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subsided faster than elsewhere, resulting in most stream channels being concentrated
there. Porosity, being dependant on the depth of burial, is almost twice as high over
the main palmo-highs as in the major troughs, and is higher still towards the basin
margins (Heath, 1989). Continued uplift along the palmo-highs after the mid-Permian
ensured the further structural growth of most Patchawarra closures before
hydrocarbon generation and migration (Heath, 1989). Thirty per cent of the gas-inplace
in the Cooper Basin is in the Patchawarra Formation, while about 40% is located
in the Toolachee - Daralingie formations.
Galilee Basin
No hydrocarbon accumulations of consequence have been found in the Galilee Basin
to date (1989). Apart from a Late Permian marine incursion across the Springsure
Shelf in the SE, the entire sequence is non-marine. The best source rock potential
occurs in the Aramac Coal Measures, and the Colinlea Sandstone and Bandanna
Formation correlatives (Hawkins, 1978; Hawkins & Harrison, 1978; Scholefield, 1989).
Sapropelic debris which collected on lake bottoms (such as the torbanite oil shale near
Alpha) is especially likely to have yielded light to intermediate hydrocarbons. Vitrinite
ref lectances and temperature gradients indicate that the sequence is in the lower half
of the oil window. Good reservoir lithologies are present in the Aramac Coal Measures
and Colinlea Sandstone, with porosities up to 23% and 16% respectively; stratigraphic
traps containing these sands may have been formed where distributary channels
became enclosed by fine-grained floodbasin deposits, or, in the LoveIle Depression,
in the form of sand wedges against syn-depositional fault scarps (Hawkins, 1978).
Intraformational shales, or the overlying Triassic beds, could provide suitable seals.
The mid-Permian period of erosion may be partly implicated in the apparent absence
of oil and gas in the pre-Colinlea section — as Hawkins & Harrison (1978) have
pointed out, during this hiatus in the Galilee Basin, sediments were being deposited
in the adjacent Cooper and Bowen Basins which now contain hydrocarbon reserves.
Torres Shelf, Olive River and Sub-Laura Basins
The Permian sequences in these small repositories contain coal in places, and
carbonaceous shales relatively rich in humic kerogen. The potential source rocks of
the Torres Shelf (and, presumably, the Olive River Basin) have reached oil generation
maturity, but the sub-Laura sequence is overmature, even though there is no evidence
for deep burial, suggesting a high geothermal gradient. No potential reservoirs are
known, and sandstones in the Jurassic-Cretaceous succession appear to be flushed
(Smart & Rasidi, 1979). In any case, as the Mesozoic is only marginally mature, it is
probable that most hydrocarbons were expelled from the sub-Laura Permian before
the later deposits were emplaced.
Bowen Basin
The Permian succession of the Bowen Basin is largely gas-prone: Allen's (1980)
listing indicates a ratio of gas to oil fields of 19:7. The organic maturity of the eastern
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