Sequence characteristics of the fourth member of Xujiahe Formation in the middle area of Western Sichuan Depression, Sichuan Basin

The sequence characteristics of the fourth member of Xujiahe Formation in the middle area of Western Sichuan Depression have been studied by using outcrop, core, logging and seismic data. The fourth member of Xujiahe Formation is a three-order sequence and was divided into lowstand system tract, transgressive system tract and highstand system tract. An isochronous sequence stratigraphic framework was established, and its internal sedimentary characteristics were clarified, mainly developing an alluvial fan-alluvial plain-delta-lacustrine depositional system. The sequence was divided into three types based on depositional settings: symmetrical type, asymmetrical type A1 and A2. Type A1 developed in the piedmont region before the Longmen Mountain, the type A2 mainly developed in Zhongjiang-Luodai area, and the symmetrical type mainly developed in the piedmont region before the Longmen Mountain and the depression between Zhongjiang and Luodai. LST and HST of the asymmetric type and symmetric type A2 sequence were advantageous for the development of high-quality reservoirs.