Quenching of labile functionalised lipids by inorganic sulphur species: evidence for the formation of sedimentary organic sulphur compounds at the early stages of diagenesis

Abstract

The bitumen of the Jurf ed Darawish Oil Shale has been analysed for organic sulphur compounds (OSC). A number of OSC are reported for the first time: several C₂₈8, C₃₇ and C₃₈ 2,5-dialkylthiolanes and -thiophenes and 2,6-di-n-alkylthianes, and C₁₉ branched thiophenes possessing the 9-methyloctadecane carbon skeleton. A number of these compounds were identified by synthesis of authentic standards. All the OSC compound classes mentioned exhibit structural isomer distributions dominated by a limited number of all theoretically possible isomers. This provides direct evidence for the formation of these OSC by abiogenic sulphur incorporation into functionalised lipids at the early stages of diagenesis. Precursors for the OSC identified are suggested.

From these observations and from data on the occurrence of other OSC and of sulphur in high molecular weight substances a general model for the incorporation of sulphur into organic matter is proposed. Sulphur incorporation into precursors with double bonds (or other reactive functionalities) will lead to formation of OSC and sulphur-rich high molecular weight substances. Only precursors with two double bonds in favourable position for intramolecular addition of intermediate thiols can yield low molecular weight OSC. Double bond isomerisations by a sequence of H₂S addition and elimination reactions may play a role in this respect.