeandthreeseafloormoundsshowninrelationtotheseismicallymappedGudridreservoir.
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Oceanslicksappearascontinuousareasoflowbackscatterandarethusdarkerwhencomparedtothehigherbackscattervaluesofthesurroundingseasurfacebecauseoilslicksdampencapillarywaves,thosecentimetrescale,windgeneratedripplesridinglargersurfacewaves,whichreducestheircharacteristicBraggscattering.Appropriatewindspeedsarealsocriticalforimagingthiscontrastastheyarethesourceforcreatingthecapillarywaves,idealwindspeedrangesaretypicallyontheorderof2e6m/s.DarkpatchesinoceanSARimages,however,canhaveseveraloriginsotherthanoilslicks.Otherenvironmentalfactors,suchasfreshwatermixing,internalwaves,raincells,ephemeralalgalbloomsandtheforma-tionandmeltingofseaicecanconspiretocreatephenomenathatlowerradarbackscattersignals.Notwithstanding,effortsweremadetoselectforthebestpossibledataintheArcticfromSeptembertoNovemberwhenminimalicecoverpersistspriortofreeze-up.WindspeedswereestimatedfromtheSeaWindsinstrumentontheQuikSCATsatellite.ThereisexcellentdailyglobalcoverageanddataacquiredwithinseveralminutesofRADARSAT-1passesareusuallyobtainable,permittingapre-selectionoftheradardatatobeexamined.Inthisstudy,over40framesofRADARSAT-1ScanSARNarrow(B)dataoutofabout70initiallyplannedimageswerechosenforevaluationbasedonfavourablewindspeed.EachScanSARimagecoversaswathwidthof30kmwithanominalresolutionof50m.Slicklikefeatureswerethenevaluatedqualitativelyandeachcandidateslickwasclassified,simplyfordescriptivepurposes,intosingleormultiplefeatures;linearityvs.arealpatches,regularvs.irregular,andapproximatesize.AfewexamplesoftheSARimagesareshowninFig.6.Thelatitudeandlongitudeofapproximateslicklocationwereextractedfromthescenesbasedongeographicreferenceinformationprovidedwiththesatellitedata.ForRADARSAT-1,thesecoordinatesaregenerallyaccuratetowithinafewhundredmetres.Asimplequalitativeapproachofthreecategorieswasmadetogradeorrankthelikelihoodoftheslick-likefeaturesintermsofaprobabilityofbeingahydrocarbonseepinsteadofsomeotherorigin.Multipleimagingofthesamelocationoveraperiodoftimeyieldingareproducibletargetareaisgenerallyconsideredbestpracticeforsearchingandsortingofpossiblenaturalseeps.Threeormoreimagesfoundundersuitableconditionsisgenerallyacceptedasareasonablefiltertoreducethenumberof‘falsepositives’fornaturalseeps.Withinthescopeofthiswork,onlyaminimalcoveragewaspossiblewhichprovidedafirst-passcoverageoverthisquitelargearea(250,000km2)ofinvestigation.Nevertheless,preliminaryresultsprovidedanumberofinterestingresultswhenexaminedincontextwiththeotherdatasets.5.CorrelationofsatelliteandseismicdataExaminingtheresultingdistributionofpotentialoilseepanomaliesfromtheRADARSAT-1dataledtoare-examinationoftheseismicdatawithspecialattentiontotheseafloortopographyandshallowsubsurfacezone.Approximately70featureswereidentifiedoverthecoverageareamappedandtheoccurrenceofoilseepanomalieswasplottedandreferencedagainsttheseismicdatabaseandbathymetrymap;fiveseepanomaliesandfour“non-seep”seafloorfeaturesarepresented(Fig.7).ThefivedistinctoilslickanomaliesfromthesatellitesurveywereconspicuouslyabsentfromtheHekjadiscoveryarea.Thefourchimney-styleseismicfeaturesthatbrackettheHekjadiscovery(Fig.5)bearsimilaritiestotheexampleshownfromtheTorbrookareaoffshoreNovaScotia(Mosheretal.,2005)on3-Dseismicdata;thesefeaturesareinterpretedasgashydrate“pipes”associatedwithbottomsimulatingreflectors(BSR)thatoccurwhenthereisfreegasdevelopmentfrommethanehydratesinthesubsurface(Fig.8).ThreeofthefourRaleghareaventfeaturesalsoappeartobeplacedoverstructuralclosuresofthemappedGudridreservoirsandstonethatareashighorhigherthantheactualRaleghwelllocation,i.e.,occurringinabetterhydrocarbontrapsetting.Thefirstactiveoilseepfeaturewhichcorrelateswithaseafloorfeatureisfoundover150kmawayfromtheHekjadiscovery,closetotheseafloorshelfbreak.Thisisaprominentseafloormoundlikestructurewithadistinctunderlyingchimneystyleseismicsignatureofstackedchevronlikereflections.AdjacenttothisstructurethereappearstobethestartofamoundfieldcomplexsimilartothoseintheRockallTroughoftheIrishSea(Kenyonetal.,2003).Thesecondactiveoilseepsite,furthersouth,hastwoseafloorfeaturesincloseproximity;thedistinctstackedchevronstylemoundisnexttoawidermoundwithamoreroundedappearance.Theroundedmoundlacksthechevronreflectionsbutdoesshowweakerreflectionsbeneathitthatmaybeduetogaschargingintheshallowsection.ComparingtheseSaglekBasinfeatureswiththehydratepipefeaturesandthecarbonatemoundexamplesfromoffshoreNovaScotiaandIreland(Fig.8)showssimilarcharacteristicsthatsuggestupwardfluidflowfromthesubsurface.Thethirdactiveseepsiteisabout60kmeastoftheGjoaG-37well,withinthevolcanically-dominatedterrainoftheGjoaeruptivecentre(Sorensen,2006)associatedwiththeextinctspreadingridgetothesouth.Theoneavailableseismiclinehereshowednoseafloorexpression,butthefaultedandfoldednatureoftherelativelythinsedimentarycoverimpliesacomplexgeologicalsetting.6.DiscussionAllpetroleumreservoirswillleakovertimeorhydrocarbonsgeneratedinthebasinwillnotfindatrapontheirupwardmove-ment.Itisnowgenerallyrecognisedthatallpetroliferousbasinswillshowsomeformofnearsurfacepetroleumsignalashydro-carbonsmigratethroughthegeologicalcolumn(Abrams,2005).Inamarineenvironment,ventedhydrocarbonscanformoilcoatedgasbubbles,whichexpandandburstuponreachingtheseasurfaceleavinganoilslick.Marinehydrocarbonseepshavebeennotedtheworldover,oftenventingfromcircularseafloordepressionscommonlyreferredtoaspockmarks.Frequently,thesepockmarkshavebeenobservedbymarinegeologistsbutwithoutanyevidenceofgasventingoroilseepformation.Thismayberelatedtothedynamicnatureofoilslickanomalies.InSaglekbasinthepresenceofseismicallyvisiblesubsurfacechimneyfeaturesinassociationwithpositivereliefseafloorstructuresemphasizetheunex-pectedleakageofthepetroleumsysteminsomepartsofthebasinandanapparentrelictsignatureclosetotheHekjapetro-leumdiscovery.Previousevidenceofnaturallyoccurringpetroleumseepagesintheregion,asopposedtoman-madeonesfrompollution,datefromthe1976observationsofslicksontheseasurfacewithgasbubblesandoildroplets,offthenortheastcoastofBaffinIsland900kilometrestothenorth(seeFig.6).ThiswasfollowedbyaseriesofdivesusingthePiscesIVsubmersibleinScottInletofBaffinIslandin1985thatdocumentedcircularseafloordepres-sionsofabout30mdiameterandseveralmetresdeep.Theyrecoveredbottomgassamplesandpiecesofbacterialmatsthathadasubstrateofinduratedsediments;pebblesofigneousrockinafine-grainedcarbonatematrix.Grabsamplesofthismaterialwerefoundtoexudeoildropletswhenexaminedatthesurface(Grantetal.,1986).1388C.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e1394
Fig.6.Top,dataswathcoveragefromRADARSAT-1usedforthisstudy,withanexampleofaSARimageshowingland,seaiceandopenwaterintheHudsonStraitarea.Middle,appearanceofanoilseepfromspaceborneSARdatacomparedtoacloserangephotoofanemergingoildroplettakennearScottInlet,BaffinIsland.Bottom,diagramofBraggscatteringfromsurfacewaveswithconstructiveinterferencecausinghigherradarbackscatter.Surfaceoildampenswavesandlowersbackscatter,permittingtherecognitionofaslickduetotheresultingimagecontrast.
ThesepositivereliefseafloorstructuresintheSaglekBasinalongwiththeirpersistentseepagesignaturesuggeststhatthesestructuresareeithermudvolcanoesdrivenbyupwellinghydro-carbonsorbioherms,likelycold-watercoralbuildups,thatareproximaltotheseafloorseepagesites(Hovland,1997,2003).ThemechanismforcreatingamudvolcanoorgasassociatedmuddiapirshasbeenhypothesisedbyHovland(1992)basedonexamplesoffshoreNorway.Giventhegeologicalenvironmentofrapidsedimentation,lateraltectoniccompression,andtheactivegenerationofC.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e13941389
Fig.7.Bathymetriccontoursshownwith100mcontourinterval,studyareaoutlinedinred.NotethecompleteabsenceofanyactiveoilseepactivityintheHekjagasdiscoveryregion.ColouredcirclesindicateRADARSAT-1oilseepanomalies.
hydrocarbonsinthebasin;theprobabilityofmudvolcanoesbeingthesourceoftheseseismic/seepanomaliesappearshigh.Whywastherean“inactive”chimneyfeatureoffsettingtheRaleghwell?Possiblythebacterialmattingdevelopedatsuchahydrocarbonventcanresultinself-sealingofthebreachasthesematsareassociatedwiththeformationofhardcarbonatecementedcrusts.Longtermepisodicventingandresealingheremaybeduetoexternaleventssuchasearthquakesbreachingthereservoiror1390C.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e1394
Fig.8.Topleftisagaschimneyfeatureseenon3-DseismicneartheTorbrookC-15welloffshoreNovaScotia.Theinsetshowsahorizontalcoherencyslicethroughonemoundshowingitscircularshape.AtrighttopacarbonatemoundfromthesouthernRockallTrough,offshorewesternIrelandwithanassociatedmoundfield.Middle,activeseafloorventsfromtheSaglekBasin.Theupperventhasawidthofabout400mandsome40mofreliefwithalowreliefmoundfielddevelopmentatright.Bottom,pairedmoundsobservedtothesouthofthemiddleimage,theleftmoundisw800macrossversustherightonewithw450mwidth.Bothmoundsrisesome10mabovetheseafloor.TheseseafloormoundfeaturesaremappedastheredandgreenstarredfeaturesinFig.7.
internalpressurebuildupfromhydrocarbonchargingreachingsufficientstrengthtoinducehydraulicfracturing.Theepisodicreleaseofpetroleumfromsubmarinemudmoundshasbeendocumentedinsimilargeologicalsettings(MacDonaldetal.,2000)inthenorthernGulfofMexico.Themechanisminthatcaseforperiodicreleaseofhydrocarbonswasafunctionofongoingpetroleumgenerationreachingacriticalsubsurfacepressurewithinthestructure,thenleakingoff.GiventhefairlylargeareaofstructuralclosureseenontheseismicinterpretationneartheRaleghstructurethisappearstobeaplausibleexplanationandthelackofanyvisibleseepagetheremaybeduetotiming,or,asthisagas-pronearea,theventingofC.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e13941391
Fig.9.MappingofSARoilseepfeaturesagainstfisheriesdataofcoralsrecoveredduringtrawlingoperations.Notetheareasofsamplingwherenocoralrecoverywasindicated.
gasandsmallamountsofcondensateleavingnodiscernableoilslickontheseasurface.RecentpetroleumsystemmodellingoftheHekjaareashowsthatpetroleumgenerationpeakedat25Maforgasand20Maforoil,butotherwisethesystemisstillactivelygeneratinghydrocar-bons(WielensandJauer,2009).Bycomparison,theScottInletseepsystemwithitswidespreadpockmarkareamaybeanunsealablesettingwherenoeffective1392C.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e1394
structuraltrapispresent,allowingforthemoreorlesscontinuousventing(MacDonaldetal.,2000)ofthesethermogenicpetroleumfluids(Fowleretal.,2005).Thereisgoodcircumstantialevidenceforabiologicaloriginforthepositivereliefseafloorstructuresseenheretobecoldwatercoralreefs.TheseafloordepthofbothofthesouthernSaglekseepagesites,about200kmsouthsoutheast,areat620mforthefirstsiteneartheshelfedgeandabout315mforthesecondsite.Thedepthofthesouthernmostsitewiththepairedstructureswouldnotprecludereefaldevelopment,usuallyviewedasashal-lowerwateroccurrenceinthephoticzone.Theexistenceofcoldwatercoralswasessentiallyunexpectedtwentyyearsago,muchlikethecommunitiesofchemosyntheticbenthicfaunaofthedeephydrothermalventsystemsseenatoceanicriftareas.These“coldseep”benthiccommunities,incontrasttothechemosyntheticcommunitiesseenathydrothermalventareasinoceanicriftzones,gainafeedingadvantagebyusingthemeth-anotrophicbacteriathataretheprimaryusersofthenutrientsavailableintheseafloorhydrocarbonseeps.Also,theformationoftheassociatedcarbonatehardgroundsprovidesthenecessarysolidsubstrateforcoralstocoloniseon.ColdwatercoralreefscomposedprimarilyofLopheliahavebeenwelldocumentedat40e350mdepthsoffshoreNorway(HovlandandThomsen,1997)andhavefrequentlybeenassociatedwithseafloorseepagesitesatseverallocations(HovlandandRisk,2003;Enachescu,2004).AreviewoffisheriesdatafromFisheriesandOceansCanadashowsthatthereisaconsiderableamountofdeepwatercoralsandspongesoverabroadoffshoreregionofeasternCanada(Wareham,2009).WhenthisdistributionwasmatchedwiththeSARmappingoftheoilslicksoneintriguingmatchofslicksandcoralscanbeseen(Fig.9)thatappearssignificantforseveralreasons.Highconcentrationsofcoralsandotherfilterfeedersareoftenseenalongslopingseafloorareaswherethetopographyacceleratescurrentflow.Thisenhancedwatercirculationisakeyfactorforfilterfeedingorganismstothrive.Thecircledareashowingwheretheseepsarepresentisnotablyflatlyingintermsofseafloortopographyonascaleoftensofkilometres.WhilerelativelyclosetotheoutletfromHudsonStrait,whichhassignificantcurrentflowouttothesewaters,theapparentconcentrationofcoralsandspongesasrecordedfromthefisheriessurveysinthisapparentclusterseemstoindicatearelationtotheseeps.ThemajorseepanomalyeastoftheGjoaG-37wellposessomeissuesforconventionalpetroleumthinking.Thisareahasrela-tivelythinsedimentarycoveroveramassivebasalticeruptivecomplexwhichwaswelldocumentedbySorensen(2006)astheGjoaeruptioncentre.VolcanicrelatedpetroleumseepageshavebeendocumentedfromonshoreWestGreenland(Bojesen-KofoedFig.10.This158kmregionalbentlineprofileshowsthetopofthelastbasaltdrilledatGjoaG-37andthemajoreruptivecentretotheeastlinkedtotheseepageanomaly.Notethepronouncedupwarddipofthereflectorsintheseepagearea.Thedetailshowingthegammarayandsoniclogsclearlyshowstheinterbeddednatureofthebasaltsandshale/clasticsectionsfoundovera1300msection.
C.D.Jauer,P.Budkewitsch/MarineandPetroleumGeology27(2010)1379e13941393
etal.,1999)thatindicatethepresenceofmultiplepetroleumsystemsbasedongeochemicalanalysisof71oilsamplestakenfromtheDiskoIslandwesternNuussuaqregions.Theextensiveoilseepsandstainsseenwerefoundtooccurinvesicularlavaflowtopsandassociatedcarbonatemineralveinsinmajorfracturesystems.Recentdiscoveriesshowthatavolcanicallydominatedterrainmayholdgreatpotentialasahydrocarbonhabitat,notjustasasealoverconventionalpetroleumbearingsediments,butasanintegralpartofthepetroleumsystem;asatthecommercialRosebankdiscoveryintheUKoffshoreregionoftheFaeroe-ShetlandBasin.ThereservoiratRosebankisaw100mthickinterbeddedsequenceofbasalticlavaflows,hyaloclastites,vol-caniclastitesandsiliclasticsandstones(Passey,2008).Thisenvi-ronmentappearsquitesimilartothedrillingresultsseenfromGjoa(Fig.10)andavariationonthisgeologicalsettingmaybetheoilprovenanceforthisinterpretedseepfeature.Althoughnoothergeologicalseepcharacteristicsarepresentontheavailableseismiclines,thereisahighdegreeoffoldingandfaultingpresentthatlikelyenhancedfluidmigration,inthiscasehydrocarbons,tobeconductedupwardstotheseafloor.7.SummaryandconclusionsOurstudyoftheseismicexpressionoftheHekjagasdiscoveryanditssurroundingsshowsthatthereareseverallargestructuralclosurespresentthatimplythatanorderofmagnitudemoreofthediscoveredgasreservesisverylikelypresent.TheHekjastructuralcomplexalsoshowsdistinctevidenceofrepeatedperiodsofhydrocarbonventingtotheseafloor,mostlikelyfromhydrocarbonsthatarestillbeinggeneratedbelowthereservoirzone.Tothesouthofthediscoveryarea,thepresenceofpersistentoil-slickfeaturesonSARdatathatareincloseproximitytopositivereliefseafloorstructuresindicateastrongpossibilitythatanoil-pronepetroleumsystemispresent,quitepossiblyasastratigraphictrap,thatalsoisleakinghydrocarbons.Thepresenceofasecondoil-slickanomalyneartheGjoaeruptivecomplexmaybeanindicatorofavolcanigenichostedpetroleumsystemsimilartotherecentRosebankdiscoveryoffshoreU.K.AlthoughtherearegeologicdifferencesbetweenthisNorthAtlanticbasinandtheonesusedasexamplesofsimilarhydro-carbonventingbehaviour,thepresenceofmorehydrocarbonsthantheonealreadydiscoveredappearsconvincing.Fisheriesdataalsoshowaprobablebiogeologicalconnectionbetweenthesecoldseepareasandhighconcentrationsofcoralandotherbenthicfaunaontheseafloor,likelyresultinginsizeablecoralreefdevelopment.Ifthisconnectioncanbeconfirmedwithaddi-tionalsurveys,suchasmulti-beamorotherhighresolutionseismicworkcombinedwithasamplingoftheseafloorstructures,aconsiderablydifferentpictureoftheresourcepotential,bothfromageologicalandbiologicalperspective,willemerge.AcknowledgementsThisworkwouldnothavebeenpossiblewithoutthesupportofstaffattheGeologicalSurveyofCanada(Atlantic)andthekindassistanceoftheNationalCoral/SpongeWorkingGroupattheDepartmentofFisheriesandOceansCanada.WethankJohnWolfeandGoranPavlicattheCanadaCentreforRemoteSensingforRADARSAT-1dataprocessingandacknowledgesupportfordatapurchasefromtheCanadianSpaceAgencythroughtheGovernmentRelatedInitiativesProgram.EarthScienceSectorcontributionnumber20090282.ReferencesAbrams,M.A.,2005.Significanceofhydrocarbonseepagerelativetopetroleumgenerationandentrapment.MarineandPetroleumGeology22,457e477.Balkwill,H.R.,McMillan,N.J.,MacLean,B.,Williams,G.L.,Srivastava,S.P.,1990.GeologyoftheLabradorShelf,BaffinBayandDavisStraitChapter7.In:GeologyoftheContinentalMarginofEasternCanada.GeologicalSocietyofAmerica,theGeologyofNorthAmerica,vol.2.Bates,J.L.,1989.In:Bell,J.S.,Howie,R.D.,McMillan,N.J.,Hawkins,C.M.(Eds.),EastCoastBasinAtlasSeriesLabradorSeaGeologicalSurveyofCanada,EastCoastBasinAtlasSeries,p.112.Bern,T.-I.,Whal,T.,Anderssen,T.,Olsen,R.,1993.OilspilldetectionusingsatellitebasedSAR:experiencefromafieldexperiment.PhotogrammetricEngineeringandRemoteSensing59(3),423e428.Bojesen-Kofoed,J.A.,Christiansen,F.G.,Nytoft,H.P.,Pedersen,A.K.,1999.OilseepageonshoreWestGreenland:evidenceofmultiplesourcerocksandoilmixing.In:Fleet,A.J.,Boldy,S.A.R.(Eds.),PetroleumGeologyofNorthwestEurope:Proceedingsofthe5thConference.GeologicalSociety,London,pp.305e314.Duk-Rodkin,A.,Hughes,O.L.,1994.Tertiary-Quaternarydrainageofthepre-glacialMackenzieBasin.QuaternaryInternational22/23,221e241.Enachescu,Michael,December2004.ConspicuousdeepwatersubmarinemoundsinthenortheaternOrphanBasinandontheOrphanKnoll,offshoreNewfoundland.TheLeadingEdge,1290e1294.Enachescu,Michael,2006a.Favourablegeology,advancedtechnologymayunlockLabrador’ssubstantialresource-HopedaleBasin.OilandGasJournalJune19,2006.Enachescu,Michael,2006b.AtlanticoffLabradorpoisedformodernexplorationround.OilandGasJournal104,36e42.Fowler,M.,Stasiuk,L.,Avery,M.,2005.PotentialpetroleumsystemsintheLabradorandBaffinShelfareas,offshorenortheasternCanada.In:OrganicGeochem-istry:Challengesforthe21stCentury.BookofAbstractsfor22ndInternationalMeetingonOrganicGeochemistry,Seville,Spain,September2005,vol.1,pp.463e464.Funck,Thomas,Jackson,H.,Ruth,Louden,Keith,E.,Klingelhofer,Frauke,2007.Seismicstudyofthetransform-riftedmargininDavisStraitbetweenBaffinIslandandGreenland:whathappenswhenaplumemeetsatransform.JournalofGeophysica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