The Geology And Hydrocarbon Potential Of The Pedirka Basin.
Giuliano, Michael F
Honours Degree, 1988
University of Adelaide
The Pedirka Basin is a Permo-Carboniferous infrabasin located in the central north of South Australia and extending into the Northern Territory. Permian sediments unconformably overlie the folded and faulted sediments of the Amadeus and Warburton basins, which in turn rest on the Archaean Musgrave and Arunta Blocks. Unconformably overlying the Pedirka are sediments of the Simpson Desert, Eromanga and Lake Eyre Basins.
The Pedirka region has undergone at least three major compressional events resulting in a highly deformed setting. These periods of tectonism, acting along a common east/west orientation, preferentially reactivated pre-existing trends initiating a series of thrust related structures. The major structurally high feature in the region is the deep seated McDills/Mayhew Trend which separates the thick localised Permian sequence of the Eringa Trough from the thinner more widespread sediments to the east.
The sediments of the Pedirka Basin have been formally divided into the Crown Point and Purni formations, and further subdivided into intraformational members. The Crown Point Formation is predominantly a glacigenic suite of sediments and exhibits characteristics of glaciofluvial, glaciolacustrine, interglacial and periglacial origins. It occurs basinwide with the thickest sequences occupying the Eringa Trough and the Eastern Pedirka depocentre northeast of Witcherrie #l.
The Purni Formation has been subdivided into three members. The Upper and Lower Members are distinguished by the coals, carbonaceous shales and silts of paludal/overbank deposits, while the Middle Member is characterised by moderate energy fluvial channel and bar sands. The Purni Formation is extensive throughout the basin, although through either non deposition or erosion is absent from some structural highs. The eastern Pedirka depocentre migrated east during Purni deposition with the thickest Upper Member section located near Colson #1. The Eringa Trough also contains a thick Purni sequence.
The hydrocarbon potential of the Pedirka Basin has been tested with twelve exploration wells intersecting Permian sediments. These wells yielded no shows and have all been plugged and abandoned. Approximately 10,000 km of seismic have been acquired in the South Australian section of the basin, delineating major structural trends and proving up several prospects.
Reservoir quality has been shown through core analysis and log interpretation to be good in both the Purni and Crown Point Formations, especially the latter to the west of the McDills/Mayhew Trend. Formational sealing is achieved by the Peera Peera, Walkandi, and Poolowanna Formations, and intraformationally by the shales and siltstones of the Purni Formation.
Although, with respect to reservoir, seal, and trap quality, the prospectivity of the Pedirka Basin appears good, source rock quality and maturity are limiting factors. Analyses have shown the most source rich areas to be in the Purni Upper Member depocentre trending southwest from Colson #1 to Mokari #1. Kerogen studies indicate these sources to be both oil and gas prone but low hydrocarbon yield figures are generally exhibited. Low present day geothermal gradients and inferred low palaeogradients have at best placed Permian sediments within the oil window, and areas of sufficient maturity have only been encountered along the eastern margin of the basin.
The Pedirka Basin on present data, appears to be unprospective in all but the eastern margin areas. Limited exploration and low data density across most of the region cannot account for the anomalies apparent to the basin, and many areas, the Eringa Trough in particular, still warrant exploration.