RECENT DEVELOPMENTS IN EXPLORATION FOR OIL IN TASMANIA

Abstract

Recent work on oil seeps, organic geochemistry, geophysics, structural geology and palaeontology suggests that there is considerable potential for onshore petroleum in Tasmania.

Archival research has shown that hydrocarbon seeps were commonly reported in the first half of this century and that wildcats produced gas (at Port Sorell in the north) and oil (at Johnson's Well on Bruny Island, in the south). Almost all of the 270 historical hydrocarbon occurrences lie on lineaments revealed independently by gravity and magnetic surveys. The thermal maturity of conodonts from Ordovician and Siluro-Devonian carbonates suggests that much of the pre-Upper Carboniferous beneath the Tabberabberan unconformity is within the oil and gas windows.

Organic geochemistry reveals a very close similarity between hydrocarbons from Ordovician limestones, those from the drill site at Bruny Island and with tar samples from the Tasmanian coast, but little similarity with the Permian Tasmanite Oil Shale, or with the Gippsland crudes and botryococcane-rich South Australian bitumens. The predominance of C27 steranes in Tasmanian bitumens suggests a widespread algal source and the abundant diasteranes imply a clay or silt-rich source that extends across much of Tasmania.

Recent geophysical and structural work suggests that a thin skinned interpretation of Tasmania's structure is reasonable. Most sightings of hydrocarbons are associated with either faults or fractures which have post-Jurassic displacements or with intersections of major high angle faults with thrusts. The delineation of reservoirs within the thrust sheets is a priority.