Evidence from the Changing Carbon Isotopic of Kerogen, Oil, and Gas during Hydrous Pyrolysis from Pinghu Formation, the Xihu Sag, East China Sea Basin

Semi-open hydrous pyrolysis experiments on coal-measure source rocks in the Xihu Sag were conducted to investigate the carbon isotope evolution of kerogen, bitumen, generated expelled oil, and gases with increasing thermal maturity. Seven corresponding experiments were conducted at 335 °C, 360 °C, 400 °C, 455 °C, 480 °C, 525 °C, and 575 °C, while other experimental factors, such as the heating time and rate, lithostatic and hydrodynamic pressures, and columnar original samples were kept the same. The results show that the simulated temperatures were positive for the measured vitrinite reflectance (Ro), with a correlation coefficient (R2) of 0.9861. With increasing temperatures, lower maturity, maturity, higher maturity, and post-maturity stages occurred at simulated temperatures (Ts) of 335–360 °C, 360–400 °C, 400–480 °C, and 480–575 °C, respectively. The increasing gas hydrocarbons with increasing temperature reflected the higher gas potential. Moreover, the carbon isotopes of kerogen, bitumen, expelled oil, and gases were associated with increased temperatures; among gases, methane was the most sensitive to maturity. Ignoring the intermediate reaction process, the thermal evolution process can be summarized as kerogen0(original) + bitumen0(original)→kerogenr (residual kerogen) + expelled oil (generated) + bitumenn+r (generated + residual) + C2+(generated + residual) + CH4(generated). Among these, bitumen, expelled oil, and C2-5 acted as reactants and products, whereas kerogen and methane were the reactants and products, respectively. Furthermore, the order of the carbon isotopes during the thermal evolution process was identified as: δ13C1 < 13C2-5 < δ13Cexpelled oil < δ13Cbitumen < δ13Ckerogen. Thus, the reaction and production mechanisms of carbon isotopes can be obtained based on their changing degree and yields in kerogen, bitumen, expelled oil, and gases. Furthermore, combining the analysis of the geochemical characteristics of the Pinghu Formation coal–oil-type gas in actual strata with these pyrolysis experiments, it was identified that this area also had substantial development potential. Therefore, this study provides theoretical support and guidance for the formation mechanism and exploration of oil and gas based on changing carbon isotopes.

Formation of ethane and propane via abiotic reductive conversion of acetic acid in hydrothermal sediments

A mechanistic understanding of formation pathways of low-molecular-weight hydrocarbons is relevant for disciplines such as atmospheric chemistry, geology, and astrobiology. The patterns of stable carbon isotopic compositions (δ13C) of hydrocarbons are commonly used to distinguish biological, thermogenic, and abiotic sources. Here, we report unusual isotope patterns of nonmethane hydrocarbons in hydrothermally heated sediments of the Guaymas Basin; these nonmethane hydrocarbons are notably 13C-enriched relative to sedimentary organic matter and display an isotope pattern that is reversed relative to thermogenic hydrocarbons (i.e., δ13C ethane > δ13C propane > δ13C n-butane > δ13C n-pentane). We hypothesized that this pattern results from abiotic reductive conversion of volatile fatty acids, which were isotopically enriched due to prior equilibration of their carboxyl carbon with dissolved inorganic carbon. This hypothesis was tested by hydrous pyrolysis experiments with isotopically labeled substrates at 350 °C and 400 bar that demonstrated 1) the exchange of carboxyl carbon of C2 to C5 volatile fatty acids with 13C-bicarbonate and 2) the incorporation of 13C from 13C-2–acetic acid into ethane and propane. Collectively, our results reveal an abiotic formation pathway for nonmethane hydrocarbons, which may be sufficiently active in organic-rich, geothermally heated sediments and petroleum systems to affect isotopic compositions of nonmethane hydrocarbons.