Some Elements of Australian Petroleum Geology

Charles D. Masters, E. W. Scott

The petroleum geology of Australia reflects the existence of a large cratonic block broken away from India and Antarctica in the early Mesozoic and early Tertiary that has resulted in a rifted passive-margin character on the northwestern, western, and southern boundaries of the continent. Pre-breakup Paleozoic sediments are widely distributed but commonly not deeply buried nor particularly thick, and hence contribute minimally to petroleum resource occurrence. Like their Asian neighbors, much of Australian petroleum geology is nonmarine and associated with marginal rift basins. The small Gippsland basin on the southeastern coast, which is responsible for more than 90% of oil and 28% of the gas discovered in Australia, derives its petroleum from nonmarine Eocene to Cretace us graben-fill sediments, sealed and buried by Oligocene marine shales. The most active play in Australia is in the Eromanga depression of the Great Artesian basin, where nonmarine oil is trapped stratigraphically in small fields in Jurassic and Cretaceous sandstones. These Mesozoic sediments are sag-fill deposits above the Permian-Triassic Cooper basin, and are responsible for some 12% of the gas reserves in Australia. Offshore of the western coast, graben basins filled with late Paleozoic to Mesozoic sediments are prolific and gas-prone--55% of reserves--owing to coaly source rocks. North Sea-type, Upper Jurassic grabens off the northwestern coast of Australia contain Kimmeridgian "hot shales," but developmental drilling, following the initial Jabiru discovery, has yet to demonstrate l rge reserves.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.