Depositlonai Architecture of Lacustrine-Delta and Fluvlal Systems of the Permian EpsiIon and Toolachee Formations at Duilingari Field, Southeastern Cooper Basin, South Australia: ABSTRACT

Abstract

The Epsilon and Toolachee Formations record a transition from lacustrine deltas to fluvial depositional systems in the southeastern part of the Cooper Basin. Gas reservoirs in these formations occur mainly in narrow (typically less than 2 mi [3.2 km] wide) belts of distributary- and fluvial-channel sandstones in structurally high areas in Dullingari field and neighboring areas. In a study by the Bureau of Economic Geology and Santos to document distribution of Epsilon and Toolachee lithotacies, we mapped multiple depositional axes and projected them into undeveloped areas in the field. We performed a high-resolution sequence-stratigraphic study, defining four operational Epsilon subunits bounded by lacustrine mudstones recording periods of delta-platform abandonment and subsidence. In contrast, six genetic Toolachee subunits are bounded by regionally extensive coal seams recording pervasive sediment-supply shutoff and peat-swamp development over broad areas. Epsilon lacustrine deltas prograded northwestward over a muddy substrate on a broad, shallow basin floor. Distributary-channel axes, defined by narrow, northwest-trending belts of upward-fining sandstones less than 20 ft (6 m) thick, are projected across the field area. However, the younger Toolachee Formation contains northeast-trending sandstones in structural troughs, suggesting tectonic control on deposition. Detailed lithofacies maps of these formations provide a framework for assessing potentialmore » development targets at Dullingari field.« less