Stable carbon isotope distribution patterns of kerogen and its derived hydrocarbons constrained by primary biomass

There exists a stable carbon isotope reversal between crude oils and kerogens from the Cambrian-Lower Ordovician source rocks of the Tarim Basin, NW China. To verify the inverse carbon isotopic pattern and probe the possible mechanism, three shale samples with low thermal maturity were used for thermal simulation experiments, two of which were taken from the Neoproterozoic Xiamaling Formation in Xiahuayuan region, North China (one from argil-laceous shale and the other from calcareous shale) and the third one was from the Permian Lucaogou Formation in Santanghu Basin, Northwest China (argillaceous shale). A reversed carbon isotope distribution pattern between kerogen residue and its pyrolysates was observed for the Xiamaling calcareous shale, which was not found for the Xiamaling argillaceous shale or Lucaogou argillaceous shale. A stronger carbon isotope fractionation was found for the pyrolysates from Xiamaling calcareous shale kerogen than those from argillaceous shale kerogens, and then accordingly a weaker carbon isotope fractionation was determined for the pyrolyzed residues from Xiamaling calcareous shale kerogen. Combined with organic petrology and thermal simulation experiments, the “Xiahuayuan algal relic” from Xiamaling calcareous shale was supposed to have a lower hydrocarbon generation capacity than the mineral bituminous matrix from Xiamaling argillaceous shale and the laminated algae from Lucaogou argillaceous shale. Combined with biomarker distribution features, it was suggested that the reversed carbon isotope pattern, between kerogen residue and its pyrolysates, may be ascribed to the contribution of some special biomass (primarily constituted by n-alkanes) of early life, or the isoprenoid compounds were less preserved in the process of biomass sedimentation.