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The Effect of Redox Conditions on Carbon Isotopes of Hydrocarbons During Hydrous Pyrolysis


The carbon isotope composition of hydrocarbons has been used extensively to evaluate their origin and thermal maturity of source rocks. However, environmental conditions may change the formation (abundances, types, kinetics, etc.) of hydrocarbons and corresponding isotope fractionations. These controlling factors include temperature, pressure, redox, and the presence of water. To elucidate the role of environmental redox conditions (with H2, sulfate, etc.) in carbon isotope values of hydrocarbons during petroleum generation, a series of hydrous pyrolysis experiments were conducted. The source rock was obtained from the Huadian Basin in Jilin Province, northeast China. It is a shale layer in the Oil Shale Formation, which is Type-1 lacustrine deposit in the middle Eocene. At 310 °C, two sets of experiments were performed for 4 days under hydrous conditions. The difference between them is: one with dissolved H2 gas in the concentration of 100 mmol/kg, while the other with He gas. Similar experiments were also conducted at 350 °C. In all experiments, the products in gas phase include CO2, linear alkanes (C1-C5), and branched isomers of C3 to C5. The carbon isotope value of straight-chained alkanes increases with carbon numbers, following the trend for thermogenic hydrocarbons. At 310 °C, C1-C4 alkanes from experiments with H2 are depleted in 13C than experiments with He. For example, the δ13C value of methane in experiments with H2 is -40.88 ± 0.1‰, which is 2.71‰ depleted than methane (-38.17 ± 0.1‰) with He. This depletion becomes less with increasing carbon numbers. Pentanes from each experiment have similar carbon isotope values. In experiments with H2 at 350 °C, however, only C1 and C2 are depleted in 13C, whereas the δ13C values of C3 and C4 are higher than the values in experiments with He. The carbon isotope depletion of light alkanes suggests that the predominant pathway of their formation, direct kerogen breakdown vs. bitumen decomposition, may be different under reducing conditions. Temperature is also a factor governing carbon isotope fractionation. The on-going isotope measurement of other products, including groups in bitumen, is necessary to fully understand carbon isotope fractionation in this process. Combined with other experiments under different redox conditions (e.g., with the presence of gypsum), it would facilitate pathway identification for oil/gas formation in different environments and effective use of carbon isotopes for exploration.