PRODUCTIVITY-INDUCED SULFUR ENRICHMENT OF HYDROCARBON-RICH SEDIMENTS FROM THE KIMMERIDGE CLAY FORMATION

This work aims to highlight the relationship between primary productiv ity, sulphate reduction and organic carbon preservation in cyclic mari ne sediments from the Kimmeridge Clay Formation. A concomitant increas e of the total sulphur content with the preserved organic content (TOC ), shows the progressive supply of both metabolisable organic matter a nd resistant organic matter is linked to primary productivity. However , variations in sulphate reduction efficiency, based on elemental abun dance and isotopic composition of sulphur, reveal that the proportion of metabolisable vs. resistant organic matter has varied along the cyc les. This is interpreted in terms of the variation in organic delivery . Organic sulphur content is found to be proportional to the organic m atter content, whereas concentrations of pyritic sulphur are constant at very high (> 10% TOC) values. This result is explained by a limitat ion of available iron for pyritisation at times of very high organic f lux. Under such conditions, HS- in excess could be responsible for the early formation of organo-sulphur compounds and thus for the preserva tion of highly aliphatic (i.e. lipid-rich) organic matter.