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marine Watermark Formation, and in the Arkarula Sandstone Member, a marine
intercalation within the Black Jack Formation; the Maules Creek Formation (a coal
measures unit), and the rest of the coal-bearing Black Jack Formation have only fair,
but still significant, oil source potential. Good highly-permeable reservoirs may be
found in the quartzose sandstones of the lower Black Jack, and the Clare Sandstone
Member, derived from westerly sources. Easterly derived sands, being of volcanolithic
composition, have very poor reservoir characteristics, unless they were cleaned up
and their porosity improved somewhat by being re-worked in littoral or tidal shallow
marine situations, such as occurred at some levels in the upper Watermark and lower
Black Jack Formations. The Permian could also have supplied hydrocarbons to
suitable reservoir lithologies in the Triassic and Jurassic. Regionally extensive shales,
laid down either by marine incursions or lacustrine episodes, cap most potential
reservoir units, and more local seals are provided by other intrafornnational shales,
particularly within the coal measures (Hamilton & others, 1988).
Sydney Basin
The search for oil and gas in the Sydney Basin has been unsuccessful, due mainly to
the tightness of putative reservoir lithologies. The cause has a palmogeographic
background, in that the sandstones are lithic and clay-rich because of their derivation
from the uplifted New England Orogen. The largest permeabilities occur in the
Illawarra Coal Measures, in the Marrangaroo Conglomerate alluvial fan / fan delta, and
in a quartzose sandstone below the Tongarra Coal, but range up to only 10 md
(Herbert, 1987). However, better reservoir units have been identified in the Triassic
Narrabeen Group (Herbert, 1987; Hamilton & Galloway, 1989), signifying that
examination of the Permian section for hydrocarbon generation is not academic. The
coals and carbonaceous shales of the coal measures units are obvious source rocks,
as are the lenses of oil shale they contain in places. Shows of free gas and oil have
been encountered in the Illawarra Coal Measures, and to a lesser extent in the shallow
marine Shoalhaven and Dalwood Groups. Coal rank studies of the top of the Permian
(Middleton, 1989) show that the centre of the basin is overmature for oil, but may be
a gas generator. Also not to be overlooked is the methane drainage potential of the
coal seams; two collieries south of Sydney are currently using their drained methane
for small-scale electricity generation to power mine plant, with any surplus being fed
into the domestic electricity grid (Bishop & Callinan, 1987).
Torbanite oil shales are known from 34 localities around the margins of the Sydney
Basin, and several of these deposits were worked in the period 1865-1952 (Mayne,
1970). All are associated with the coal-bearing sequences: 30 occur within the Late
Permian coal measures, 3 in the Early Permian Greta Coal Measures, and 1 in the
Early Permian Clyde Coal Measures. The deposits are developed as lenses in coal
or carbonaceous shale, and originated as algal muds at the bottoms of open bodies
of water within peat swamps. The largest, in the Glen Davis - Newnes district, is
continuous over an area of 35 km 2 and has a maximum thickness approaching 2 m
(Morris, 1975); it lies within the Glen Davis Formation of the Illawarra Coal Measures
(Permian 6).
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