Composition, Origin, and Accumulation Model of Coalbed Methane in the Panxie Coal Mining Area, Anhui Province, China

To investigate the geochemical characteristic, genetic types, and accumulation model of coalbed methane (CBM), 16 samples from a burial depth of 621-1494 m were collected in the Panxie Coal Mining Area of Huainan Coalfield. The results indicate that the samples are dominated by methane, and the concentrations are distributed in the range of 73.11-95.42%. The dryness coefficient is 0.77-1.00 (average, 0.93), and the ratio of methane to the sum of ethane and propane (C₁/(C₂ + C₃)) is 3.18-242.64 (average, 36.15). The δ¹³C_CH values are distributed in the range of -65.44 to -32.38‰ (average, -45.22‰), the δD_CH values are in the range of -226.84 to -156.82‰ (average, -182.93‰), and the δ¹³C_CO values are in the range of -19.7 to -10.1‰ (average, -15.51‰). CBM samples in the study area are dominated by thermogenic gases, followed by secondary biogenic gases with CO₂ reduction. For the percentages of different genetic gases, the distribution range of thermogenic gas is 70.11-97.86%, whereas that of biogenic gas is 58.65-77.86% for five samples from Zhangji, Panyi, Pansan, and Panbei Coalmines. Moreover, desorption-diffusion fractionation and the effect of groundwater dissolution occurred in the Panxie Coal Mining Area, and higher δ¹³C_CH values mostly existed in the deeper coal seams. Furthermore, the biogenic gases are more likely to be secondary biogenic gases generated by CO₂ reduction on the basis of data comparison, which is related to the flowing water underground. Accumulation models of different genetic types of CBM are correlated with the burial depth of coal seams, location, and type of faults and aquifers.