Geochemical Characteristics of Coalbed Methane and Its Multifactor Control in High-Rank Coal Reservoirs, Southern Qinshui Basin, China

The investigation of the origin, occurrence, and migration mechanisms of coalbed methane (CBM) in high-rank coal reservoirs was reported in this study via a comprehensive analysis of CBM geochemical data from the Zhengzhuang and Fanzhuang blocks to establish the distribution pattern of methane carbon isotopes. Combined with reservoir information, geological characteristics, and hydrodynamic conditions, the CBM genesis and its control factors were elucidated and further quantitatively evaluated by gray correlation analysis. The results demonstrate that the carbon isotope ratio of methane (δ¹³C₁ value) mainly varies between -57.80‰ and -27.35‰ with an average of -34.21‰, indicating that CBM is dominated by thermogenic gas in the study area. While the δ¹³C₁ value is light at the center and heavy in the surroundings of the plane. When the maximum reflectance of vitrinite (<i>R</i> _o,max) is between 3.00% and 4.50%, the δ¹³C₁ value slightly decreases with an increase in thermal evolution degree. In addition, the δ¹³C₁ value exhibits a positive correlation with vitrinite, gas content, coal thickness, and burial depth. Further, by means of the gray correlation analysis, the order of the influence of various factors on methane carbon isotopes is determined as follows: thermal evolution degree > vitrinite content > gas content > coal seam thickness > gas saturation > inert matter content > burial depth. Thus, the distinctive geochemical characteristics of high-rank coal reservoirs were formed via the synergistic effects of thermal evolution, macerals, and gas content. On the one hand, during high thermal evolution, massive micropores are generated to promote the partial retention of ¹²CH₄ within the coal matrix, resulting in a relatively light trend in δ¹³C₁. Meanwhile, a high vitrinite content implies the presence of excess aromatic macromolecular condensates to facilitate the occurrence of more ¹³CH₄, thus appearing as a heavy δ¹³C₁ value. On the other hand, considerable burial depth, complex geological structures, and strong hydrodynamic activity are highly prone to induce the fractionation effect of methane carbon isotopes, including diffusion, migration, water solubility, and secondary biogenic gas.