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Comparative Study of Hydrogen and Carbon Isotopic Composition of Gases Generated from the Pyrolysis of a Peat under Saltwater and Freshwater Conditions
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This study was supported by the National Natural Science Foundation of China (Grant nos. 41772108 and 41472121). The authors appreciate and acknowledge the analytical work at the Analytical Service Center，Research Center for Oil and Gas Resources, Northwest Institute of Eco-Environment and Resources, CAS. We also thank two anonymous reviewers and Dr Degan Shu for valuable suggestions and critical comments that immensely contributed to the improvement of this manuscript.

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    Abstract:

    To understand the influence of the diagenetic water medium on the isotopic compositions of thermogenic coalbed gas, both hydrous and anhydrous closed-system pyrolyses were performed at temperatures of 250°C to 650°C on an herbaceous marsh peat. Compared to the results of anhydrous pyrolysis, the hydrocarbon gases generated from hydrous pyrolyses have very different hydrogen isotopic compositions. However, the carbon isotopic compositions of the hydrocarbon gases became only slightly heavier in hydrous pyrolysis, compared to that from anhydrous pyrolysis. With the progress of thermal evolution from peat to a more advanced thermal maturity of vitrinite reflectance values (Ro) of 5.5% during the pyrolysis, the difference in the average δD value increased from 52‰ to 64‰ between the hydrous pyrolysis with saltwater and anhydrous pyrolysis and increased from 18‰ to 29‰ between the hydrous pyrolysis with freshwater and anhydrous pyrolysis, respectively. The difference in the average δ13C value was only 1‰–2‰ between the hydrous and anhydrous pyrolysis. The relationships between the δD values of the generated hydrocarbon gases and Ro values as well as among δD values of the hydrocarbon gas species are established. The close relationships among these parameters suggest that the water medium had a significant effect on the hydrogen isotopic composition and a minimal effect on the carbon isotopic composition of the hydrocarbon gases. The results of these pyrolyses may provide information for the understanding of the genesis of coalbed gas from herbaceous marsh material with the participation of different diagenetic water media.

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WU Yingzhong, DUAN Yi, ZHAO Yang, CAO Xixi, MA Lanhua, QIAN Yaorong, LI Zhongping, XING Lantian.2018. Comparative Study of Hydrogen and Carbon Isotopic Composition of Gases Generated from the Pyrolysis of a Peat under Saltwater and Freshwater Conditions[J]. Acta Geologica Sinica(),92(5):1879-1887

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History
  • Received:November 06,2017
  • Revised:January 23,2018
  • Online: October 19,2018
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