The sedimentology of the Permian Gidgealpa Group of the southern Cooper basin currently is being evaluated to ascertain the tectono-sedimentologic evolution of the basin and to determine the architecture of the clastic suite in order to generate exploration plays.

The study has produced a new understanding of the relationship between the hydrocarbon-rich Tirrawarra Sandstone, of dominantly fluvial origin, and the, as yet little explored, glacigenic Merrimelia Formation. The facies states and transitions of both formations interdigitate, and typical porous Tirrawara-type fluvial facies are clearly evident within glaciolacustrine Merrimelia-type sediments.

The Merrimelia Formation was examined regionally in 29 cored wells. The formation attains a maximum thickness of 300 m (1,000 ft), and representative facies include glaciofluvial outwash, terrestrial and subaqueous diamictites, and glaciolacustrine, wave-affected, and ripple-laminated sandstones, with thick, monotonous mudrock sequences containing clay-dominant rhythmite horizons.

The Tirrawarra Sandstone, analyzed in 32 cored wells, comprises four major facies associations throughout its maximum 75 m (250 ft) thickness. These associations indicate a temporal and spatial evolution of a fluvioglacial to predominantly fluvial system. Initial deposition on low slope, outwash fans, where braided processes operated is indicated. This sedimentation style evolved into a low sinuosity, bedload-dominant, sandy braided system, with high width-to-depth ratio channels. Allocyclic control mechanisms are invoked for "late Tirrawarra" sedimentation as the facies reveal proximal-distal patterns and the fluvial style changes to a mixed-load channel system.

The interfacing and evolutionary pattern of the deposystem indicates that additional reserves potential exists for reservoirs developed locally within the Merrimelia Formation.

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