Hydrogen isotopic characteristic of hydrocarbon gas pyrolyzed by herbaceous swamp peat in hydrous and anhydrous thermal simulation experiments

In order to understand the influence of diagenetic water medium on hydrogen isotope of thermogenic coalbed gas, the hydrous and anhydrous pyrolytic simulation experiments have been carried out on herbaceous peat. We studied hydrogen isotope composition and its difference and evolution pattern of the pyrolysis hydrocarbon gases It was observed that diagenetic water medium exerts a significant influence on hydrogen isotopic composition of the pyrolysis hydrocarbon gases. The experiment added water with heavier hydrogen isotopic composition than peat-formed environmental water generated hydrocarbon gases with heavier hydrogen isotopic compositions. When peat was subject to continuous pyrolysis until 5.5%Ro, average δD values of the generated methane, ethane and propane were increased by 74‰, 42‰ and 66‰, respectively. It was considered that the reason for hydrous simulation experiment to increase hydrogen isotopic value of pyrolytic hydrocarbon gases is due to isotopic exchange between water-derived hydrogen and organic hydrogen. Mathematical models between the δD values of the hydrocarbon gases generated under the participation of freshwater with higher δD values and Ro values as well as between δD values of the generated hydrocarbon gases were established. These research results provide a scientific basis for the genetic study of thermogenic coalbed methane pyrolyzated by coal-forming materials formed in herbaceous marsh under the participation of diagenetic water media.