The Bass, Gippsland, and Otway basins lie mainly offshore southeast of Australia. Offshore fields in the Gippsland basin contain recoverable reserves of approximately 8.0 Tcf of gas and 1.7 billion bbl of oil and currently supply two thirds of Australia's oil requirements. No commercial hydrocarbons have been discovered in the adjacent Bass and Otway basins.

The basins were created during the Jurassic to mid-Cretaceous continental breakup of Antarctica, Australia, and New Zealand. Their initial tectonic framework was primarily tensional with basining achieved by normal faulting.

End_Page 1440------------------------------

The Otway basin has a classic "pull-apart" structural style with a series of normal faults parallel with the spreading axis. The Gippsland and Bass basins are similar but their complete pull-apart was prevented by the influence of the Tasmanian continental block (subplate). The separation of New Zealand from Australia had only a minor effect on the structural framework of the Gippsland basin in the shallow-slope areas.

Following the continental breakup and margin collapse, the Otway basin remained structurally quiescent but the Bass and Gippsland basins continued to subside and accumulate thick Tertiary sections. However, during the late Eocene and Miocene all basins were subjected to an east-west right-lateral-shear deformation. In Gippsland, the shear generated large en echelon anticlines and rejuvenated some of the old tensional faults. Hydrocarbons accumulated in these anticlines or their eroded remnants. In Bass and Otway the shear was less severe and confined to the northern margins. The deformation in these basins was correspondingly less intense.

However, structural styles in each basin are similar enough that their varying intensity does not explain fully the lack of success in the areas explored in Bass and Otway. Other factors such as the presence or absence of organic matter, maturation, and migration must have influenced hydrocarbon distribution.

End_of_Article - Last_Page 1441------------