Abstract:

Heavy hydrocarbon gas (HHG) in the coal seam of the Enhong syncline is extremely high in concentration but the gas
source is still unclear. Samples of the gas naturally desorbed from the Enhong’s coal sample was intensively collected and tested
in order to discuss the origin of the abnormal HHG. It was shown that the fractionation of the desorbed gas components is obvious
during the desorption. As the desorption time extends, the HHG concentration becomes obviously higher, the carbon isotope of
the methane becomes heavier. The carbon isotopic composition of the alkane always presents a positive carbon isotope series,
but the carbon isotopic composition of the HHG did not change significantly. Gas component concentration is related well to the
carbon isotopic composition but the opposite trend occurs for the methane and HHG. It was suggested that the CBM in the Enhong
syncline originated mainly from the thermogenic gas of the organic matter in coal but the possibility that the part of the methane is
secondary biogenic can not be excluded.