Genesis and Accumulation Modes of Coalbed Methane at the Eastern Margin of Ordos Basin, China: Evidence from Its Geochemistry

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.