THE CENTRAL AUSTRALIAN BASINS

Abstract

The Amadeus and Ngalia Basins are two of several intracratonic basins situated in the central region of the Australian Continent and underlain by Upper Proterozoic and Lower Palaeozoic sedimentary rocks.

In the Amadeus Basin, the preserved sedimentary section has been deformed by several orogenic events through geological history, with salt tectonics playing an important role in the structural evolution. The Ordovician System is the primary exploration objective. The Cambrian and Proterozoic sequences, which also carry rock strata having source, reservoir and sealing properties, are secondary targets. However, these latter units are sparsely explored, and only limited information is available on their petroleum prospectiveness. Three of the four petroleum accumulations found to date are in Ordovician sandstones, with the fourth accumulation contained in Cambrian sandstones.

The initial drilling phase in the Amadeus Basin in the early 1960s was concentrated on geologically defined surface antic :nes, with seismic surveying becoming the principal technique employed in subsequent exploration phases. The ongoing work has demonstrated a major untested structural play associated with a regional thrust fault system — in particular, combination dip and fault closures developed on the underthrust blocks. Stratigraphic prospects also are present in the Amadeus Basin, but none of these yet has been drilled.

The Ngalia Basin is similar stratigraphically and structurally to the Amadeus Basin and is considered prospective for oil and gas. Much less work has been done in the Ngalia than in the Amadeus, with only one well drilled in the entire basin. The well yielded a gas snow from a Proterozoic formation, and other direct hydrocarbon indications have been recorded elsewhere in the basin. Rock units having source, reservoir and sealing parameters are present, as are structures capable of forming traps. Again, these are associated largely with a complex regional thrust fault system.