Shale pore evolution and variation are very important to evaluate shale gas reservoirs. Hydrous pyrolysis experiment in a semi-closed system, low-pressure gas (N₂ and CO₂) adsorption and high-pressure methane adsorption, FE-SEM analysis, and geochemistry experiments were applied on Upper Permian organic-rich shale from Dalong formation, Sichuan Basin, to understand the nanopores and adsorption capacity evolution during maturation process, and to explore the role of organic matter (OM) pores on porosity and adsorption capacity. Analysis of pore structure parameters revealed that micropores primarily contribute to the specific surface area (SSA), while macropores dominate the total pore volume (PV). The overall PV and SSA increase with thermal maturity (%R_o), exhibiting two peaks associated with the last stages of oil generation and kerogen conversion to gas, and the cracking peak of liquid hydrocarbon to dry gas. Shale porosity formation and development are primarily influenced by diagenesis and hydrocarbon generation, while OM pores evolve with thermal maturity, particularly during the thermal cracking of kerogen and secondary OM into hydrocarbon. Most organic pores are micropores, especially within the diameter range of 0.3 to 0.9 nanometers. The PV and SSA of organic micropores significantly contribute to the overall pore characteristics, especially at over-mature stage. The presence of OM pores influenced by TOC content and thermal maturity, significantly enhances porosity and adsorption capacity. Mineral composition has a minor influence, with thermal maturity often overshadowing its impact. The conversion of OM during maturation has a pivotal role in influencing methane adsorption capacity, with high maturity stages favoring adsorption in organic-rich shale. The majority of adsorbed methane was hosted by organic pores. The organic pores, predominantly micropores, are characterized by a substantial SSA, making a significant contribution to the adsorption capacity. This emphasizes the crucial influence of organic pores, especially micropores, on the gas adsorption dynamics within shale reservoir.
 

Hydrothermal pyrolysis, Organic matter pores, Porosity, Kerogen, Thermal maturity