Abnormal concentration and origin of heavy hydrocarbon in upper permian coal seams from Enhong syncline, Yunnan, China

Abstract

Coalbed gas (CBG) in Enhong (恩洪) syncline, eastern Yunnan (云南), China, is characterized by high concentration of heavy hydrocarbon with the highest content of ethane, which is more than 30%. Some previous investigators paid much attention to the abnormal concentration of heavy hydrocarbon in the CBG of Enhong, but few have researched on its origin. This article describes the characteristics of abnormal high concentration of heavy hydrocarbon in Enhong syncline and analyzes its reason from the aspects of origin and evolution of heavy hydrocarbon by carbon isotope, coal petrography, and coal rank. Features of gas carbon isotope composition display that there is no inorganic gas or oil components in the CBG, which is classified to thermogenetic gas produced by humic material, with characteristic of secondary biogenic gas in shallow coal seam. The concentration of heavy hydrocarbon in Enhong syncline increases with the increase of vitrinite, vitrinite/inertinite ratio, and hydrogen/carbon ratio and decreases with the increase of inertinite, so hydrogen-rich vitrinite may be a very important factor resulting in the abnormal concentration of heavy hydrocarbon. It also increases with the increase of degree of coalification of coking to lean coals during which the peak of heavy hydrocarbon generation is reached. Therefore, we think that high concentration of heavy hydrocarbon originated from the coupling effect of higher content of the hydrogen-rich vitrinite in the coal and the coal rank of coking to lean coals.

Keywords

coalbed gas / heavy hydrocarbon / abnormal concentration / gas isotope / hydrogenrich vitrinite

References

[1] Alsaab D., Elie M., Izart A., . Comparison of Hydrocarbon Gases (C₁–C₅) Production from Carboniferous Donets (Ukraine) and Cretaceous Sabinas (Mexico) Coals. International Journal of Coal Geology, 2008, 74: 154-162.

[2] Alsaab D., Suarez-Ruiz I., Elie M., . Comparison of Generative Capacities for Bitumen and Gas between Carboniferous Coals from Donets Basin (Ukraine) and a Cretaceous Coal from Sabinas-Piedras Negras Basin (Mexico) during Artificial Maturation in Confined Pyrolysis System. International Journal of Coal Geology, 2007, 71: 85-102.

[3] Bertrand P.. Geochemical and Petrographic Characterization of Humic Coals Considered as Possible Oil Source Rocks. Organic Geochemistry, 1984, 6: 481-488.

[4] Cheng K. M., Zhang C. F.. Research of Coal-Formed Oil in Tulufan-Hami Basin. Science in China (Series B), 1994, 24(11): 1216-1222.

[5] Dai J. X., Qi H. F., Song Y., . Coalbed Methane Composition and Carbon Isotope Types in China and Their Cause and Significance. Science in China (Series B), 1986, 12: 1317-1326.

[6] Dai J. X.. Carbon and Hydrogen Isotope Characteristics of Natural Gas and Authentication of Various Natural Gases. Natural Gas Geoscience, 1993, 2–3: 1-40.

[7] Dai J. X., Xia X. Y., Hong F.. Natural Gas Geology Accelerated the Growth of Natural Gas Reserve in Large Scale in China. Xinjiang Petroleum Geology, 2002, 23(5): 357-365.

[8] Dai J. X., Ni Y. Y., Li J., . Carbon Isotope Types and Significances of Alkane Gases from Junggar Basin and Tarim Basin. Xinjiang Petroleum Geology, 2008, 29(4): 403-410.

[9] Fu X. H., Qin Y., Wei C. T.. Coalbed Methane Geology, 2007, Xuzhou: China University of Mining and Technology Press

[10] Gao B., Tao M. X., Zhang J. B., . Distribution Characteristics and Controlling Factors of δ¹³C₁ Value of Coalbed Methane. Coal Geology and Exploration, 2002, 30(3): 14-17.

[11] He T. C., Qin Y.. Exploration and Development of Coalbed Methane and Its Utilization Technology, 2007, Xuzhou: China University of Mining and Technology Press

[12] Kotarba M. J.. Composition and Origin of Coalbed Gases in the Upper Silesian and Lublin Basins, Poland. Organic Geochemistry, 2001, 32: 163-180.

[13] Killops S. D., Funnell R. H., Suggate R. P., . Predicting Generation and Expulsion of Paraffinic Oil from Vitrinite-Rich Coals. Organic Geochemistry, 1998, 29(1–3): 1-21.

[14] Killops S., Walker P., Wavrek D.. Maturity-Related Variations in the Bitumen Compositions of Coals from Tara-1 and Toko-1 Wells. New Zealand Journal of Geology and Geophysics, 2001, 44: 157-169.

[15] Li X. Q., Ma A. L., Zhong N. N.. Research Method and Application of Organic Petrology of Source Rock, 1997, Chongqing: Chongqing University Press

[16] Petersen H. I., Rosenberg P.. Reflectance Retardation (Suppression) and Source Rock Properties Related to Hydrogen-Enriched Vitrinite in Middle Jurassic Coals, Danish North Sea. Journal of Petroleum Geology, 1998, 21(3): 247-263.

[17] Petersen H. I.. The Petroleum Generation Potential and Effective Oil Window of Humic Coals Related to Coal Composition and Age. International Journal of Coal Geology, 2006, 67: 221-248.

[18] Petersen H. I., Nytoft H. P.. Oil Generation Capacity of Coals as a Function of Coal Age and Aliphatic Structure. Organic Geochemistry, 2006, 37: 558-583.

[19] Qi H. F.. Preliminary Comment on the Variation of Carbon Isotopic Composition of Methane in Coal Measure Gas and Seam Gas, and Its Controlling Factor. Experimental Petroleum Geology, 1985, 7(2): 81-84.

[20] Qin Y., Tang X. Y., Ye J. P.. Stable Carbon Isotope Composition and Desorption Diffusion Effect of the Upper Paleozoic Coalbed Methane in North China. Geological Journal of China Universities, 1998, 4(2): 127-132.

[21] Rice D. D.. Law B. E., Rice D. D.. Composition and Origins of Coalbed Gas. Hydrocarbons from Coal, 1993, Halifax, N1S, Canada.: AAPG Special Publication 159 184

[22] Shen B. J., Huang Z. L., Liu H. W., . Advances in the Study of Gas and Source Rock Correlation. Natural Gas Geoscience, 2007, 18(2): 269-274.

[23] Suggate R. P.. Application of Rank (Sr), a Maturity Index Based on Chemical Analyses of Coals. Marine and Petroleum Geology, 2002, 19: 929-950.

[24] Wang S. Q.. Geochemical Characteristics of Jurassic-Sinian Gas in Sichuan Basin. Natural Gas Industry, 1994, 14(6): 1-5.

[25] Wilkins R. W. T., George S. C.. Coal as a Source Rock for Oil: A Review. International Journal of Coal Geology, 2002, 50: 317-361.

[26] Wu, G. Q., Lin, Y. C., Li, Y. B., 2003. Occurrence Characteristic of Coalbed Methane Resouse in Enhong and Laochang Mine Area. Jiangsu Coal, (3): 26–27 (in Chinese)

[27] Wu J.. Theory and Applications of Coal-Derived Hydrocarbon in China, 1994, Beijing: China Coal Industry Publishing House

[28] Yu L. C., Li W. F.. Study on Composition of Hydrocarbons in Coal Seams Liable to Outbursts. Journal of China Coal Society, 1981, 4: 1-8.

[29] Zhang J. B., Tao M. X.. Geological Significances of Coal Bed Methane Carbon Isotope in Coal Bed Methane Exploration. Acta Sedimentologica Sinica, 2000, 18(4): 611-614.

[30] Zhang X. M., Zhuang J., Zhang S. A.. Coalbed Methane Geology and Resource Evaluation in China, 2002, Beijing: Science Press