The genesis and accumulation mechanism of CBM in the typical mid- to low-rank coal-bearing basins

The composition and isotopic characteristics of coalbed methane (CBM), as well as the genesis of CH₄ and CO₂, associated geological process and migration-accumulation model of CBM in typical mid- to low-rank coal-bearing basins were studied. The genesis of CBM is jointly influenced by the degree of coalification and biochemical processes, which in turn determine the composition and isotopic characteristics of CBM. Biogenic gas is extensively developed in mid- to low-rank coal-bearing basins, but its genesis varies. In the Baode area, China, and the San Juan Basin, USA, CBM is mainly secondary biogenic gas and thermogenic gas. In the Miquan area, China, CBM is characterized by primary biogenic gas. However, CBM in the Jiergalangtu area, China, Surat Basin, Australia, and Power River Basin, USA, are characterized by secondary biogenic gas. Microbial CO₂ reduction occurs in these coal-bearing basins, but with significant gas generation by acetate fermentation in some areas of these basins. Moreover, CO₂ in the Power River Basin, Surat Basin, and Jiergalangtu area mainly originates from microbial degradation of organic matter. However, in other basins studied, CO₂ initially derives from coal pyrolysis and is subsequently supplemented by CO₂ from microbial methanogenesis. The generation and isotope fractionation of CBM are affected and controlled by associated geological processes. Additionally, under the control of tectonic morphology and hydrogeological conditions, the genesis and migration-accumulation models of CBM in mid- to low-rank coal-bearing basins can be summarized into two models—the hydrodynamic active monoclinic model and the hydrodynamic differential syncline model—or a combination of the two.