Gas isotope variation characteristics and genesis identification of coalbed methane in Yuwang Block

The research on the geochemical characteristics and genesis of coalbed methane (CBM) is of great significance to the formation and resource exploration-development. In order to investigate the genesis and geochemical characteristics of CBM in the Yuwang Block in eastern Yunnan, the gas samples from six typical CBM wells in the area were collected, and the composition determination, carbon isotope and rare gas isotope tests were carried out. The genesis of CH₄, N₂ and other gases in coal seam were systematically analyzed. Combined with the hydrogeological environment, the isotope variation of gases and their controlling factors during the CBM drainage process in Yuwang Block were investigated. The results shown that, the volume fraction of CH₄ in the gas produced from CBM wells in the Yuwang Block averaged 73.95%-92.90%, and those of ethane, CO₂ and N₂ were 0.20% - 0.40%, 0.05% - 1.05% and 6.15% - 25.32%, respectively. In addition, helium was only detected on the first day of gas production, while hydrocarbons with a carbon atom number greater than 2 were not detected. The drying coefficient (C₁/C_{1 ~ 5}) of coalbed methane was 0.999 5 - 0.999 7, which belonged to high mature dry gas. Theδ¹³C value of CH₄ was −36.7‰ - −27.6‰, which was an organic matter thermogenic gas. Theδ¹³C value of CO₂ was −19.1‰- −6.6‰, with the main distributed from −18.0‰ to −14.3‰, which was of organic origin. The volume fraction of nitrogen averaged 12.63%, with relatively high content, and itsφ(N₂)/φ(Ar) value was much larger than 84, dominated by organic genesis. The average content of argon was 0.06%, much larger than 0.017%, dominated by atmospheric genesis. The volume fraction of helium averaged 0.02%,φ(³He)/φ(⁴He) was 0.29×10⁻⁸ - 19.6×10⁻⁸, with the main distributed from 2.00×10⁻⁸ to 9.38×10⁻⁸, the ratio ofφ(³He)/φ(⁴He) to atmospheric helium was 0.01 ~ 0.09, which was typical shell source helium, and the variation trends ofφ(⁴He) andφ(²⁰Ne) were consistent withφ(⁴He)/φ(²⁰Ne). The alkane isotope values of the produced gas shown a tendency to become progressively lighter as the increasing discharge time and gas purity. The study concluded that the “drying and lightening” of CBM in Yuwang Block was the result of the comprehensive influence of gas desorption, diffusion and water dissolution.