Sequence stratigraphy and sedimentary evolution of Changxing—Feixianguan Formations in Yilong—Pingchang area, Sichuan Province

The Yilong—Pingchang area in Sichuan, especially the Changxing—Feixianguan Formations in the region surrounding the Kaijiang-Liangping trough, contains a large amount of oil and gas resources, but the third class sequence classification scheme needs to be unified. Based on the comprehensive analysis of the latest drilling core, well logging, and field outcrop data, two types of type II sequence interfaces, including the lithologic facies abrupt interface and the partial exposure unconformity interface, were identified. The Changxing—Feixianguan Formations in the study area were divided into 4 third class sequences, and a sequence stratigraphic framework was established. The target stratum in the study area mainly developed a typical rimmed platform sedimentary system. In the late stage, the trough disappeared, and the open platform and evaporate platform of the epicontinental sea platform system developed. During the deposition of the Changxing Formation, the Kaijiang-Liangping trough gradually expanded, and the highstand system tract was a favorable stage for the development of reefs. Due to the rapid decline in sea level in the late SQ2 period, the scope of the reefs was reduced compared to the SQ1 period. The sea level rose rapidly in the early depositional stage of the Feixianguan Formation, and it was difficult to form oolitic shoals, and then entered the overall regression stage, and to the highstand system tract of SQ3, the oolitic shoals developed contiguously. The sequence classification scheme matches the development characteristics of reef and shoal reservoirs and can be applied to the next step of oil and gas exploration.