C-S-Fe geochemistry of some modern and ancient anoxic marine muds and mudstones

Abstract

Most sedimentary C, S and Fe occurs in marine muds and is originally present as an unstable mixture of dissolved sulphate, organic matter and detrital Fe minerals. During early diagenesis, key reactants are either destroyed (reduction of Fe(III) and SO 2- 4 to form pyrite), created (organically bound sulphur (OBS)) or preserved (Fe-bearing silicates). Pyrite formation is commonly limited not by sulphide availability but by the rate at which detrital Fe minerals react with sulphide. At low temperatures, detrital Fe(II) and Fe(III)-bearing clay minerals react very slowly with sulphide and are buried intact. The uptake of sulphur into organic matter is not, as commonly assumed, sulphide-limited. We believe that polysulphides react rapidly with organic matter and that the availability of reactive polysulphides may also influence the sulphur content of kerogen. Polysulphide formation is favoured at stable aerobic-anaerobic interfaces and sediments deposited under weakly oxygenated bottom waters are most likely to contain sulphur-rich kerogen. Deep burial (greater than 70 °C) C-S-Fe diagenesis is characterized by the loss and partial reduction of Fe(III) from smectitic and illitic clays and the conversion of limited amounts of obs to lower molecular mass S compounds. Pyrite formation is volumetrically insignificant. More than 90% of obs remains within organic-rich mudrocks during petroleum generation and expulsion, preserving a sulphur-enriched residual kerogen. The extent to which obs is eventually converted to H 2 S, and at what level of thermal maturity, is unclear. If, as may be likely, much of the sulphur in residual kerogen is thiophenic, it will be stable to at least 200 °C.