Ten bitumen samples collected between 1880 and 1915 from the west and southwest coasts of Tasmania, three from Flinders and King Islands in Bass Strait and one each from Preolenna in northwestern Tasmania, Cape Jaffa in South Australia and a beach in the south of Western Australia were analysed as part of a study to re-evaluate the petroleum prospectivity of Tasmania and adjacent areas in southeastern Australia. The bitumens dissolved completely in chloroform and contained aliphatic hydrocarbons (12.3-17.3%), aromatic hydrocarbons (4.4-10.6%), and uncharacterized asphaltenes (75.6-82.9%). The distributions of aliphatic and aromatic biomarkers were remarkably similar in all of the bitumens indicating that they were derived from a common or very similar source. Biomarker data confirmed that the bitumens are not related to land-plant derived oils presently recovered from the nearby Bass, Otway and Gippsland Basins of southern Australia. The high proportions of C27 steranes and presence of C30 steranes including dinosterane suggested that the bitumens were derived from a marine source rock containing mainly marine organic matter. Biomarker maturity parameters were also very similar in all bitumens and consistent with those found in crude oils generated at an equivalent vitrinite reflectance of about 0.75. Hopanes showed a high proportion of C29 and C30 17α(H), 21β(H)-hopanes with very minor amounts of the corresponding moretanes. The unusual absence of 2α-methylhopanes and low abundance of tricyclic alkanes ruled out carbonates or the tasmanite oil shales respectively as sources. If the source of the bitumens can be related to local geological features, these new findings could be significant for future oil exploration in Tasmania and adjacent offshore areas.
- aliphatic and aromatic biomarkers
- coastal bitumens
- oil seeps
- petroleum exploration