Coexistence and Enrichment Models of Coal-Measure Gas in Complex Faulted Basins: A Case Study of the Carboniferous–Permian in the Dagang Exploration Area, Huanghua Depression

[Objective] This study aims to clarify the superimposed gas-bearing systems and coexistence-enrichment models of coal-measure gas in complex faulted basins. [Methods] Taking the Carboniferous–Permian Taiyuan and Shanxi formations in the Dagang exploration area of the Huanghua Depression as the research object, we comprehensively analyzed source–reservoir assemblages, key barrier layers, and gas-bearing system architectures based on geochemical characteristics of source rocks, reservoir properties, gas-bearing potential, carbon isotope signatures of natural gas, and mercury injection capillary pressure tests. [Results] The results show that coal, mudstone/shale, and tight sandstone in the study area form three types of configurations: source–reservoir integration, source–reservoir juxtaposition, and composite arrangements. Coal-measure reservoirs are underpressured at shallow depths, while both normal pressure and overpressure coexist at depth; gas-bearing potential fluctuates but generally increases with burial depth. Significant geochemical differences in gas composition are observed between the Taiyuan and Shanxi formations. Mudstone and tight siltstone effectively limit vertical fluid exchange, resulting in three types of superimposed gas-bearing systems: mixed superimposed, independent superimposed, and unified superimposed systems. Furthermore, three coexistence-enrichment models are developed: coalbed methane–tight sandstone gas, coalbed methane–shale gas–tight sandstone gas, and coalbed methane–shale gas. [Conclusion] The coexistence and enrichment of coal-measure gas are jointly controlled by lithological associations, source–reservoir proximity, reservoir connectivity, and barrier conditions. This work provides a theoretical basis for identifying favorable intervals of coal-measure gas in complex faulted basins.