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川西坳陷中段上三叠统-侏罗系天然气成藏地球化学研究.docx

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川西坳陷中段上三叠统-侏罗系天然气成藏地球化学研究 1.Introduction TheSichuanBasinisoneofthemostprolificregionsfortheexplorationandproductionofnaturalgasinChina.TheUpperTriassicandLowerJurassicformationsintheSichuanBasinhaveyieldedsignificantamountsofnaturalgas.Theobjectiveofthisstudyistoinvestigatethegeological,geochemical,andreservoircharacteristicsoftheUpperTriassicandLowerJurassicformationsinthemiddlesectionoftheSichuanBasin,inordertobetterunderstandthenaturalgasaccumulationmechanismsintheregion. 2.GeologicalSetting ThemiddlesectionoftheSichuanBasinischaracterizedbythepresenceoftheSichuanFoldBelt,whichisaseriesofcomplexlyfoldedandfaultedstructuresthatformedduringtheMesozoicera.TheSichuanFoldBeltiscomposedofseveralnorthwest-southeasttrendingstructuralbelts,includingtheLongmenShan,DabaShan,andQinlingShan. TheUpperTriassicandLowerJurassicformationsinthemiddlesectionoftheSichuanBasinareprimarilycomposedofsandstones,shales,andcarbonates.Theseformationsweredepositedinavarietyofenvironments,includingfluvial,deltaic,lacustrine,andmarine. 3.GeochemicalCharacteristics ThenaturalgasintheUpperTriassicandLowerJurassicformationsinthemiddlesectionoftheSichuanBasinisprimarilycomposedofmethane,withlesseramountsofethane,propane,andbutane.Thecarbonisotopiccompositionofthemethaneintheseformationsistypicallybetween-41‰and-46‰,indicatingathermogenicorigin. ThebiomarkercompositionsoftheUpperTriassicandLowerJurassicformationsareconsistentwithatypeII-IIIkerogensource,whichsuggeststhatthenaturalgaswasgeneratedfrommarineorganicmatter.Thematurityofthesourcerocksintheregionisinthehightoover-maturestage,indicatingthatthenaturalgaswasgeneratedduringdeepburial. 4.ReservoirCharacteristics ThereservoirsintheUpperTriassicandLowerJurassicformationsinthemiddlesectionoftheSichuanBasinareprimarilycomposedoftightsandstonesandshales.Thereservoirsarecharacterizedbylowporosityandpermeability,whichmakesnaturalgasextractionchallenging. Inordertoenhanceproductionfromthesereservoirs,hydraulicfracturingandotherstimulationtechniquesare