Secondary biological coalbed gas in the Xinji area, Anhui province, China: Evidence from the geochemical features and secondary changes

In order to ascertain the origin of the coalbed gas in the Xinji area, Anhui Province of China, the paper examined the geochemical features and secondary changes of CH4, C2H6, CO2 and N2 from the coalbed gas. The related gas composition, carbon isotope and tracer geochemical data are as follows: 0.993 to 1.0 for C1/C1−n, 188.6 to 2993.7 for C1/C2, < 2% for CO2, 0.64 to 3.06% for [CO2/(CO2 + CH4)]100%, − 50.7‰ to − 61.3‰ for δ13C1 with the average value of − 56.6‰, − 15.9‰ to − 26.7‰ for δ13C2, − 10.8‰ to − 25.3‰ for δ13C3, − 6.0‰ to − 39.0‰ for δ13CCO2 with the average value of − 17.9‰, 30.7‰ to 43.9‰ for Δδ13CC2−C1, and 17.2‰ to 50‰ for Δδ13CCO2−C1, − 1‰ to + 1‰ for δ15NN2, 1.13 × 10− 7 to 3.20 × 10− 7 for 3He/4He with R/Ra ratios range from 0.08 to 0.23. The Ro values of the coal range from 0.88% to 0.91%. The trends of the δ13C1 values and δ13CCO2 values downward in the stratigraphic profile are opposite: the former appears as a slight light–heavy–light trend, but the latter appears as a heavy–light–heavy trend. The δ13C1 values have a negative correlation with the δ13CCO2 values. However, the δ13C2 values have no correlation with the δ13C1 values due to its complicated variation. The thermal evolution of the coal in the Xinji area is in the phase of a lot of wet gas generation, but most of the CO2 and heavy hydrocarbons have been reduced or degraded by microbes and have changed into biogenic methane. The coalbed gas is comprised of secondary biogenic methane, thermogenic methane, the residual thermogenic CO2 and heavy-hydrocarbons, and nitrogen (mainly derived from the atmosphere). Based on the binary mixed mode, the estimated portion of thermogenic methane is 31.5% to 39.9% while that for the secondary biogenic methane is 60.1% to 68.5%. Intense tectonic uplift, faults and erosion in the studied area have created favorable conditions for the infiltration of surface water, the abundance of microbes and the formation of the secondary biogenic methane. The coalbed gas in the Xinji area is a mixed type made of a greater portion of secondary biogenic gas and a smaller portion of thermogenic gas.