Genetic mechanism and accumulation model of coalbed methane in the Midong Block, Xinjiang

Located in the Bogda Mountain fold-and-thrust belt on the southern margin of the Junggar Basin, the Miquan area presents significant challenges for coalbed methane (CBM) development due to its complex tectonics and high CO₂ concentrations in deep coal seams. By systematically collecting coal, gas, and water from various depths within the Xishanyao Formation in the Miquan area, and comprehensively applying coal petrology, gas carbon isotope, and hydrochemical testing and analysis technologies, the spatial distribution characteristics, genetic mechanisms, andaccumulation models of CBM in the two primary structural units (the Badaowan syncline and the Qidaowan anticline) in this area have been revealed. The results show that: The average coalbed methane content of the main coal seams is primarily between 2−12 m³/t. Vertically, the gas content increases with the increasing of depth, showing vertical zonation. Horizontally, the distribution is characterized as being higher in the west and south than in the east and north. The average CO₂ content in the produced gas reaches 22% and increases with increasing depth. At depths below 1400 m, the CO₂ content can reach up to 75%. The CO₂ on the north flank of the Qidaowan anticline and on both flanks of the Badaowan syncline is primarily associated with microbial methanogenesis. In contrast, the CO₂ in the core of the Badaowan syncline is shown to be generated during the process of coalification. Due to the downward penetration of Tianshan snowmelt and atmospheric precipitation, microbial degradation is active, and biogas accumulates in the central region due to hydraulic blockage. Therefore, the northern wing of the Qidaowan anticline exhibits a hydraulic blockage-type gas accumulation pattern. Due to the migration of deep thermal hydrocarbons toward the structural high areas on the northern and southern wings, and the continuous generation of secondary biogenic gases by methane-producing bacteria in the central area through CO₂ reduction, combined with lateral capping by thrust faults, a structurally controlled composite gas source enrichment zone has been formed. Therefore, the Badaowan syncline exhibits a composite gas source structural capping type gas enrichment pattern. A valuable reference is provided for the exploration and development of CBM in the Miquan area and other similar regions with complex structures, low-rank coal, and high CO₂ content.