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The Pedirka Basin's petroleum prospects are also poorly known. The coals and
carbonaceous shales of the Purni Formation constitute source rocks which are
probably gas-prone, and in the deeper eastern part of the basin they may have
reached organic maturity (Youngs, 1976). In Mokari 1, for example, the main coalbearing
section lies between 1800 to 2000 m depth, with the rest of the Permian
extending down to about 2250 m. At least some of the fluvial sandstones in the Purni
Formation would qualify as reservoirs, because they form good aquifers, but this in
turns means that most of the hydrocarbons may have been flushed from them. Many
potential reservoir units may not have been flushed, however. Shales in the fluvial
sequence could act as seals, as could any overlying Triassic shales. The Purni
Formation warrants greater attention, especially as it is situated between the gas fields
of the Amadeus and Cooper Basins, and could be connected to the distribution
pipelines of either fairly easily (Youngs, 1976). The glaciofluvial Crown Point
Formation, like the Grant Group in the Canning Basin, may provide high-risk targets.
Cooper Basin
The Cooper Basin is the most prolific hydrocarbon-bearing Permian basin in Australia,
and is the sole source of natural gas for New South Wales and South Australia. To
date (1989), 18 oil fields and 121 gas fields have been discovered (L. Pain, BMR, pers.
comm.). As well as reservoiring hydrocarbons, the Permian sequence is the source
of the bulk of the oil in the overlying Mesozoic Eronnanga Basin (Gilby & Mortimore,
1989; Jenkins, 1989; Heath & others, 1989). Gas accumulations occur in every
formation, except the glacigene Merrimelia Formation and the lacustrine Murteree and
Roseneath Shales. Oil has been recovered mainly from the braided-fluvial Tirrawarra
Sandstone, and in lesser amounts from the meandering-fluviodeltaic Patchawarra and
Toolachee Formations.
The Merrimelia Formation contains a variety of sub-facies, including glaciofluvial
outwash, tillite, glaciolacustrine muds and rippled sands, and paraglacial eolianites.
Some of the lacustrine muds might be rich enough in algal remains to be source rocks
(Battersby, 1976), and some of the better-sorted sands may form locally developed
reservoirs (including the tops of porous eolianites as suggested by Williams & others
(1985)), but detailed palaeogeographic studies will be required to delineate such plays.
The formation interfingers with the Tirrawarra Sandstone, leading to the possibility that
tongues of hydrocarbon-bearing Tirrawarra facies may be stratigraphically trapped in
seal lithologies of Merrimelia Formation in such zones (Williams & Wild, 1984a). The
Tirrawarra Sandstone holds over 95% of Cooper Basin oil, and 12% of the gas (Heath,
1989); it is dominated by thick, multistorey sandstones typical of low-sinuosity,
braided, bedload fluvial channels, but also contains source beds such as minor coals.
The Patchawarra, Epsilon, Daralingie, and Toolachee Formations are coal measures
units which were deposited as lacustrine deltas or fluvial systems. As such, they
contain an abundance of organic source material, as well as channel and delta front
sandstone reservoirs, and intrafornnational seals. The lacustrine Murteree and
Roseneath Shales provide regional seals to the Patchawarra and Epsilon Formations
respectively. Palogeographic controls operated to determine the locations of
channel belts, and thus of potential reservoirs: for example, the Nappermerri Trough
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