Variation characteristic and mechanism of carbon isotope composition of coalbed methane under different conditions and its tracing significance

Abstract Coalbed gas (CBG) is a form of new energy with a large resource potential. The δ13C1 is an important indicator in trace CBG origins and thermal evolution degrees. However, regarding CBG δ13C1 as a genetic tracer still exists uncertainty. Here, we systematically studied the distribution characteristics, variation mechanisms and applications of the δ13C1 of CBG samples produced under different conditions. The study shows CBG δ13C1 are highly variable and complex. The δ13C1 are from −79.5‰ to −10.1‰, and the correlation between δ13C1 and Ro is week. The δ13C1 of the production gas and coal core desorption gas are stable and could be used as an effective genetic tracer. The δ13C1 of the mining coal desorption gas are significantly higher than the theoretical δ13C1, however, the δ13C1 of the drained coalmine gas are lower, suggesting that the mining coal desorption gas represents the residual CBG, however, the drained coalmine gas represents the first desorbed CBG. Therefore, their δ13C1 cannot be used as genetic tracer, but have great potential in tracing the CBG prevention condition. The mixture of secondary biogenic gas (SBG) and thermogenic gas makes δ13C1 high variability. It cannot be directly used as tracer, but is of great significant in study the mixing ratios of the two kinds of gases. The main reasons for the high variability and complexity of CBG δ13C1 are the isotope desorption fractionation and the mixing of SBG and thermogenic gas. This study provides a new scientific basis for geochemical tracing and resource evaluation of CBG.