Geochemical characteristics and identification of thermogenic CBM generated during the low and middle coalification stages

Abstract

Six woody peat samples were heated to 150, 250, 300, 350, 370, and 400°C to determine their geochemical characteristics and identify whether the coal bed methane (CBM) was thermogenic or secondary biogenic. The experiments were conducted in a closed pyrolysis system, from which the generated pyrolysis gas was collected. The composition of the gas and its carbon and hydrogen isotope concentrations were also determined. The results indicate that methane and carbon dioxide were the main hydrocarbon and non-hydrocarbon gases generated from the samples. The carbon isotopic compositions of the generated methane, ethane, and propane became initially lighter and then heavier with increasing experimental temperature; conversely, the hydrogen isotopic composition of the generated methane and the carbon isotopic composition of the generated carbon dioxide became gradually heavier. Therefore, a strong positive correlation exists between the aforementioned parameters. In addition, relationships have been established between carbon and hydrogen isotopic contents and R_o values, and a model for the identification of the maturity and relative content of thermogenic CBM is proposed. Then, the model was applied in a case study to evaluate the gas source of CBM in the Luling CBM field in Anhui Province, China.