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

Abstract 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 CH 4 (95.9–99.4 mol%, air-free basis) with minor amounts of N 2 (average: 1.07%), CO 2 (average: 0.25%), and ethane (average: 0.02%). The carbon isotope ratios of the production CH 4 range from − 40.8‰ to − 33.2‰, with an average of − 37.1‰, and the corresponding hydrogen isotope ratios of CH 4 − 196‰ to − 178‰, with an average of − 186‰ ( 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%. The type of CBM production water is Na–HCO 3 , and the concentrations of total dissolved solids range from 1282 mg/mL to 1718 mg/mL, with an average of 1417 mg/mL. The δ D values of the water samples range from − 74.8‰ to − 61.1‰, the δ 18 O values from − 9.6‰ to − 8.0‰. The isotope compositions of water samples fall to the right of the GMWL, suggesting a combination of fluid–rock interaction under high temperature conditions and evaporation. There is a good correlation between the distribution of KCCs and the isotopic compositions of CBM production gases and waters in the study area. KCCs in this area can be the free pathways going through the surface, the limestone beds and the 15# coal seam of Taiyuan Fm. KCC presence makes the CH 4 carbon isotopic composition become more enriched in 13 C in shallower areas, because of the stronger desorption–diffusion–migration isotope fraction effect of CBM in the north of Sijiazhuang mining area. In contrast, the δ 13 C 1 and δ 13 C CO 2 values are lighter in deeper areas in the southwest of the study area. This could be due to the stronger groundwater stripping process in that area. The results can be used for CBM exploration and exploitation in KCC areas.