THE SHOWGROUNDS FORMATION, ITS SETTING AND SEAL, IN ATP 145P, QLD

Abstract

The Middle Triassic Showgrounds Formation is the major reservoir in the permit ATP 145P, situated on the western flank of the Mimosa Syncline of the Surat Basin and including the Roma Shelf and Wunger Ridge. Proven and probable reserves stand at 2 billion cubic metres (70 billion cubic feet) of gas and 635 000 kilolitres (4 million barrels) of liquid hydrocarbons.

The Showgrounds Formation lies directly on either the Lower Triassic Rewan Formation, the Upper Permian Kianga Formation or older Palaeozoic rocks. The underlying lithology and palaeotopography contribute to the nature and distribution of the Showgrounds Formation. Source rock potential in the Permian Kianga and Back Creek Formations is very good, but good Permian reservoirs are lacking. The Rewan Formation, where present in the eastern half of the permit, acts as a barrier to the vertical migration of hydrocarbons from the Permian source.

Although the Showgrounds Formation is widespread, it is not homogeneous and exhibits pronounced lateral variation. It can be subdivided, on the basis of vertical profiles seen in cores and on log character, into three major facies of fluvial origin, one of which shows evidence of marine modification in a deltaic environment. Reservoir qualities vary considerably in each facies but are best developed in the proximal fluvial facies. This facies exhibits high porosities and permeabilities, with secondary porosity developed by dissolution of carbonates and flushing of clays being an important feature. This sequence is ideally situated to trap westerly migrating hydrocarbons, since it is situated west of the Rewan Formation pinchout. The widespread marginal marine to marine Sub-Unit 9 shale, always present above the Showgrounds Formation, provides the seal. The typical trapping mechanism is a combination of structural and stratigraphic components.