Asphaltites From The Southern Australian Margin Submarine Oil Seeps Or Maritime Artefacts?
Smart, Susannah Mary
Honours Degree, 1999
University of Adelaide
A black pitch-like substance, known as asphaltite, has been collected from along the coast of southern Australia for at least one hundred and fifty years. The origin of the asphaltites is in question since previous geochemical studies have failed to correlate the asphaltites to a source. Oils from the southern margin basins (Gippsland, Bass, and Otway) have been tested, as well as oils from further away. This lack of correlation has prompted the suggestion that they may be sourced anthropogenically, from residues of bitumen used to waterproof ships during the time when the whaling and sealing industries were operating in the Southern Hemisphere. An alternate natural source for the asphaltites is suspected to be from the Bight Basin, a frontier basin in which there is indirect evidence of hydrocarbon seepage.
This study selected samples of bitumens from 17th, 18th and 19th century shipwrecks to compare with the asphaltites. Bulk geochemical analysis allowed initial differentiation between two different types of anthropogenic material, and the asphaltites. More detailed biomarker analysis characterised the types of compounds present in each sample. The first type of anthropogenic material was found to be a pine tar. A suite of tricyclic diterpenoids, compounds that are derived from conifer resins, dominated the pine tar samples. They are different from coal tars, the second type of anthropogenic material The coal tars were dominated by di-, tri- and polycyclic aromatic hydrocarbons. They were more like the asphaltites than the pine tars, as they contained the same types of biomarkers. However, the constituents common to the coal tar and the asphaltites occurred in very different relative amounts, and so it can be concluded that the asphaltites are not sourced from anthropogenic material.
With the evidence that the asphaltites are unlikely to be sourced anthropogenically, this study reviews the possibility of natural seepage occurring in or around the Bight Basin. This study illustrates how organic geochemistry can be used to constrain the source of apparent petroleum seepage in a frontier basin.