CHARACTERISTICS AND ORIGIN OF 13C(CH4) IN COAL-BED GAS IN SOUTHERN QINSHUI BASIN.

Exploring the cause and origin of coal-bed gas serves as a guide on the exploration, evaluation and development of coal-bed gas. Based on test of coal-bed gas samples, this paper analyses the composition of coal-bed gas in southern Qinshui basin and the distribution of methane carbon isotope [13C(CH4)] and CO2 carbon isotope [13C(CO2)], and discusses the origin of coal-bed gas in southern Qinshui basin in consideration of the thermal maturity of coal, groundwater dynamic conditions and the generation of biogas through bacteria degradation. The following conclusions are drawn from the research: first, the coal-bed gas in southern Qinshui basin is an extremely dry gas mainly made up of methane, plus a very small amount of ethane and non-hydrocarbon components; second, δ13C(CH4) follows unimodal distribution, falling in the range between -38.8 and -23.52%.; third, the change of δ13C(CH4) has a positive correlation with the variation of groundwater salinity, and has nothing to do with the change of coal rank; fourth, the generation of biogas through bacteria degradation is one of the possible causes to the phenomena that δ13C(CH4) gets heavier from the groundwater recharge zone to the central stagnant zone, while δ13C(CO2) changes in the opposite way. [ABSTRACT FROM AUTHOR]