Hydrocarbon Potential of Basins along Australia's Southern Margin

WILLINK, R. J., SAGASCO Resources Limited, Adelaide, South Australia

Seven discrete sedimentary basins are recognized along the southern margin of the Australian continent; namely, from east to west, the Gippsland, Bass, Sorell, Otway, Duntroon, Bight, and Bremer. All formed since the Late Jurassic in response to the separation of Australia and Antarctica, and to the opening of the Tasman Sea. Only the Gippsland basin, which has proved initial oil reserves exceeding 3.6 billion barrels, is a prolific oil province. The search for oil in the other basins has been virtually fruitless despite many similarities between these basins and the Gippsland in terms of stratigraphy and structural geology.

Rift and drift components are discernible in the sedimentary successions of all basins but the precise tectonic controls on respective basin formation remain conjectural. The early rift history of the basins, for example, has been variably explained in terms of left-lateral wrenching, right-lateral wrenching, oblique extension, and staged simple extension. Most of the basins are complexly faulted. Early Cretaceous faults are clearly observed to detach in basement, but Late Cretaceous faults variably detach in basement, top of basement, or within the sedimentary succession itself. Evidence for Late Tertiary reactivation of older structures is also widespread. In consequence, potential trapping mechanisms are diverse.

The lack of drilling success in the Bremer, Bight, Duntroon, Otway, and Sorell basins has been attributed mainly to the paucity of mature, oil-prone source rocks. The common occurrence of stranded bitumens along the entire coastline, however, indicates oil generation.

The Bass and Gippsland basins are both characterized by excellent oil-prone source rocks developed in Late Cretaceous to Early Tertiary sediments. Limited exploration success in the Bass basin is due to poorer reservoir development.

The Gippsland basin is at a mature stage of exploration whereas the other basins are moderately to very sparsely explored. Consequently, there is a comparable potential for undiscovered hydrocarbons in all basins. Success in the under-explored basins will come only to those prepared to challenge the perception of low prospectivity. Many play types remain to be tested by the drill. Further success in the Gippsland is more likely to result from small discoveries that are likely to follow after a relaxation of government imposts.

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