Carbon and hydrogen isotopic compositions and their evolutions of gases generated by herbaceous swamp peat at different thermal maturity stages

Coalbed methane (CBM) accumulation models include continuous gas accumulation and staged gas accumulation. However, studied on the geochemical characteristics and indices to evaluate staged accumulation CBM are lacking. This study for the first time obtained the carbon and hydrogen isotopic compositions of methane and ethane generated at different evolution stages using thermal simulation of samples prepared by treating an herbaceous swamp peat at different temperatures. The results showed that the carbon isotopic compositions of methane and ethane were obviously affected by the thermal evolution level of the starting sample, while the hydrogen isotopic compositions were closely related to the maturity of gases. The carbon and hydrogen isotopic compositions of gases generated by coal-forming organic matter with R _o values from 1.2%, 1.7%, 2.4%, 3.2% and 3.7% to 5.2% were determinated. The relationship between R _o values and the carbon and hydrogen isotopic compositions of gases generated by coal-forming organic matter at different evolution stages as well as the carbon or hydrogen isotopic relationships between methane and ethane were established. The results provide a scientific basis for studying the genesis of CBM generated at different maturity intervals and understanding the geochemical characteristics of staged accumulation CBM. These results were applied to a case study on CBM from the southern Qinshui basin, and it was found that the CBM accumulated after the Middle Jurassic and was characteristic of staged gas accumulation. This is consistent with the result of geological studies, and further showed that the results of thermal simulation experiments are very important for evaluating the genesis of natural CBM.