| Abstract: | Carotenoid pigments are ubiquitous components of the biosphere but have rarely been reported in geological samples. Oils from the Kelamayi oilfield of northwestern China have been shown by GC-MS analysis to contain abundant carotenoid-derived alkanes including β- and γ-carotanes, 1,1,3-trimethyl-2-alkylcyclohexanes and a compound tentatively identified as saturated lexene. In addition, a seepage oil contained a possible C40 alkene with a structure similar to that of β-carotane except for a carbon-carbon double bond in one of the cyclohexyl rings. The distributions of biomarker compounds in these oils are very similar to those observed in the few other reported samples that contain abundant carotenoid-derived compounds, indicating that the Kelamayi source rock was probably deposited under very specific environmental conditions. The distribution of carotenoid-derived compounds, particularly the ratio of γ-carotane to β-carotane, appears to be sensitive to both thermal maturation and biodégradation. This ratio increases with maturation and decreases with increasing biodegradation. |
