Evolution of Pore Structure and Methane Adsorption in Lower Silurian Longmaxi Shale: Implications for Uplifted Shale Gas Reservoirs

Pore volume, pore size and adsorption capacity are fundamental factors that control gas storage and production in shale reservoirs. We investigated the evolution of pore structure and adsorption capacity as functions of lithology and present-day burial depth. Nineteen shale samples from the Wufeng-Longmaxi Formation from shallow depths (present depths < 700 m) in the Sichuan Basin, China, were analyzed to elucidate the evolution of the pore system, applying N 2 /CO 2 isotherm adsorption, high-resolution FE-SEM, FIB-SEM, and CH 4 isotherm adsorption. Two comparison basin models with 700 m and 4000 m present-day burial depth were constructed to illustrate the impact of uplift on pore structure and methane adsorption. The results indicate that mineral structure, rock fabric, and TOC content are essential factors affecting the pore structure. Pores of various sizes are preserved through different mechanisms during the compaction. The development of micropores is mainly controlled by thermal maturity and TOC content. In contrast, mesopores and macropores are stress-sensitive and prone to be compacted during deposition. Methane adsorption capacity is highly related to the burial depth. During the process of reservoir uplift, the methane adsorption capacity of the reservoir gradually increases with decreasing burial depth, with a transition point occurring between 700 and 1300 m. Furthermore, as the reservoir continues to uplift to burial depths below 700 m, the methane adsorption capacity of the reservoir in normally pressured shale gas formations rapidly decreases. However, for overpressured shale gas reservoirs, there is no significant change in the methane adsorption capacity of the reservoir.