Identification and geochemical significance of demethylated highly cyclized regular polyprenoids in oils and source rocks

Polycyclic polyprenoid hydrocarbons and related demethylated compounds have been identified in severely altered bitumen from oil stained rocks of Upper Jurassic/Lower Cretaceous age. They occur as three series, one homologue of which has been isolated and structurally characterized from nuclear magnetic resonance (NMR) studies. They comprise hydrocarbons demethylated at the A/B ring junction, compounds demethylated at rings E/F/G and a novel series of compounds demethylated at both the ring A/B junction and the ring E/F/G junction, identified for the first time. The occurrence in a number of unaltered and biodegraded oils and source rocks of Upper Jurassic/Lower Cretaceous age, of non-demethylated hydrocarbons, together with related series demethylated at rings E/F/G, indicates that the demethylation process is unlikely to be related to a late degradation process, but rather results from the early transformation of precursors bearing functionalised methyl groups on rings E/F/G. In contrast, polycyclic polyprenoid hydrocarbons demethylated at the A/B ring junction occur only in the most biodegraded samples, suggesting that this demethylated series is formed at later stages of alteration by further demethylation of both non-demethylated compounds and compounds demethylated at rings E/F/G. In addition, a novel series of carboxylic acids related to the polycyclic polyprenoids has been tentatively identified using gas chromatography–mass spectrometry (GC–MS) and comprises both non-demethylated and demethylated structures at rings E/F/G. It is proposed that the acids represent alteration products of polyprenoids bound to aromatic sub-units from macromolecular material that have been released upon severe oxidation during exposure to biodegradation/weathering.