Abstract
There are few studies on the origins of coalbed gas in the northern Qinshui Basin, therefore, the coalbed gas in the Taiyuan Formation of the Sijiazhuang mine field in the northern Qinshui Basin was extracted to test the chemical composition and isotope of effluent gas from six coalbed gas wells, and thus the origins of coalbed gas can be explored. The results show that the average volume fractions of CH4, C2H6, CO2 and N2 in the coal seam of the northern Qinshui Basin are 94.98 %, 0.11 %, 0.25 % and 4.8 %, respectively, with no hydrocarbons heavier than ethane detected. The carbon and hydrogen isotopic values of methane of coalbed gas are within the range of -42.5 ‰ ~ -36.7 ‰ and - 200.06 ‰ ~ -196.33 ‰, respectively; and d13C (CO2) is between -17.1 ‰ and -15.1 ‰. The solution fractionation of groundwater is the main cause of the methane carbon isotope’s becoming lighter with increasing depths. The chemical composition and isotope test data of the Sijiazhuang mine field indicate that the coalbed gas has been subjected to the secondary transformation effect, mainly from coal cracking thermo genesis and supplemented by secondary biogenesis of CH4.
