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side of the outcropping portion of the Bowen Basin is even beyond the peak of gas
generation (based on the vitrinite isoreflectance maps of Beeston (1981) and Middleton
(1989)), but nevertheless this region still has noteworthy potential for methane
drainage from the Late Permian coal seams (Bell, 1987). The organic maturity
decreases to the south, until south of Meandarra, and northwards from there along the
basin edges, the sequence lies within the oil window (Thomas & others, 1982). The
known hydrocarbon accumulations are small, and occur in three areas - the Moonie
district, the Roma Shelf, and the Denison Trough. However, most, if not all, of the
discoveries in the overlying Surat Basin were also sourced from the Permian (Thomas
& others, 1982).
Source rocks are generally within coal-bearing intervals of the Reids Dome beds and
the Back Creek and Blackwater Groups. The Bandanna Formation contains a 0.7 m -
thick oil shale (torbanite). The shallow marine muds of the Denison Trough sequence
(Black Alley Shale, Peawaddy Formation, Ingelara Formation, and particularly the
Cattle Creek Formation), also have fair to good source potential (Jackson & others,
1980). But although lying within the oil-generative zone, the Denison Trough has a
preponderance of humic kerogen, and is therefore gas-prone (Paten & others, 1979).
Below the Surat Basin, Permian sandstones generally lack good porosities and
permeabilities, due to their lithic and argillaceous natures; in places where better
reservoir properties do occur, hydrocarbons have been produced (Butcher, 1984).
Locally, palmochannels have been concentrated between isolated highs, and
Cosgrove & Mogg (1985) have interpreted potential alluvial fan reservoirs in the upper
Tinowon Formation. Gas has flowed from the Camboon Andesite in Burunga 1 and
Scotia 1 (Elliott, 1989). In the Denison Trough, gas has been discovered in all the
sandstone-dominated formations between the upper Reids Dome beds (a coal
measures unit) and the high-energy nearshore Mantuan Productus bed at the top of
the Peawaddy Formation, with the thick, fluviodeltaic Aldebaran Sandstone being an
important reservoir formation in the succession. The Late Permian Bandanna
Formation, also gas-bearing, should contain a variety of reservoir sand types in its
meandering-fluvial, deltaic, and shoreline facies. In general, these units are not as
permeable as those of the Roma Shelf. Zones with good reservoir characteristics in
the upper Reids Dome beds are associated with marginal marine settings (Draper &
Beeston, 1985b). Sealing lithologies are provided regionally by the marine shelf mud
formations and, in some areas, by the Early Triassic Arcadia Formation of the lower
Rewan Group. Intraformationally, shallow marine sandstones can be entirely encased
in shales, and flood plain silts can enclose the coarser sediments of channel belts and
alluvial fans.
Gunnedah Basin
No commercial hydrocarbons have been located in the basin so far, but a subcommercial
gas field has been discovered at Wilga Park 1. Other gas shows have
been encountered in the Porcupine, Watermark, and Black Jack Formations, and an
oil show has been observed in the Early Permian Boggabri Volcanics. The Permian
sequence is usually in the early part of the oil generation zone, and has reached
greater maturity near some igneous intrusions, so that the hydrocarbon potential is
considered high (Hamilton & others, 1988). The best source rocks are in the shallow
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