The Denison Trough is a geological feature that has developed into a hydrocarbon province. Located in Central Queensland, it covers much of the western Bowen Basin. Its sediments were laid down during various geologic eras, and at its deepest the trough contains of deposits from the Permian and Triassic. The Denison Trough shares some hydrocarbons with other parts of the Bowen Basin, but has become particularly notable for its natural gas reserves, some of which have been commercially extracted.
The main part of the Denison Trough formed during the Permian and Early to Mid-Triassic. Geological formations from this period overlie sediment from the Devonian, with little or no Carboniferous material in between. The formations found provide evidence of different environmental periods, as what became the Denison Trough was at various times a riverine, perhaps swampy, environment, and at other times was coastal or part of a shallow sea. The shifting environment is reflected both in the sediment material of different formations, and the fossils found within these sediments.
Geologic surveys of the area began in the 1920s, and accelerated in the 1960s after oil was found nearby. Various hydrocarbon pockets were found by different companies in the decades following, a majority of which was found within one formation referred to as the Aldebaran Sandstone. The completion of a gas pipeline to Gladstone in 1990 allowed for gas to be more easily transported and then exported. The natural gas produced in this trough often has high levels of carbon dioxide, although the reasons for this have not been determined.
Location
The Denison Trough makes up the western part of the Bowen Basin, elongated in a roughly north–south direction. The Denison Trough is one of the two depocentres of the Bowen Basin, although the Denison Trough is less thick than the other depocentre, the Taroom Trough. The two troughs are separated by the Comet Platform. To the west of the Denison Trough are the Springsure Shelf and Nebine Ridge. The south is overlaid by the Surat Basin, where the southern border of the Denison Trough meets the Roma shelf.
This area covers around of Central Queensland. It is traversed by the Dawson Highway, and significant settlements include Injune and Rolleston. The climate shifts between wet summers from November to March, in which rainfall is often over , and dry winters with some frost.
Formations + Rough stratigraphy of the Denison Trough (and Comet Platform) Period Formation Triassic Middle Moolayember Formation Early Clematis Group Rewan Group Permian Late Bandanna Formation Black Alley Shale Mantuan Formation Peawaddy Formation Early Catherine Sandstone Ingelara Formation Freitag Formation Aldebaran Sandstone Cattle Creek Formation Reids Dome Beds Carboniferous None Devonian Timbury Hills Formation Most geological formations within the trough date from the Permian and Triassic. At the deepest point of the trough, these stack to a depth of . The various formations overlie in a series of north–south folds. These folds bring layers of different ages to the surface at different points. For example, at the Inglis Dome formation in the centre-west of the trough, formations ranging temporally from the Aldebaran Sandstone to the Bandanna Formation are exposed. At least one older interpretation of the geologic data divided the Denison Trough into four smaller troughs. While the upper and lower Permian layers are similar to those of surrounding areas, the middle Permian layers within the Denison Trough are distinct.
A series of deposits from the Permian include sediment from coastal areas and shallow seas, perhaps only deep. Some formations show evidence of flooding, with peat swamp forest and other non-marine deposits being interrupted by small sections of marine deposits. Minerals within the formations include quartz, feldspar, kaolinite, illite, pyrite, calcite, and ankerite. The presence of clays has meant the sandstone has remained porous. Pyrite is present in coal deposits, likely related to flooding. The Catherine Sandstone formation shows evidence of oil formation from the coal deposits. Temperatures range from at the surface to at deep.
The formations overlap with changes in brachiopod and microorganism fossil compositions. The Reids Dome beds contain many plant fossils, such as Glossopteris, as well as other fossils such as ostracods and bivalves. The Cattle Creek Formation has shell and plant fossils. Fossils found in the Aldebaran Sandstone include Cordaites, Skolithos burrows, shells, and vertebrate remains. Fossils found within the Freitag Formation include Terrakea, Ingelarella undulosa, Conulariida, and animal burrows. The Ingelara Formation includes animal burrows, as well as a wide variety of marine fossils, including bivalves, brachiopods, bryozoans, crinoids, and tabulate corals. The Catherine Sandstone formation contains similar fossils. The Peawaddy Formation also holds similar marine fossils, as well as Glossopteris and other wood fossils. The Black Alley Shale contains a large number of plant fossils, including representatives of Glossopteris, Neomariopteris, Phyllotheca, and Paracalamites. Lower parts of the Black Alley Shale contain marine fish scales, acritarchs, and foraminifera. Similar plant fossils are also found in the Bandanna Formation. Plant species fossils found in the trough resemble flora associated with India, reflecting biological links between former parts of Gondwana.
Hydrocarbon resources
The Denison Trough is considered a hydrocarbon province, as it contains natural gas and coal. Most resources are locked within the Aldebaran Sandstone formation. Natural gas is the most distinctive feature of the Denison Trough itself, while coal is found throughout the Bowen Basin. Most of the Permian sediment in the Denison Trough is considered to be of the right age for hydrocarbon presence, although the deeper and older areas, such as the Reids Dome beds, may have been too hot. As of 1998, there was thought to be of recoverable gas in the trough. Some of the gas from the Denison Trough is exported to Southeast Asia.
The Reids Dome beds have a number of natural gas pools, although some have not been commercially viable. The Cattle Creek Formation, Aldebaran Sandstone, and Rewan Formation also have natural gas. In the Cattle Creek Formation, there is a large accumulation within sandstone deposits, especially in the western and southern areas of the trough. These makes those areas possible locations of more hydrocarbon reserves. Natural gas from the Aldebaran Sandstone has a high concentration of carbon dioxide, particularly in the south and southeast, which is also the richest area for gas. Many pockets of natural gas from the Aldebaran Sandstone formation within the Denison Trough have been commercially extracted. Some gas has also been commercially extracted from the Freitag Formation. The fine-grained sand of the Ingelera Formation formed a seal over the layers below it, and also contains some minor gas reserves. The Catherine Sandstone contains gas reserves in the trough's north, some of which has been commercially extracted. The overlying marine Peawaddy and Mantuan Formations have some commercially viable gas pockets in their northern non-marine areas. The marine Black Alley Shale forms a seal over these layers. As of 1996, only one gas pocket was found in the Rewan Formation within the Denison Trough, although the formation has additional gas pockets elsewhere. No hydrocarbons have been found in the Denison Trough portion of the Moolayember Formation.
Overall gas wetness in the Freitag Formation, Aldebaran Sandstone, and Cattle Creek Formations ranges from 8-12%. Gas in the Catherine Sandstone and Reids Dome beds may be drier. Theories for the high levels of carbon dioxide within the natural gas of the trough include biodegradation and the dissolution of calcium carbonate in groundwater. However, it is thought that it is not due to temperature changes.
The Arcturus gas accumulation contains 92-96% methane (considered "dry"), with only 0.25%-0.83% carbon dioxide. By June 1996, of the estimated had been extracted from the site. The Glentulloch gas accumulation is 84.5% methane and 10.75% carbon dioxide. Total gas in this accumulation has been estimated at . Gas from the Aldebaran Sandstone portion of the Merivale gas accumulation (estimated to be ) is 83.2% methane and 4.04% carbon dioxide, however gas from the Reids Dome beds portion (perhaps only 10% of the total accumulation) has almost no carbon dioxide. The Rolleston gas accumulation consists of three separate reservoirs, one in the Mantuan Formation (94% methane), one in the Freitag Formation (86% methane, 2.70% carbon dioxide), and one in the Aldebaran Sandstone (similar methane to the Freitag Formation reservoir, but only 0.20% carbon dioxide). The Mantuan and Freitag reservoirs make up 95% of the estimated in the accumulation, of which had been extracted by June 1996. The Aldebaran Sandstone reservoir at Westgrove is 67% methane and perhaps 30% carbon dioxide. The Reids Dome beds reservoir at Westgrove is only 1.5% carbon dioxide. A total of is thought to exist in these reservoirs. The Aldebaran Sandstone reservoir of the Yellowbank accumulation is 60% methane and over 30% carbon dioxide, whereas the Rewan Formation reservoir there has almost no carbon dioxide. 97% of the estimated initially recoverable gas is in the Aldebaran Sandstone reservoir. A Reids Dome beds reservoir found at the site was thought small and was not commercially developed. As of 1998, the Aldebaran Sandstone held over half of the trough's known natural gas reserves.
Gas from southern accumulations including Springvale, Myrtleville, Merivale, and Yellowbank is processed in the Denison Trough Central Treatment Plant. This has specialised equipment to prevent the high carbon dioxide levels of the gas from causing carbonic acid corrosion. Northern accumulations (including Rolleston, Springton, Arcturus, and Moorooloo) are processed in the Rolleston Gas Plant.
Coal deposits in the trough and wider basin are irregular and varied, reflecting a complicated geologic history. The maceral (components) of this coal are mostly vitrinite (predominantly as collotelinite), with smaller amounts of inertinite (mostly semifusinite and inertodetrinite) and liptinite (including sporinite, resinite, alginite, and cutinite). The Reids Dome Beds are known to have significant coal reserves. Thin seams of coal can also be found in the Cattle Creek Formation. The Bandanna Formation contains a large amount of coal in some areas.
Geologic history The deepest layer of the Denison Trough is a heavily eroded layer of metasedimentary rock from the Devonian, referred to as the Timbury Hills Formation. The earliest evidence of geologic activity is a number of half-grabens from either the late Carboniferous or the early Permian. It is possible that there is some Carboniferous sediment, the Roma Granite, but only in the very south and southwest edges of the trough. There may also be some volcanic sediment from the Early Permian that pre-dates the Reids Dome beds.
During the Permian, the area was around latitude 60°–65°. Today this correlates to a conifer forest environment, and the area at the time likely had a similar temperate or cool temperate climate. It was a coastal area, including both non-marine and marine deposits. It likely saw repeated flooding and erosion, which interrupted the formation of deposits such as the Aldebaran Sandstone and may have eventually halted its formation altogether.