Clarence-Moreton Basin - Geological Overview
The Clarence-Moreton Basin is an extensive intra-cratonic Mesozoic sedimentary basin located primarily onshore in north-eastern New South Wales and southern Queensland and possibly eastwards offshore across the adjacent continental shelf. In Queensland, the arealy greater portion of the basin merges with the Surat Basin. Despite stratigraphic units in New South Wales being correlated to hydrocarbon productive units in Queensland, the 16 000 square kilometre New South Wales portion of the basin is lightly explored with approximately 30 petroleum exploration wells drilled to date.
The sediments of the Clarence-Moreton Basin are continental (non-marine) in origin and range in thickness from 2.5 km to 4 km. Triassic-Jurassic sediments of the Clarence-Moreton Basin unconformably overlie Ordovician to Triassic sedimentary and metasediments and igneous intrusions.
Hydrocarbon shows have been reported throughout the onshore basin mainly in association with quartzose sandstones of Marburg Group. The Triassic to Jurassic rocks of the basin contain abundant oil-prone organic matter of terrestrial origin, particularly in the Walloon Coal Measures and to a less extent in the Koukandowie Formation. Older units in the basin are generally lean in organic matter, but coal-rich sediments occur in older sequence.
The Clarence-Moreton Basin developed as a result of thermal relaxation following a period of oblique extension of a basement. It is cut by major long-lived strike-slip faults and intruded by extensive plutonic suites. Initial sedimentation in the basin is comprised of coal measures development (Nymboida Coal Measures) in the south-east portion of the basin, thinning towards the margins. At the same time in the north-east of the basin the Chillingham Volcanics were deposited. This was followed by deposition of the Evans Head and Red Cliff Coal Measures along the eastern margin of the basin. These equivalents of the Ipswich Coal Measures in Queensland are strongly folded and faulted, and form an angular unconformity with the underlying Nymboida Coal Measures. This sedimentation may represent an initial syn-rift phase of basin development.
Uplift in the Late Triassic led to an unconformity between the Nymboida and Ipswich equivalent Coal Measures sequences and the Bundamba Group, which may represent post-rift basin development. The Bundamba Group consists of a thick sequence of mainly conglomerate and sandstone deposited in a fluvial to lacustrine environment. An unconformity toward the top of the Bundamba Group separates out an upper unit, the Marburg Subgroup, which consists of interbedded sandstone, siltstone and claystone formed in meandering stream environment. It appears to be transitional with the overlying Walloon Coal Measures.
The Walloon Coal Measures represent a major episode of widespread fluvial and lacustrine to paludal deposition which covers not only the Clarence-Moreton Basin, but also the Surat and Eromanga Basins. They consist of grey siltstone, thick banded coal horizons and fine to medium grained lithic sandstone. The Walloon Coal Measures are overlain by the distinctive medium to coarse grained Kangaroo Creek Sandstone. This is in turn overlain by the Grafton Formation, a fluvial to lacustrine claystone and sandstone unit.
Tertiary intrusives and extrusives occur throughout the basin but are concentrated in the north of the basin associated with the Mount Warning Complex and the Main Range and Lamington Volcanics.
Structure and tectonics
The Clarence-Moreton Basin is a broad Mesozoic intracratonic basin extending from the Kumbarilla Ridge in the west, to the east coast of Australia and overlies the mid to late Palaeozoic rocks of the New England Orogen in south-east Queensland and north-east New South Wales. It consists of three sub-basins. These are from the west, the Cecil Plains Sub-basin, the Laidley Sub-basin and the Logan Sub-basin. The Cecil Plains Sub-basin is separated from the Surat Basin in the west by the Kumbarilla Ridge, and from the Laidley Sub-basin to the east by the Gatton Arch. The Laidley Sub-basin is separated from the Logan Sub-basin by a complex structural high called the South Moreton Anticline. A zone of strike slip fault was active since the Early Triassic or Late Permian. The West Ipswich and East Richmond Faults form the west and eastern boundaries of the South Moreton Anticline.
Within the sub-basins are subsidiary structures. The deepest part of the Cecil Plains Sub-basin is a half graben, probably formed by strike-slip faulting, called the Horrane Trough. The Logan Sub-basin consists of a complex series of highs and troughs that probably reflect the trend of older faults. In addition, the Logan Sub-basin is divided along the Queensland-New South Wales border by a group of imposing intermediate intrusions of Tertiary age. Smaller dykes are abundant in this region and extensive basalt shield obscures the Mesozoic geology. The structure contour map of the base of the Clarence-Moreton Basin shows steep upturning by the Mount Barney intrusion that also brings Palaeozoic basement and Ipswich Coal Measures to the surface.
The tectonic history of basin formation in the region can be summarised as follows:
- Late Permian dextral transtension on the West Ipswich Fault formed a basin beneath the Laidley Sub-basin.
- In the Early Triassic, transtension stepped eastward to the Logan Sub-basin and the Esk Trough formed by thermal relaxation subsidence.
- Thermal subsidence and continued minor strike slip faulting formed the Ipswich Basin in the east and the Horrane Trough in the west. The area of the Esk Trough-Laidley Sub-basin was a region of non-deposition.
- From the Late Triassic to probably the Cretaceous, thermal subsidence high across the region saw deposition of the Clarence-Moreton Basin. Minor dextral strike slip movements along the basin forming faults produced locally enhanced subsidence or uplift.
- Compression or transpression during the Late Cretaceous formed minor thrusts with hanging wall anticlines, flower structures and inverted some normal faults.
- Initiation of rifting and sea floor spreading along the eastern Australian continental margin in the palaeogene saw heating and uplift of the eastern part of the Clarence-Moreton Basin and the end of dextral transpression.
- Intermediate to basic volcanics and intrusions produced extensive heating and disruption of parts of basins during the Miocene.
In summary, the tectonic model for the Clarence-Moreton Basin is almost pure dextral strike slip along the Coraki Fault. Such a stress direction would produce transpression and hence thrusting on the East Richmond and Pillar Range Fault, whereas flower structures formed on the eastward side steps or restraining bends on the Coraki Fault. The deeper parts of the Basin occupy zones of dividing faults. The Yamba Trough is a pull-apart basin formed between splay faults, and the Martin and Shannon Faults south of Grafton are thrust faults splaying from the Coraki Fault. The basin would thus offer some areas of extensional tectonics with perhaps improved permeability.
In the onshore Clarence-Moreton Basin, there are abundant oil-prone source rocks in the Walloon Coal Measures and Koukandowie Formation. Older units are generally lean in organic matter, but coal-rich sediments occur in older sequences. The organic matter is rich in liptinite, and the hydrogen indices of coals are commonly as high as 450. The kerogen is largely Type II/III.
Isothermal contours map indicates eastward exhumation of the thermal maturity zones. In the far west, west and central areas, rocks at or near the surface contain SOM that is immature or marginally mature for oil generation, i.e. MVR < 0.45-0.6% and Tmax < 440oC. In the north-east area the surface rocks contain SOM that is thermally marginally mature for oil generation, i.e. MVR=0.6-0.7% and Tmax= 440o-450oC. In the south-east and far south-east areas, the surface rocks contain SOM that is thermally mature to overmature for oil generation, i.e. MVR > 0.7% and Tmax > 450oC.
Coal samples of the Nymboida and Ipswich equivalent Coal Measures have TOC values ranging between 1% and 20% (mostly 3%) and vitrinite reflectance (Vr) values of between 0.95% and 4.0%, indicating that hydrocarbons generated by these units are likely to be gas and/or condensate. Given the net coal present, there is likely to have been considerable quantities of gas generated which are possibly now contained within either coal seams, sandstones within the coal measures, or sandstones in the overlying Bundamba Group. Gas peaks have been logged whenever the Nymboida Coal Measures have been penetrated in petroleum wells. Gas has been noted seeping from coal in cores and cuttings. The Nymboida I coal hole blew out while coring the upper Nymboida Coal Measures and continued to flow gas for at least 4 years. In 1956 a large explosion in the Nymboida Colliery caused by seeping methane killed several miners and resulted in closure of part of the mine.
Within the deeper portions of the basin the Raceview Formation (within the Bandamba Group) is very shaly with considerable coal intersected in at least one well. The dominance of vitrinite with secondary inertinite, Kerogen Type III, HI values less than 100 and Vr values from 0.8% to 2.2% suggest that hydrocarbons generated from this unit are likely to be gas prone with a minor oil fraction. The best potential is likely to be in the Casino Trough to the north rather than the Grafton Trough in the south.
The Koukandowie Formation, within the upper Marburg Subgroup, has TOC values averaging 3%. Macerals are mainly vitrinite, but with substantial amounts of liptinite and minor inertinite. Together with Kerogen Types II and III and HI values less than 200 this unit would be expected to yield gas with minor oil. These results together with other values suggest that the Koukandowie Formation is a marginal to good source of gas with some potential for oil. Most of the significant hydrocarbon shows within the basin have come from this horizon. Gas flows have been essentially all methane. However, during drilling mud logging equipment generally indicate the presence of gas from C1 to C5.
The Walloon Coal Measures are well known for their high volatile, sub-bituminous coals. However, the unit also contains claystones, shales and siltstones rich in carbonaceous material. TOC values range from 1% to 20%. S1+S2 values of 7.79 to 43.96 have been recorded and HI range from 119 to 282. The Walloon Coal Measures are mostly Kerogen Types II and III with minor Type I. These and other data suggest that there is little doubt that the Walloon Coal Measures would generate both oil and gas at appropriate maturation levels; in fact oil and dry and wet gas shows have been observed in this unit.
The Ripley Road Sandstone, in the lower Bundamba Group, is the most widespread and most quartzose sandstone dominated unit within the basin. It has only been tested by one successful drill stem test (DST) which recovered 46 m of gas cut mud with a trace of oil. Reservoir data is limited; however, porosities of 3% to 29% and permeabilities of 0.21 to 57md have been recorded, and log-derived porosities generally range from 15% to 25%.
The Heifer Creek Member, in the middle of the Koukandowie Formation, has the best reservoir properties of any unit in the Clarence-Moreton Basin. Porosities of 7.8% to 20.5% and permeabilities of 0.13 to 69md have been measured. Core analyses and DST results indicate that the Heifer Creek Member typically has fair to good porosity but with generally low permeability. The best gas flow in the unit was 500 000 cf/d (14 000 cubic metres per day) and other DST data suggests if hydrocarbons were encountered they would flow at substantial rates. 0.5% of the pore volume of one sidewall core, in a unit with 24% porosity and 26md of permeability, contained oil with a strong yellow cut.
The Nymboida Coal Measures, Raceview Formation, Gatton Sandstone (in the middle Bundamba Group) and the Walloon Coal Measures are also considered potential reservoirs if areas of favourable facies or structural porosity/permeability can be recognised.
Traps & seals
The relationship between timing of thermal maturation and the formation of structural traps in the Clarence-Moreton Basin are still not well understood. It has been concluded that thermal maturity at the base of the sequence may have been attained not long after the deposition of the lower Bundamba Group. The major basin-wide structural event probably took place following deposition of the Ipswich Coal Measures equivalents and prior to deposition of the Bundamba Group. However, trapping of hydrocarbons in the Nymboida and Ipswich equivalent Coal Measures probably occurred in reservoirs within the coal measures themselves. Any hydrocarbons surviving this deformational event, or generated later, would be sealed by the overlying lowest Bundamba Group. Based on analogues from the Surat/Bowen Basin, most generation occurred since the Late Cretaceous.
It has been inferred that the subsequent major deformation in the overlying sequence occurred as a result of the Late Cretaceous and Tertiary reactivation of basement structures, by which time the Marburg Formation and Walloon Coal Measures would presumably have reached thermal maturity. Major traps are likely to include anticlines and high angle reverse fault traps associated with the Late Cretaceous-Early Tertiary deformation or stratigraphic traps.
Lithologies capable of acting as seals are widespread throughout the onshore Clarence-Moreton Basin section, and by analogy the offshore section as well. In the onshore area, potential seals are the floodbasins present within most units. The most extensive are the Ma Ma Creek Member of the Koukandowie Formation and Calamia Member of the Gatton Sandstone. The Ma Ma Creek Member may provide a seal for the scattered porous sandstone bodies found at top of the Gatton Sandstone and the Calamia Member for the Ripley Road Sandstone. Of these units, the Ma Ma Creek is the most consistently developed. In places, the Calamia Member is predominantly siltstone and fine sandstone and therefore will not always act as a seal.
For other potential reservoirs in the Koukandowie and Raceview Formations, sealing is dependent on local development of suitable floodplain facies. Any porous and permeable bodies in the Walloon Coal Measures could be sealed by the typical, highly labile sandstone of the Walloons as well as by fine-grained facies.
Coal seam methane
Exploration for coal seam methane in the Clarence-Moreton Basin has grown since the first well was drilled in 1997. The limited available data on the Nymboida and Ipswich equivalent Coal Measures indicate that they probably meet most of the requirements for the production of coal seam methane. The depth of cover is excessive in the deeper parts of the basin; however, this is not always a problem. There is ample evidence of gas within the Nymboida Coal Measures, particularly in the shallower coal mining area in the south-west of the basin.
The Casino area of the Clarence-Moreton Basin has been found to be prospective for both conventional and coal seam gas. Metgasco has proposed development of the gas powered Richmond Valley Power Station which will be fed by coal seam gas extracted from the Walloon Coal Measures and conventional gas extracted from deeper reservoirs.
Despite being only lightly explored for petroleum there are numerous oil and gas shows in the Clarence-Moreton Basin and a sub-commercial gas field (Hogarth). The deeper parts of the basin have provided favourable conditions for maturation and generation of oil and gas as evidenced by recent discoveries. The Kingfisher Gas Field (PEL16) is the first conventional gas discovery in the Clarence-Moreton Basin and the largest conventional gas discovery in NSW. Metgasco is continuing evaluation, however contingent resource estimate of the field is 80 PJ(P50) and 298 PJ(P10). There is ample evidence of the source rock potential and although reservoirs are generally tight there is certainly potential for localised improved reservoir quality and/or reservoir stimulation. The basin is located close to a large regional population base (approx. 300 000 people). The Moomba-Roma-Brisbane gas pipeline and Jackson-Brisbane oil pipeline are located less than 100 km to the north of the New South Wales portion of the Clarence-Moreton Basin. The basin is ideally suited for exploration by a company willing to exploit smaller fields, rather than a large multinational explorer.
Products & Publications
- Clarence-Moreton SEEBASE™ and Structural GIS Project
- Onshore Clarence-Moreton Basin Petroleum Data Package
- Petroleum Prospectivity – Clarence-Moreton Basin, NSW (Book)
- Clarence-Moreton Basin Seismic Survey 2008
- New South Wales Petroleum Potential (Book)
- NSW Coal Seam Methane Potential (Petroleum Bulletin No. 2)
- New South Wales Coal Seam Methane Potential (Brochure)
- NSW Well Wireline Logs Petroleum (oil, gas & CSM) Wells Data Package
- NSW Well Wireline Logs Stratigraphic/corehole wells (drilled prior to 2003) Data Package
- Facilitating the Development of Natural Gas in Northeastern NSW Report
- Geoscience Product Catalogue (GPC) (for all Mineral Resources products)
- Shop NSW (for online ordering and payment for some Mineral Resources products)
Exploration data - Clarence-Moreton Basin (onshore and offshore)
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