ABSTRACT

Three, small, Tertiary-aged onshore basins in north-central Tasmania (the Wesley Vale, Sassafras and Port Sorell) were analyzed with an emphasis on source rocks and reservoir quality. The basins are similar in age, structural history, and sedimentary/volcanic stratigraphy to the nearby, offshore Bass Basin. A geological model, using data from the study of the onshore basins, was constructed to assist in Bass Basin oil and gas exploration.

The onshore basins were initiated by Late Cretaceous rifting associated with the breakup of Gondwana. The resulting structural style is a horst and graben geometry with planar and rotational normal faults forming the basin margins and high-angle cross-basin faults.

Differential adjustment along fault planes during the Paleocene elevated some areas as subaerial, topographic highs. Erosion of these highs under tropical conditions resulted in large volumes of sediment accumulating in the adjacent lows in fluvial/lacustrine environments. Detrital grains in the resulting sandstones and mudrocks were derived from pre-tectonic palaeosols and contemporaneous soil profiles and consisted of well-ordered kaolinite clay. These sandstones lack any porosity or permeability; however, because of rapid deposition they are a potential Type II source rock.

Uplift on cross-basin faults during the Eocene produced restricted shallow water lakes, anoxic ponds and swamps. Within the restricted lakes there was an abundant accumulation of oil-prone organic matter that resulted in an oil shale deposit onshore and a potential oil source rock in the deeper subsurface.

Eocene fault reactivation also produced alluvial fans, slope deposits and eruptions of basaltic lava, which filled valleys and depressions. The fractured and jointed vesicular lava provides an aquifer for irrigation water onshore and a potential unconventional hydrocarbon reservoir in the Bass Basin.

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