GEOLOGIC HISTORY OF THE BASS BASIN

Abstract

The Bass Basin is located offshore between the southern coast of Victoria and the northern coast of Tasmania. It is bounded on the west by King Island and on the east by Flinders Island and the Bassian Rise. Water depths throughout the basin rarely exceed 270 feet (82 metres) and the area has been actively explored for hydrocarbons since 1963.

The oldest sedimentary rocks encountered whilst drilling are Early Cretaceous, but the greatest volume of sediment was deposited during the Tertiary. Lithologies vary from continental sandstone, siltstone, shale, and coal in the older, non-marine Cretaceous to Middle Eocene section to limestone, marl, mudstone, and shale in the younger, marine Late Eocene to Recent section. Drilling and seismic data indicate that there was a considerable amount of volcanic activity in the Bass Basin throughout its history.

Three distinctively different structural provinces can be recognised in the basin. These provinces are referred to as- a) southeastern area, b) central area, and c) northwestern area.

The southeastern area exhibits the earliest structural growth (Early Cretaceous) whereas the structural growth in the central and northwestern areas occurred in Early and Late Tertiary respectively. Structural style also varies from tilted fault blocks with thousands of feet of vertical displacement in the southeastern area, to low relief, small anticlinal folds and minor faults in the northwestern area. Most of the prominent structural trends are oriented in a northwest-southeast direction which is parallel or sub-parallel to the present basin axis.

Seismic and E-log correlations within the non-marine Cretaceous and Lower Tertiary section are extremely difficult and palynology is used to differentiate time-rock units. Five separate zones are identifiable within the Eocene and Paleocene, and the Cretaceous has been sub-divided into ten zones which can be related to the time-rock units in the adjacent Otway and Gippsland Basins.

Non-commercial accumulations of hydrocarbons have been found in three different structural features: Pelican, Cormorant and Bass −3. These accumulations are from within the Lower Tertiary non-marine sequence known as the Eastern View Group.