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

摘要

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 CH_4, C_2H_6, CO_2 and N_2 from the coalbed gas. The related gas composition, carbon isotope and tracer geochemical data are as follows: 0.993 to 1.0 for C_1/C_(1-n) 188.6 to 2993.7 for C_1/C_2, < 2% for CO_2, 0.64 to 3.06% for [CO_2/(CO_2 + CH_4)]100%, -50.7per thousand to -61.3per thousand for δ~(13)C_1 with the average value of-56.6per thousand, -15.9per thousand to -26.7per thousand for δ~(13)C_2, -10.8per thousand to -25.3per thousand for δ~(13)C_3, -6.0per thousand to -39.0per thousand for δ~(13)C_(CO_2) with the average value of-17.9per thousand, 30.7per thousand to 43.9per thousand for Δδ~(13)C_(C_2-C_1), and 17.2per thousand to 50per thousand for Δδ~(13)C_(CO_2-C_1), -1 per thousand to + 1 per thousand for δ~(15)N_(N_2), 1.13 x 10~(-7) to 3.20 x 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 δ~(13)C_1 values and δ~(13)C_(CO_2) 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 δ~(13)C_1 values have a negative correlation with the δ~(13)C_(CO_2) values. However, the δ~(13)C_2 values have no correlation with the δ~(13)C_1 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 CO_2 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 CO_2 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.