Main controlling factors and accumulation model of shale gas enrichment in the Permian Dalong Formation, Western Hubei-Eastern Chongqing complex structural zone, China

To clarify the main enrichment-controlling factors and accumulation mechanisms of shale gas in the Permian Dalong Formation within the Western Hubei–Eastern Chongqing complex structural zone, this study systematically reveals the enrichment patterns and accumulation model through analysis of typical drilling data, geochemical testing, scanning electron microscopy (SEM), methane isothermal adsorption experiments, numerical simulations, and research on tectonic evolution and preservation condition. The results are obtained in two aspects. First, the enrichment of shale gas in the Dalong Formation is synergistically controlled by four factors, i.e. rift troughs controlling shale development, provenance controlling reservoir heterogeneity, temperature and pressure controlling gas occurrence, and structure controlling differential enrichment. The geometry and scale of rift troughs (Chengkou-Western Hubei, and Kaijiang-Liangping) determine the development of organic-rich shale (average TOC>6%, thickness of 15–50 m). Multi-source materials lead to strong heterogeneity of the reservoir, with endogenous minerals as the main component (accounting for 74.31%), and the pores mainly organic matter pores (micropores and mesopores accounting for 93.4%). The formation temperature and pressure control the occurrence state of shale gas, with adsorbed gas (higher than 50%) dominantly in 500–2 750 m depth, while free gas (higher than 50%) prevailing at depth deeper than 2 750 m depth. The uplift, erosion, and fault systems associated with the Yanshanian tectonic activity result in differential enrichment of shale gas, with three structural styles—broad gentle anticlines, residual synclines, and low gentle slopes—exhibiting relatively high shale gas enrichment. Second, the self-sealing mechanism of medium-shallow shale gas in the Western Hubei–Eastern Chongqing complex structural zone is revealed. Specifically, the Dalong Formation shale aquifer forms a lateral seal for shale gas in the downdip direction via water films and capillary forces, and it combines with the overlying Daye Formation limestone and underlying Xiayao Formation tight layers to establish a synclinal/monoclinal self-sealing accumulation model. The geological insights, such as “four-factor synergistic control” and self-sealing accumulation model, provide a dynamic coupling evaluation framework for shale gas in complex structural zones, promoting the transition of shale gas exploration and evaluation from static descriptions to integrated reservoir-tectonic-fluid analysis.