Hydrocarbon generation-expulsion-retention simulation experiments of coal rock with different roof lithology combinations and its implications for exploration

To investigate the controlling mechanism of roof lithology on coalbed methane (CBM) formation and enrichment, low-rank coals from the Carboniferous Taiyuan Formation in the Qingshuihe area, northeastern Ordos Basin, were selected as the research objects. Based on the actual geological background of exploration area D in the central-eastern Ordos Basin, temperature-pressure simulation conditions were designed. Using a hydrocarbon generation-expulsion simulation apparatus, hydrocarbon generation-expulsion-retention simulation experiments were conducted on coal rocks under three roof lithologies (sandstone, limestone, mudstone). The results showed that: (1) Under the three roof conditions, the total hydrocarbon gas yield continuously increased with thermal maturity. It increased rapidly after the vitrinite reflectance (R_o) reached 1.2% (simulated temperature 370 ℃), and then the growth rate slowed down after R_o reached 2.0% (simulated temperature 450 ℃). Among the three roof conditions, limestone roof yielded the highest gas production. (2) The gas retention capacity was significantly positively correlated with roof breakthrough pressure. The limestone roof (breakthrough pressure 80.12 MPa) had the strongest sealing capacity, and its coal rock exhibited the highest ratio of retained hydrocarbon gas and the highest retained gas content per unit rock at all evolution stages, especially when R_o ≥ 1.50% (simulated temperature > 400 ℃). The mudstone roof (breakthrough pressure 25.43 MPa) ranked second, and the sandstone roof (breakthrough pressure 8.77 MPa) was the lowest. At R_o = 1.50%, the retained gas content under limestone and mudstone roof combinations was 1.33 and 1.23 times that of the sandstone roof combination, respectively. Roof lithology controlled the maximum enrichment degree of CBM at the source by governing the expulsion efficiency of overpressured fluids during the hydrocarbon generation stage. High-breakthrough-pressure roofs (limestone, mudstone) effectively sealed free gas and maintained reservoir pressure, thereby ensuring high gas content. Exploration practices confirmed that in exploration area D of the Ordos Basin, the gas content in coal seams with limestone/mudstone roofs was significantly higher than that of those with sandstone roofs. This study clarifies the critical role of roof in the "source-reservoir-seal" system and suggests that areas with "limestone-coal" and "mudstone-coal" combinations in regions such as the Ordos Basin, western Hubei, and eastern Chongqing are favorable targets for CBM exploration.