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Coal measures in South Australia are primarily of Permian, Triassic, Jurassic and Tertiary age (South Australia Department of Mines and Energy, 1987). Almost all known deposits have been evaluated for coal extraction potential, but not for coal seam gas potential (CSG). The depth and maturity of the coal deposits and distance to infrastructure and markets have prevented economic exploitation of all except the Leigh Creek Coalfield.

Coal seam gas formation

During coalification, large quantities of methane are generated. This gas is adsorbed onto the coal surface in cleats and pores, and is held in place by reservoir and water pressure. Coal rank, reservoir pressure (related in part to depth) and temperature are important factors controlling the amount of methane held in a coal seam.
For more information on coal seam gas formation see Chapter 1.2.5 of the "Roadmap for Unconventional Gas Projects in South Australia".

The South Australian coals at potentially exploitable depths for CSG (generally between 100–1500 m; Poynton and Simon, 2001) have maturities significantly <0.8% Ro which is the approximate threshold needed for economically significant thermogenic methane production (200–300 scf/t in Scott, 2002). As such all South Australian CSG prospects may need a contribution from a biogenic methane source similar to the Powder River Basin (SanFilipo, 2000) to have economic gas contents.

Coal geology

Extensive and thick Permian coal measures occur in the intracratonic Arckaringa, Pedirka and Cooper basins. These deposits are similar in age to proven eastern Australian CSG producing basins (i.e. Bowen and Surat basins). Coals in the Cooper Basin are a proven source of conventional oil and gas and occur at depths from 2000–3500 m (bituminous to anthracite rank). The Early Permian Patchawarra Formation contains a major coal seam up to 30 m thick and this forms an important seismic reflector in the Cooper Basin. Coals in the Pedirka and Arckaringa basins are shallower and less mature for oil and gas generation (sub-bituminous) than Cooper Basin coals.

Triassic coal measures are intracratonic remnants of broader deposystems and was mined at Leigh Creek. This Triassic coal is sub-bituminous in rank, and was mined at the margins of the Telford Basin, but extends to depths in excess of 1000 m in the basin centre. Jurassic coal measures form a deposit at Lock in the Polda Basin. Jurassic coals also occur in the deeper parts of the Eromanga Basin in the Poolowanna and Birkhead formations. Cretaceous coal measures are known from the Otway Basin (e.g. Eumeralla Formation) and Eromanga Basin (Winton Formation); the latter is a potential exploration target for CSG. South Australia also has significant deposits of very low-grade lignite found in shallow Tertiary basins

Cooper Basin

The Weena Trough in the southern Cooper Basin contains the shallowest occurrences of thick Patchawarra Formation sub-bituminous coal seams (e.g. ~1500 m depth in Tinga Tingana 1 and Weena 1). Minor mud gas indications have been recorded while drilling through coal seams in Kumbarie 1, and the water chemistry in Tinga Tingana 1 suggests some methane is present (Nitschke, 2006).

Higher rank, thick Permian coal seams in the deeper parts of the Cooper Basin are now being assessed as a source of deep coal gas.  In the US, the deepest CSG production is from 2500m in the Piceance Basin and 2000m is generally considered the floor for CSG production due to cleat closure and permeability reduction at these depths.  However the Cooper Basin coals are characterized by a high inertinite content. Inertinite is essentially non-reactive during the carbonization process, and the cellular structure of the component plant material is preserved. As a result these coals contain significant macro-porosity indication considerable free-gas storage potential, in addition to gas storage by adsorption.

A fracced deep Patchawarra Formation coal in the Moomba 77 vertical well flowed gas at 0.1 mmscf/d. More recently Senex Energy’s Paning 2 vertical exploration well in the northern Cooper Basin flowed gas from a fracture stimulated Toolachee coal seam at peak flows of up to 0.09 mmscf/d. Senex estimates that the dry Toolachee coal has 2.1 tcf of gas in place (>1 tcf 3C resource estimate) across the 9,000 acre Paning structure. Paning 2 intersected 70 metres of Permian coals with excellent gas contents of 25 cubic metres/tonne (~ 800 scf/ton) Given the very high gas content of the coals, the application of drilling technologies to maximize the surface area of coal exposed to production may significantly improve flow rates.

For more information on the deep coal seam gas potential of the Cooper Basin see Chapter 2.1.3 of the “Roadmap for Unconventional Gas Projects in South Australia”.

Pedirka Basin

The Pedirka Basin in South Australia comprises a shallow western depocentre and a deeper eastern depocentre. The Early Permian succession is thickest in the western depocentre (up to 1000m). Coal seams are present in the upper part of the Early Permian Purni Formation, a lateral equivalent of the Patchawarra Formation in the Cooper Basin.

For more information on the deep coal seam gas potential of the Pedirka Basin see Chapter 2.5.1 of the “Roadmap for Unconventional Gas Projects in South Australia”.

Arckaringa Basin

The sub-bituminous coals in the Arckaringa Basin have features (coal thickness, continuity and suitable depth) which make them appealing for CSG feasibility projects, although no significant gas shows were recorded by the two petroleum exploration wells in the basin drilled with mud gas detection equipment (Birribiana 1 and Weedina 1).

For more information on the geology of the Arckaringa Basin see Chapter 2.2 of the "Roadmap for Unconventional Gas Projects in South Australia".

Leigh Creek

There are five discrete basins in the area: North Field (Lobes D and C), Telford Basin (Lobe B), Copley Basin (Lobe A) and, the latest to be discovered, Lobe E.

Lobe B, the largest of the five basins, was mined around the margins, where the overburden is as thin as 10 m. However, the seams have a moderate dip (10–30º) and the depth to coal reaches up to 1000 m in the centre of the basin. The moisture content of the coal is quite high (33%), which may have a negative effect on the amount of gas that could be stored within the coal, but the other characteristics of the deeper sections of Lobe B have good potential for CSG.

Polda Basin

The Jurassic Lock and Mullaquana coal/oil shale deposits occur at mineable depths in the Polda Basin. Overburden ranges from 35 to 230 m but is generally between 50 and 130 m, which may be too shallow to store significant amounts of gas. The sub-bituminous coal has low levels of inertinite and higher levels of liptinite. This makes the composition of the coal very similar to the Walloon Coal Measures of Queensland, which are being successfully exploited for CSG (but typically at greater depths).

Tertiary Coals

South Australia also has significant deposits of very low-grade lignite (<100 m deep) that occur in both intracratonic and structurally controlled basins located along Australia’s southern margin. In South Australia this includes the Gambier, Pirie-Torrens, St Vincent and Murray basins. Tertiary coal deposits that were evaluated for coal extraction for power generation in the 1980s include Bowmans, Lochiel and Clinton (St Vincent Basin), and Kingston, Sedan and Moorlands (Murray Basin). Mining some of these deposits for coal gasification projects is also currently being evaluated. Tertiary lignites are typically high in moisture, sulfur, sodium and chlorine and are overlain by unconsolidated Tertiary and surficial sediments. Although thick seams are developed in some deposits, all Tertiary coals are generally shallower than 100 m and this limits reservoir pressure and likely gas content.

Since all of the coal is very low-grade lignite, biogenic activity would be required to generate CSG. Hydrological conditions may provide the trapping and overpressure required to store economic amounts of biogenic gas in some of the coal seams, which have other appealing properties (thickness, continuity etc.).

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