Reservoir Characteristics and Controlling Factors of the Upper Permian Changxing Formation Reef‑Shoal Carbonate Reservoirs in the Huanglongchang Structure, Eastern Sichuan Basin

Abstract

The Upper Permian Changxing Formation reef-shoal carbonates in the Huanglongchang Structure, eastern Sichuan Basin, are important gas reservoirs, yet their characteristics and controlling mechanisms remain poorly understood. We integrated 481 core samples and 126 cast thin sections from 8 wells with logging, petrophysical, and structural data to characterize reservoir properties and controls. The results show that: (1) Reservoirs are dominated by oolitic, reef breccia, and rudstone dolomites, with secondary dissolution pores as the main storage space. They exhibit low porosity, low permeability, and strong heterogeneity, with porosity of 4%-12% and permeability of 0.1-10 mD. (2) The thermogenic dry gas contains 92.86%-97.41% methane. CaCl$_2$-type formation water has an average TDS of 110.15 g/L, indicating a strongly confined accumulation environment. (3) Platform-margin reef-shoal facies are most favorable for reservoir development, particularly back-reef shoals, with an average effective porosity of 8.75%. (4) Multi-stage dolomitization and dissolution are the principal constructive diagenetic processes. Burial dolomitization creates a compaction-resistant intercrystalline pore framework, while karstification and burial dissolution enlarge storage space. Yanshanian and Himalayan fractures further enhance permeability, with fracture intensity increasing toward structural highs and greater fold curvature. Therefore, overlap zones of platform-margin reef-shoal bodies and structural highs are most favorable for high-quality reservoirs. These results establish a coupled model of ``sedimentary facies constraint-diagenetic modification-tectonic fracture enhancement'' and provide guidance for reef gas exploration around the Kaijiang-Liangping Trough.