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 C-1/C1-5 value higher than 0.99, indicating that the gas is part of dry gas; (ii) the delta C-13(1) value is in the range of -61.7%, to -47.9 parts per thousand, mostly lower than -55%, which is much lower than the estimated delta C-13(1) value of thermogenic methane according to the thermal evolution degree of the coal rocks (with R. value from 0.87% to 1.43 parts per thousand, showing the characteristics of the secondary biogenic gas; (iii) the delta D value of methane ranges from -244 parts per thousand to -196%; (iv) delta C-13(2) value ranges from -26.7 parts per thousand to -15.9 parts per thousand and delta C-13(3) value ranges from -1 0.8 parts per thousand to -25.3 parts per thousand, indicating that the heavier hydrocarbons have a thermogenic origin; (v) the content Of CO2 is very low, and delta C-13(CO2) value changes greatly, reflecting a characteristic of secondary change; (vi) delta N-15(2) value ranges mainly from -1 parts per thousand to +1 parts per thousand, indicating N-2 derived significantly from air. The negative linear correlation between the contents of N-2 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.