Controlling factors of coalbed methane occurrence below redbeds in Xutuan mine: Caprock thickness below redbeds

Coalbed methane (CBM) occurrence is highly significant in gas drainage design and gas disaster prevention. The redbeds of Xutuan mine, red clastic sedimentary rocks, distributed in the southeastern region of the mine, generally make gas contents lower than normal area, which could make CBM occurrence more complicated and ambiguous to determine. To explore the CBM occurrence law and its controlling factors in the redbeds-covered area (RCA), this study constructed geological profiles based on geological exploration drillings, and then analyzed the geological evolution and the critical periods of gas emission in the RCA combining regional geological evolution and sedimentary history of the Guzhen Basin. Additionally, coal samples obtained from the RCA and the non-redbeds covered area (N-RCA) were analyzed through testing the proximate analysis, adsorption constant, and vitrinite reflectance. There are two critical gas emission periods, the Yanshan tectonic movement period when the Huaibei strata lifted and denuded and the deposition of the redbeds in the Guzhen Basin. During the two periods, the caprock below redbeds played a role in gas emissions. The differences in the proximate analysis, adsorption capacity, and metamorphosis between the coal samples from the two regions were relatively small, indicating that redbeds deposition had a minimal effect on the coal properties of the coal seams. Finally, the linear correlation between the in-situ gas contents and caprock thickness below redbeds was determined to be higher than that between the contents and buried depth, proving that the controlling factor of CBM occurrence is the caprock thickness below redbeds in the RCA.