A Study on Geochemical Characteristics and Genesis Mechanisms of Coalbed Methane in the Dafosi Well Field, Huang-Long Jurassic Coalfield

The Dafosi well field is a typical Huang-Long Jurassic low-rank coalbed methane (CBM) field. Clarifying its CBM geochemical characteristics and the mechanisms of its formation is of significant importance for deepening the understanding of the formation mechanisms of low-rank CBM in China and for the scientific assessment of its resource potential. A total of eight gas emission samples from six coalbed methane wells in the Dafosi coalfield were collected, along with 22 coal samples from the 4# coal seam. Detailed analyses of microscopic coal petrographic components, gas chemical compositions, and carbon isotopes were performed. By integrating data from the 20 relevant literature sources on coalbed gas composition and isotopic characteristics within the study area, a comprehensive dataset comprising 28 sets was utilized to examine the carbon isotope characteristics and genesis types of both CH4 and CO2 in the coalbeds, as well as elucidate the mechanism behind CH4 carbon isotope depletion. The findings indicate that in the primary 4# coal seam’s microscopic petrographic composition, the organic matter content is considerably higher, averaging 93.2%. Among these, the inertinite group is dominant, averaging 68.2%; the vitrinite group is the next most abundant, averaging 22.8%. The CBM composition is predominantly CH4, with concentrations varying from 68.753% to 98.006%, averaging 80.276%. N2 concentrations range from 1.259% to 29.926%, averaging 17.476%. CO2 concentrations vary from 0.04% to 2.380%, averaging 1.032%. The average concentration of heavier hydrocarbons C2 and above is less than 0.078%, indicative of typical dry gas characteristics, C1/C1~n > 0.999. The concentration of CH4 and N2 was negatively correlated. δ13C1 ranges from −87.200‰ to −62.400‰, averaging −75.802‰. CH4 is composed of secondary biogenic gas with dominant content and a small amount of thermogenic gas. δ13CCO2 ranges from −41.693‰ to −7.065‰, averaging −20.016‰. CO2 is an organic gas, mainly derived from thermal degradation and microbial degradation of organic matter. The mechanism responsible for the light carbon isotopic composition of δ13C1 lies in the fact that most of CH4 is produced by CO2 reduction, and a small amount is produced by acetic acid fermentation. In the gas generation process of these two pathways, biogenic methane will eventually enrich light carbon isotopes, resulting in light δ13C1.