Objective The Xintian Coal Mine, a critical coal-producing mine in western Guizhou Province, faces high risks of coal and gas outbursts and the challenge of low underground coalbed methane (CBM) extraction efficiency, necessitating surface CBM extraction. However, the complex geological conditions in the area limit well placement and the fracturing and gas drainage effectiveness. Therefore, there is an urgent need to systematically elucidate the occurrence characteristics and enrichment patterns of CBM in the area.

Methods Based on data from exploration boreholes in the Xintian Coal Mine, combined with experiment and test results and log interpretations, this study revealed the physical properties, structural conditions, and hydrodynamic characteristics of the No.9 coal seam—a major minable coal seam in the coal mine. Accordingly, the CBM enrichment patterns were clarified, and the CBM accumulation models integrating the coupling effects of structures, hydrodynamics, sedimentation, and coals were determined.

Results The No.9 coal seam is dominated by anthracite with a low ash content, a low volatile content, a medium-to-high fixed carbon content, a low water content, and a low-to-medium sulfur content. The coal seam is characterized by considerable thicknesses, moderate burial depths, and a relatively high gas content, representing coal reservoirs featuring medium-to-low permeabilities and normal pressure. In the coal seam, the gas content exhibits negative correlations with the ash, volatile, and water contents but shows positive correlations with the fixed carbon content, burial depth, and reservoir pressure. The coal seam roof is composed predominantly of thinly layered mudstones and medium-to-thick silty mudstones, demonstrating excellent sealing performance. The northwestern limb of the Baiyanpo anticline exhibits simple structural features and the presence of concealed reverse faults, which are conducive to gas enrichment. The axis of the Yantoushang syncline shows low groundwater potential, with the resulting groundwater stagnation zone favoring CBM accumulation. In contrast, the southwestern portion of the syncline exhibits tensional faults characterized by steep dip angles and long extended distances, leading to gas escape. The hydrodynamic control of gas exhibits zoning in the study area. Specifically, the southwestern part of the study area primarily shows hydrodynamic-controlled gas trapping and escape. In addition to the above two control types, the hydrodynamic-controlled gas sealing can also be observed in the northeastern part.

Conclusion The CBM enrichment in the study area is primarily governed by structural and hydrodynamic conditions, as manifested by gas trapping induced by compressional faults and hydrodynamics, along with gas sealing attributed to synclines and hydrodynamics. The coupling effects of physical properties and sedimentary characteristics result in two CBM reservoir types: (1) the gas trapping type governed by physical properties, structures, and hydrodynamics and (2) the gas sealing type attributed to sedimentary and hydrodynamic conditions. Additionally, there exist two gas escape models: tensional fault-controlled gas escape and gas escape governed by sedimentary and hydrodynamic conditions. The results of this study provide insights into well placement in the Xintian Coal Mine, further guiding efficient CBM production and surface gas control.