Abstract

The extensional basins of the southeast Australian margin were initiated by rifting in the Late Jurassic to Early Cretaceous, with the rifting progressing from west to east. Sea-floor spreading in the mid-Cretaceous, when Australia separated from Antarctica, initiated the development of an elongate passive margin adjacent to the southern coastline. The geological history, hydrocarbon potential and exploration activity of the under-explored nearshore and deepwater areas associated with three of the basins will be discussed.

The Otway basin has onshore, shallow-water and deepwater components, and is a complex series of rift half-grabens, influenced by north-south strike-slip faulting. The onshore part of the basin has reached exploration maturity, and contains numerous small gas fields. The shallow-water part of the basin contains several moderate-sized gas fields which will soon supply the southeast Australian energy market. This area is only partly explored. The deepwater part of the basin and its southern extension, the Sorell Basin, are virtually unexplored.

The Duntroon Sub-basin consists of a series of half-graben, and contains up to 12,000m of Late Jurassic and Early Cretaceous sediments. Only a handful of wells have been drilled in the shallow-water northern part of the basin, all prior to 1993. Most of the wells have had hydrocarbon shows. To the south of these wells, in water depths of 1,500-3,000m, lies the eastern extension of the Ceduna Sub-basin, which is currently undrilled – several extremely large structures with stacked objectives have been mapped in this frontier area.

The 95,000 sq km Ceduna Sub-basin contains 15,000m of Late Jurassic syn-rift and Cretaceous post-rift deltaic sediments. Most of the hydrocarbon-prospective parts of the basin lie in water depths greater than 1,000m. Only one well has been drilled in the western and central basin area, which contains the thickest sediments. Recent analysis has revealed the presence of large, fault-dependent, seismic amplitude-supported prospects, lying in water depths of 1500-2200m.

All three deepwater areas associated with these basins are prospective for hydrocarbons, and are covered by exploration permits with relatively high levels of exploration commitments. Thus, deepwater exploration in the next five years will determine the extent to which the known plays extend into the deepwater, and should establish the prospectivity of several newly defined plays.