Application of supercritical fluid extraction to organic geochemical studies of oil shales

Oil shales were subjected to supercritical fluid extraction (s.f.e.) with carbon dioxide, utilizing the ability of supercritical fluids to extract rapidly specific solute classes from complex matrices. Yields of the extracts obtained were comparable with those of the hydrocarbon fractions of Soxhlet chloroform extracts of oil shales but depended greatly on the inorganic matrix. The lowest s.f.e. extract yield was obtained from Krasava oil shale with mainly a ceolite mineral matrix, and the highest from kukersite with a predominantly carbonate mineral matrix. The relative distribution of n-alkanes and the pristane/phytane ratio in both s.f.e. and Soxhlet extracts were similar, whereas the absolute concentrations of isoprenoid and polycyclic hydrocarbons (steranes and triterpanes) were lower in the s.f.e. extracts. The extraction with the more polar carbon dioxide-methanol fluid system was more exhaustive in the case of the above-named compounds used as biomarkers in organic geochemical studies of oil shales.