01 - 1903 Segmentation of an obliquely rifted margin, Campos and Santos basins, southeastern Brazil - 图文

Meislingetal.1919

Figure11.Summarymapofmainfeaturesin?uencingpetroleumsystems,CamposandSantosbasins.salt,outboardoftheTubara?o,Merluza,andMarlim?elds(Figure11).

Thedriftphasethinsoversynrifthighs,andlacus-trinefaciesareinferredtoundergofacieschanges,whichmightimpactlocalsourcequality.EvidenceforfaciesvariationswithinthelacustrinedriftphasecanbefoundinvariabilityoflithofacieswithintheCamposBasinencounteredinwells(Diasetal.,1988).ThisinferenceissupportedbyfacieschangesobservedintheoverlyingAptianevaporitesection,inwhichbrightre?ectionssuggestanhydritedepositiononpresalthighs.

InthesouthernSantosBasin,thereisahighriskthatvolcanicswillbefoundinsubsaltstrata(WilliamsandHubbard,1984),whichareneartheTrista?odaCunhahot-spottrackandAvedisvolcanicchain(Fig-ures6,11).NeartheAvedisvolcanicridge,EarlyCre-taceousvolcanicsareobservedtooverlieandinter-?ngerwithAptianevaporites,andtheymayinter?ngerwithriftanddriftphasestrataaswell.SubsaltMigrationPathways

MostoftheprovenreservesintheCamposBasinarecontainedinLateCretaceous–Miocenedeep-waterturbiditicsandstonereservoirs(Guardadoetal.,1990).Forhydrocarbonstoreachsuprasaltreservoirs,theymustmigratefromriftlacustrinestrataacrosssalt(E.M.Meister,1982,unpublisheddata).Transferzones,whichareassociatedwithobliquerifting,canprovideamechanismforsubsaltfocusingandmigra-tionofhydrocarbonstowindowsatbasesalt(seeCob-boldetal.,2001).

Transferzonescompartmentalizetheriftbasin.Theyaretransverse,structurallycomplexintrarifthighsandmayacttofocusstratalhydrocarbonmigra-tionfromintrariftlows.Cross-stratalhydrocarbonmi-grationmaybeenhancedalongtransferzones,wherefracturingandstructuralcomplexityareassociatedwithdistributedshear,allowinghydrocarbonstoreachbasesalt.EarlyCretaceouscarbonatesofthedriftphasearelocalreservoirtargetsintheCamposBasin(Guardadoetal.,1990).Barremianlacustrinecoqui-nas,associatedwithshoalingonintrarifthighs,mightbepreferentiallydevelopedalongtransferzones,wherefracturingcouldfurtherenhancereservoirquality.Fracturedtransferzonesmayalsoofferpathsforlong-distancesubsalthydrocarbonmigrationintheabsenceofastratigraphiccarrierbedatbasesalt.

Oncehydrocarbonshavereachedbasesalt,amechanismmustbefoundfortraversingit.Figure101920

showsthegeneralregionaldistributionofsalttectonicdomains.Continuoussaltactsasabarriertohydrocar-bonmigration,butnumerouswindowscanberecog-nizedthroughwhichmigrationmightoccur.Windowslocatedabovetransferzonesintheriftsystemwouldbebetterpositionedtoreceivechargeviasubsaltfo-cusingandverticalmigrationpathways.Landwardofthelimitofcontinuoussalt,saltevacuationproducedaweld,enhancingthepotentialformigration.Notethatmajor?eldsintheCamposandSantosbasinslieattheedgeofcontinuoussalt.Inacompanionarticle(Cobboldetal.,2001),wesuggestthattheneotectonicreactivationoftransferzonesmaybeanimportantfactorinfacilitatingsubsaltfocusingandmigrationofhydrocarbonsthroughsaltviafaultedweldsandwindows.

ConstraintsonPetroleumSystemRisk

Figure11summarizesthemainfeaturesofthelacus-trinepetroleumsystem.TheCampos,Santos,andEs-p?′ritoSantobasinscanbesubdividedintoregionaldo-mainsthathavevariableriskforpresenceoflacustrinesourcerockandpotentialformigrationacrosssalt.AlackofsourcelimitsthepetroleumsystemalongtheshorewardhingelineandvolcanicTorresArch;supra-saltvolcanicshinderinterpretationbeneaththeAbrol-hosPlateau;andthicksaltandquestionablesourcelimitpotentialwithinthecompressionalsaltdomainandSa?oPauloPlateau.Theaxisofthecentralriftsys-temhasthegreatestpotentialforpresenceoflacustrinesourcerock.Migrationriskisgreater,however,sea-wardofthelimitofcontinuoussaltthanitiswithinthesaltwelddomain(Cobboldetal.,2001).Totra-versesalt,hydrocarbonsmigratingseawardofthelimitofcontinuoussaltmustpassthroughwindowsatgroundedsynclines.

SUMMARYANDCONCLUSIONS

1.ThepassivemarginofsoutheasternBrazilinitiatedasariftsystemintheNeocomian.Riftingpropa-gatednorthwardintothecentralSouthAtlantic.Riftarchitecturewasstronglyin?uencedbypre-existingfabricandstructuresoftheLateProtero-zoic(Brasilianoorogeny).BetweentheSantosandCamposbasins,theAtlanticmargininheritedaneast-northeast–west-southwestorientation,sorift-ingwasobliquetothemargin,assumingeast-westextension.

ObliquelyRiftedMarginSegmentation(SoutheasternBrazil)

2.OnaregionalmapofBouguer-correctedgravity,anearshorebeltofpositiveanomaliescorrelateswithabroadinterpretedMohoupliftinthefootwallofNeocomianextensionalfaults.

3.Fartheroffshore,asecondbeltofpositivegravityanomaliescorrelateswithapresaltridgeoferodedvolcanicandbasementanticlines,whicharecov-eredbythinAptianevaporites,especiallyanhydrite.Weinterpretthisridgeasafailedspreadingcenter.4.AninterveningbeltofnegativegravityanomaliescoincideswiththemainSantos-Camposriftsystemaxis.

5.Allthreebeltsofgravityanomaliesshowapparentoffsetsalonglinearzones,trendingwest-northwest–east-southest.WeinterprettheseastransferzonesthatdevelopedduringNeocomianrifting.

6.Thevergenceofhalfriftstendstochangeacrosstransferzones,compartmentalizingtheriftedmar-ginintosubbasins.TransferzonesexertedcontrolsonthethicknessesandfaciesoftheNeocomianriftsequence.

7.Attheeasternlimitofcontinentalcrust,transferzonesbecomealignedwithoceanridgetransformfaults,whichtrendeast-westacrosstheSouthAtlantic.

8.TheCampos,Santos,andEsp?′ritoSantobasinscanbesubdividedintoregionaldomainsthathavevari-ableriskforpresenceoflacustrinesourcerock.9.Transferzones,whichareassociatedwithobliquerifting,canprovideamechanismforsubsaltfocus-ingandmigrationofhydrocarbonstowindowsatbasesalt.

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