Geology and Exploration in Southwest Pacific Australian Region: Western and Northwestern Margin Basins of Australia

WOOLLANDS, MICHAEL ALAN, BHP Petroleum Pty Ltd., Melbourne, Victoria, Australia

The Marginal basins of Western and Northwestern Australia extend approximately 3800 km along the coast and comprise both continental shelf and adjacent deepwater plateau areas. From north to south, the principal basins are the Sahul/Malita, Browse, Carnarvon, and Perth basins.

Their overall development and tectonic history are related primarily to rifting associated with the Mesozoic break-up of the Australian continental margin.

The stratigraphic sequence within each basin is broadly similar, with initial widespread Triassic-Early Jurassic deposition in broad regional pre-rift sags. Middle Jurassic to Late Jurassic rifting on the Northwest Shelf and early Cretaceous rifting in the Perth basin varies in intensity across the region, and provides the primary control of hydrocarbon distribution through trap type, reservoir, and source rock quality. The best-developed oil source rocks of the region are found in these Jurassic rifted sequences. A post-rift passive margin sequence developed from the early Cretaceous-Recent, with most hydrocarbon discoveries being found beneath the regional base Cretaceous (Valanginian) post-rift unconformity.

Although drilling to date has been generally sparse, with only some 824 wells throughout the entire area, significant accumulations of gas and oil have been found in the offshore Carnarvon basin and the Vulcan Graben area of the Browse basin.

Recent discoveries of commercial oil at Jabiru in the Vulcan graben and Griffin in the Carnarvon basin have again focused activity on the area's oil potential. Many structural traps remain to be drilled and the potential for stratigraphic trapping has been only slightly explored.

Large, poorly explored frontier areas, between the established producing areas and in the Perth basin, as well as adjacent deepwater, are available. However, present data suggest that not all key success factors, notably mature oil source rocks and good reservoirs, may be present in these areas.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)