The genesis, occurrence, and accumulation of coalbed methane (CBM) are critical to the methane exploration and development. Combining the geological and geochemical data from CBM exploration wells and basin modeling, the genesis and accumulation characteristics of CBM in different regions and depths at the eastern margin of Ordos Basin were elucidated. Regional-scale gas content is controlled by depth and coal rank without turning depth occurs, but blocks perform variably. Three CBM genetic types, that is, secondary microbial, thermogenic, and mixed genesis, were identified based on data of gas components and isotopic composition. Under the influences of tectonic and hydrogeological conditions, secondary microbial gas is widely distributed in shallow coal seams (depth <1350 m) showing mixed gas characteristics, while secondary thermogenic gas dominates in deep coal seams. The extensive variations of carbon isotope of methane raised from desorption-diffusion induced by tectonic uplift, dissolution in flowing groundwater, secondary microbial gas generation, and rapid gas generation enhanced by magmatic thermal. Three CBM accumulation modes have been concluded depending on the depth and coal rank. The Baode mode is characterized by a high-ratio secondary microbial gas replenishment, typically occurring at lower ranks and shallow depths. The Linxing mode is distinguished by magmatic thermal influences, and the Daji mode features massive secondary thermogenetic gas associated with higher ranks and deeper depths. Every mode has the potential to form high-gas-content zones.

Summary: The anisotropy and dynamic variation in permeability of gas-adsorbing coals have a significant influence on fluid flow behavior in the cleat system. The assumption of a constant anisotropy coefficient (the ratio between permeability components in orthogonal directions) has been traditionally made to simplify the seepage-stress coupling analytical model. In this approach, the pressure drop of the coalbed is separated into desorption and nondesorption areas. To evaluate the effective stress, pore pressure, permeability distribution, and variable anisotropy coefficient more accurately, analytical formulas were developed that consider elastic mechanics and methane sorption. The results show that the anisotropy coefficient can be dynamic when cleat compressibility anisotropy exists. Pressure contours are a set of ellipses that increase in eccentricity from the near-wellbore area to the pressure drop boundary, leading to corresponding anisotropy changes in effective stress and permeability. The gas desorption-related matrix shrinkage effect causes a discontinuous pressure drop gradient at the boundary between desorption and nondesorption areas, resulting in nonsmooth pressure drop curves. The pressure gradient difference changes with the radius of the desorption area and is nonisotropic, with the high-permeability direction showing a greater difference than the low-permeability direction. These results indicate that the dynamic anisotropy coefficient has a significant impact on coalbed drainage and extraction. Compared to previous mathematical models, which assumed permeability isotropy or constant anisotropy coefficient in cleat systems, the proposed model provides a more accurate method to evaluate pressure and permeability distribution.