Spectroscopic, kinetic and pyrolytic studies of kerogen from the dark parallel laminae facies of the sulphur-rich Orbagnoux deposit (Upper Kimmeridgian, Jura)

Kerogen isolated from a representative sample of the dark parallel laminae facies of the Orbagnoux deposit, characterized by very high hydrogen and sulphur indices, was examined to derive information on its chemical structure and source organisms. The study of this extremely sulphur-rich material was carried out via a combination of kinetic (Pyromat), spectroscopic (FTIR, solid-state 13C NMR) and pyrolytic (‘off-lineʹ pyrolysis) methods. The highly complex pyrolysate obtained consisted chiefly of organic sulphur compounds (OSC). Analyses were focused on the highly polar and/or high molecular weight constituents, including the non-column chromatography (CC)-eluted and non-gas chromatography (GC)-amenable macromolecular components which account for the bulk of the pyrolysate. This study showed that two types of structural units, exhibiting differences both in chemical structure and source organisms, occur in the Orbagnoux kerogen. The predominant type corresponds to macromolecular units based on intermolecularly sulphur-linked, normal, C14–C24 hydrocarbon chains with a marked even-over-odd predominance and a sharp maximum at C18. These chains exhibit a high degree of cross-linking via (poly)sulphide bridges; they probably originated from cyanobacterial lipids, especially n-alcohols. The second type (whose occurrence was recognized in previous studies through analysis of bitumen and of apolar pyrolysis products) is based on longer, normal, hydrocarbon chains, up to C31, and is characterized by a lower degree of cross-linking. These chains, of algal origin, were likely sourced from lipids of the coccolithophorid Cyclagelosphaera margereli, whose mineral remains built up the bulk of the rock matrix. Both types of units were chiefly formed via the sulphurization of lipids and of diagenetically transformed lipids. Owing to extensive, early, intermolecular sulphurization, such lipids underwent only moderate bacterial degradation, as shown by the nature and distribution of the fatty acids released upon pyrolysis.