Abstract

A broad classification of petroleum systems has been established in Australia, linking individual systems that share the same age and facies of source rock together into a petroleum supersystem. This has been a useful concept in exploration and has been verified by analysis of the oils, which cluster into families mirroring the supersystem framework. Petroleum supersystems and oil families occur regionally across basins, and from Australia into other parts of the plate in Indonesia and Papua New Guinea (PNG).

Six Phanerozoic petroleum supersystems have been recognised in Australia and three of these are also found in eastern Indonesia. The Australian supersystems are developed in Mesozoic and Paleozoic sequences with source rock intervals in the Cambrian, Ordovician, Late Devonian, Early Carboniferous, Permian, Triassic, Late Jurassic and Cretaceous. The shift in exploration focus in Indonesia to pre-Tertiary objectives, provides an opportunity for the understanding gained of Australian petroleum systems to be usefully applied across international borders.

The Joint Zone of Cooperation (ZOC) in the Timor Sea is an obvious place where synergy between Australian and Indonesian petroleum systems is being exploited. The Late Jurassic marine source rocks of the Westralian Supersystem have produced a similar suite of oils along the Australian margin from the Carnarvon Basin, through the Timor Sea to the Papuan Basin in PNG (Figure 1). Westralian accumulations are characteristically found in Mesozoic sandstone reservoirs sealed by Cretaceous marine shales.

Successful Paleozoic petroleum systems in Australia also have their parallels in eastern Indonesia. The Petrel gas and condensate field in the Bonaparte Basin is an example of a Gondwanan Supersystem accumulation. It is contained in Late Permian sandstones, sourced from underlying Permian deltaic sediments, and the regional seal for the play is provided by earliest Triassic marine shales. Geochemical and other evidence points to a similar system operating in the Bintuni Basin, with Permian deltaic sequences sourcing hydrocarbons to pre Tertiary reservoirs.

Paleogeographic and paleoclimatic conditions were at their optimum for source rock deposition in Australia in the Early Paleozoic when the continent straddled the equator and was cris-crossed by shallow epeiric seas (the Larapintine Seaway). Some of the most organic-rich rocks in the Australian record are Cambrian and Ordovician in age; and to date there have been discoveries in the Amadeus (Mereenie, Palm Valley) and Canning (Pictor) Basins (Figure 1). One of the most prospective areas to further explore this Larapintine Supersystem is in the Arafura Basin, on both sides of the Australia/Indonesia border.