Isotopic composition and content of coalbed methane production gases and waters in karstic collapse column area, Qinshui Coalfield, China

摘要

The Qinshui Coalfield is one of the most important coalbed methane (CBM) coalfields in China with vast CBM resources, in which the Carboniferous Taiyuan Formation is one of the main coal-bearing sequences. Eleven CBM production gas samples and associated production waters were collected from the Taiyuan Fm. in the karstic collapse column (KCC) area of Sijiazhuang mining area, Qinshui Coalfield. The gas molecular and isotopic compositions and water quality, water carbon and hydrogen isotope compositions were analyzed. Results in this study reveal that CBM from Taiyuan Fm. of Sijiazhuang is dominated by CH4 (95.9-99.4 mol%, air-free basis) with minor amounts of N-2 (average: 1.07%), CO2 (average: 0.25%), and ethane (average: 0.02%). The carbon isotope ratios of the production CH4 range from -40.8 parts per thousand to -33.2 parts per thousand, with an average of -37.1 parts per thousand, and the corresponding hydrogen isotope ratios of CH4 -196 parts per thousand to -178 parts per thousand, with an average of -186 parts per thousand (n = 11). Thermogenic methane is the primary source of CBM from the Taiyuan Fm. of Sijiazhuang, and its estimated proportion is calculated to range from 72% to 95%.