Abnormal Concentration and Origin of Heavy Hydrocarbon in Upper Permian Coal Seams from Enhong Syncline, Yunnan, China

Coalbed gas （CBG） in Enhong syncline, eastern Yunnan China, is character- ized 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 hydrocar- bon 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 off 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 coalifica-tion 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.