Petroleum Prospectivity of Australia's Southern Margin

J. B. Willcox, H. M. J. Stagg

Australia's southern margin extends for almost 4,000 km, from the Naturaliste Plateau in the west to the South Tasman Rise in the southeast, and developed from the Gondwana rift system which also gave rise to the Bass basin and prolific Gippsland basin of southeastern Australia. It incorporates six major sedimentary basins--Bremer, Eyre, Great Australian Bight (GAB), Duntroon, Otway, and Sorell--which are the products of Gondwana fragmentation. Basin development began as northwest-southeast extension in the pre-Middle Jurassic--the Bremer basin and Eyre subbasin being almost purely extensional; the GAB and Duntroon basins being transtensional, bounded by major transfer faults; and the Otway and Sorell basins occurring within a strike-slip zone. The latter two basins were urther developed in the Early Cretaceous by north-northeast-south-southwest extension, and in the Eocene and Oligocene they were subjected to wrenching due to continued contact between the Australian and Antarctic plates. Exploration targets generated by this complex tectonic history include fault blocks and associated rollovers; wrench-related anticlines and flower structures, often developed over reactivated transfer faults; strongly eroded unconformities; and deltas of Neocomian and Maastrichtian-Cenomanian age.

The southern margin basins contain several sandstone units with good reservoir potential. Hydrocarbons are being actively generated, at least within the Otway and Sorell basins, as indicated by abnormally high concentrations of thermogenic gases in surface sediment samples and the presence of free oil in the Cape Sorell 1 well. Deposits containing oil-prone type I organic matter occur in the Lower Cretaceous section of the Otway basin.

To date, exploration has been light, with 17 of the total 27 wells having been sited in the Otway basin. The huge GAB basin (~ 400 × 200 × 10 km thick) contains only one exploration well, sited near the northern margin of the basin, and this basin is an obvious focus for further exploration effort.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.