Evidence for sulphide links in a crude oil asphaltene and kerogens from reductive cleavage by lithium in ethylamine

Reduction of a sulphur-rich asphaltene from a crude oil (Rozel Point) with lithium in ethylamine (Li/EtNH2) affords an alkane fraction similar to that obtained from desulphurisation with Raney nickel, but in higher yield. Using Li/EtND2, deuterium-labelled alkanes were obtained, thereby confirming the presence of sulphide linkages in the asphaltene fraction. Reduction of two sulphur-rich kerogens with similar sulphur contents, Serpiano shale (M. Triassic) and Jurf Ed Darawish (JED, U. Cretaceous) afforded significant quantities of polar solvent-extractable material. Both kerogens also yielded low amounts of hydrocarbons although the amount released from JED kerogen was significantly higher. The results indicate that, whereas the kerogens contain hydrocarbon moieties intermolecularly linked by sulphide bridges, the distributions of the precursors lipids which were incorporated were different. For example, GC-MS analysis of the labelled products from JED kerogen revealed them to be dominated by steranes of algal origin, with the position(s) of deuterium labelling providing evidence of sulphide linking and cross-linking. On the other hand, the labelled products from Serpiano kerogen were dominated by hopanes of bacterial origin which were linked terminally via the side chain.