Institute of Exploration and Development, PetroChina Southwest Oil and Gasfield Company, Chengdu, 610051 Sichuan
Abstract  The sedimentary environment of the Early Feixianguan Stage of Early Triassic is mainly inherited the pattern of the Late Changxing Stage of Late Permian. The northeastern Sichuan area is evaporate carbonate platform. The Feixianguan Formation of evaporate carbonate platform is a set of sulfate-bearing evaporate tidal sediments with aboundance of dolomite. The lower part is micro-tidal difference evaporate tidal sequence constituted by thin-bedded gypsum-bearing crystal, muddy-limestone and dolomicrite. The middle part is a set of great-tidal difference tidal sequence including oolitic shoal facies which is made up of bedded-oolite dolomite, oolite limestone, gypsum-tidal facies in sabkha environment which is constituted by bedded gypsum and gypsum-bearing gypsum-dolomite tidal faecies. The upper part is a set of evaporite quasi-tidal sequence with clear cycles which is made up of middle-thin-bedded mud abounding, gypsum-bearing muddy limestone and dolomicrite. Vertical combination of these three squences can be well correlated in evaporite platform area even though thicknesses of each squence are obviously different. Strongly dolomized oolite dolomites are the main reservoir of middle-large scale gas reservoir of the Feixianguan Formation in Sichuan Basin. According to its facies squence features, the oolitic rocks is divided into platform marginal oolitic bar and inner-platform oolitic bank. Platform marginal oolitic bar along the west margin of the evaporite platform was during the Feixianguan Stage, while the platform marginal oolitic bar along the eastern margin greatly shifted towards the basin during the Middle-Late Feixianguan Stage.