Pyrolysis experiments of forest marsh peat samples with different maturities: An attempt to understand the isotopic fractionation of coalbed methane during staged accumulation

In order to understand the isotopic features of coalbed gas formed in staged accumulation processes, five starting materials with different maturity were prepared from a peat at temperatures of 250 °C, 300 °C, 350 °C, 400 °C and 450 °C, respectively. Closed-system isothermal pyrolysis experiments were performed on these samples at various temperature intervals. The carbon and hydrogen isotopic compositions of methane and ethane generated during the pyrolysis of the samples with different thermal maturities were determined. The results showed that the higher the initial thermal maturity of the starting sample, the heavier the carbon and hydrogen isotopic compositions of the generated methane and ethane. It was observed that the carbon isotope compositions of the generated methane differed significantly depending on initial thermal maturity of the samples, whereas the hydrogen isotopic compositions did not differ as much. The relationships of the δ13C and δD values of gases generated from the samples with maturity intervals of 0.8–4.8%, 1.1–4.8%, 1.9–4.8%, 2.3–4.8%, 3.1–4.8% Ro, respectively, were established. These results may provide a basis for studying on the isotopic geochemistry of coalbed gas formed in staged accumulation processes.