ABSTRACT: Petroleum Maturation, Tectonic and Subsidence History of the Offshore Otway Basin: Implications for Exploration

WILLIAMSON, P. E., and G. W. O'BRIEN, Bureau of Mineral Resources (BMR), Canberra, Australian Capital Territory, Australia

The Otway Basin formed by rifting and subsidence in the earliest Early Cretaceous and mid-Cretaceous, associated with the separation of the Australian and Antarctic margins. Maximum subsidence of more than 7 km occurred under the present continental shelf in the Voluta Trough and the eastern Mussel Platform.

Geochemical results suggest that a significant oil-prone source interval occurs in the basal Early Cretaceous Pretty Hill Sandstone. On the Crayfish Platform of the western basin, this interval entered the oil window in the Early Cretaceous, prior to trap formation. The interval moved into the thermal gas regime over much of the Crayfish Platform. The upper Pretty Hill Sandstone reservoir sequences are presently within the oil window, and consequently the relationship between the timing of trap formation and hydrocarbon generation is critical to the preservation of oil, gas, or gas-condensate accumulations.

By latest Cretaceous time source rocks of the probably gas prone Early Cretaceous Eumeralla Formation had entered the oil window over most of the Voluta Trough. The overlying Waarre Sandstone reservoir thus could have been charged in essentially its present structural configuration in the eastern and central Voluta Trough. In parts of the western Voluta Trough, probably gas-prone source rocks of the Belfast Mudstone could possibly source the underlying Waarre Sandstone and the overlying late Cretaceous Curdies and Paaratte Formations.

The integration of geohistory and geochemical data suggests that a significant, untested, Early Cretaceous oil exploration play exists on the offshore Mussel Platform, shoreward of the Prawn-1 and Pecten-1 wells.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)