HYDROCARBON BIOMARKERS, THERMAL MATURITY, AND DEPOSITIONAL SETTING OFTASMANITE OIL SHALES FROM TASMANIA, AUSTRALIA

This study represents the first geological and organic geochemical inv estigation of samples of tasmanite oil shale representing different th ermal maturities from three separate locations in Tasmania, Australia. The most abundant aliphatic hydrocarbon in the immature oil shale fro m Latrobe is a C-19 tricyclic alkane, whereas in the more mature sampl es from Oonah and Douglas River low molecular weight n-alkanes dominat e the extractable hydrocarbon distribution. The aromatic hydrocarbons are predominantly derivatives of tricyclic compounds, with 1,2,8-trime thylphenanthrene increasing in relative abundance with increasing matu rity. Geological and geochemical evidence suggests that the sediments were deposited in a marine environment of high latitude with associate d cold waters and seasonal sea-ice. It is proposed that the organism c ontributing the bulk of the kerogen, Tasmanites, occupied an environme ntal niche similar to that of modern sea-ice diatoms and that bloom co nditions coupled with physical isolation from atmospheric CO2 led to t he distinctive ''isotopically heavy'' delta(13)C values (-13.5 parts p er thousand to -11.7 parts per thousand) for the kerogen. delta(13)C d ata from modern sea-ice diatoms (-7 parts per thousand) supports this hypothesis. Isotopic analysis of n-alkanes in the bitumen (-13.5 to -3 1 parts per thousand) suggest a multiple source from bacteria and alga e. On the other hand, the n-alkanes generated from closed-system pyrol ysis of the kerogen (-15 parts per thousand) are mainly derived from t he 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 wit h 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.