Amir-ea-05-modelling-Paris-Basin

www.elsevier.com/locate/tecto

Effectofthethermalgradientvariationthroughgeologicaltimeonbasinmodeling;acasestudy:TheParisbasin

L.Amira,L.Martineza,T,J.R.Disnarb,J.-L.Vigneressea,R.Michelsa,T,

F.Guillocheauc,C.RobindbG2R-UMR7566/CNRS,Universite´HenriPoincare´,Nancy,France

ISTO,UMRCNRS6113,Ge´osciences,Universite´dTOrle´ans,BP6759,45067Orle´anscedex2,France

cUPR-4661,Ge´osciencesRennes,Universite´deRennesI,France

dESA-7073,Pale´ontologieetstratigraphie,Universite´PierreetMarieCurie,Paris,France

Received30January2004;accepted1March2005

Availableonline9April2005

aAbstract

ManystudiesinvestigatedthethermalmodelingoftheParisbasinforpetroleuminterestsduringthe1970s.Mostofthesoftwaresdevelopedbyoilcompaniesorresearchcenterswerebasedontheassumptionofaconstantthermalgradient.Inordertotakeintoconsiderationthevariationofthethermalgradientduringbasinevolution,wedevelopedtheTherMO’sVisualBasic1Dprogram.Weappliedourmodeltotwentyboreholeslocatedalongacross-sectionroughlyrunningEWover150kminthecenteroftheParisbasin.Thenumericalresultswerecalibratedwithorganicmattermaturitydata.TherMO’ssimulatestheamountofheatprovidedtothesedimentaryorganicmatter.Theheatparametersimulatedshowslateralvariationalongthecross-section.ItdecreasesfromRambouillettoTrouAuxLoupsboreholes(87–66mW/m2)atabout100kmmoretotheeastwhereastheheatfluxvaluesimulatedis73mW/m2inSt.Loupborehole.Themeanthermalgradientcalculatedforliassichorizonsat87MyfortheRambouilletwellis50.48C/km.Thisvalueissimilartopreviouslypublishedresults.Byintegratingthecalculationofthethermalgradientsandconductivitiesrelatedtotheburialofeachstratigraphicsequence,ourapproachpointsoutvariationsinthethermalregimesthesedimentaryorganicmatter(SOM)hasbeensubjectedtothroughgeologicaltime.

D2005ElsevierB.V.Allrightsreserved.

Keywords:Parisbasin;Thermalmodeling;Sequencestratigraphy;Sedimentaryorganicmatter;Basinalheatflux

1.Introduction

Thermalmodelinginsedimentarybasinshelpsdeterminingthelocationofoiland/orgasgenerativeformations,andadditionallythetimingofhydro-carbongeneration(YuklerandKokesh,1984).In

TCorrespondingauthors.

E-mailaddresses:luis.martinez@g2r.uhp-nancy.fr

(L.Martinez),raymond.michels@g2r.uhp-nancy.fr(R.Michels).0040-1951/$-seefrontmatterD2005ElsevierB.V.Allrightsreserved.doi:10.1016/j.tecto.2005.03.004

228L.Amiretal./Tectonophysics400(2005)227–240

additiontothequalityoftheinputdata,thereliabilityoftheoutputinformationalsodependsonthemodelingresolutionscale.

TheParisbasinhasforlongbeenwidelystudiedbytheoilindustryandacademicresearchbothasaneffectiveexplorationtargetandaconvenientareaforcheckingnewapproaches.Asamatteroffact,themodelingofitsthermal(Espitalieetal.,1987)andsubsidencehistory(BrunetandLePichon,1982)gaveinformationonpetroleumgeneratedfromthelowerToarcianblackshaleswellknownassourcerocksforoil(PouletandEspitalie,1987;Bessereauetal.,1995;Disnaretal.,1996a,b).Takingintoaccountpresentdaythermalgradientsandpresentdaythermalconductivities,modelingapproachesprovidedthermaldatasuchasheatfluxvaluesandpalaeotemperatures.

A

50 kmNCHANNELARDENNESMASSIFMELINARMORICANMASSIFVOSGESMORVANCENTRAL MASSIFTertiaryUpper CretaceousLower CretaceousMalmDoggerLiasTriasBasementRambouilletWellZoom on the studied sectorFig.1.(A)GeologicalmapoftheParisBasin(fromDemars,1994).(B)Zoomonthestudiedsection.LettersAtoTrepresentthestudiedboreholes.DottedlinescorrespondtofaultsidentifiedintheParisBasin.(FS=bSeinefaultQ;FB=bBrayFaultQ;AC=bChaunoyAccidentQ;Fsy=bSennelyFaultQ;FSMB=bStMartindeBossenayFaultQ).

L.Amiretal./Tectonophysics400(2005)227–240229

B

NFBFSREIMSPARISMEAUXTA, BC,DGE,FCHARTRESHJKIFSyMELUNACSLNMO, PQ, RFSMB50 kmTROYESFig.1(continued).

InsuchawayGaulierandBurrus(1994)reportedpresentdaythermalgradientsvaluesaround55–658C/kmforLiassicseries.TheyalsonoticedthatthosevalueswerehigherthanthoseusedtodescribethethermicityoftheDoggerformationthatispresentlycharacterizedbyhotwatercirculations.Inamoregeneralway,theresultsofclaymineralogystudiessuggestthatthermalconditionshavevariedduringtheParisbasinhistory(Liewigetal.,1987;GuilhaumouandGaulier,1991).

Allalongtheirformationandevolution,sedimen-tarybasinsaresubmittedtoburialataratethatdependsonthegeologicframeworkandongeo-dynamics.Duringburial,depositionalsequencesaresubmittedtoincreasingPressure–Temperaturecon-ditions.Asaconsequence,thetemperatureandtheenergyprovidedtothesedimentaryorganicmattercontainedintheconsideredformationsincrease,thuscausingkerogenthermaltransformationandsubse-quenthydrocarbongeneration.

Inthepresentpaper,wepresentanattemptattakingintoaccounttemporalvariationofthethermalgradientsandthermalconductivitiesin1Dmodelinginaseriesoflocationsdeterminingacross-sectionintheParisbasin.Inthataim,insteadofapproachingthethermalhistorythroughthereconstructionoftheburialofapileofsedimentarylayers,wesuggestintegratingthestratigraphicanalysisoperatedatatemporalresolutionscalearound1–5Myduringcalculations.Theresolutionscaleallowstoconsiderminorandmajortransgressive–regressivecyclesthatoccurredthroughouttheParisBasinevolution(Guil-locheauetal.,2000).

InafirsttimeourmodelnamedTherMO’scal-culatespalaeodepthsandthecorrespondingpalae-oporosityandthermalpalaeoconductivityforallthe

230L.Amiretal./Tectonophysics400(2005)227–240

stratigraphichorizonsstudied.Inasecondpartitest-imatesthethermalparameters,calibratedagainsttheactualmaturitystateoftheorganicmatterusingRock-Evalandakineticmodel.Theprinciplesofdevelop-mentoftheTherMO’sprogramaredescribedinthispaper.Itisappliedtoacross-sectioninanE–Wprofile,alongthelongaxisofthebasin.Itinvolves20bore-holes(seeFig.1)andrunsfromtheRambouilletrefe-rencewelltoa150-kmeastwheretheSt.Loupwell.

2.GeodynamicandstratigraphicframeworkTheParisBasin,initiatedduringthePermo-Triassic(Megnien,1980;BrunetandLePichon,1982;CurnelleandDubois,1986;BourquinandGuillocheau,1996;Bourquinetal.,2002).Fromthattime,itexperiencedseveralepisodesofsubsidencefromJurassictoTertiary,mostlymarkedinitscenter,followedbyupliftofitsnorthernandsouthernrimsduringNeogenetimes.ThemajorsedimentarycyclesidentifiedintheParisBasin(firstordercycles)documentthegeo-dynamiceventsrelatedtotheTethysevolutiontotheSouthandtheopeningoftheNorthAtlantictotheWest(Guillocheau,1991).Accelerationinthesubsidencerateismarkedbytransgressivephases,whereasslowersubsidencecorrespondstoregressivephases.

3.Methodology

Throughoutitshistory,asedimentarybasinis

submittedtoburialwhichcanvaryaccordinglytotherateofsedimentsdepositionandtectoniccontext.Toreconstructthethermalhistoryofsedimentarybasins,theburialhistoryforeachstratigraphicsequenceisfirstdetermined.Then,thethermalparametersarecomputedbytakingintoaccounttheburialparametersofeachstratigraphicsequence(Fig.2).

3.1.Burialhistoryofsedimentarybasins:burialprocedure

3.1.1.Palaeodepth

TheTherMO’sprogramincludestheclassicalmethodsusedforsolvingtheproblemofsedimentaryunitssubmittedtocompactionanddecompaction(BrunetandLePichon,1980;AllenandAllen,

1990).Thecodewaswrittenbyconsideringtheem-piricallawwhichlinksporositytodepth.ThesurfaceporosityandthecoefficientporosityparametersusedinthepresentworkarelistedinTable1.Themath-ematicalequationfordeterminingburialhistoryis:

y/eÀcjyj1ÀeÀcjyyj2

jV2jV1¼yj2Àyj1À0j

cj

ecjyjV1ÀeÀcjyjVþ/2

0jcþeustþbathyð1Þ

j

j

Where:yjV1,\\yjV2:\\newpalaeodepth\\(base\\and\op\\of\he\\sequence\\j);\\yj1,yj2:presentdepthofthesequencej;/0j:surfaceporosity;cj:porositycoef-ficient;eust:eustaticvalue;bathyj:bathymetriccorrectionforthesequencej.

3.1.2.Palaeoporosity

Inordertoestimatenewpalaeoporosityvalueswhichareafunctionofthenewpalaeodepths,weintroducedthefollowingexpressioninthesubsidenceprocedure(AllenandAllen,1990):

eÀcjy/jV1ÀeÀcjyjVjV¼/0jcjÀyjV2ÀyjV1Á2

ð2Þ

WhereUjVisthenewpaleoporosityforthesequencej.3.1.3.Thermalpalaeoconductivity

Weusethegeometricmeanmodel(Vasseuretal.,1995)wherethebulkconductivityisdefinedby:KYn¼

Kð1ÀPij

iÞ

Â0:6Pi

ð3Þ

j¼1

whereKijistheexperimentalthermalconductivityofthejthlithologiccomponentfortheconsideredsequencei,PiisthepaleoporosityoftheconsideredsequenceiestimatedfromtheEq.(2).Avalueof0.6W/m/Kistakenforthethermalconductivityofwater.3.2.ThemodelingofthethermalparametersThermaltransformationsoperatingonorganicorinorganicmatterdependontheheataccumulatedineachstratigraphicsequence.Ineachsequencetheheatingofthesedimentaryorganicmatteriscon-trolledbythetemporalvariationofthetemperaturedifferencebetweenitstopandbase.Theheatisprovidedtoageneticunitwithinaspecificperiodof

L.Amiretal./Tectonophysics400(2005)227–240

Sequential lithostratigraphic database Decompaction 231

Palaeo porosity Palaeo thermal conductivity Heat Flux Surface Palaeo temperature Palaeo temperature Thermal Palaeo gradient Palaeo Tmax Palaeo activation energy Checking Test: Comparison of Tmax measured at present time and Tmax simulated for the latest time interval No Tmax simulated − Tmax measured = +/- 10ºC Yes End of the simulation Fig.2.FlowChartofTherMO’Smodel.

time.Thesedimentaryorganicmatter(SOM)involvedineachgeneticsequenceisthentrans-formedthankstothePressure–Temperaturecondi-tionsvaryingallalongtheburialofthebasin.Inthepresentwork,wedonotdistinguishtherespectivepartofconvectionversusconductionduringheattransportandthegenerationofheatduetoradio-activedisintegration.TheaimofourapproachistoestimatetheamountofheatingnecessaryforthermalcrackingoftheSOM.

3.2.1.Computingtheamountofheating

Themodelsupposesthatthermalconductivityandthicknessvaryfromonesequencetoanotheraccord-ingtotheburialhistoryparameterssimulatedatthestratigraphicresolutionscale.Thetemperaturediffer-

encebetweentopandbaseofeachstratigraphicsequenceisestimatedfromtheFourier’slaw:dTðt;sÞ¼Q4

thicknessðt;sÞKðt;sÞ

ð4Þ

inwhich:dT(t,s)isthedifferenceoftemperaturebetweenthetopandthebaseoftheconsideredsequencesattimet,expressedin8C.K(t,s)isthethermalconductivityoftheconsideredsequencesattimet,expressedinW/m/8C.Qistheheatflux.Itrepresentstheheatprovidedandtransferedtothesedimentaryorganicmatterbymeansofthethermalconductivity.Anumericalvalueismanuallyintro-ducedbytheuserandisthesameforthewholelithologiccolumnandconstantforthestudiedbore-hole.ItisexpressedinmW/m2.

232L.Amiretal./Tectonophysics400(2005)227–240

Table1

Mainnumericalconstantsusedforthemodelingprocedures(surfaceporosity,etc.)(Martinezetal.,2000;Durand,M.,personnalcommunication)Lithology

SurfaceLithologicDepthDensityporosityconstant(km)U(kg/m3)U0(%)c(kmÀ1)Shale1772.5zb0.32720400.3zN0.3Shale2380.20bzb62720Silt490.3zb62650Chalk710.70bzb62710Limestone800.5zb0.52710500.5zN0.52710Halite

2010bzb62160Gypsumanhydrite6010bzb62920Sandstone

490.30bzb62650Shalysandstone

56

0.4

0bzb6

2680

3.2.2.Calculationofthepaleotemperaturesandthermalpaleogradients

Themainassumptionsforestimatingthepalae-otemperaturesaretheknowledgeofthesurfacepalaeotemperaturesandthecalculationofthetemper-aturedifference.Paleotemperaturesarededucedfromthefollowingequation:

Tbottomðt;sÞ¼Ttopðt;sÞþdTðt;sÞ

ð5Þ

WheredT(t,s)isthetemperaturedifferencecalcu-latedfromEq.(4).T_bottom(t,s),T_top(t,s)arethetemperatures(in8C)simulatedatthebottomandthetopofthesequencesattimet.

Onthefirststepofthecalculation,thetopofthelithologiccolumncorrespondstothesurfacepalae-otemperature.Temperatureofthebottomofthesommitalsequencecanthenbeeasilydeduced.Then,basedonthesimpleassumptionthatthetemperatureofthebaseofasequenceisequaltothetemperatureofthetopofthepreceedingsequence,themodelestimatesprogressivelythetemperatureateachinter-faceuntilthebaseofthelithologiccolumnconsid-ered.Thermalpalaeogradientsarededucedfromthetemperaturedifferenceusingthefollowingformula:

Hðt;sÞ¼dTðt;sÞ

thicknessðt;sÞð6Þ

3.2.3.Calibrationofthemodeling

InordertocontrolthethermalparameterssimulatedbyTherMO’s,weintroducedacodebasedonthethermalcrackingoforganicmatter.Basically,likeother

modelsofthatkind,thislatteroneisbasedontheclassicalArrhenius’empiricallaw(Arrhenius,1909)whichrelatestherateofareactionatagiventemper-aturetoconcentrationsfactors,throughtheamountofenergyrequiredforthereactiontoproceed.Infactthisamountofenergyisdefinedastheactivationenergy.TheArrheniusequationformulatesasfollows:k¼AÂexpðÀEa=RÂTÞ

ð7Þ

wherekistheratecoefficient,AistheArrheniusconstant,Eaistheactivationenergy,Ristheuniversalgasconstant,andTisthetemperature(inK).

Thematurationoftheorganicmatterisanirreversibleprocesswhich,canbereproducedexper-imentallybyRock-Evalpyrolysis(Disnar,1986;Disnar,1994;Lafargueetal.,1998).FromthesingleArrheniusequation,amodelwasdevelopedaimingtothedeterminationofmaximumpaleotemperaturesofburialfromorganicmatterexperimentaldata(Disnar,1986,1994).Thismodelsupposesthatprogrammedlaboratorypyrolysiscontinuesthesuccessiveelimi-nationofkerogenmoleculeswhereithadstoppedduringpreviousburialdiagenesis.TheexperimentaltemperatureTmin,graphicallydeterminedattheonsetoftheS2Rock-Evalpyrolysispeak,isusedtocalculatethecorrespondingmaximumpaleotemper-aturesofburial,takingintoaccountdifferentvaluesoftheexperimentalandnaturalthermalgradients.Thiscalculationisoperatedwiththefollowingequationdirectlyderived1󰀄fromtheArrhenius’󰀅

law:

T¼R2lnT2TÀlnB2þ

1

ð8Þ2Ea1B1T1whereT1istheexperimentaltemperature(inK),B1istheexperimentalgradient(heatingrateoftheRock-Evalpyrolysisexpressedin8C/min),T2isthemaximumpaleotemperatureofburial(inK)andB2isthenaturalgradient(8C/My).Inthismodel,thenaturalgradientistheproductoftherateofsubsidencecalculatedfromtheburialhistorywiththemeanpresentthermalgradient.

WeconsiderEq.(8)forcalibrationoftheTher-MO’smodel.Forthatpurpose,thepaleotemperaturesandthethermalpaleogradientscalculatedpreviously(seeSection3.2.2)wereheretransferredasknownvariablesinordertodeducesimultaneouslytheenergiesnecessaryforthethermalcrackingofthesedimentaryorganicmatterandthetemperaturefor

L.Amiretal./Tectonophysics400(2005)227–240233

theRock-Evalpyrolysis.Then,theanalyticaltemper-aturesestimatedfromthemodelingforthepresenttimearecomparedwithvalueseffectivelymeasuredoncoressamplesbyRock-Evalpyrolysis.Theassumedheatparameterismanuallyadjustedbytheuseruntilthefollowingconditionisreachedforeachstudiedstratigraphicsequence:jTsimulatedÀTmeasuredj¼0þ=À108C

ð9Þ

4.Datausedinthisstudy4.1.Stratigraphicdata

Thedifferentwavelengthsoftectonicmovementsareconstrainedbyhigh-resolution2Dand3Dgeo-metriesofstratigraphiccycles.Therespectivelithol-ogyandfaciesobservedwithinthoseperiodsalsocontrolthenature,geometryandhierarchyofthestratigraphiccycles(Guillocheauetal.,2000).Theprovidedstratigraphicdatabasewasestablishedintheframeworkofapreviousscientificprogram(Robinetal.,1996;Robin,1997;Guillocheauetal.,2000).Theapproachusedclassicallytookintoaccount:(1)Theidentificationofsequencestratigraphic

isochrones(maximumfloodingsurfacesforinstance)ondiagraphicrecords.

(2)Thebiochronostratigraphicdata(ammonites

zones)reportedintheliterature;eachisochronbeingdatedwiththehelpofOdin(1994)datationscale.Forexample,theisochronidentifiedatthebaseoftheHettangianisafloodingsurfacemarkedbytheplanorbisammonitezone.Itcorrespondsto205Myinage,accordingtotheOdin(1994)datationscale.

(3)Theverticalstackingofparasequences:thereco-gnitionoftheverticalfaciesevolutionondiagra-phicrecordsleadstodeterminethetransgressive–regressivecycles.Italsopermitstodefinegeneticunits(firstorder,secondorder,etc.).

(4)Thecorrelationofthegeneticunitsdelimitedbytheidentifiedisochronsfromboreholetobore-hole,allalongthecross-section.Sequencesaredelimitedbyfloodingandmax-imumfloodingsurface.Concerningthepresent

work,theycorrespondtothirdordereventsintermsofsequencestratigraphy.Thethicknessofeachsequencerangesfrom5to20manditdefinesthedepthresolutionofourmodel.Thecorrespond-ingtimeresolutionrangesfrom1to5My.About78stratigraphiclimitshavebeenselectedforeachofthe20selectedboreholes.Theydelineate49sequencesforwhichthemineralpercentage(shale,carbonate,sand,etc.)wasestimatedfromwelllogrecordings(Robinetal.,1996;Robin,1997).Theapproachisbasedontheidentificationofhomoge-neouselectrofacies.Theanalysisofgammaray,resistivityandneutronlogsleadtotherecognitionofdiagraphicpropertiesspecifictolithologiccomponents.

Theupperstratigraphiclimitofourmodelcorre-spondstoanimportanterosionepisodethatoccurredduringCretaceoustime(Santonien,83–87My).Themodelingisnotperformedtoupperlevels,TherMO’sbeingnotyetabletotakeerosionaleventsintoaccount.Thisprovidestheuppertimeconstraint.ThelowerlimitisfixedastheTriassic(Scythian,232.5My)consideredasthebasementofthemodelledseries.ThelithologicvariationinpercentageofshalecontentrecordedbytheMesozoicsedimentspermittedtodelimitthebeginningandtheendoftransgressiveandregressivestages.Forexample,fortheHettangian,aminortransgressivecyclestagewasdelimitedbythefloodingsurfacenamedH1andthemaximumfloodingsurfacenamedH2.Thosestrati-graphichorizonsarerespectivelycharacterizedby30%and80%ofshalesintheRambouilletwell(Robin,1997).

4.2.Organicmatterdata

TheexperimentaltemperaturesTminandTmaxcorrespondtodifferentpointsofthepyrolysisS2signalrepresentingthehydrocarbonsgeneratedfromtheexperimentalheatingofthekerogen.TminandTmaxparametersaredeterminedattheonsetandtopoftheS2curve,respectively.Boththeseobeytothesamekineticlawswithdistinctactivationenergies.OnlytheactivationenergiesarenecessarilydifferentaccordingtothepointlocationontheS2signal(Disnar,1986;Disnar,1994;Lafargueetal.,1998).

TheParisBasinwasafairlyshallowepicontinentalbasin.Thedepositionalconditionswhichprevailed

234L.Amiretal./Tectonophysics400(2005)227–240

Table2

Surfacepalaeotemperaturesusedforthesimulationprovidedandextrapolatedfromtheliterature(Bowen,1966)Timeinterval(My)Surfacetemperature(8C)205to18724.5187to18025180to17620176to17221172to16717From167

12

duringMesozoicledtothedepositionofgoodpotentialsourcerocks(Espitalieetal.,1987;Ungereretal.,1991).ThebestsourcerocksarelocatedbetweentheHettangianandtheBajocian.TheOMthatmainlyoriginatesfrommarineplanctonicorgan-isms(typeII)wasburiedatsufficientdepthinthecentreofthebasintoproducesomeoil(Espitalieetal.,1987).Forourmodelingpurposes,weusedTmaxdatadeducedfromtheiso-TmaxcurvesdrawnbyEspitalieetal.(1987)basedontheanalysisofaconsiderablesetofsamples,forHettangian,Sinemur-ianandlowerToarciansequences(Espitalieetal.,1987).

4.3.Surfacepaleotemperaturesdata

Stableisotopegeochemistryofcarbonatescanbeusedtoestimatethesurfacepaleotemperatures.Inthemodeling,weintroducedthesurfacepaleotempera-turesdeterminedfromtheisotopicanalysisofd18Oandd13CbyBowen(1966)(seeTable2).

5.Limitsofthemodeling

Asaconsequenceoftheuncertaintiesspecifictoourcodeandforreasonsthataregeneraltoallmodels,thenumericalvaluesprovidedbyTherMO’smustbeinterpretedwithcaution.Ononehand,resultsofsimulationarestronglyconstrainedbythedataimplementedinthemodel.Asforthestrati-graphicconstraintswehadtoconsider,weobservedthatthelimitsofthesequencescanbedifferentdependingontheauthorsandonthewaytheyestablishedtheirstratigraphicdatabase.Thestrati-graphicdatabaseusedandimplementedforthepresentworkisdifferentfromthestratigraphic

databaseestablishedbyJacquinandGraciansky(1997)fortheParisbasin.Itcouldbeinterestingtocompareourresultstothoseissuedfromsimulationsoperatedwithotherstratigraphicdata-basesestablishedfortheParisbasin.Furthermorethemodelispresentlyunabletotakeerosionandpossibleupliftintoaccount.However,nomajorerosionwasidentifiedbeforeCretaceousintheParisBasin.

Uncertaintiesforthepresentevaluationofthethermalparametersarealsorelatedtothefactthatwedonottakeintoaccount,duringthecalculation,thevariationofheatfluxthroughtime.Forexample,theproceduredealingwiththeestimationoftheheatfluxshouldbeadaptedtotakeintoaccounthotterregimeduetoriftingperiods.Threatstovalidityofthemodelingoperatedalsocomefromtheuncertaintyofthesurfacepaleotemperaturesestimationsfromthestableisotopicanalysis.

Consequently,thelimitsofthemodelingconcerntheprecisionatwhichthecalibrations(stratigraphy,lithology,geochemistry)isperformed.Forestimatinguncertaintiesvalues,thenextstepsaretouseotherstratigraphicdatabaseandcomparetheresultsobtained.Weshouldalsointegrateinthemodelingsurfacepaleotemperaturesestimatedfromindepend-entmethodsinordertochecktheimpactofanypossibleerrorintheestimationofthethermalparameters.

6.Results

TheheatfluxsimulatedinthecentralpartoftheParisBasinshowslateralvariationsalongthecross-section(Fig.3).ItdecreasesfromRambouillettoTrouAuxLoupsboreholes(87–66mW/m2)locatedabout100kmmoretotheeastwhereastheheatfluxsimulated2inSt.LoupBoreholeishigher(73mW/m).TheresultsobtainedareinagreementwithheatflowsvaluesdeterminedfromapreviousstudyrealisedinthecenterpartoftheParisbasinwiththeTemispacksoftware(GaulierandBurrus,1994).Thosevaluesvarylaterallybetween70and90mW/m2.LucazeauandVasseur(1989)alsopresentnumerical2heatflowvaluesvaryingfrom50mW/monthewesternsideoftheParisBasintoabout80mW/m2inthecenter.

L.Amiretal./Tectonophysics400(2005)227–240235

A

959085ABQ(mW/m2)807570ITS6560020406080100DISTANCE (KM)120140B

455

450

Tmax (ºC) 445

Tmax measured (ºC)T440

Tmax simulated (ºC)435

430

A425

020406080100Distance (km)

120140160Fig.3.(A)HeatfluxsimulatedwithTherMO’S.(B)ComparisonbetweenTmaxdatameasuredfortheHettangiansequenceintheParisbasintoTmaxvaluescalculatedwithTherMO’sforthecalibrationprocedure.

Thestudyofthethermalpropertiesofthestrati-graphichorizonsH1andH2permitstocharacterizethedepositionoftheHettangianminortransgressivecycle.Theevolutionofthethermalgradientsandconductivitiesobeytotheburialofthesedimentarybasinallalongthecross-section.Wehavenotincludedinourcalculationsconsiderationsdealingwiththermalrelaxation.Wecansee,fromtheresultsobtainedfortheliassicsequencesintheRambouilletwell(Fig.4),thatthethermalparametersareaffectedbythevariationoftheburialrateofthestratigraphichorizons.DuringMalm,thesuddendecreaseofthethermalgradientsvaluesobservedisdifferentaccordingtothehorizonconsidered.ForthehorizonH1,thethermalgradientdecreaseis5.2Â10À38C/mwhereasitis3.3Â10À38C/mforthehorizonH2.Thermalgradientsvaluesrangefrom1008C/km(at205My)to57.338C/km(at154My)forH1andfrom1118C/km(at205My)to59.438C/km(at154My)forH2.FromtheMalmstage,thethermalgradientsdecreasemoreslowly.Thoseresultscon-firmthatduringpasttimes,thethermalgradient

236L.Amiretal./Tectonophysics400(2005)227–240

recordedbyMesozoicsedimentsmighthavebeenhotterthanduringpresentdays.

At87My,thethermalgradientfortheRam-bouilletwellvariesfrom45to578C/kmaccordingtothesequencesconsidered.Themeanthermalgradientcalculatedfor87myis50.48C/km,aresultintherangeofvaluesforthermalgradientsobtainedfrompreviousstudiesontheParisbasin.GaulierandBurrus(1994)publishedthermalgradientsforliassicunitsrangingfrom55to658C/km.Consequently,itwasalsoveryinterestingto

notefortheresultsherepresentedthatthemeanthermalgradientcomputedforeachhorizonsfrom205to87Myrangedbetween58.33and67.038C/km(seeTable3).

7.Discussion

ThethermalhistoryrecordedbytheMesozoicsedimentsmaybelinkedto(1)thegeodynamiccontextand(2)thesyn-sedimentarytectonic.Liter-

AJURASSICLIAS7,11,97,52,952019191818CRETACEOUSMALM3,71,36,60,57,531415151416DOGGER4,48,3161716LOWERCRETACEOUS3,66,78,15,85,18121111111110101UPPER CRET.Age (My)910200THERMAL DOMING IN THE NORTH SEAOCEANIC ACCRETION -TETHYSRIFT BISCAY BAY400600Depth (m ) 8001000CENTRAL ATLANTIC RIFTRIFT -TETHYSOCEANICACCRETION BISCAY BAY120014001600ProgradationRetrogradationH1 : Lower limit of the Hettangian mid transgressive cycle.H2: Upper limit of the Hettangian mid transgressive cycleFig.4.(A)SimulationoftheburialhistoryfortheHettangian’ssequencesattheRambouilletWellandgeodynamiceventsassociated.H1isthelowerlimitoftheHettangianmidtransgressivecycle;H2istheupperlimitoftheHettangianmidtransgressivecycleandthelowerlimitoftheHettangianregressivecycle.(B)SimulationoftheevolutionofthepalaeothermalgradientGtfortheLiassichorizonsattheRambouilletWell.H1,...,T4areisochrons’nameforremarkablessurfaces.Inparticularly,thosesurfacesdelimitmid-transgressiveandregressivecycle.H1isthelowerlimitoftheHettangianmidtransgressivecycle;H2isthethelowerlimitoftheHettangianmidregressivecycle;S1isthelowerlimitoftheSinemurianmid-transgressivecycle;S2isthelowerlimitoftheSinemurianmid-regressivecycle;Pl1isthelowerlimitofthePliensbachianmid-transgressivecycle;Pd4isthelowerlimitofthePliensbachianmid-regressivecycle;Pd7isthelowerlimitoftheToarcianmidtransgressivecycle;T2isthelowerlimitoftheToarcianmidregressivecycle.

L.Amiretal./Tectonophysics400(2005)227–240237

BJURASSICLIASGt(ºC/km)110,00CRETACEOUSMALMLOWERCRETACEOUSUP.CRET.DOGGER100,00THERMALDOMING IN THE NORTH SEAOCEANIC ACCRETION -TETHYS90,00RIFT BISCAY BAY80,0070,0060,00CENTRALATLANTIC RIFT RIFT-TETHYSOCEANIC ACCRETION BISCAY BAYH1H2S1S2Pl1Pc2Pc3Pd4Pd5Pd7T1T2T3T450,0040,005200,4195,6191,1898,1187,2182,9176172,5168,3164,8162,3160,5158,5154151,3148,6145,6143141120,1178,1117,2116,1135,8115,3114108104,298,5912087Age(My)Fig.4(continued).

atureontheisotopicdatingofburialbasedonillitization(Claueretal.,1995)suggeststhattheTriassicwasprobablyaffectedbyahydrothermalevent.Geochemicalanalysesonclaymineralsindicatedifferentdiageneticeventscharacterizedbyunusuallywarmthermalregimesatabout190My,150Myand80My(Liewigetal.,1987;GuilhaumouandGaulier,1991;Spo¨tletal.,1996).

SinceTriassic,sedimentarysequencesrecordedseveralphasesofacceleration–decelerationofthesubsidence.TheseepisodeswerelinkedtomajorgeodynamiceventsaffectingtheWestEuropeanplate(Guillocheau,1991;Prijacetal.,2000).Prijacetal.(2000)analysedthetectonicsubsidenceoftheParisbasinasresultfromthedecayofathermalanomalyduetothecollapseoftheVariscanbelt.Forthatpurpose,theycomputedthethermalevolutionofthelithospherebyconsideringthePlateandtheChablismodel.CurvesoflargescalebulksubsidenceintheParisBasin(Prijacetal.,2000)showalongtermcomponentreflectinglithospherecoolingonwhichashortertrendreflectsthechangesinthetectonicstresses.Thelong-termvariationisexponential(BrunetandLePichon,1982)andreflectsthecoolingrateaftertheHercynianorogeny.Forwardmodelingindicatesthatsubsidencerateisbyevidencethefastestduringthefirst40Myimmedi-atelyaftertheendoftheorogeny(230My),withabout600minamplitude.Duringtheperiodinwhichweobserveanomalousrates(190–140My)thebulksubsidenceisabout400m.Itslowsdowntoabout400minthenext100My(Prijacetal.,2000).Theobservedsubsidencevaluesduringtheanom-alousperiodoftime(190–140My)areindeficitbyabout150mcomparedtothebulksubsidencerate.Itdocumentsaslowertectonicsubsidence,evidencing,eitheradecreaseintheheatconductionlossbythelithosphereorbyastiffercrustalresponse.Theamplitudeintimeoftheanomaly,appearstooshortcomparedtothetimewavelengthoflithospheric

238L.Amiretal./Tectonophysics400(2005)227–240

Table3

NumericalmeanthermalgradientvaluescalculatedforeachliassichorizonsfortheRambouilletwellIsochronMeanvalue/isochron(8C/km)H158.33H261.38S163.67S259.64Pl162.73Pc263.16Pc363.63Pd463.27Pd563.78Pd761.97T161.87T267.03T367.50T4

62.81

H1,...,T4areisochrons’nameforremarkablessurfaces.Inparticular,thosesurfacesdelimitmid-transgressiveandregressivecycle.H1isthelowerlimitoftheHettangianmid-transgressivecycle;H2isthethelowerlimitoftheHettangianmid-regressivecycle;S1isthelowerlimitoftheSinemurianmidtransgressivecycle;S2isthelowerlimitoftheSinemurianmidregressivecycle;Pl1isthelowerlimitofthePliensbachianmidtransgressivecycle;Pd4isthelowerlimitofthePliensbachianmidregressivecycle;Pd7isthelowerlimitoftheToarcianmidtransgressivecycle;T2isthelowerlimitoftheToarcianmidregressivecycle.Themeanthermalgradientvalueforallliassichorizonsis62.91(8C/kmandthemeanthermalgradientvalueat87Myforallliassichorizonsis50.59(8C/km).

thermalloss.Inconsequence,atectoniccauseshouldbeconsideredthatvariesintimethesubsidenceoftheParisBasin.

Gable(1984)pointedoutarelationshipcouldbesuggestedforexplainingtheoriginofthermalanomaliescharacterizedbyhighheatflowsmeasuredintheParisBasinfromgeophysicalmethodsandpossiblegeologicalstructuressuchasgraniticmassif(Armoricanmassif)ortheupwardingoftheMohor-ovicicdiscontinuityandcrustalthinning.LefortandAgarwal(1996,2000)studiedapossiblecorrelationbetweenheatfluxandgeophysicalmeasurements(gravityandseismicdata)fortheParisbasin.ThisapproachledthemtoidentifyMohoundulationsunderthebasin.ThismethodcouldbeaswellconsideredtoestablishtherelationshipsbetweenthelateralvariationoftheheatflowsobtainedfromthemodelingherepresentedandthegeometryoftheParisBasinsubstratum(CazesandTorreilles,1998a,b).

Concerningtheeffectofthesyn-sedimentarytectoniconthevariationofthethermalparameterssimulated,itisveryinterestingtopointouttheaccelerationofthedecreasingofthermalgradientsforLiassichorizonsduringthePliensbachianstage.Atthatparticularperiod,theaccelerationofburialmayberelatedtothereactivationoftheSeine-Loirefaultandconsequentlyhaveanimpactonthethermalgradientsevolutionforthattime.

8.Conclusion

Manyofthethermalsedimentarybasinmodelsusedinpetroleumindustryimposethermalgradientsasaconstantthroughgeologicaltime.Theestimationofthethermalgradientisderivedfromtheknowledgeofthebottomboreholetemperature.

TherMO’scalculatesthermalpaleogradientsfromthesimulatedpaleodepthsandpaleotemperatures.Takingintoaccountthetemporalvariationofthethermalpaleogradientsassociatedtotheburialhistoryofeachstratigraphicsequences(timescale:1–5My),wesimulatedtheheatfluxprovidedforthethermalcrackingofthesedimentaryorganicmatterbymeansoftheconductivityandcalibratedtheresultsobtainedwithorganicmatterdatareflectingitspresentstateofmaturity.

Theheatflowthroughundisturbedsedimentarybasinsisusuallyeitherconstantordecreasingasfunctionofgeologicaltime(TissotandWelte,1984).ThethermalhistoryoftheParisbasiniscomplexandhasnotbeenconstantthroughgeologicaltime.Itisonlyforsimplicityofprogrammingthatthe1DTherMO’scodeconsidersaconstantvaluefortheheatflux.ThereconstitutionoftheParisbasinthermalhistoryledtodifferentresultsdependingonthemethodofstudy.But,onthewhole,TherMO’scalculationresultsareingoodagreementwithpreviousestimates.

TheoriginalityoftheapproachdescribedinthepresentpaperwastointroducethethermalgradientvariationthroughgeologicaltimeonbasinmodelingandexaminetheeffectforthereconstitutionoftheParisbasinthermalhistory.

Theknowledgeofthisthermalenergyalsocalculatedduringthecalibrationprocedureallalongtheburialhistory,atthestratigraphicresolutionscale,maybeaveryinterestingissueinthePetroleum

L.Amiretal./Tectonophysics400(2005)227–240239

Industryresearchprograms.Thismethodcouldhelpintheestimationofthegeologicaltimingofthehydrocarbonsgeneration.Acknowledgments

ThispaperisbasedonthethesisworkofL.AmiratH.Poincare´UniversityforthePhDdegree.ItwassupportedbythePNRHwhichisgratefullyacknowl-edged.WealsothankF.Malartreforsuggestionsthatallowedustoimprovethemanuscript.

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