Differences in the mode of incorporation and biogenicity of the principal aliphatic constituents of a Type I oil shale

Compound-specific stable carbon isotope (δ) measurements on the aliphatic hydrocarbons released from an immature Tertiary oil shale (Göynük, Turkey) via hydropyrolysis, following solvent extraction and a milder hydrogenation treatment, have further highlighted that significant compositional differences may exist between the principal aliphatic constituents of the solvent extractable (bitumen) phase and the insoluble macromolecular network (kerogen) comprising the bulk of sedimentary organic matter. Whilst inputs from diverse sources; including algae, bacteria and terrestrial higher plants, were implied from analysis of solvent-extractable alkanes, the much larger quantities of kerogen-bound n-alkyl constituents released by hydropyrolysis had a uniform isotopic signature which could be assigned to (freshwater) algae. Remarkably, the aliphatics bound to the kerogen by relatively weak covalent bonds, liberated via catalytic hydrogenation, appeared to comprise mainly allochthonous higher plant-derived n-alkanes. These results provide further compelling evidence that the molecular constituents of bitumen and, indeed, of low-yield kerogen degradation products, are not necessarily reliable indicators of kerogen biogenicity, particularly for immature Type I source rocks. The isotopic uniformity of aliphatic n-hydrocarbons released by the high-conversion hydropyrolysis step for the ultralaminae-rich Göynük oil shale, lends further support to the theory that selective preservation of highly resistant aliphatic biomacromolecules is an important mechanism in kerogen formation, at least for alginite.