Abstract
Gases generated by laboratory anhydrous closed-system pyrolysis of hydrogen-rich Eocene Yubari subbituminous coal were distinguished as expelled gas, desorbed gas, residual free gas, and residual adsorbed gas, and the compositional and isotopic changes in these gases during laboratory maturation were investigated. The relative abundances of methane and H2 in the expelled gas were higher than those in the residual free and adsorbed gases, showing compositional fractionation during gas expulsion from the coal fragments. The δ13C and δ2H values of ethane, propane, and CO2 in all the gas fractions were nearly the same. However, the δ13C and δ2H values of methane in the residual free gas were significantly higher than those of methane in the expelled and desorbed gases, demonstrating a large isotopic fractionation associated with the expulsion of methane from the coal fragments. The carbon and hydrogen isotopic fractionations of ethane, propane, and CO2 during their expulsion were almost negligible.The evolution paths of the differential δ13C values of hydrocarbon gases generated from hydrogen-rich Eocene Yubari coal during laboratory maturation were rather close to those of hydrocarbon gases from Type II kerogen in the early stage of maturation (VRr%<1.2), but these shifted closer to those of hydrocarbon gases from Type III kerogen in the higher maturation stage (VRr%>1.6). The similar δ13C values of hydrocarbon gases from Eocene Yubari coal and Type II kerogen are partly due to the abundant and 13C depleted aliphatic structures in the Eocene Yubari coal. The δ2H of methane in the expelled gas from Yubari coal also increased systematically with laboratory maturation, although it can be influenced by various factors. Estimation of the maturity level and type of source organic matter of thermogenic methane based on isotopic composition requires caution, especially in the Cenozoic sedimentary basins.
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