A New Understanding of Reef and Beach Facies in the Maokou Formation of Southwest Sichuan Basin
The gas reservoirs in Maokou Formation of Middle Permian in Sichuan Basin are mostly fracture vuggy limestone reservoirs with limited scale.In recent years,high-energy shoal facies reservoir of Maokou Formation have been drilled succes-sively in wells YB7 and JT1 in northern Sichuan Basin and high-yield industrial gas flow is obtained,which made an important breakthrough in platform margin exploration of oil and gas of Maokou Formation in Sichuan Basin.Based on the basic re-search on sedimentation,geology and paleogeomorphology,combined with the latest high-quality 3D seismic,time-frequency gravity and magnetic data,it is found that:1)Maokou Formation in southwest Sichuan Basin develops open platform-rimmed platform margin-depression edge-slope-intra platform depression sedimentation,depression edge reef beach facies distribute in large scale,and the reflection characteristics of"mound like,disordered,intermittent moderate weak amplitude"in the three-dimensional seismic sections are obvious;2)the NE trending basement strike slip faults developed in the southwest Sichuan Basin,mainly formed in the Xingkai rifting movement period,reactivated in the Emei rifting period,resulting in the graben horst pull apart pattern,which is one of the main controlling factors for the formation of the rimmed platform margin of Maokou Formation;3)the depression edge reef beach facies of Maokou Formation overlapped with the fresh water karst in the Dongwu Period,which has the basis of forming a large-scale and high-quality reservoir.It is located in the high part of the structure in the period of oil and gas accumulation and is the long-term direction area of oil and gas migration and accumulation.The study has a certain guiding significance for expanding the exploration field and increasing reserves and production in southwest Sichuan Basin.