Resource potential and exploration targets of low-rank coalbed methane in China

The low-rank coalbed methane (CBM) is distributed in both the northwest and northeast regions of China, as dominated by the Jurassic, Cretaceous, and Paleogene with coal seams exhibiting large quantities and considerable thicknesses but low gas content. With resources estimated at approximately 14.7 × 10¹² m³, the low-rank CBM at burial depths of 2000 m or less holds tremendous potential for exploration and production. Through a systematic analysis of lowrank CBM resources in four typical basins in China, we investigate the typical accumulation characteristics and the exploration and production potential of low-rank CBM under various burial depth-geology combinations. The results indicate that low-rank CBM reservoirs in China manifest low permeability (<1 × 10^-3 μm²), and relatively high salinity of coal seam water (>5000 mg/L). An analysis of the factors governing the accumulation of low-rank CBM under the synergistic effects of tectonism and hydrodynamic, temperature, and pressure fields reveals a gas enrichment-controlling pattern consisting of sedimentary microfacies-controlled coal occurrence, hydrogeology-controlled gas generation and preservation, burial depthcontrolled reservoir properties, and tectonism-controlled gas accumulation. Six enrichment patterns of low-rank CBM are identified based on the analysis of typical zones. In combination with the geological characteristics and production practice of low-rank CBM in the Powder River Basin of the United States and the Surat Basin of Australia, we propose two enrichment patterns of low-rank CBM in China featuring high productivity, that is, the accumulation of gas from multiple single thinbedded coal seams and deep CBM multiple factor-controlled storage. An assessment methodology and index system for the selection of CBM target areas are developed based on the analysis of five crucial factors for the efficient production of low-rank CBM: coal composition, resource potential, preservation conditions, production conditions, and fracability. Future exploration targets include the footwalls of piedmont overthrust faults along basin margins and the deep-central uplifted areas of slope zones within basins.