The Permian Longtan Formation in the Songzao coalfield, Southwest China, has abundant coalbed methane (CBM) stored in high-rank coals. However, few studies have been performed on the mechanism underlying the differences in CBM gas content in high-rank coal. This study focuses on the characterization of coal geochemical, reservoir physical, and gas-bearing properties in the coal seams M₆, M₇, M₈, and M₁₂ based on the CBM wells and coal exploration boreholes, discusses the effects of depositional environment, tectono-thermal evolution, and regional geological structure associated with CBM, and identifies major geological constraints on the gas-bearing properties in high-rank coal. The results show that high-rank coals are characterized by high TOC contents (31.49~51.32 wt%), high T_max and R₀ values (averaging 539 °C and 2.17%), low HI values (averaging 15.21 mg of HC/g TOC), high porosity and low permeability, and high gas-bearing contents, indicating a post-thermal maturity and a good CBM production potential. Changes in the shallow bay–tidal flat–lagoon environment triggered coal formation and provided the material basis for CBM generation. Multistage tectono-thermal evolution caused by the Emeishan mantle plume activity guaranteed the temperature and time for overmaturation and thermal metamorphism and added massive pyrolytic CBM, which improved the gas production potential. Good geological structural conditions, like enclosed fold regions, were shown to directly control CBM accumulation.

The Permian Longtan Formation (burial depth >2000 m) is a major hydrocarbon-producing unit in central Sichuan Basin, China, and an important heterogeneous unconventional gas reservoir. However, the control of sedimentary environment on organic matter enrichment in its coal-bearing strata remains unclear, hindering deep coalbed methane (CBM) exploration in this region. This study analyzed core samples from NT1H Well using petrological and geochemical methods (e.g., thin-section, X-ray diffraction, trace elements, total organic carbon, FE-SEM, and macerals) to characterize the middle-lower Longtan Formation. It examined how sedimentary environments influence organic matter enrichment and generating-gas capacity (abundance, maturity, and component), revealing key controls. Results show the lithologies in the middle-lower Longtan Formation include mudstone/shale, silty mudstone, and tuffaceous-muddy siltstone-standstone interbedded with multiple thin coal seams, mainly containing clay minerals, quartz, pyrite, and siderite. Horizontal bedding, horizontal-wavy bedding, and parallel bedding are common. Coal seams and their surrounding mudstones/shales have higher organic matter abundances than muddy siltstones-sandstones, indicating strong capacities of organic matter accumulation and gas generation. Coal seams, dominated by vitrinite with minor inertinite, show the highest organic abundance and maturity (TOC 43.76 wt% to 77.97 wt%, R ₀ >2.5%) than surrounding mudstones/shales. The coal-bearing strata is mainly assigned to lagoon deposits, with two microfacies: sandy-muddy lagoon and peat swamp. The peat swamp is formed by a rapid transformation of muddy lagoon, with sufficient supply of terrestrial materials. These results reveal that the peat swamp microfacies, characterized by weak hydrodynamics and anoxic-to-reducing conditions, is the primary control on the development of multiple thin coal seams, mudstone/shale, and carbonaceous shale, constraining the degree of organic matter enrichment. This study clarifies the sedimentary environmental constraint on organic matter enrichment in the coal-bearing strata, providing a geological basis for evaluating deep CBM resources in central Sichuan Basin.