Sedimentary characteristics and geological significance of reef-shoal deposits in carbonate shelf settings: a case study of the Upper Permian Changxing Formation in the Mianyang–Yanting area, northwestern Sichuan Basin

Abstract

Conventional views suggest deep-water, low-energy deposition in intra-platform depressions, limiting exploration in transitional zones. However, new data from the Pengxi–Wushen area reveal extensive high-energy reef-shoal complexes along the carbonate ramp margin during the Changxing Stage. This study characterizes these deposits and their reservoir significance. During the Upper Permian Changxing period, the Sichuan Basin exhibited a sedimentary framework characterized by “three uplifts and three depressions.” Reef–shoal reservoirs developed in the high-energy facies zone associated with transitions between uplifts and depressions, particularly along the platform-margin reef–shoal gas reservoirs on both sides of the Kaijiang–Liangping trough, where exploration breakthroughs have continued. However, an unclear paleogeographic framework in the transitional area from platform depressions to deep-marine settings in central Sichuan has constrained the exploration and deployment of reef–shoal reservoirs in this region. Based on previous studies, thin-section identification, drilling and logging data, and seismic interpretation, the paleogeographic framework of the Upper Permian Changxing period in the Yanting–Nanchong area of western Sichuan was investigated. The uplift–depression configuration and the development patterns of reef–shoal deposits under a carbonate shelf setting were clarified. The results indicate that open-platform facies, fore-platform slope facies, shelf facies, and basin facies successively developed from central to northwestern Sichuan. Overall, the study area is located in the transitional zone from the Pengxi–Wushengtai Depression to the northwestern Sichuan Basin. This framework was controlled by regional extensional tectonics and secondary interplatform tectonic processes, resulting in localized uplift–depression sedimentary patterns within the platform. The results reveal the presence of high-energy zones favorable for reef–shoal development within the shelf area and provide a geological basis for the deployment of risk exploration wells in northwestern Sichuan.