Sequence stratigraphy and isotope geochemical response and development pattern of reef and shoal: a case study of Changxing Formation in the marginal zone of Eastern Sichuan Chengkou-Western Hubei Oceanic Trough

The Upper Permian Changxing Formation reef and shoal in the eastern Sichuan Basin has great potential for oil and gas exploration. The analysis of sequence stratigraphic and isotopic composition response as well as reef and shoal development patterns can provide a reliable geologic basis for reef and shoal exploration in this area. Observation of the typical outcrop section and the sequence stratigraphic analysis of the margin of the Chengkou-Western Hubei Oceanic Trough, combined with the carbon isotope composition of the Changxing Formation, show the sequence stratigraphic and geochemical response characteristics of the Changxing Formation and the reef and shoal development patterns. The Changxing Formation was divided into two third-order sequences and five fourth-order sequences. The carbon isotope distributions in the sequence framework have a good relationship with sea level fluctuation. There are obvious differences in the development patterns of reef and shoal in different sedimentary facies belts. The Panlongdong section on the windward side is in the highest energy platform margin belt, where reef and shoal are the most developed, the dolomitization of reef and shoal is obvious, and the reservoir is developed. In the Yanggudong section, which is on the leeward side of the platform margin, the development of reef and shoal takes the second place, and the dolomitization is weak. However, only the intra-platform grain shoal is developed in the Dukou section in the open platform, and reservoir is not developed.