theSeismicArchitecture,SantaAguedaandPozaRicaFields,Mexico

Figure4.CoredescriptionofwellA.FromLoucksetal.,inpress.

styleofcarbonateslopetobasin-floordeposition.

Duringlatetransgressivesystemstract(TST)and

earlyhighstandsystemstract(HST)deposition,

platformsbuildsteepmarginsthatbecomeunstable

andshedthebulkofmaterialthatformedthe

megabrecciacomplexes.Inaddition,platform–marginfailureatthistimesetsupacorrugatedplatformmarginandtendstochannelizedebrisinthebasin,yieldingahighlylaterallyheterogeneousreservoirstratigraphy.Duringtheremainderofhighstanddeposition,decreasedaccommodationleadstorapidsheddingofcarbonatesandsalong

the

Jansonetal.113

lengthofthemargin,producingamorehomoge-neousstrike-continuousblanketofgrain-dominatedturbidites.Giventhissimplemodel,theoverallstrat-igraphicarchitectureofPozaRicadepositscanbesubdividedintofoursequencesthatfitremarkablywellwiththestratigraphicarchitecturerecognizedelsewhereinMexicoandsouthTexasfortime-equivalentcarbonatedeposits,describedindetailbyLoucksetal.(inpress)(Figure2).

Loucksetal.(inpress)interpretedthepartoftheTamaulipasthatoccursatthebaseoftheTa-mabraFormationastimeequivalentwithgulfwidetransgressionduringdepositionoftheAptianLaPenaFormationelsewhereanddepositionofretro-gradingcarbonaterampssuchastheCupidoinnorthernMexicoandtheAptianCowCreekandPearsallFormationinTexas(Goldhammer,1999).Itthereforemustcorrespondtothemaximumfloodingsurface(MFS)oftheirAptian–Albian1sequence.

TheearlyAlbianwasaperioddominatedre-gionallybyverticalaggradationofcarbonateplat-formsoftheGoldenLane,Valles,Coahuila,andStuartCity–Comanche.Theverticalaggradationintervalismostpronetodevelopmentofover-steepenedmarginsandmostlikelywassynchro-nouswithdepositionofabundantmegabrecciasinthebasalintervaloftheTamabraFormationatPozaRica(Figure4).Loucksetal.(inpress)equatedthisepisodeofplatformdevelopmentwiththelatestTSTtoHSToftheAptian–Albian1sequence.Thesecondcompositesequence,Albian2,isrepresentedbyaninitiallowstandsystemstract(LST)phaseofslowsedimentationfollowedbytransgression.TheAlbianupperGlenRoseinTexasandpartofthemiddleAlbianAcatitaintheCoahuilaareareflectsaperiodofstrongprogradation(HST)intheAlbianStuartCitymargin,whichislikelytobecoevalwiththefirstperiodofmajorgrainstone-dominatedhyperconcentratedtoconcentratedandturbiditedepositionintheTamabra(Figure4).

ThenextwidespreadeventinnorthernMex-icoandTexasistheAlbian3compositesequence,whichreflectsaregionalchangeinthestyleoftheStuartCitymarginfromprogradingtoaggradingandcorrespondstotheLehmannetal.(1998)Co3sequenceinnorthernMexico.Thenextdebris-

114flowintervalthatoverliesthefirstAlbian2HSTgrainstoneinterval(Figure4)wasinterpretedbyLoucksetal.(inpress)torepresentearlyTSTde-positionassociatedwithshelfandshelf-marginaggradation.Thefollowinggrain-dominatedhy-perconcentratedtoconcentrateddepositsandtur-biditedepositsaretheninterpretedastheAlbian3HST.TheterminalAlbiansequence(Albian4)hasagreen-graydeep-watershale(atadepthof2012m[6601ft]inFigure4)atthebasethatisin-terpretedasanLSTcorrespondingtoaGOM-wideevent(McFarlanandMenes,1991;Goldhammer,1999;KeransandLoucks,2002).TheupperpartoftheAlbian4sequencerepresentsaperiodofret-rogradationthatcarriesthroughintotheCenoma-nian.AcrossTexas,intrashelfbasinsformabovetheshallow-waterplatformtopoftheAlbian3se-quence.InthePozaRicaarea,thissequenceequateswithdepositionofanintervalthatrecordscon-tracteddepositionofreefalbrecciasandrudistde-brisinanapronclosetotheshelfmargin(Loucksetal.,inpress).DATAANDMETHODSDataUsedSeismicanalysisisbasedon425km2(164mi2)of3-Dseismicdataand50km(31mi)of2-Dseismicdata.Two3-Dseismicsurveys(Figures1,3)coverthedeep-waterPozaRicafieldandtheadjacentGoldenLaneplatform,aswellasSantaAguedafield.Inaddition,two2-Dseismiclinesthatconnectthetwo3-Dblockswereusedtobuildregionalseismicsections(Figure3).SeismicdataqualityisoverallfairlygoodinthePozaRicadataset.IntheSantaAguedadataset,ruggedtopographyatthetopoftheplatformhasinducedseverediffractionsoftheincomingseis-micwaveenergythathavenotbeenentirelyre-movedbythemigrationprocess.Inaddition,be-causeofthestrongimpedancecontrastbetweenthemostlysiliciclasticTertiaryformationsandtheunderlyingCretaceouscarbonateplatform,mostoftheincomingacousticenergyhasbeenreflectedbacktothesurface,andonlyasmallpartofthisSeismicArchitecture,SantaAguedaandPozaRicaFields,Mexico

Figure5.ExamplesofseismiccharacteracrosswestwardmarginoftheGoldenLaneplatformintheSantaAguedafieldarea.(A,B)Faintsteeplyinclinedreflectionsuggestsasteep-sidedmarginwithbypassslope.InC,someoftheslopereflectionsareinterfingeringwiththeplatforminterior.Foreachofthethreeseismicsections,thedashedlineindicatestheinterpretedtopofthecarbonateinterval.TWT=two-way

traveltime.

Figure6.Verticalseismicprofile(VSP)dataatwellXoverlainonseismicdata.MatchbetweenVSPandseismicdataisgenerallygoodexceptwithinthereservoirintervalwhereVSPdatashowoneadditionalreflectioncomparedwiththatof3-Dseismicdata.SeeFigure3forlocationofseismiclinesandwellX.MFS=maximumfloodingsurface;TWT=two-waytraveltime.

Jansonetal.115

Figure7.Verticalseismicsectionillustratingagoodmatchbetweenwell-log-basedformationtopsandcorrespondingseismichorizonsinthedistalpartofPozaRicafield.MFS=maximumfloodingsurface;TWT=two-waytraveltime.

acousticenergyhasbeenrefractedintotheunder-

lyingplatform.Asaresult,onlythelow-frequency

high-energypartoftheplatforminteriorseismic

responseisvisibleintheseismicprofile.Thehigher

frequencypartoftheseismicsignalissocontam-

inatedbydestructivediffractionthatitisindistin-

guishablefromnoiseand,therefore,unavailablefor

theinterpretationprocess.Inaddition,seismicex-

pressionofthewestwardslopeoftheGoldenLane

platformisnotconstantthroughoutthedataset.

Insomelines,theedgeoftheplatformconsistsofa

steeplyinclined,faint,low-amplitudereflectionthat

separatestheplatforminteriorfromonlappingre-

flectionsagainsttheslope(Figure5).Inotherlines,

upperslopereflectionsappeartointerfingerwith

platforminteriorreflections.Giventheclassicin-

terpretationthattheGoldenLaneplatformisan

isolatedplatform(Galicia,2000;Chenetal.,2001),

asteepplatformmarginwitherosionalandbypass

slopeshouldbeexpected.Consequently,theplat-

116formedgeisalsosometimesdifficulttointerpretontheseismicprofile.Well-to-SeismicCalibrationWell-to-seismiccalibrationwasperformedusingoneverticalseismicprofile(Figure6)andaddi-tionalcheckshotsinseveralwellsandbygener-atingalimitedsetofsyntheticseismicprofilesofwellswithdensityandsonicwelllogs.However,seismic–welltiesarenotoptimalbecauserelativelyfewwellswithtime-depthconversioninformationareavailableascomparedwiththetotalnumberofwellsinthefield(>800).Inaddition,aconcen-trationofwellswithreliabledepth-timeconversionlocatedontopofthestructuralhighexists,wherethemainhydrocarbonproductioninPozaRicafieldislocated.Asaresult,agoodtieispresentbetweenthewell-log–pickedtopandthe

seismicSeismicArchitecture,SantaAguedaandPozaRicaFields,Mexico

datainthewestpartofthefieldthatlieinthehigherpartofthestructure(Figure6).

MappedHorizons

Thefollowinghorizonsweremappedthroughoutthe3-Dseismicdatainthebasinarea(e.g.,PozaRicafield)(Figure7):topAlbian4sequence(topTamabra),topAlbian2sequence(topBC),andtopAlbian2MFS(topF).Inaddition,areflectionin-terpretedasthetopoftheupperTamaulipaswaspickedwithmoderateconfidence.ThechangebetweentheTamabraFormationandtheupperTamaulipasFormationisalateralfacieschangefromgravity-flowdominatedtopelagicdominatedanddoesnotcorrespondtoatimeline.Itdoes,however,correspondtoaclearimpedancecontrast,creatingamappablediachronousseismicreflectionthrough-outPozaRica3-Dseismicdata.AccordingtoEnos(1977)regionalcrosssection,lateralpinch-outoflimestone-dominatedredepositedsedimentoftheTamabraFormationintopelagicsedimentoftheupperTamaulipasisoutsidetheareacoveredby3-Dseismicsurveys.Loucksetal.(inpress)showedthattheTamabraisstillmorethan130m(492ft)thickinawellonly1.5km(0.9mi)landwardofthedistaledgeofthe3-Dseismicdata.ThetopAlbian1sequence(topTamabra)isawell-markedpositivereflectionthatcanbemappedovertheentirePozaRica3-Ddataset.ThetopAlbian2MFS(topF)andthetopAlbian2sequence(topBC)arebothmappedatazerocrossing.TopAlbian2(topBC)isapositive-to-negativezerocrossing,whereastopAlbian2MFS(topF)correspondstoanegative-to-positivezerocrossing.Thisrelationshipappearstoberelativelyconsistentthroughoutthestud-iedarea.TheupperTamaulipashorizonisalsoanegative-to-positivezero-crossingreflection.IthasbeenmappedthroughoutthePozaRica3-Dseis-micdataset,althoughsomedifficultyinmappingwithconfidenceispresentinthenortheasternpartofthedatasetclosetotheplatformtoeofslope,whereseismicreflectionsbecomemoreandmorecontortedanddiscontinuous,affectingcontinuityoftopTamabraFormationreflections.ThetopAlbian3surface(topD)andtheMFSoftheAl-bian4sequencewerenotmappedbecausese-