Discovery of isolated shoals in the Permian Maokou Formation of eastern Sichuan Basin and their natural gas exploration potential
The Middle Permian Maokou Formation has long been one of the key targets for natural gas exploration in the Sichuan Basin,and the exploration and development of natural gas in the Maokou Formation have mainly focused on karstic fractured-vuggy limestone reservoirs in southern Sichuan Basin and dolomitized shoal facies reservoirs in central Sichuan Basin successively.In order to ascertain the prospects of natural gas exploration of the Maokou Formation,a lot of basic research efforts have been taken in eastern Sichuan Basin where the exploration maturity is relatively low.On the basis of further studying the stratigraphic framework,a systematic analysis is conducted on the sedimentation,reservoir and hydrocarbon accumulation of the Maokou Formation,the existence of isolated shoals of lower Mao-2 Member below the silicite of Gufeng Member is pointed out.Finally,the exploration potential of natural gas in the Maokou Formation is clarified.The results show that before the sedimentation of event silicite in Gufeng Member of Maokou Formation in eastern Sichuan Basin,isolated carbonate shoals had been developed in the lower part of the second member of Maokou Formation(lower Mao-2 Member)in the Xuanhan-Wanzhou area,with an area of up to 6 500 km2.Shoal facies matrix pore reservoirs have been discovered in the lower Mao-2 Member of the Xuanhan-Wanzhou area:the porous dolomite reservoir developed at the bottom of lower Mao-2 Member is 3.0-12.0 m thick,and the porous limestone reservoir developed in the middle-upper part of lower Mao-2 Member is 5-18 m thick.The hydrocarbon accumulation conditions of shoal facies reservoirs in the Maokou Formation are excellent in eastern Sichuan Basin:the source rock mainly consists of thick shale of underlying Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation,and the faults connecting the source rocks are developed,while the regional caprocks are mainly composed of silicite of Gufeng Member and marine mudstone and shale of Wujiaping Formation,the preservative conditions are good.It is concluded that the discovery of isolated shoals and favorable hydrocarbon accumulation conditions of Maokou Formation in eastern Sichuan Basin will further accelerate the process of natural gas exploration and development in the region,and the high-energy shoals widely distributed in the Maokou Formation are expected to become the next replacement option of natural gas exploration and development.
Eastern Sichuan BasinPermianMaokou FormationGufeng MemberIsolated shoalReservoir characteristicsNatural gas exploration potential
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