Origin and accumulation mechanisms of coalbed methane in low-rank coals in gas-rich sags in the Erlian Basin

Inner Mongolia boasts abundant coalbed methane (CBM) resources in low-rank coals. Investigating the origin and accumulation mechanisms of CBM is crucial for the siting and assessment of these CBM resources. This study examined the CBM in low-rank coals in major gas-rich sags of the Erlian Basin through various experiments, including tests of the composition and carbon/hydrogen isotopes of CBM, as well as tests of the water quality, hydrogen/oxygen isotopes, and radioisotopes (³H and ¹⁴C) of coal seam water. By analyzing the geochemical characteristics of both CBM and coal seam water, this study revealed the origin, sources, and accumulation mechanisms of CBM in low-rank coals within the study area. Key findings are as follows: (1) The CBM in the Erlian Basin is all dry gas and dominated by methane (CH₄). With an increase in the burial depth, the CH4 volume fraction increases, whereas the CO₂ volume fraction first increases and then decreases, peaking at depths between 300 and 500 m. The CH₄ exhibits generally lighter carbon and hydrogen isotopic values, with δ¹³C(CH₄), δD(CH₄), and δ¹³C(CO₂) values ranging from −70.3‰ to −48.0‰, from −285.5‰ to −189.0‰, and from −37.6‰ to 1.94‰, respectively. (2) The coal seam water primarily has hydrochemical types of HCO₃-Na and Cl·HCO₃-Na. The current coal seams feature a relatively stable water environment, with weak hydrodynamic conditions. The coal seam water, with apparent ages between 1020 and 47490 a, principally originates from Quaternary meteoric water, with no or little recharge from modern surface water. (3) The CBM in the Erlian Basin is predominantly composed of original biogenic gas, mixed with a minor amount of early thermogenic gas. With an increase in the burial depth, the stratigraphic environment and types of methanogenic archaea change, with the generation paths of biogenic methane also shifting. In the early stage, the biogenic methane in the Jiergalangtu sag was primarily derived from acetic acid fermentation. In the late stage, it predominantly originated from CO₂ reduction in the late stage, mixed with a small quantity of low-maturity thermogenic gas. In contrast, the biogenic methane in both Bayanhua and Huolinhe sags was principally sourced from acetic acid fermentation. Besides, the biogenic methane in the Huolinhe sag contains a trace amount of biogenic gas from methyl fermentation. (4) The study area has coal seam conditions favorable for biogenetic gas generation, such as low geotemperatures, low total dissolved solids (TDS) content, and low thermal maturity. The Jiergalangtu sag exhibits a graben-type biogenetic-gas accumulation mode in shallow thick coal seams, while the Bayanhua and Huolinhe sags display a half-graben-type biogenetic-gas accumulation mode with hydraulic sealing in moderately deep confined areas. Identifying favorable target areas for the formation and enrichment of biogenic gas should be the focus of subsequent exploration and production of CBM in the Erlian Basin. Besides, it is practically significant for reserve growth and production addition of CBM in low-rank coals in the basin.