Secondary biogenic coalbed gas in some coal fields of China

The secondary biogenic coalbed gas, a new genetic and energy source type of coalbed gas in China, has been found in Xinji, Liyazhuang and Enhong areas. The essential characteristics of this type of gas are: (i) the major component of the gas is methane, with C1/C1–5 value higher than 0.99, indicating that the gas is part of dry gas; (ii) the δ13C1 value is in the range of −61.7‰ to −47.9‰, mostly lower than −55‰, which is much lower than the estimated δ13C1 value of thermogenic methane according to the thermal evolution degree of the coal rocks (with Ro value from 0.87% to 1.43%), showing the characteristics of the secondary biogenic gas; (iii) the δD value of methane ranges from −244‰ to −196‰; (iv) δ13C2 value ranges from −26.7‰ to −15.9‰ and δ13C3 value ranges from −10.8‰ to −25.3‰, indicating that the heavier hydrocarbons have a thermogenic origin; (v) the content of CO2 is very low, and δ13CCO2 value changes greatly, reflecting a characteristic of secondary change; (vi) δ15N2 value ranges mainly from −1‰ to + 1‰, indicating N2 derived significantly from air. The negative linear correlation between the contents of N2 and CH4 reflects the activity of bacteria bearing surface water infiltrating into coal beds. The comprehensive tracing indices show that the coalbed gas in the studied areas is the mixed gas of primarily secondary biogenic gas and a part of remnant thermogenic gas. The uplift of coal beds and the development of faults in the studied areas create favorable conditions for the formation of the secondary biogenic gas.