Sequence Stratigraphy of the Middle Permian strata in northeastern Sichuan Basin

Four sequence boundaries in the Middle Permian strata in northeastern Sichuan Basin are identified:uplift erosion unconformity, subaerial sequence unconformity, transgressive (coastal) onlap unconformity and lithologic-lithofacies unconformity on the basis of outcrops, well logs, seismic and palaeontological data. Three third-order sequences encompass SQ1 corresponding to the Liangshan and Qixia Formations with a time duration of ca. 7 Ma; SQ2 corresponding to the first member of the Maokou Formation with a time duration of ca. 3 Ma, and SQ3 corresponding to the second and third members of the Maokou Formation with a time duration of ca. 5 Ma. The controlling factors include:(1) the controls of the sequences on sedimentary facies, which exert an important influence on the development of primary porosity in the reservoirs, and (2) the controls of carbonate rocks development by the eustatic fluctuations in the third-order sequences. The excellent reservoirs are apparently constrained by the transgressive semi-cycles.