118quence4istoothintobeseparatedfromsequence3onseismicdata.DefinitionsofSeismicFaciesSeismicfaciesaredistinguishedonthebasisofthefollowingseismicreflectioncharacters:(1)reflectionconfiguration;(2)reflectioncontinuity;(3)reflec-tionamplitudeandfrequency;(4)boundingre-lationships,thatis,types(s)ofreflectiontermina-tionorlateralchange;and(5)externalgeometryofthereflectionpackage(Mitchumetal.,1977;BrownandFisher,1980;Fontaineetal.,1987).IntheCretaceousintervalofinterest,sevenseismicfacieswereidentified(Table2)andmappedonre-gionalsections(Figures8,9).Seismicfaciesinter-pretationintermsofdepositionalenvironmentisdifficultwithseismicalonebecausesedimentarysuccessiondepositsinverydifferentdepositionalenvironmentscanleadtosimilarseismicfacies(Fontaineetal.,1987;Macurda,1997;Eberlietal.,2004).However,combinedwiththegeneralmor-phologyofthesystemandregionalknowledgeofthegeology,cores,andwelllogs,agoodcorrelationbetweenseismicreflectioncharacterandfaciesorlithologycanbeachieved.REGIONALSEISMICARCHITECTURE:PLATFORM-TO-BASINCORRELATIONSTworegionalseismicsectionsfromtheSantaAguedaarea(GoldenLaneplatform)tothePozaRicaba-sinalareawereconstructedusingasmallpartoftwo2-Dseismicprofilestoconnectthetwo3-Dseismicsurveys(Figures8,9).ThesetworegionalprofilesshowthearchitectureoftheGoldenLaneplatformandslopeandPozaRicatoe-of-slopeandbasinde-posits.Severalprominentseismicreflectionscanbetracedfromtheplatformareadowntothetoe-of-slopeandbasinalarea.Onthebasisofwell-logcor-relation,thesereflectionsareinterpretedtocorre-spondtothefollowingformationtops(bottomup):(1)topofthesubstratum,(2)topofthePimientaFormation,(3)topoftheTamabraFormation,(4)topoftheElAbraFormation,(5)topoftheMendezFormation,and(6)topoftheChicontepecForma-SeismicArchitecture,SantaAguedaandPozaRicaFields,Mexico

Figure8.CompositeregionalcrosssectionshowingregionalstratigraphicorganizationfromtheSantaAguedaplatformdowntothePozaRicatoe-of-slopearea.Coloredthicklines=horizonsthatcorrespondtoselectedformationtops.Inclinedyellowlines=interpretedsmallfaults.Redboxes=locationofenlargementsshowninFigure17C,D.LocationofcrosssectionshowninFigure3.TWT=two-waytraveltime.

Figure9.CompositeregionalcrosssectionshowingregionalstratigraphicorganizationfromtheSantaAguedaplatformdowntothePozaRicatoe-of-slopearea.Coloredthicklines=horizonsthatcorrespondtoselectedformationtops.Inclinedyellowlines=interpretedsmallfaults.LocationofcrosssectionshowninFigure3.TWT=two-waytraveltime.

tion.Thesehorizons,exceptforthetopTamabraandtopElAbraformationshavebeenmappedonlyonthesecompositesections.Thesecompositere-gionalseismicsectionsshowplatform-to-basinar-chitecturethroughtheSantaAguedafieldontheGoldenLaneplatformmarginandPozaRicafieldinthebasinalarea.ThemostprominentfeatureonthesetwoseismicsectionsistheGoldenLaneplatformtotheeastanditsassociatedwedgeex-tendingtothewest.

TheGoldenLaneplatformaccumulateddur-ingmostoftheAlbianmostlybyverticalaggrada-tion(ViniegraandCastillo-Tejero,1970;Galicia,2000;Chenetal.,2001).Theplatform-to-basinreliefattheendoftheAlbianisestimatedtobeupto1200m(3937ft)(giventhe600-mstwo-waytraveltime[TWT]ofreliefusinga4000-m/sav-eragevelocitywithnocorrectionforcompaction).Figure10showsa3-DviewoftheElAbraFor-mationhorizonandthetime-equivalentTamabraFormationtophorizon(light-bluesurface)overly-ingthetopPimientaFormationhorizon(purplesurface)andthebasementhorstblock(redsur-face).TheGoldenLaneplatformconsistsofathicksuccessionofmostlychaoticcontortedreflectionscappedbyseveraldiscontinuous,mounded,high-amplitudereflections.ThebaseoftheCretaceousplatformisrepresentedbyasemicontinuoushigh-amplitudereflectionthatisinterpretedtocorrelatewiththetopoftheJurassicPimientaFormationinthebasinalarea.Inclinedhigh-amplitudesemicon-tinuousreflectionsdippingtowardthenortheastrepresentthetransitionbetweenthetopographicrimoftheGoldenLaneplatformintotheplatforminterior.ThereliefbetweentheGoldenLanerimandtheplatforminteriorismorethan50ms,equatingto75m(246ft)ormore,dependingontheacousticvelocityofthesecarbonatesediments.Givencurrentdata,whetherthisreliefrepresentsatruetransitionfromanelevatedcarbonatemargintoadeeperplatforminteriororwhetheritissimplytheresultofdifferentialcompactionbetweenthece-mentedmarginandthemuddierplatforminteriorisunknown.

ThewestwardslopeoftheplatformintotheMislanta–ChicontepecBasinisrepresentedonseis-micdataby(1)onlappingreflections;(2)discon-

120tinuousinclinedreflections;or(3)(occasionally)continuous,inclined,high-amplitudereflections.Thesereflections,whichcanbemappedusingthePozaRica3-Dsurvey,correspondtothetopTamabraFormation,whichwasinterpretedfromthewelltie.TheintervalbetweenthePimientaandtopElAbra–TamabrahorizoncomprisestheAlbiansectionthatinthebasinincludesboththeTamabraFormation(redepositedcarbonatesediment)andtheupperTamaulipasFormation(pelagicsediment).ThebasementhorstunderPozaRicafieldisseismicallyimagedbyachaoticlow-amplitudere-flectioncappedbyasemicontinuoushigh-amplitudereflectionthatbecomesdifficulttotraceaccuratelybelow2s(TWT).ViniegraandCastillo-Tejero(1970)locatedanotherbasementhorstbelowtheGoldenLaneplatformonthebasisofasinglecore,butitcannotbeclearlyseenonavailableseismicdata.BetweenthebasementhighunderPozaRicafieldandtheoneunderlyingtheGoldenLaneplatform,thetopographiclowisfilledwithJurassicsedi-ment(Salas,1949;ViniegraandCastillo-Tejero,1970;Enos,1977)thatcorrespondstosemicontin-uousmoderate-tohigh-amplitudereflectionsinregionalcompositeseismicsections(belowthebluehorizoninFigures8,9).Thesereflectionsseemtoonlaponthebasementhigh.OverlyingtheAlbianinterval,severalhigh-amplitudecontinuousreflectionsinthebasinalarearepresenttheUpperCretaceousAguaNueva,SanFelipe,andMendezformations,whichconsistofpelagicchalkandshalydeposits.Aseriesofeastward-inclined,semicontinuous,low-amplitudereflec-tionswithnumeroustruncationgeometriesrep-resenttheeastwardprogradationofTertiarysilici-clasticsedimentsassociatedwiththeforelandbasinfilloftheSierraMadreOriental(Soegaardetal.,2003).Thehigh-amplitudecontinuousreflection,easilyrecognizedonseismicdata(yellowhorizoninFigures8,9),correlatestothetopoftheChicon-tepecFormationinwell-logandcoredata(Figure6).TheseTertiaryreflectionsonlapanddrapeoverre-liefoftheGoldenLaneplatform.DetailedseismicarchitectureoftheGoldenLaneplatformintheSantaAguedafieldareaandtoe-of-slopeandbasindepositsinthePozaRicafieldareawillbedescribedinthenextsections.SeismicArchitecture,SantaAguedaandPozaRicaFields,Mexico

Figure11.(Top)Uninter-

pretedseismicsectionthrough

theGoldenLaneplatform.(Mid-

dle)Interpretedseismicsection.

Purpleline=topofElAbraFor-

mation.Blueline=topofPi-

mientaFormation.Dashedgreen

line=intersectionwithamplitude

timeslicedisplayedinFigures13,

15.Theyellowdashedlines

representtheinterpretedtopof

theplatformbelowtheElAbra

horizon.Redbox=locationof

enlargementpresentedbelow.

(Bottom)Enlargedviewofseis-

micexpressionofthetopofthe

ElAbraFormation.Thisirregular

surfaceshowsreliefupto200m

(656ft)(usinga400-m/sseis-

micvelocity),whichisinterpreted

asapost-Albiankarstfeature

ontheplatformtop.TWT=two-

waytraveltime.

DETAILEDSEISMICARCHITECTURE

SantaAguedaPlatform

PlatformTop

Becauseofthehigh-impedancecontrastbetween

platformcarbonatesandoverlyingTertiarysilici-

clasticdeposits,fewifanyreflectionscanbecarried

fromtheplatforminteriortotheslope.Theplat-

formtopshowshigh-amplitude,mounded,semi-

continuousreflectionswithabundantdiffraction

hyperbolas(seismicfacies7inTable2)(Figure11).

Theruggedtopographyimagedbythe3-Dseis-micsurveysismostlikelyproducedbyintensivekarstificationoftheGoldenLaneplatformtop.KarstificationofthetopoftheGoldenLaneplat-formhasbeenknownaboutsincediscoveryoftheGoldenLanemarginreservoirs(Salas,1949).Inaddition,coreobservationsinSantaAguedafieldbytheauthorsrevealarangeofkarst-relateddeposits,suchaslaminatedterrigenoussedimentinterpretedascavefillandarangeofcarbonatebrecciafromcrackletomosaictochaoticinter-pretedascavebreccia(ViniegraandCastillo-Tejero,1970;Cooganetal.,1972;Jansonetal.,

2004).

Jansonetal.121