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

The Bayanhua Sag in Erlian Basin is a key experimental area for the development of low-rank coalbed methane(CBM) in Inner Mongolia. However, the exploration degree is relatively low and the understanding of CBM genesis is insufficient, which restricts exploration and development to a certain extent. Through carried out laboratory experiments of gas components, stable isotope, hydrochemical and radioisotope dating of CBM well gas samples and water samples in Bayanhua Sag. And comprehensive use the classical natural gas to clarify genesis. Further analyze potential and identify the key elements of gas-generation. The results show that C₁/C_1-5>0.99, CDMI is basically less than 5%, the drying coefficient C₁/C₂₊ is between 224 and 5 540. The volume fraction of CH₄ is high, while heavy hydrocarbons and CO₂ is low. The δ¹³C(CH₄) ranging from -51.80‰ to -67.70‰, δD(CH₄) ranges from -226.20‰ to -291.00‰, and δ¹³C(CO₂) ranges from -20.30‰ to -37.60‰, indicating a terrestrial biogenic gas characteristic. In the identification chart, most samples are located in the regions of biogenic gas, indicating that methane pathways are acetic acid fermentation and methyl fermentation, and CO₂ is mainly the associated product of microbial methanogenesis. Coal seam water comes from atmospheric precipitation, mainly NaHCO₃ type weakly alkaline water, δ¹³C_DIC value is -2.6‰, δ¹⁸O is -16.4‰. Combined with ¹⁴C chronology shows that the water is quaternary water, non-primary water, but modern mixed water. Based on the analysis of geological structure and hydrogeological conditions in the whole area, the runoff area is conducive to acetic acid fermentation to produce gas, while the slow flow area is conducive to biogas enrichment and accumulation. The low-rank coal reservoir has good porosity and permeability, suitable temperature reservoir and superior hydrogeological conditions, which are conducive to the generation of biogas and the development of hydraulic plugging type biogas in the pressurized area. Hydrogeological conditions are the key to the formation of biogas, and should be paid more attention to the exploration area of CBM.