Thermal simulation study on the influence of coal-forming material on the isotopic composition of thermogenic coalbed gas

摘要

In order to understand the influence of coal-forming materials and slight differences in coal maceral composition on the isotopic composition of thermogenic coalbed gas, closed-system isothermal pyrolysis experiments were performed on forest and herbaceous swamp peats and on coals with slight differences in maceral compositions. The carbon and hydrogen isotopic compositions of the hydrocarbon gases (methane, ethane and propane) generated during the pyrolysis of the samples were determined. The results showed that the carbon and hydrogen isotopic compositions of the hydrocarbon gases generated from the herbaceous swamp peat were lighter than those generated from the forest swamp peat. At pyrolysis intervals from peat to vitrinite reflectance values (Ro) of 2.5%, 3.5% and 5.5%, this difference in the average delta D value of the generated methane was 18 parts per thousand, 11 parts per thousand and 9 parts per thousand, and in the delta C-13 value of methane was 3.4 parts per thousand, 3.8 parts per thousand and 1.8 parts per thousand, respectively. The reason for the difference of trend can be explained as follows. The methane generated from the coal containing slightly higher exinite content had noticeably lighter carbon isotopic composition. However, the hydrogen isotopic composition of the generated methane was primarily related to coal-forming environmental factors, such as latitude, delta D value of rainwater, and diagenetic fluid of freshwater or seawater. These results provide a basis for studying the isotopic geochemistry of coalbed gas.