The evolutionary history of methane adsorption capacity with reference to deep Carboniferous-Permian coal seams in the Jiyang Sub-basin: Combined implementation of basin modeling and adsorption isotherm experiments

Carboniferous-Permian (C-P) coal seams experienced multiple burial and uplift events, which result in complex gas adsorption and desorption processes. Therefore, the understanding of the adsorption history and its dominant factors contribute to determining present-day coalbed methane (CBM) storage and enhance the development of deep CBM. In this paper, we first reconstructed the burial history using the basin model and synchronously conducted the isothermal adsorption experiment. Then an adsorption model was achieved from regression analysis about Langmuir parameters, and therefore, the adsorption history was rebuilt, accompanied by temperature, pressure, and maturity. Lastly, incorporated with the hydrocarbon generation history and adsorption history, we divided the actual adsorbed stages and further analyzed the dominant factors controlling the actual adsorption amount. The results indicate that: (1) based on the adsorption model calculated using regression analysis, a real critical depth of adsorption capacity (approximately 2000 m) is obtained; (2) coal-bearing sequences experience three temperature, pressure, and hydrocarbon generation processes, and the Himalayan epoch is the main stage for producing coal-derived gas; (3) the actual adsorption history contains a maximum depth stage (250-208 Ma), reburial stage (208-110 Ma), re-uplift stage (110-50 Ma), and present stage (50 Ma-present); (4) the critical event (208 Ma) represents that generating the amount of gas is equal to the adsorption capacity in the actual adsorbed amount, and before and after the event, the dominant factors controlling the actual adsorbed amount are maturity and temperature, respectively.