BASS AND GIPPSLAND BASINS: A COMPARISON

Abstract

After two decades of exploration, one wireline test of oil, one of light oil and several of gas and gas/condensate have been recovered from the Bass Basin while the adjacent Gippsland Basin has established an estimated ultimate recoverable reserve of the order of half a billion kilolitres of liquids and a quarter of a trillion cubic metres of gas. Geologically, the basins are similar.

The alluvial and nearshore deposits at the top of the Latrobe Group in Gippsland are as porous and permeable as similar deposits at the top of the Eastern View Group in Bass.

The Eastern View and Latrobe Formations are regionally sealed by the Upper Eocene Demons Bluff and Oligocene Lakes Entrance Formations respectively. The intra-Latrobe section in Gippsland has significant but regionally not very extensive sealing units, whereas the Lower Eocene to Paleocene sequence in Bass is increasingly shale prone with depth, sometimes over-pressured, and constitutes an extensive seal for a base of Tertiary play. This play comprises Paleocene shales sealing Upper Cretaceous clastics with hydrocarbons potentially sourced from both units.

Maturation studies (Saxby, 1980) indicate that the Upper Cretaceous is the principal source for hydrocarbons in Gippsland with possible lesser contributions from the Lower Paleocene and Lower Cretaceous. Limited data indicate the same is true in Bass and that the Paleocene and parts of the Lower Eocene are mature sources for gas/condensate and light oil. Normal faults assist vertical migration in Gippsland. In Bass, relatively few normal faults penetrate the Paleocene and Lower Eocene shales to reach the top of the Eastern View, greatly restricting the chances of vertical migration over much of the basin. Vertical migration is more likely beyond the margins of the depocentre.

Eroded anticlines at the top of the Latrobe form large traps for the bulk of Gippsland's hydrocarbons. Small anticlines, wrench-related features and intra-Latrobe closures are more difficult to find. The normal fault blocks in Bass at the top of the Eastern View are wrench-modified and have proven difficult to define.

The recent recognition in Bass of the base of Tertiary play and the need for careful structural and seismic interpretations is expected to lead to discoveries of oil and gas.