Coalbed methane accumulation characteristics and type classification in Hancheng mining area

Hancheng mining area is a hot spot of coalbed methane (CBM) development and research in China.A comprehensive analysis of the data acquired from the CBM exploration and exploitation will improve the CBM accumulation theory and promote the CBM development.On the basis of the key reservoir parameters of gas content and permeability, and the macro-scale geological conditions of tectonic deformation and hydrogeology, the CBM accumulation conditions and their coupling effect are revealed in this study, and the CBM accumulation types are classified.The following conclusions are obtained: the Hancheng mining area is located within a monoclinal structure with northeast trend andnorthwest dip.The groundwater recharges from the shallow strata in southeast, and then flows to the northwest.The hydrodynamic intensity weakens with increasing depth, and results in the hydro-sealing effect and CBM accumulation in the deep coal seam.The gas content of coal seams increases along the dip direction generally, secondary biogenic gas generates in the shallow zone as a result of the active groundwater, and results in the high gas content in some local area.Based on the structural curvature analysis, the shallow tectonic zone and Dongzecun tectonic zone deform the coal seam severely.Under its control, meager coal occurs locally in the shallow area, which coincides with the highly deformed area, indicating the dynamic metamorphism of coal seams in the shallow strata.The critical depth of 400 m and 800 m controls the vertical differences of gas content and permeability, and three CBM accumulation types can be identified including the gas loss type in the shallow tectonic zone, the gas enrichment type in the deep slope zone, and the middle transitional type.The middle transitional zone realizes the optimum match between gas content and permeability and favors the CBM development, which is verified by the actual CBM productivity.