Extractable aromatic hydrocarbons in a short-term organic cycle of the Kimmeridge Clay formation, Yorkshire (U.K.): relationship to primary production and thermal maturity

Abstract

Seven core samples from a 120 cm section of the Kimmeridge Clay formation, Cleveland Basin, north-eastern England, comprising a short-term organic cycle were analyzed for extractable aromatic compounds. Yields and ring-class distributions of C 11+ aromatic hydrocarbon plus C 9+ aromatic sulphur compound fractions were determined by liquid chromatography while distributions of benzohopanes, C 2C 3 naphthalenes, C 0C 2 phenanthrenes, C 1C 3 benzo[ b]thiophenes, and C 0C 2 dibenzothiophenes were quantified by capillary gas chromatography. Early thermal evolution of the organic matter, corresponding to the top of the oil-generation window, was inferred from aromatic parameters (Methylphenanthrene Index, methyldibenzothiophene ratio). It was verified by yields of total methylphenanthrenes, C 1C 3 benzo[ b]thiophenes, and C 0C 2 dibenzothiophenes in the 30–40 μg/g total organic carbon (TOC) range comparable to those in the Posidonia Shale (Toarcian) formation, western Germany, at 0.5% vitrinite reflectance ( R r). Variations in total aromatics yields are paralleled by TOC and Hydrogen Index variations with depth thus suggesting an origin of aromatics mostly from algal material. Enhanced contributions of biological source materials other than algae to the sediments are indicated by elevated yields of benzohopanes (C 32C 35), alkylnaphthalenes, and alkylbenzo[ b]thiophenes at the edges of the section investigated.