Analysis, structure and geochemical significance of organically-bound sulphur in the geosphere : State of the art and future research

This paper reviews the developments of the 1980s in the characterisation of organically-bound sulphur in the geosphere and summarises the geochemical significance of the results obtained by these studies. The identification of more than 1500 novel OSC (organic sulphur compounds) with structures related to biochemical precursors in mostly immature sediments and crude oils has led to a better understanding of the process by which inorganic sulphur is incorporated into organic matter. The sulphur enrichment of organic matter probably takes place via 'quenching' of functionalised lipids by sulphur at the early stages of sediment diagenesis or even in the water column. This results in the formation of OSC with specific structures which may be applied as palaeoenvironmental indicators. Hydrocarbon biological marker distributions may be heavily biased by diverting functionalised potential hydrocarbon precursors in OSC. Thermal evolution of sedimentary organic matter also affects the composition of OSC and when these changes are completely understood these compounds may be applied as maturity parameters as well. Recent developments in the characterisation of organically-bound sulphur present in macromolecular substances, i.e. kerogen, asphaltenes and high-molecular-weight fractions of crude oils and bitumens, show that the sulphur-containing moieties in these substances are formed in a similar way to the low-molecular-weight OSC. Sulphur incorporation into functionalised lipids in an intermolecular fashion should be considered as an important pathway for the formation of macromolecular substances. The abundance of organic sulphur in these substances also has a marked influence on their thermal evolution. Hiatuses in our present knowledge are numerous and stimulate future studies. Priorities are: the unravelling of the actual mechanisms of sulphur incorporation into organic matter and a better understanding of the diagenetic pathways of OSC and sulphur-containing macromolecular substances