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

► Isotopic geochemistry of CBM formed in staged accumulation processes. ► δ13C and δD of gases generated by the samples with different maturity intervals. ► δ13C1 and δ13C2 are mainly affected by the evolutional level of starting sample. ► δD of methane and ethane are closely related to the maturity of these gases. ► Relationships of the δ13C/δD of the generated methane and ethane.