Deep coalbed methane resources in the Shenmu-Jiaxian block, Ordos Basin, China: Geological characteristics and potential for exploration and exploitation

China boasts abundant coalbed methane (CBM) resources in deep layers (burial depths: greater than 2000 m), with considerable potential for CBM exploration and development. Compared to medium and shallow CBM, deep CBM differs significantly in enrichment and accumulation patterns and exploitation means. Therefore, there is an urgent need to dissect key CBM blocks. The Shenmu-Jiaxian block, situated in the northeastern Ordos Basin, is currently in the early stages of deep CBM exploration, with enrichment characteristics and exploration potential remaining undefined. Based on recently obtained seismic data, logging data from over 300 wells, and analytical results from four cored wells, this study conducted a comprehensive analysis of the fundamental geological characteristics of coal reservoirs, identified the primary controlling factors and patterns for CBM enrichment, and analogically dissected the potential for deep CBM exploration in the study area. The No.8 deep coal exhibits vitrinite reflectance of 0.7%‒1.8%, indicating that the coalification in most parts of the study area is currently in the peak stage of pyrolytic gas generation. The No.8 deep coal seam exhibits regional net thicknesses of up to 7‒8 m, stable distribution, comparably simple structures, highly closed hydrodynamic conditions, and favorable roof and floor sealing conditions, which create favorable conditions for widespread, continuous CBM accumulation. Across the study area, the southern portion of Jiaxian County demonstrates the optimal CBM resource conditions. Compared to other deep blocks in the eastern Ordos Basin, the No.8 coal seam in the study area exhibits significantly higher proportions of free gas (15.21%‒46.47%). Moreover, its adsorbed gas is primarily governed by adsorption pressure-induced trapping, whereas its free gas is subjected to the dual controls of capillary pressure and buoyancy-gravity differentiation. Both trapping mechanisms jointly determine the vertical and planar zoning of gas content in the deep part of the coal seam. Taking the southern area of Jiaxian County as an example, this study summarized a typical deep CBM enrichment pattern, which consists of the predominant control of the adsorption pressure for adsorbed gas, followed by the control of the coupling of capillary pressure and buoyancy-gravity differentiation for free gas. The comparative analysis with the Linxing and Daning-Jixian blocks indicates that the Shenmu-Jiaxian block enjoys a solid resource foundation of CBM, favorable reservoir stimulation, and great potential for high production, hinting at a promising prospect for large-scale exploitation of deep CBM in this block. In addition, the deep part of the Daning-Jixian block is characterized by high-rank coals, while that of the Shenmu-Jiaxian block features medium-rank coals. Therefore, this study contributes to the enrichment and improvement of the theory about deep CBM enrichment, serving as indicators for the exploration and exploitation of deep CBM in China.