Extractable and kerogen-bound hopanoids from typical Eocene oil shales in China

This study conducted a comparative analysis of extractable hopanoid hydrocarbons and those released via stepwise pyrolysis of typical Eocene immature oil shales in China, namely the Huadian, Maoming, and Fushun shales. Both the Huadian and Maoming shales exhibit immature indicators in extractable and kerogen-bound hopanoids (notably, high abundance of C29 to C32 17β,21β-hopanes and unsaturated hopenes). In contrast, the Fushun oil shale's hopanoids from extracts and pyrolyzates suggest a higher maturity level. The absence of neohopenes in the pyrolyzates of the shales underpins that the kerogen-bound hopanoid skeletons resist rearrangement. However, the Huadian oil shale's asymmetric distribution of C29 and C30 hopenes and neohopenes hints at the presence of an additional source. Novel unsaturated hopenes, such as hop-20 (21)-enes, identified in pyrolyzates of the three kerogens at various pyrolysis temperatures, reveal the occurrence of double bonds in kerogen-bound hopanoid skeletons without methyl rearrangements. The absence of hop-20-(21)-ene in extracts suggests that it might act as an intermediate of these novel hopenes during the epimerization of hopanoid skeletons within kerogen. The extractable and pyrolytic hopanoids' stereochemical alignment indicates that epimerization may occur in both ring systems and alkyl side chains of kerogen-bound hopanoid skeletons. Sequential stepwise pyrolysis proves to be a quick screening method for geological hopanoids without causing any significant alteration to the original skeletons even when cracking multiple covalent bonds is necessary.