Sulphur Incorporation in Jurassic Marine Mudrocks and Their Bitumens at Low Thermal Maturity, Cleveland Basin, England

The distribution of sulphur between pyrite, kerogen, elemental sulphur, and the aromatic fraction of mudrock bitumens has been quantitated in two cored sections of immature marine Kimmeridgian strata which have experienced different levels of thermal stress. Strata in the two boreholes (Ebberston and Reighton) were deposited in different basinal settings (basinal marine and platform, respectively) and occasional euxinic (laminated) horizons are present at Ebberston, otherwise dysoxic deposition was the norm at both localities.Pyrite, the dominant sulphur carrier in all the mudrocks, has a sulphur isotopic composition consistent with mineralisation in a system mainly open to sulphate diffusion. Kerogens are mostly low sulphur (atomic S/C < 0.02) and account for between 5 and 10% of rock sulphur although occasionally up to 30%. Kerogen sulphur is closely correlated with TOC and highest in laminated mudrocks consistent with most effective sulphurization of kerogen under anoxic conditions. Although kerogen compositions in both cores vary with organic richness, no systematic differences in organic input between the borehole localities are recognised geochemically. Some thermal effects on sulphur geochemistry are, therefore, interpreted from a comparison of stratigraphically equivalent sections from the two cores.Bitumen yields and abundances of aromatic sulphur in the more thermally-stressed Ebberston section, although variable, are generally much higher than in the Reighton section and are partly related to kerogen sulphur content. This is tentatively attributed to thermal release of additional extractable bitumen from macromolecules. Elemental sulphur (S-0) in bitumen is unrelated in abundance to other sulphur species and isotopically heavier than coexisting pyrite. Overprinting of minor preserved diagenetic S-0 by secondary oxidation of pyrite appears likely. The distributions of individual sulphur-aromatic compound classes have features related both to maturity and stratigraphy.