Formation mechanism of coalbed methane in Dafosi mine field, Huanglong coalfield

Dafosi mine field is a typical low-rank coalbed methane (CBM) field in Binchang mining area of Huanglong Jurassic. There are many coalbed methane wells in the field, but few researches are performed on the formation mechanism of coalbed methane. Thus, it is of great guiding significance to clarify the geochemical characteristics and formation mechanism of coalbed methane (CBM) in the mine field, which could provide important basis for the analysis on productivity difference of high- and low-yield CBM Wells. For this reason, the samples of drained gas at the wellhead of six CBM wells, 22 coal samples of No.4 coal seam, 1 water sample from coal seam and one from surface water were collected from the study area. Then, the coal maceral, gas chemical composition, carbon isotopes and sample water quality were detected, and the carbon isotope characteristics, genetic types and lightening mechanism of coal seam methane of (CH₄) in Dafosi mine field were analyzed with reference to the relevant literature data in some research areas. According to the results, the content of organic components in coal maceral of the main coal seam No.4 in Dafosi mine field is obviously higher, with an average of 93.2%. Specifically, the inertinite is the most dominant with an average of 68.2%, and the vitrinite ranks the second with an average of 22.8% and an average reflectance R_max of 0.65%. The coalbed methane is mainly composed of CH₄, which ranges from 73.805% to 98.006%, with an average 83.753%, N₂ with a volume fraction of 1.259% to 25.735% and 15.220% on average, CO₂ with a volume fraction of 0.040% to 2.380% and an average of 1.023%, C₂ and above heavy hydrocarbon with an average fraction less than 0.005 4%. It is shown that C₁/C_1—n is greater than 0.999, the content of CH₄ and N₂ is negatively correlated, and the components of coalbed methane are obviously affected by air in the later stage of reservoir formation. The δ¹³C₁ ranges from −80.516‰ to −62.400‰, with an average of −73.000‰, while δ¹³C_CO2 ranges from −41.693‰ to −7.065‰, with an average of −18.660‰. The coalbed methane in Dafosi mine field is secondary biogenic gas, which is obviously marked by light δ¹³C₁ and low content of heavy hydrocarbon, thus presenting the typical characteristics of extra-dry gas. The lightening mechanism lies in that most of the coalbed methane is produced by CO₂ reduction and a small amount by acetic acid fermentation, and in the process of gas production in these two ways, the enrichment of light carbon isotopes in biomethane will be resulted in eventually, further leading to lighter δ¹³C₁.