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structure in this area, ideally suited to trap hydrocarbons, seems to be unprospective.
The latest Permian Chinty Formation, a basal transgressive sand, has good quality
reservoir sandstones, and may host hydrocarbons from the adjacent Locker Shale, or
perhaps from lower in the Permian section (Parry & Smith, 1988).
Shales in the Byro Group (Permian 4) were deposited in shallow marine reducing
conditions, and may have generated hydrocarbons. Shallow water sandstones are
common at the top of the unit, and have porosities of 15-22%, but the seal formed by
the Kennedy Group is poor (Bentley, 1988).
The basal Permian Lyons Group, though argillaceous, is generally believed to lack
source potential because the prevailing glacigene depositional setting is assumed to
have been inimicable to the development of organic-rich sediments (Delfos & Dednnan,
1988). However, Domak (1988) has shown that a time of sediment starvation follows
the retreat of a polar marine ice shelf, so that organic-high sediments can accumulate.
Glacioeustatic falls in sea level might restrict circulation in some areas, resulting in
kerogen-rich deposits. Also, coals and organic-rich mudstone can accumulate on the
distil parts of glaciofluvial outwash plains (Boothroyd & Ashley, 1975; Thornton, 1979).
Subglacial tunnel valley infills could in theory form reservoirs large enough to be
possible targets, and any sand or gravel facies in the outer glacial or periglacial zones
may have good reservoir characteristics (Brakel & others, 1988). Thus although
glacigene fades represent high risk for petroleum exploration, their potential should
not be overlooked. The Lyons Group may also act as a seal for the underlying
Carboniferous Quail Formation.
Permian igneous activity, interpreted as volcanism, occurred around Enderby 1 and
Edel 1 in the northern and southern offshore Carnarvon Basin respectively, and may
have locally raised the geothermal gradient and affected organic maturity.
Seismic work suggests the existence of Palmozoic rocks in the Exmouth Plateau,
which are inferred to include equivalents of the marine Permian in the Carnarvon Basin
(Exon & Willcox, 1978, 1980). Such rocks would have been buried deeply enough to
have sourced hydrocarbons, in contrast to the younger immature sequence deposited
since the Mesozoic continental break-up.
Perth Basin
In the northern Perth Basin, gas has been produced from Permian levels in three
fields. The reservoirs in the Dongara district include the Early Permian fluvial Irwin
River Coal Measures and the Late Permian fluvial Wagina Sandstone, the source of
the gas in both cases being the associated coal and carbonaceous shales. These
units are productive despite low permeabilities, and, in the case of the Wagina, low
porosities of only 5 -15% (Hall, 1989). Thin, discontinous sandstones with 11 -20%
porosity in the nearshore Early Permian Carynginia Formation also produce gas. The
main reservoir of the Woodada Gas Field is coquinitic limestone of the same formation,
but because of extensive diagenesis, production has to rely on secondary fracture
porosity. Of the other possible reservoirs in the sequence, sands in the glacigene
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