BMR Cruise 067 : Otway Basin and West Tasmanian Sampling

In early 1987, scientists aboard R.V. Rig Seismic carried out a 29 day research cruise over the Otway Basin and the Sorell Basin of the west Tasmanian margin, to provide new geological, geochemical and heatflow data, in an area with considerable petroleum potential. Altogether, 130 sampling stations were occupied using dredges, corers, grabs and a heatflow probe.

Among the rocks recovered were: Palaeozoic volcanics and metasediments; Late Cretaceous sandstones, siltstones and mudstones; early Tertiary siltstones; and late Tertiary carbonates. All samples were taken along seismic profiles, so that the results can be easily incorporated into the regional geological framework. In general, the further down the continental slope, the older the rocks. Palaeontological results indicate that there has been very considerable post-Eocene subsidence of the slope. A great variety of Quaternary sediments were recovered, and these have allowed a detailed sedimentation model to be developed.

Heatflow calculations from 20 stations suggest that the present zone of thermal maturation of hydrocarbons is 2-4 km deep. Headspace gas analyses of many cores indicate that thermogenic hydrocarbons are widespread, with particularly high readings in both the eastern and the western Otway Basin and on the west Tasmanian margin. Thus, mature hydrocarbon source rocks must also be widespread.

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