Sedimentary facies and sand body distribution of the Middle Jurassic Upper Shaximiao Formation in the middle segment of western Sichuan depression

Abstract：Guided by the theory of high-resolution sequence stratigraphy, based on the classification of base level cycle sequence, and aimed at the distribution characteristics of sand bodies, this paper has divided Upper Shaximiao Formation into base level cyclic sequences of one long period (L1) and four moderate periods (M1，M2，M3，M4). Sequence-based lithofacies and paleogeographic maps of Upper Shaximiao Formation were compiled on the basis of mapping units of middle period base level cyclic sequence. The result shows that meandering river delta facies and lake facies were developed during the stage of M1 and M2. During the stage of M3 and M4, the depositional setting was changed and characterized by the development of alluvial fan facies, alluvial plain facies, braided river delta facies and lake facies. The sand bodies were distributed in two areas, i.e., Dayi-Wenjiang area and Zhongjiang area: the former occurred during the stage of M1, M3 and M4, whereas the latter came into existance during the stage of M1，M2 and M3.