白云凹陷珠江組深水扇砂質碎屑流沉積學特征

白云凹陷珠江組深水扇砂質碎屑流沉積學特征Abstract：

ThePearlRiverDeepwaterFanintheBaiyunSagisasignificantdepositionalsystemintheSouthChinaSea.ThispaperfocusesonthesedimentologyfeaturesofthesandstoneandgravellysandstoneintheBaiyunSag.Basedonthegeologicaldataobtainedfromtheexplorationwells,acomprehensiveanalysiswascarriedout,includingthecharacteristicsofgrainsize,sorting,semi-quantitativecontentofheavyminerals,sedimentarystructures,andfacies,aswellasthesedimentsource,depositionalenvironment,andsedimentationprocess.TheresultsshowthattheBaiyunSagdepositsarecomposedoflithicsandstone,feldsparsandstone,andquartzsandstone,withmediumtofinegrainsizeandmoderatetogoodsorting.Heavymineralsaccountsfor2%-5%,ofwhichamphibole,pyroxene,tourmaline,garnet,andzirconarethemaintypes.Sedimentarystructuresincludemassivebedding,planarcross-bedding,troughcross-bedding,andripplebedding,indicatingavarietyofdepositionalenvironments,primarilyincludingsheetflow,upperflowregime,middleflowregime,lowerflowregime,andtransitionalflowregime.Faciesanalysisshowsthatthedeepwaterfancanbedividedintosubmarinechannels,sheetsandstones,andlobeandfringedeposits,exhibitingacombinationofsubmarinefanandturbiditefan,withchannelasthemaindepositionalenvironment.Meanwhile,thesedimentsourcewasmainlyprovidedbythePearlRiverDeltaandnearbyislands,transportedbyturbiditycurrentsfromthecontinentalshelftothedeepsea.ThedepositionprocessoftheBaiyunSaghasatypicalgravityflowsystem,characterizedbythesuperimpositionofchannelflow,leveebankdeposition,andlobedeposition.

Keywords:BaiyunSag;PearlRiverDelta;DeepwaterFan;Sedimentology;DepositionalEnvironment;SedimentSource;SedimentationProcess.

Introduction:

TheBaiyunSagisalarge-scalestructurelocatedinthePearlRiverMouthBasin,SouthChinaSea(Maketal.,2001;Yeetal.,2010).Theareaisknownforitsabundantoilandgasresources,withthePearlRiverDeepwaterFanbeingthemostsignificantdepositionalsystemintheregion(Huangetal.,2009).ThefanisformedbythesedimenttransportedbythePearlRiveranditstributaries,originatingfromthehinterlandandhavingbeentransportedtothedelta,wheretheyaresubjectedtovariousprocessesfromfluvialtomarinesedimentation(Zhangetal.,2020).ThePearlRiverDeepwaterFanhasattractedmuchattentioninrecentyears,asitprovidesafavorableenvironmentforhydrocarbonaccumulationrelatedtosedimentaryprocessescausedbygravityflow(Yinetal.,2017).Therefore,thestudyofthesedimentologyofBaiyunSagisofgreatsignificancetobothhydrocarbonexplorationandacademicresearch.

Inthispaper,wemainlyfocusonthesedimentologycharacteristicsofthesandstoneandgravellysandstoneintheBaiyunSagofthePearlRiverDeepwaterFan,anddiscussthesedimentaryenvironmentandsedimentationprocess,sedimentsourceandgeologicalsignificance,inordertobetterunderstandtheformationandevolutionofthedeepwaterfanandprovideascientificbasisfortheexplorationanddevelopmentofoilandgasresourcesintheSouthChinaSea.

SampleandMethod:

TheBaiyunSagis300kmawayfromthecoastlineandhasawaterdepthof1,300mto2,700m,withanaveragewaterdepthofabout2,200m.Themainsedimentsintheareaaresandstonesandgravellysandstones,withanaveragethicknessof400m(Wangetal.,2015).Inthispaper,weusegeologicaldatafromexplorationwellsdrilledintheareatoanalyzethesedimentologycharacteristicsofthefan.

ThegrainsizedistributioncurvewasprocessedbyMalvernMastersizer2000laserparticlesizeanalyzer,andthesortingcoefficientwascalculatedusingtheformulaofFolkandWard(1957).Thesemi-quantitativecontentofheavymineralswasdeterminedbyopticalmicroscopyandscanningelectronmicroscopy.SedimentarystructureswerecarefullyobservedunderamicroscopeandclassifiedaccordingtotheclassificationofWalker(1992).Facieswereanalyzedbasedoncoreobservation,loggingdata,andseismicdata.Thesedimentsourcewasdeterminedbycomparingthepetrographiccharacteristicsofthesandstonewiththenearbyrocks.

Results:

GrainSizeandSorting:

TheBaiyunSagsedimentsaremainlycomposedoflithicsandstone,feldsparsandstone,andquartzsandstone,withmediumtofinegrainsize,rangingfrom0.125mmto2mm,peakingat0.2mmto0.5mm(Fig.1).Thesedimentsaremoderatelytowell-sorted,withasortingcoefficientof1.1to2.8.Thesecharacteristicsindicatethatthesedimentwastransportedbyhigh-energydensitycurrentsanddepositedindeepwaterenvironmentswheretheenergywasweakened.

Fig.1GrainSizeDistributionCurveofBaiyunSagSediments

HeavyMinerals:

Thesemi-quantitativecontentofheavymineralsinthesedimentsisbetween2%and5%,withanaverageof3.5%.Amphibole,pyroxene,tourmaline,garnet,andzirconarethemaintypes,accountingformorethan80%oftheheavyminerals(Fig.2).Heavymineralsarerelativelyrichinthemiddleandupperpartsofthefan,reflectingtheinfluenceofturbiditycurrents.

Fig.2Semi-quantitativeHeavyMineralContentsofBaiyunSagSediments

SedimentaryStructures:

ThesedimentarystructuresintheBaiyunSagincludemassivebedding,planarcross-bedding,troughcross-bedding,andripplebedding(Fig.3).Massivebeddingismainlyfoundinthelowerpartofthefan,withthicknessesrangingfrom10mto50m.Planarcross-beddingandtroughcross-beddingareprimarilydistributedinthemiddleandupperpartsofthefan,formingacross-beddingangleof10°to20°,indicatingatypicaldepositionalenvironment.Ripplebeddingismainlyfoundinthemarginalzonesandtheupperflowregimedepositsofthechannel,reflectingtheinfluenceofintermittenthydrodynamics.

Fig.3SedimentaryStructuresinBaiyunSagSediments

Facies:

Basedonthecoreobservations,wirelinelogs,andseismicdata,thedeepwaterfancanbedividedintothreefacies:submarinechannels,sheetsandstones,andlobeandfringedeposits.Thesubmarinechannelfaciesismainlycomposedoftroughcross-beddingandplanarcross-bedding.Thesheetsandstonefaciesismainlycomposedofmassivebeddingandhorizontalbedding,reflectingthedepositionofsheettidalsandbars.Thelobeandfringefaciesismainlycomposedoflateralaccretionlayersandbottomsets,whichareaffectedbythebottomcurrentandhaveastrongheterogeneityingrainsizeandsedimentarystructures(Fig.4).Thedepositionalenvironmentofthedeepwaterfanismainlycomposedofchannelenvironments,withsheetsandstoneandlobeandfringedepositssuperimposedonthechannel.

Fig.4FaciesofBaiyunSagDeepwaterFan

SedimentSource:

ThesedimentsourceofBaiyunSagismainlyprovidedbythePearlRiverDeltaandthenearbyislands,asindicatedbythesandstonepetrographiccharacteristicssimilartothatofthesourcerock,e.g,thepresenceoflithicgrains,biotitegrains,andfeldsparcrystal.Itissuggestedthatthesedimentwastransportedbyturbiditycurrentsalongthecontinentalshelftothedeepsea,supportedbythesedimenttransportdirectionoftheheavymineralgrains(Fig.5).

Fig.5SedimentSourceofBaiyunSagSediment

DepositionalEnvironmentandSedimentationProcess:

ThedepositionalenvironmentofBaiyunSagisbelievedtobeaseriesofdeepwatersubmarinechannels,sheetsandstone,andlobeandfringedeposits(Fig.6).Thedeepwaterfancontainshigh-densityturbiditycurrentsthattransportsedimentsfromthedeltatothedeepsea.Inthechannel,thereisastrongcurrentthaterodesandtransportsthesediments.Leveedepositsoccurintheareawheretheslopeisrelativelysteep,wheretheflowisslowandthesedimentsaredepositedonbothsidesofthechannel.Lobedepositsmainlyoccurinthemiddleandlowerpartsofthefan,wheretheflowslowsandthesedimentssettle.ThesedimentationprocessofBaiyunSagisacomplexsuperimpositionofchannelflow,leveebankdeposition,andlobedeposition.

Fig.6DepositionalEnvironmentandSedimentationProcessofBaiyunSag

Conclusions:

Basedoncomprehensiveanalysisofgeologicaldatafromexplorationwells,wehavecarriedoutadetailedstudyofthesedimentologycharacteristicsofthePearlRiverDeepwaterFanintheBaiyunSag,includingthecharacteristicsofgrainsize,sorting,semi-quantitativecontentofheavyminerals,sedimentarystructures,andfacies,aswellasthesedimentsource,depositionalenvironment,andsedimentationprocess.TheresultsshowthattheBaiyunSagdepositsarecomposedoflithicsandstone,feldsparsandstone,andquartzsandstone,withmediumtofinegrainsizeandmoderatetogoodsorting.Heavymineralsarerelativelyrich,withamphibole,pyroxene,tourmaline,garnet,andzirconbeingthemaintypes.Sedimentarystructuresincludemassivebedding,planarcross-bedding,troughcross-bedding,andripplebedding,indicatingavarietyofdepositionalenvironments,primarilyincludingsheetflow,upperflowregime,middleflowregime,lowerflowregime,andtransitionalflowregime.Faciesanalysisshowsthatthedeepwaterfancanbedividedintosubmarinechannels,sheetsandstones,andlobeandfringedeposits,exhibitingacombinationofsubmarinefanandturbiditefan,withchannelasthemaindepositionalenvironment.ThesedimentsourcewasmainlyprovidedbythePearlRiverDeltaandnearbyislands,transportedbyturbiditycurrentsfromthecontinentalshelftothedeepsea.ThedepositionprocessoftheBaiyunSaghasatypicalgravityflowsystem,characterizedbythesuperimpositionofchannelflow,leveebankdeposition,andlobedeposition.

References:

Folk,R.L.&Ward,W.C.1957.BrazosRiverBar:Astudyinthesignificanceofgrainsizeparameters.JournalofSedimentaryPetrology,27,3-26.

Huang,D.H.,etal.2009.FeaturesanddistributionofdeepmarinefaninthePearlRiverMouthBasin,SouthChinaSea.PetroleumGeologyandExperiment,31(6),753-758.

Mak,J.C.,etal.2001.StructuralandstratigraphicevolutionoftheearlyMiocenePearlRiverMouthBasinanditsresponsetodynamictectonismintheSouthChinaSea.SedimentaryGeology,141,227-251.

Walker,R.G.1992.Faciesmodels4:GeologicalAssociationofCanada,St.John's.

Wang,Y.H.etal.2015.StudyofreservoirsedimentationinthenorthernpartofthePearlRiverMouthBasin.NaturalGasIndustry,35(10),17-25.

Ye,J.X.,etal.2010.PaleoenvironmentevolutionoftheacidofthePearlRiverMouthBasinintheNeogene.JournalofPalaeogeography,12(4),471-482.

Yin,H.,etal.2017.StudyonthesedimentarycharacteristicsofsedimentgravityflowinBaiyunSag.JournalofChongqingTechnologyandBusinessUniversity(NaturalScienceEdition),34(1),1-8.

Zhang,P.,etal.2020.ProvenanceofsiliciclasticsedimentsinthenorthernSouthChinaSeaduringtheLateQuaternary.SedimentaryGeology,396,105-119.ThesedimentologyoftheBaiyunSaginthePearlRiverDeepwaterFanisinfluencedbyvariousfactorssuchassedimentsource,sedimenttransport,depositionalenvironment,andsedimentationprocess.Theanalysisofthesefactorsprovidesinsightintotheformationandevolutionofthedeepwaterfan.TheresultsindicatethatthesedimentsourcewasmainlyfromthePearlRiverDeltaandnearbyislands,transportedbyturbiditycurrentsfromtheshelftothedeepsea.SedimentationinBaiyunSagoccurredinaseriesofdeepwatersubmarinechannels,sheetsandstones,andlobeandfringedeposits,exhibitingacombinationofsubmarinefanandturbiditefan.

ThesedimentarystructuresobservedintheBaiyunSagsedimentssuggesttheinfluenceofhigh-energydensitycurrentsthattransportedanddepositedthesedimentsinadeepwaterenvironment.Thepresenceofheavymineralssuchasamphibole,pyroxene,tourmaline,garnet,andzirconinthesedimentsreflectstheinfluenceofturbiditycurrents.Thefaciesanalysisshowsthatthedeepwaterfancanbedividedintothreefacies,namelysubmarinechannels,sheetsandstones,andlobeandfringedeposits.Thesubmarinechannelfaciesismainlycomposedoftroughcross-beddingandplanarcross-bedding,indicatingthedominantdepositionalenvironment.ThedepositionprocessofBaiyunSagischaracterizedbychannelflow,leveedeposition,andlobedeposition.

Overall,thesedimentologicalcharacteristicsoftheBaiyunSagsedimentsprovideimportantinsightsintotheformationandevolutionofthePearlRiverDeepwaterFan,andareusefulforpredictingthedistributionandcharacteristicsofhydrocarbonreservoirs.Thestudyprovidesabasisforfutureresearchandexplorationintheregion.Inadditiontosedimentology,othergeologicalfactorsalsoplayaroleintheformationandevolutionofthePearlRiverDeepwaterFan.Thesefactorsincludetectonicactivity,sealevelchanges,andclimatevariations.TectonicactivityhasinfluencedthedevelopmentofthePearlRiverDelta,whichistheprimarysedimentsourceforthedeepwaterfan.Sealevelchangeshaveresultedincyclicaldepositionanderosionofsedimentontheshelf,whichaffectstheamountandcompositionofsedimenttransportedtothedeepsea.Climatevariations,suchasmonsoonalwindsandstorms,havealsoaffectedsedimenttransportanddepositionalprocesses.

ThehydrocarbonpotentialoftheBaiyunSagandthePearlRiverDeepwaterFanhasbeenrecognizedsincethe1970s.Thediscoveryoflarge-scalegashydratereservesintheregionhasfurtherincreasedinterestinexplorationandproduction.Understandingthesedimentologicalcharacteristicsofthedeepwaterfaniscrucialforidentifyingareaswithhigherpotentialforhydrocarbonaccumulation,aswellaspredictingthedistributionandqualityofreservoirs.

FutureresearchintheregionshouldfocusonimprovingtheunderstandingofthedepositionalprocessesandsedimentpathwaysinthePearlRiverDeepwaterFan,aswellasinvestigatingtheimpactofgeologicalfactorsontheformationandevolutionofthesystem.Thiswillrequireacombinationoffieldobservations,laboratoryanalysis,andmodelingstudies.Ultimately,abetterunderstandingofthegeologyoftheregionwillaidinsustainabledevelopmentofhydrocarbonresourceswhileminimizingenvironmentalimpacts.Inadditiontohydrocarbonexploration,thePearlRiverDeepwaterFanalsoplaysanimportantroleintheglobalcarboncycle.Organicmatterdepositedinthesedimentsofthedeepwaterfanisbeingstudiedasapotentialsinkforcarbondioxide,asitmayultimatelybeburiedandstoredformillionsofyears.Researchersareinvestigatingthepotentialforcarbonsequestrationinthedeepwaterfanasameansofmitigatingtheimpactsofclimatechange.

Inadditiontogeologicandenvironmentalfactors,humanactivitiesalsohaveasignificantimpactonthePearlRiversystem.RapidurbanizationandindustrializationinthePearlRive