Secondary biogenic coalbed gas in some coal fields of China
Tóm tắt
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.
Tài liệu tham khảo
Rightmire, C. T., Eddy, G. E., Kirr, J. N., Coalbed Methane Resources of the United States, AAPG Studies in Geology Series #17, Tulsa:AAPQ 1984, 1–14.
Rice, D. D., Composition and origins of coalbed gas, in Hydrocarbons from Coal, AAPG Studies in Geology Series #38 (eds. Law B. E., Rice, D. D.), Tulsa: AAPG, 1993, 159–184.
Tao, M. X., Research progress in studies on the coalbed gas geochemistry, Progress in Natural Science (in Chinese), 2005, 15(6): 648–652.
Schoell, M., The hydrogen and carbon isotopic composition of methane from natural gases of various origins, Geochimica et Cosmochimica Acta, 1983, 67(12): 2225–2238.
Whiticar, M. J., Faber, E., Schoell, M., Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation—Isotope evidence, Geochimica et Cosmochimica Acta, 1986, 50(5): 693–709.
Rice, D. D., Claypool, G. E., Generation, accumulation, and resource potential of biogenic gas, AAPG Bulletin, 1981, 65(1): 5–25.
Scott, A. R., Kaiser, W. R., Ayers, W. B. et al., Thermogenic and secondary biogenic gases, San Juan Basin, AAPG Bulletin, 1994, 78(8): 1186–1209.
Smith, J. W., Pallasser, R. J., Microbial origin of Australian coalbed methane, AAPG Bulletin, 1996, 80(6): 891–897.
Ahmed, M., Smith, J. W., Biogenic methane generation in the degradation of eastern Australian Permian coals, Organic Geochemistry, 2001, 32(6): 809–816.
Kotarba, M. J., Composition and origin of coalbed gases in the Upper Silesian and Lublin basins, Poland. Organic Geochemistry, 2001, 32(1): 163–180.
Dai, J. X., Qi, H. F., Sun, Y. et al., Composition, carbon isotope characteristics and the origin of coalbed gases in China and their implications, Science in China, Ser. B (in Chinese), 1987, (12): 1324–1337.
Xu, Y. C., Shen, P., A study of natural gas in China, AAPG Bulletin, 1996, 80(10): 1604–1614.
James, A. T., Correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components, AAPG Bulletin, 1983, 67(7): 1176–1191.
Katz, B. J., Narimanov, A., Huseinzadeh, R., Significance of microbial processes in gases of the South Caspian basin, Marine and Petroleum Geology, 2002, 19(6): 783–796.
Li, M. Z., Zhang, H. N., Liu, H. et al., Advances in simulated test of biogas, Oil and Gas Geology (in Chinese), 1996, 17(2): 117–122.