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PorosityandpermeabilityoftightcarbonatereservoirrocksinthenorthofIraqF.Rashidn,P.W.J.Glover,P.Lorinczi,R.Collier,J.LawrenceSchoolofEarthandEnvironment,UniversityofLeeds,UKarticleinfoArticlehistory:Received17October2014Accepted11May2015Availableonline19May2015Keywords:TightcarbonatesPorosityPermeabilityReservoirqualityPetrofaciesKometanFormationIraqPermeabilitypredictionabstractThedistributionofreservoirqualityintightcarbonatesdependsprimarilyuponhowdiageneticprocesseshavemodifiedtherockmicrostructure,leadingtosignificantheterogeneityandanisotropy.Thesizeandconnectivityoftheporenetworkmaybeenhancedbydissolutionorreducedbycementationandcompaction.Inthispaperwehaveexaminedthefactorswhichaffectthedistributionofporosity,permeabilityandreservoirqualityintheTuronian–CampanianKometanFormation,whichisaprospectivelowpermeabilitycarbonatereservoirrockinnorthernIraq.Ourdataincludesregionalstratigraphy,outcropsections,welllogsandcorematerialfrom8wellsaswellasalargesuiteoflaboratorypetrophysicalmeasurements.ThesedatahaveallowedustoclassifytheKometanFormationintothreelithologicalunits,twomicrofaciesandthreepetrofacies.PetrofaciesAischaracterizedbydenseandcompactedandcementedwackstone/packstonewithnanometersizeintercrystallineporesandstylolitesandpresentsapoorreservoirquality(porosityrange0.00570.01to0.09970.01,permeabilityrange65nD–51μD).OccasionalopenfracturesinPetrofaciesAimprovereservoirqualityresultingina2–3orderofmagnitudeincreaseinpermeability(upto9.75mD).PetrofaciesBisadissolvedwackstone/packstonethatcontainsmoldicandvuggypores(porosityrange0.19770.01to0.29370.01;permeabilityrange0.087–4.1mD),withbothpresentinggoodreservoirquality,whilePetrofaciesCisacarbonatemudstonethathasundergonedissolutionandpossiblysomedolomitization(porosityrange0.12370.01to0.25570.01;permeabilityrange0.065–5mD).AllthreepetrofaciescanbedistinguishedfromwirelinelogdatausingporosityandNMRmeasurements.AporopermplotofallofthedataisfittedbyapowerlawoftheformkðmDÞ¼aϕbwitha¼28.044andb¼2.6504withcoefficientofdetermination,R2¼0.703.WhenthepermeabilityispredictedwiththeRGPZmodeloftheformkðmDÞ¼d2ϕ3m=4am2withmeangraindiameterd¼10μm,andmeancementationexponentm¼1.5anda¼8/3abetterfitispossiblewithR2¼0.82.&2015ElsevierB.V.Allrightsreserved.1.IntroductionTheMiddleTuroniantoLowerCampanianrocksuccessioninthecentralpartofIraqisrepresentedbytheKhasib,TanumaandSa’diFormations(Aqrawi,1996).TheseformationshostproducingfieldsincludingtheEastBaghdadfieldsinanimportantreservoir-sealsystem,whichcontainsanestimated9billionbarrelsofoilin-place(Al-Sakini,1992;Aqrawi,1996).TheequivalentoftheMiddleTuroniantoLowerCampanianrocksuccessioninNorthIraqistheKometanFormation,whichmayalsobeproductivewhereitissufficientlyfractured(JassimandGoff,2006).Fig.1showsthepalaeogeographymapoftheKometanFormationanditsequiva-lentrocksinIraq,whileFig.2showsthepositionsofthevariousgeologicalstructures,majorfaults,fieldsandwellsreferredtointhispaper.TheKometanFormationisafracturedreservoirunitthatproducescommercialoilinsomeoilfieldsinthenorthofIraq(Aqrawi,1996).TheTaqTaqoilfield,forexample,isafracturedCretaceousreservoirthatincludestheKometanFormationandproduceslightoil(41API)withestimatedrecoverablereservesof700–750millionbarrels.Ithasbeenpredictedthatthefieldwillproduce200,000–250,000barrelsperdaywhenitisfullydevel-oped(TTOPCO,2007).IntheKirkukembayment,theKometanFormationisrecog-nizedasaproductiveformationintheoilreservoirsattheAvanahandBabaDomesoftheKirkukstructureandintheBaiHassanfield,aswellasproducinggasintheJamburoilfield(Aqrawietal.,2010).TheequivalentformationsinthecentralandsouthernpartsofIraq,whicharecharacterizedbythechalkyunitsoftheKhasibandSa’diFormationswithintercalationofshaleandmarloftheContentslistsavailableatScienceDirectjournalhomepage:www.elsevier.com/locate/petrolJournalofPetroleumScienceandEngineeringhttp://dx.doi.org/10.1016/j.petrol.2015.05.0090920-4105/&2015ElsevierB.V.Allrightsreserved.C3Correspondingauthor.Tel.:þ447445291315.E-mailaddress:eefnr@leeds.ac.uk(F.Rashid).JournalofPetroleumScienceandEngineering133(2015)147–161Fig.1.PalaeogeographicalmapoftheKometanFormationanditsequivalentformationinIraq(JassimandGoff,2006).Fig.2.TectonicdivisionofIraq(afterAqrawietal.,2010),showingtheinvestigationareaandincludingthewellsusedinthisworkaswellasthepositionoftheDokanout-cropsection.F.Rashidetal./JournalofPetroleumScienceandEngineering133(2015)147–161148TanumaFormation,haveporositiesintherange18–23%andpermeabilitiesabout10mD(Aqrawi,1996).AqrawiidentifiedtheKhasibandTanumaFormationsintheMesopotamianbasinasgoodreservoirunitsthankstotheeffectsofdissolutiondiagenesisandtectonicactivity.Morerecently,Sadooni(2004)haspointedoutthatthepresenceofachalkymatrix,bioturbationandthecreationofmicro-fracturesallcombinetoenhancethereservoirpropertiesoftheKhasibFormationincentralIraq.Al-Qayim(2010)usedpreviousstudiesofsomeofthecentraloilfieldsofIraqtodividetheKhasibFormationintofourreservoirunits.Heshowedthatdiagenesisandmicro-fracturingenhancedreservoirquality,withhighqualitybeingcharacterizedbyabundanceofmoldic,vuggyandintercrystallineporositywithvaluesgreaterthan20%andpermeabilityintherange1–25mD.Garlandetal.(2010)havealsoidentifiedthatdolomitizationcausedlocaldevel-opmentofporosityintheKometanFormation.Hesaidthat,asoneoftheCretaceoustargets,highproductivityhadbeenachievedfromtheKometanFormation,andhehasinterpretedthereservoirsystemasafracturedreservoir,wherestorageanddeliverabilityareonlycontrolledbyfracturing.Currently,thereisalackofpubliclyavailabledataconcerningtheevaluationoftheKometanFormation,withmostinformationresidinginconfidentialreportsbelongingtooilcompanieswork-inginnorthernIraq.Thelackofalargeamountofgood,freelyavailableinformationaboutthecometandformationmakesitdifficulttobeginthispaperwithonlyashortintroduction.InsteadwewillfirstintroducetheroleoftheKometanFormationinthelightoftheregionalgeologicandtectonicsettingandthenmoveontohowweobtainedourdata,theresultsandtheinferencesthatwecanmake.Theheartofthispaperconsidersthelithofacies,porosity,porositydistribution,permeabilityandtheeffectoffracturesonthepetrophysicalpropertiesoftheKometanForma-tioninnorthernIraq,inanattempttoassessitsreservoirqualityandtomakethesedatamorewidelyavailableintheliterature.2.RegionalsettingTheZagrosOrogenicBelt(ZagrosMountains),whichtrendsapproximatelyNW–SEthroughnorthernIraqandIran,wasformedduringtheCretaceousandTertiarycollisionofArabiaandEurasia,resultinginarangeofstructures.Presently,theZagrosdeformationzoneischaracterizedbystrike–slipandcontractionalmovements.Thesemovementsresultfromstrainbeingpartitionedintodextralstrike–slipmovementsalongmainlyNW–SEfaultsandashorteningcomponentinaNE–SWdirection(Vernantetal.,2004).TectonicevolutionduringtheEarlyCretac-eouswascharacterizedbydiscontinuationandterminationofthewestwardmotionoftheArabianPlateandcentralIranianplatesasaresultoftheopeningoftheSouthAtlanticOceanandtheclosureofthePalaeo-Tethys,respectively(IranpanahandEsfandiari,1979;Sattarzadehetal.,2000).IntheCretaceous,theeasternshelfplatformoftheArabianPlatewascoveredbytheshallow,neritic,passivemargincarbo-natesandlocalclasticsthatrepresenttheLowerCretaceousreservoirintheKirkukEmbaymentZone,whichincludesexistingandnewlydiscoveredoilfieldsinKurdistan.AforelandbasinwasformedonthenorthernmarginoftheArabianplateduringtheTuronian–Eoceneinresponsetoloadingofthecrustbyathrustsheetformedasaresultofcompressiononthenorth-eastmarginoftheArabianPlatebytheIranianPlate(JassimandGoff,2006).AmajoreventoftheLateCretaceoustectonichistoryinvolvedthecollisionofthetwocontinentalpartsoftheArabianandIranianplates,followedbythedepositionoftheKometanForma-tiononthenorth-eastmarginoftheArabianPlate(KarimandTaha,2009).Fig.3showsthestructuresatthetimeofthedepositionoftheKometanFormation.Thetrenchbetweenthetwoplateswasfilledwithradiolaritesandophiolitesslightlybeforethecollision,andthesetrenchmaterialswereupliftedandthrownontothecontinentalpartoftheArabianPlaterisingabovesealevelnearthesuturezoneoftheplates.TheearlyCretaceousrocksthathadbeendepositedontheArabianPlate,thelatestofwhichwastheQamchuqaFormation,weredeformedintoaforebulgebytheweightoftheaccretionaryprismandthrustingIranianPlate.TheKometanFormationbegantobedepositedintheresultingdepression,directlyontopoftheQamchuqaFormation.SubsidenceofthesuturezonethencontinuedwiththewaterdepthincreasingandtheKometanFormationpassingthroughatransitionalfaciestoadeepermarinedepositionalenvironmentinwhichplanktonicforaminiferaandlimemudsweredepositedaspartoftheKometanFormation(KarimandTaha,2009).3.TheKometanFormationBuday(1980)classifiedtheCretaceousrockunitsinIraqintoseveralcyclesandsub-cyclesbasedonbreaksinthesedimentationprocessandtectonicactivityduringtheperiodofdeposition.TheTuroniantolowerCampaniansub-cyclerockunitsweredepositedasapartofamiddleCretaceousrockunitoverahugeareainIraq.ThecarbonaterockoftheoutershelfandbasinalKometanFormationinthenorthofIraqiscorrelatedchronostratigraphicallywithdeepinnershelfandlagoonalcarbonatesandtheclasticrockunitsoftheKhasib,TanumaandSa’diFormationsincentralandsouthIraq(Aqrawi,1996).Sadooni(2004)hasstatedthatthemiddleTuroniantolowerCampaniansuccessioninIraqiscomprisedofhomogeneouscarbonatesedimentswithalackofsandstoneandevaporites,andincludestheBalambo,Dokan,GulneriandKometanForma-tionsinKurdistanandtheKhasib,TanumaandSa’diFormationsintheMesopotamianBasin.TheKometanFormationconsistsofarangeoffinegrainedcarbonatelithologiesdepositedinshallowshelf,restrictedsettings(oligosteginalfacies)toopenmarine(globigerinalfacies)settings(Buday,1980;AbawiandMahmood,2005;JassimandGoff,2006).InKurdistan,theformationiscomposedentirelyofglobigerinalandoligosteginalfacies,buttowardthewestandsouth-westargillaceousfaciesincrease.ThedeepmarinepelagiclimestoneoftheKometanFormationinKurdistanandthewesternpartoftheZagrosbasinchangeslaterallyintothebioturbatedchalkylime-stone,shaleandmarlylimestoneoftheKhasibFormation,lagoo-nalshaleandcarbonateoftheTanumaFormation,andopenshelfglobigerinallimestoneoftheSa’diFormationintheMesopotamianbasinofcentralIraqandinsouthIraq(Al-Qayim,2010)asshowninFig.4.AccordingtovanBellenetal.(1959)andDunnington(1958),theCretaceouscarbonaterocksoftheKometanFormationwerefirstrecognizedin1953byDunningtonatKometanVillagenearEndezah,north-eastofthetownofRanianearthecityofSulaimani,whichisclosetothecontactbetweentheBalamboTanjeroZoneandtheHighFoldedZoneintheKurdistanregionofIraq.Inthetypelocality,theKometanFormationisdescribedas36mofwhiteweathered,lightgrey,thinandwell-beddedglobigerinal–oligosteginallimestones.Itislocallysilicifiedwithchertnoduleconcentrationsinoccasionalbeds,andglauconitichorizons,especiallyatthebaseoftheKometanFormation.ThisformationwassubsequentlyidentifiedinawiderangeoflocalitiesintheImbricatedZone,HighFoldedZoneandintheLowFoldedZone,inoutcropandsub-surfacesections.TheKometanFormationcanbedistinguishedlithologicallyfromotherCretac-eoussuccessionsinoutcropsoftheareaandintheLowFoldedZonewells.Itbecomesmarlytowardthewestandsouth-westofF.Rashidetal./JournalofPetroleumScienceandEngineering133(2015)147–161149Iraq(vanBellenetal.,1959;Dunnington,1958),anditsbiofacieschangeslaterallyfromamixtureofglobigerinallimestoneandoligosteginalintercalationtooligosteginalfacies(Buday,1980).Thethicknessofthiscarbonaterockunitisvariable,varyingfromitstypesectiontothesurroundingareaanddifferenttectoniczonesofKurdistan,andinthenorth-eastofIraqeveninthesametectonicdivision.Ingeneralithasavariablethicknessupto100–120mbut“averages40–60m”(Buday,1980).Itsthicknessisabout105mintheDokansection;78kmsouth-westofSulaimani,and110mintheTaqTaqOilField.ItsthicknessincreasesagaintowardtheKirkukembayment,anditreaches120minwellK-109and178minK-116oftheKirkukOilField.However,thethicknessis145minCH-2oftheChamchamalField,whichis60kmsouth-westofSulaimaniand50kmnorth-eastofKirkuk.UpperCenomanianoligosteginalfaciesoftheBalamboForma-tionunderlietheKometanFormationunconformably,butwithalackofangulardiscordance.Buday(1980)recognizedunconform-ablelowercontactsoftheKometanFormationwithboththeCenomanianDokan,AlbianUpperQamchuqaandTuronianGulneriFormations,whileKaddouri(1982)hasshownconform-ablecontactbetweenthebaseofthemarlysiltyglauconiticlimestoneoftheKometanFormationandthepebbly,sandydetritallimestoneoftheTelHajarFormation.JassimandGoff(2006)havealsoconfirmedthatthelowercontactsoftheKometanFormationwiththeunderlyingAlbian–CenomanianFormationsintheareaareunconformable.TheShiranishFormationoverliestheKometanFormation,againwithanunconformablecontact,butwithoutangulardis-cordance.AglauconiticdepositexistsatthebaseoftheShiranishFormationandcanbeusedasamarkerbed(vanBellenetal.,1959;Dunnington,1958).Thecontactoccasionallyappearsconformable(Buday,1980)orasadisconformity(JassimandGoff,2006).4.MaterialsandmethodsThedatausedinthispaperhavetwodifferentprovenances.Provenance1.DataandrocksamplesfromtheKurdistanRegion,whichencompassestheDokanoutcrop,80kmnorth-eastofSulaimanicity.InthisareatheKometanFormationismostcompleteandhaswell-differentiatedboundaries.ThisgroupofdataandsamplesalsocoverstheMiranWestexplorationlicense,located12kmwestofSulaimani,andtheTaqTaqfield,whichislocatedintheZagrosFoldedThrustBeltwithintheKirkukEmbayment.TheTaqTaqanticlineliesinthefoldedfoothillstothesouthwestoftheMountainFrontFault,whichseparatestheHighZagrosMountainsfromtheKirkukEmbay-mentandabout60kmnorth-eastoftheKirkukoilfield.ThisgroupalsocontainstheBardaRashexplorationlicenseintheLowFoldedZonetowardthenorthernmarginoftheinvesti-gatedarea,andabout20kmsouth-westofthecityofErbil.Fig.3.Tectonicevolutionofthenorth-eastmarginoftheArabianPlate(afterKarimandTaha,2009),wheretheterminology‘postdowning’usedbytheseauthorsreferstothesituationaftersubsidencehasoccurred.Fig.4.ChronostratigraphicdivisionofCretaceousrockinIraq(Al-Qayim,2010).F.Rashidetal./JournalofPetroleumScienceandEngineering133(2015)147–161150Provenance2.ThesecondgroupisdefinedasthatdataprovidedbytheNorthOilCompany,whichcoverstheKirkukembay-mentoftheLowFoldedZone,andincludestheKirkuk,Khabaz,BaiHassanandJamburfieldsinthecityofKirkuk,north-eastofIraq.TheKirkukoilfieldislocatedgeographicallyinthecentreofKirkukandtectonicallyispartoftheKirkukembaymentoftheZagrosFoldedThrustBelt.TheKhabazanticlineisoneoftheKirkukembaymentstructureslocated23kmsouth-westofKirkukcity,withrelativelyminorsurfaceexpressionbycom-parisontotheadjacentBaiHassan,JamburandKirkukanticli-nes.TheBaiHassanfieldislocatedinKirkukcity,trendingnorth-westtosouth-eastandparalleltotheKirkukfieldand20kmtoitssouth-westernside.TheJamburfieldislocatedsouth-eastofKirkuk.Thematerialthatwehavegatheredforanalysisinthispaperincludes173coreplugsamplesrepresenting99mofcorefromvariouswellsand95mofwholeoutcropintheDokanarea.Thesecoresampleswereusedinarangeoflaboratorypetrophysicalandpetrographicaltests.Petrophysicalwirelinedatafromsevenwellswerealsoanalyzed.Inaddition,weusedsomeexistingpetrophy-sicalmeasurementsthathadbeenmadepreviouslybytheresearchdepartmentoftheNorthOilCompany.TheresearchmaterialsaresummarizedinTable1.Approximately55coreplugsampleswereprovidedfrom5wells(Table1)bytheNorthOilCompany-Kirkuk,whileafurthersetofsamplesfromtheDoka
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