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Hydrocarbon accumulation conditions of the weathering crust paleokarst gas reservoir of Leikoupo Formation,West Sichuan,China

MENG Yu-zhang;XU Guo-sheng;LIU Yong;YANG Cheng;YUAN Hai-feng;LIANG Jia-ju;State Key Laboratory of Oil and Gas Reservoir Geology and Exploration,Chengdu University of Technology;  
Good evidences of oil and gas appear in the top of Middle Triassic Leikoupo Formation in West Sichuan in recent drillings.In order to look into the accumulation conditions of the oil and gas in West Sichuan,this paper analyzes the accumulation geologic conditions of the weathering crust paleokarst gas reservoir by the methods including the rock thin section observation,organic geochemical analysis,and isotope analysis on the basis of the previous study,and reconstructs the process of hydrocarbon accumulation of the paleokarst reservoir of the top of Leikoupo Formation in West Sichuan.The result shows that the type of gas reservoir has two sets of hydrocarbon source ofMaantang Formation and Xiaotangzi Formation with sufficient oil and gas source.The weathering crust paleokarst reservoirs of Leikoupo Formation have good behaviors and develop secondary pores.The source-reservoir-caprock association of the gas reservoir is as follows.The source rocks of Maantang Formation and Xiaotangzi Formation,the weathering crust paleokarst reservoirs of Leikoupo Formation,and the cap rock of the Upper Triassic muddy rocks match well and have good preservation conditions.In the process of migration and accumulation,the hydrocarbon is affected by the Indosinian paleouplift tectonic belt and the unconformity surface of Leikoupo Formation.The trap type of this gas reservoir is a tectonics-strata combination trap.In the process of formation and evolution,the gas reservoir has experienced four important stages,that is the building stage of epidiagenesis and pores in the end of Middle Triassic,the formation stage of the paleouplift tectonic belt beneficial to hydrocarbon migration and accumulation in the middle and late period of Late Triassic,the stage of the paleouplift tectonic belt inheritance and development and large-scale oil-gas accumulation in Yanshanian,and the final shaping stage of the gas reservoir in Himalayan.The gas reservoir accumulation conditions of the top of Middle Triassic Leikoupo Formation in West Sichuan are favourable and have extensive natural gas exploration prospects.
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