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Exploration history
Petroleum exploration in the Pedirka Basin was initiated in 1959 when Delhi and Santos were granted OELs 20
and 21. The Pedirka area was farmed out to the French Petroleum Company (later Total Exploration) and
exploration during the first half of the 1960s included gravity, magnetic and seismic surveys and four wildcat wells.
A second round of activity occurred from 1969 to 1979, when five seismic surveys were carried out and one well
was drilled. In 1977 there was a non-commercial flow of oil from basal Jurassic and Triassic sediments in
Poolowanna 1, located in the adjacent Simpson Basin. Exploration activity intensified in the 1980s in response to
the Poolowanna discovery. Six seismic surveys were carried out (Fig. 1) and five wells were drilled, three in 1985
and two in 1988.
Seismic reinterpretation by Santos indicated that five of the previously drilled wells were not valid structural tests
being outside closure at all objectives. Of the 1985 wells, Oolarinna 1 had no independent fault closure and Glen
Joyce 1 had little closure.
Stratigraphy
Two Permo-Carboniferous formations are present in the subsurface and crop out on the basin margin in the
Northern Territory. The lowermost unit (Crown Point Formation) consists of fluvioglacial and glaciolacustrine
sediments. The overlying Purni Formation was deposited in a floodplain environment containing meandering river
systems and extensive swamps in which coal developed (Fig. 4).
Three facies suites are distinguishable on the basis of relative proportions of sandstone, shale and coal.
Equivalents of the Stuart Range and Mount Toondina Formations of the Arckaringa Basin are interpreted in Mount
Hammersley 1.
Source rocks
The Purni Formation contains extensive coal-rich organic shale which appears to be both oil and gas-prone. It
contains up to 4% dispersed organic matter, with vitrinite and exinite macerals present in moderate abundance.
New data from Dalmatia 1 and Mount Hammersley 1 indicate poor to good source potential for oil in the Purni
Formation. In many areas geothermal gradients are too low for the sediments to have generated significant
quantities of hydrocarbons. In South Australia thermal maturity appears to increase from west to east with VR
approaching 0.9% maximum, equivalent to peak oil generation.
Crown Point shale is organically lean, average TOC ~0.2%. There is potential for richer source rocks if lacustrine
facies are developed in deeper parts of the thick Eringa Trough section.
Reservoirs and seals
Sandstones of excellent reservoir quality are locally developed in both the Crown Point and Purni Formations.
Channel belt and point bar sands of the Purni Formation middle member have porosities from 6–12% and fair to
good permeability.