Comparative study of oil shales and shale oils from the Mahogany Zone, Green River Formation (U.S.A.) and Kerosene Creek Seam, Rundle Formation (Australia)

Abstract

Oil-shale samples from the Mahogany Zone, Green River Formation (Piceance Creek Basin), and the Kerosene Creek seam, Rundle Formation, were examined petrographically to determine maceral/mineral composition and then pyrolysed using a Fischer assay procedure to obtain the corresponding retort oils. The oils were analysed by gas chromatography—mass spectrometry and their composition related to the petrology of parent-shales. Both shales are Eocene in age, lacustrine in origin and contain organic matter predominantly in the form of alginite B. The Green River sample contained bitumen lenses and a matrix of carbonate minerals. Bitumen was absent from the Rundle sample and the matrix was largely silica. The retort oils were both highly aliphatic ( H C = 1.7 ). Over 1000 compounds were identified in the oils, including homologous series of n-alkanes, linear alkenes, isoprenoid compounds, alkylbenzenes, substituted naphthalenes and pyridines, nitriles, carbazoles, amides, linear alkanones, alkenones, phenols, phthalate esters, thiophenes and benzothiophenes. The chain-length distributions of the n-alkanes, 1-alkenes and nitriles indicate separate algal (major) and higher-plant (minor) inputs to the shale-oil kerogens. The oils differed markedly in many respects. The Green River oil contained a greater abundance of acyclic isoprenoid compounds, indicating bacterial re-working of the shale during diagenesis. 2- and 3-alkenes were absent from the Green River oil and this may be related to the differences in mineral matrix. Complex molecules of biological origin such as steranes, di- and triterpenoids were detected in the oil and are potential indicators of kerogen type and maturation history. This study indicates the feasibility of relating the composition of a shale oil to the petrology of the parent organic matter and the mineralogy of the inorganic matrix.