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

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.