Mechanisms and Predictive Modeling of Deep Coalbed Methane Enrichment: Insights from the Jinzhong Block, Qinshui Basin

To elucidate the mechanisms of deep coalbed methane (CBM) occurrence and enrichment patterns in the Jinzhong Block of the Qinshui Basin, coal and rock characterization, methane isothermal adsorption experiments, and in situ gas content measurements were integrated in this study. By incorporating external geological sealing conditions (structural, stratigraphic, and hydrokinetic), a four-dimensional conceptual framework for CBM reservoir accumulation was established: “coalification evolution–adsorption mechanism–quantitative prediction–enrichment zoning”. It has been demonstrated that the coal seams in this area were formed in a strongly reducing, deep-water, stable sedimentary environment and are characterized by a highly enriched vitrinite fraction. Subsequent high-temperature evolution produced coal with a very low moisture content, low to medium ash content, extremely low volatile matter, and a distinctly developed pore structure. The methane adsorption capacity is controlled by the ternary interaction of temperature, pressure, and coalification degree: pressure exerts a positive influence, temperature exerts a negative influence, and increasing coal rank markedly enhances the adsorption capacity by optimizing coal quality and pore structure. An in situ adsorption gas volume prediction model was developed using temperature, pressure, and Ro,max as parameters. The results indicated that its evolution with burial depth is characterized by a “two-stage response” and a “coal rank compensation effect”. By comparing in situ predicted values with measured gas content, the controlling role of geological sealing in gas preservation was clarified, and four enrichment patterns were identified: stable sealing, roof diffusion, anticline trapping, and fault leakage. This study offers theoretical support and practical implications for the assessment and efficient exploitation of deep CBM resources in the Jinzhong Block.