Genetic type and gas-generating potential of coalbed methane in the Bayanhua Sag, Erlian Basin

The Bayanhua Sag in the Erlian Basin is a critical experimental area for coalbed methane (CBM) development from low-rank coals in Inner Mongolia. However, the relatively low exploration level and the limited understanding of the CBM origin in the sag restrict the CBM exploration and development to some extent. This study tested the gas components, stable isotopes and hydrochemistry of the gas and water samples from CBM wells in the Bayanhua Sag. It also conducted radioisotopic dating for these samples. By combining the classic charts for the identification of natural gas origin, this study clarified the gas origins, further analyzed the gas-generating potential, and identified the critical gas-generating factors. The results show that the CBM from the sage is characterized by C₁/C_1-5 ratios greater than 0.99, CO₂-CH₄ coefficient CDMI=φ(CO₂)/φ(CO₂+CH₄)×100% roughly less than 5%, drying coefficient (C₁/C₂₊) between 104‒5540, high CH₄ contents, and low heavy hydrocarbon and CO₂ contents. Furthermore, the δ¹³C(CH₄), δD(CH₄), and δ¹³C(CO₂) values of the CBM range from −51.80‰ to −67.70‰, from −226.20‰ to −291.00‰, and from −20.30‰ to −37.60‰, respectively, suggesting the characteristics of continental biogenic gas. In the identification charts, the CBM samples mostly fell within the biogenic gas zone, indicating that methane originated from acetic-acid and methyl fermentation and that CO₂ is primarily the associated product of microbial methanogenesis. Coalbed water, originating from meteoric water, is NaHCO₃ type of slightly alkaline water primarily, with δ¹³C_DIC and δ¹⁸O(H₂O) values of −2.6‰ and −16.4‰, respectively. Based on these findings, as well as the ¹⁴C dating results, the coalbed water is inferred to be the Quaternary water and modern mixed water rather than primary water. By combining the analysis of the structures and hydrogeological conditions in the Bayanhua Sag, it can be concluded that the runoff areas in the sag are favorable for gas production from acetic acid fermentation and that areas with weak runoff are conducive to the enrichment and accumulation of biogenic gas. The low-rank coal reservoirs in the sag exhibit high pore permeability, suitable geotemperature, and superior hydrogeological conditions, which are beneficial for the generation of biogenic gas. Furthermore, biogenic gas reservoirs of the hydraulic sealing type have developed in the confined area. Therefore, hydrogeological conditions, which play a vital role in the formation of biogenic gas, should be emphatically concerned in selecting CBM exploration targets in the sag.