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 C-19 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 sea-ice. 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'' delta(13)C values (-13.5 parts per thousand to -11.7 parts per thousand) for the kerogen. delta(13)C data from modern sea-ice diatoms (-7 parts per thousand) supports this hypothesis. Isotopic analysis of n-alkanes in the bitumen (-13.5 to -31 parts per thousand) suggest a multiple source from bacteria and algae. On the other hand, the n-alkanes generated from closed-system pyrolysis of the kerogen (-15 parts per thousand) are mainly derived from the preserved Tasmanites biopolymer algaenan. The tricyclic compounds (mean -8 parts per thousand) both in the bitumen and pyrolysate, have a common precursor. They are consistently enriched in C-13 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.