Aromatic isoprenoids from the 25–65Ma saline lacustrine formations in the western Qaidam Basin, NW China

Abstract: Abundant aryl isoprenoids have been detected in source rock extracts from the 25–65Ma saline lacustrine formations in the western Qaidam Basin, NW China. Identification was based mainly on comparison of mass spectra and gas chromatographic behaviour with literature data. Two pseudohomologous series in the range C13–C40 were assigned as 2,3,6- and 2,3,4-trimethyl monoaryl isoprenoids. The C40 2,3,4-trimethylaryl isoprenoid, okenane, was the most abundant component of the series, while its C40 2,3,6-trimethylaryl isomer, chlorobactane, was present in low abundance. Two C40 monoaromatic isoprenoids with a cyclohexyl ring were tentatively assigned as β-isorenieratane and β-renierapurpurane. One C40 diaryl isoprenoid was identified as isorenieratane. Identification of okenane, chlorobactane and isorenieratane was confirmed by co-elution experiments using synthetic standards. In addition, two novel series of aryl isoprenoids in the C12–C39 range were tentatively assigned as 2,6- and 2,3-dimethylaryl isoprenoids. The C39 2,3-dimethylaryl isoprenoid homologue was the most abundant component of the series, while the C39 2,6-dimethylaryl isomer was only present in low abundance. Two C39 monoaromatic bicyclic compounds were tentatively assigned as 2,6- and 2,3-dimethylaryl isoprenoids with a terminal cyclohexyl ring. Furthermore, a minor C38 component was tentatively assigned as a diaryl isoprenoid with a 2,3-/2,3-dimethyl substitution pattern. These dimethylaryl isoprenoids seem to be formed by demethylation of their parent trimethylaryl isoprenoid counterparts, the location of the demethylated position being exclusively at C-3, C-4 or C-6 of the aromatic ring. This specificity points to a biologically-mediated process (as opposed to an abiotic, maturity-related transformation) that needs to be further investigated. [Copyright &y& Elsevier]