Hydrocarbon biomarkers, thermal maturity, and depositional setting of tasmanite oil shales from Tasmania, Australia

This study represents the first geological and organic geochemical investigation of samples of tasmanite oil shale representing different thermal maturities from three separate locations in Tasmania, Australia. The most abundant aliphatic hydrocarbon in the immature oil shale from Latrobe is a C₁₉ tricyclic alkane, whereas in the more mature samples from Oonah and Douglas River low molecular weight n-alkanes dominate the extractable hydrocarbon distribution. The aromatic hydrocarbons are predominantly derivatives of tricyclic compounds, with 1,2,8-trimethylphenanthrene increasing in relative abundance with increasing maturity. Geological and geochemical evidence suggests that the sediments were deposited in a marine environment of high latitude with associated cold waters and seasonal seaice. It is proposed that the organism contributing the bulk of the kerogen, Tasmanites, occupied an environmental niche similar to that of modern sea-ice diatoms and that bloom conditions coupled with physical isolation from atmospheric CO₂ led to the distinctive "isotopically heavy" δ¹³C values (-13.5‰ to -11.7‰) for the kerogen. δ¹³C data from modern sea-ice diatoms (-7‰) supports this hypothesis. Isotopic analysis of n-alkanes in the bitumen (-13.5 to -31‰) suggest a multiple source from bacteria and algae. On the other hand, the n-alkanes generated from closed-system pyrolysis of the kerogen (-15‰) are mainly derived from the preserved Tasmanites biopolymer algaenan. The tricyclic compounds (mean -8‰) both in the bitumen and pyrolysate, have a common precursor. They are consistently enriched in ¹³C compared with the kerogen and probably have a different source from the n-alkanes. The identification of a location where the maturity of the tasmanite oil shale approaches the "oil window" raises the possibility that it may be a viable petroleum source rock.