Coal Seam Gas

Unconventional Gas Plays in Cooper Basin, SA: Deep Coal Seam Gas - APPEA 2011 poster

This information is based on a draft article on the coal seam gas potential in South Australia prepared for MESA Journal 47 by Alan Sansome (Petroleum and Geothermal Group) and Lisa Nitschke and Peter Tingate (Australian School of Petroleum).

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. Over the last 20 years, coal research in the state has focused on proving up known deposits for mining and power generation, and only limited greenfield coal exploration has taken place. CSG is now an important source of gas in eastern Australia and is being produced primarily from the Bowen and Surat basins in Queensland and New South Wales. CSG exploration is under way in South Australia, Tasmania, Victoria and Western Australia.

Interest has been shown by a number of companies to explore for CSG in South Australia over the last five years, as a result of increasing CSG production from black coals in the eastern states and success achieved in producing biogenic methane from low-rank coal in the United States. Currently there are nine PELs and 6 PELAs under consideration for exploration rights to evaluate the CSG potential and/or underground coal gasification potential (UCG) of South Australian coals (Fig. 1).

On-the-ground CSG exploration is in its infancy in South Australia, however drilling and sampling in the next few years are important elements of work programs for current PELs and PELAs. The technology involved in CSG and UCG has been evolving over the last decade — optimal conditions based on numerous criteria can be developed for each application. Characteristics requiring consideration include coal seam depth, thickness, continuity, maturity, vertical distance to aquifers, maceral content, gas content indications based on water geochemistry, and coal seam permeability.

A collaborative research project between PIRSA and the Australian School of Petroleum was established in 2006 to undertake a desktop evaluation of the state’s CSG and UCG potential (Nitschke, 2006). Nitschke’s paper summarises the results of that evaluation.

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.

Gas content is a major economic consideration in assessing CSG potential, however, as yet, there are no desorption measurements of South Australian coals. Estimates of coal gas contents without these measurements are difficult and a range of indirect indicators has been used, such as mud gas measurements while drilling and water chemistry (Van Voast, 2003).
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 are currently being mined at Leigh Creek (the only operating coal mine in South Australia). This Triassic coal is sub-bituminous in rank, and is 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.

Tertiary coals are typically low-grade shallow lignites (<100 m deep) that occur in both intracratonic and structurally controlled basins located along Australia’s southern margin. In South Australia this includes the 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 to gas or coal to liquids projects is also currently being evaluated.

Eromanga Basin
AGLis currently exploring for CSG in PELs 101 and 103 on the Innamincka Dome in joint venture with Acer Petroleum. Three cored drillholes evaluated the coals in the Early Cretaceous Winton Formation.  A gas kick was recorded on the mudlog through Winton Formation coals in the Aspen 1 exploration well.

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). Linc Energy is now the owner and operator of seven PELs in the basin which were initially granted to SAPEX in October 2006 after the successful negotiation of access agreements with native title claimants and the South Australian Government. Linc is exploring for conventional hydrocarbons, shale oil and CSG in the basin.

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).

The Weena Trough is currently under exploration by Strike Energy Limited (PEL 96). The company applied for the block in March 1999 as part of the CO98 acreage release, and the licence was granted in May 2009. AGL drilled 3 exploration wells in the northern Cooper Basin to test the shallow Winton Formation coals.

Thick Early Permian coal seams may be a future target for deep coal seam 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 Santos has flowed gas to surface at 100,000 scf/day from a fracced Patchawarra coal at approximately 2900m in the Moomba 77 well.  At Wimma 1, in the centre of the Patchawarra Trough, very high gas readings (2800 units total gas) were recorded during drilling of a 14m coal seam in the Patchawarra Formation.  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.

Pedirka Basin
Most of the coal in the eastern Pedirka Basin appears to be too deep for CSG extraction using current technologies. However, in the western part of the basin, Purni Formation sub-bituminous coals are well developed at suitable depths in Mt Hammersley 1 (~600 m) and Purni 1 (~1500 m). Slightly elevated mud gas was recorded in the Purni Formation coals during drilling of these wells.

The Pedirka Basin is now fully under application for conventional oil and gas as well as CSG by Merlin Energy Pty Ltd, Stuart Petroleum Limited and Tri-Star Energy Company.

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).

The Polda Basin is currently under application by Energy Exploration Limited with PELAs 153 and 126..

Leigh Creek
The Triassic coal deposit at Leigh Creek has been mined since 1943. 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, is currently being 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.

Springfield and Boolcunda basins
The Triassic Springfield and Boolcunda basins contain thin discontinuous coal seams. Overburden does not exceed 77 m, so it would be unlikely for economic gas volumes to be found in the seams without extreme hydrological conditions. The seams of the Springfield Basin rarely reach >0.3 m in thickness (although the thickest section is found at the greatest depth — 3.6 m at 77 m). The coals of the Boolcunda Basin are thin, discontinuous and of poor quality.

The Springfield and Boolcunda basins are currently under application by Greenpower Energy Limited.

Tertiary coals
South Australia also has significant amounts of very low-grade lignite found in shallow Tertiary basins. 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.).

Linc Energy is the operator of PEL 120 over Tertiary coal deposits of the St Vincent and Walloway basins. Hybrid Energy SA Pty Ltd. is the operator of PEL 127 over the Kingston Coal deposit.  Other Tertiary coals are under application by Corporate Developments Pty Ltd (Sedan), Merill Gray (Moorlands) and Planet Gas (Willochra and Lake Eyre basins).

Conclusion

While it is early days in the evaluation of the CSG potential of South Australia, six companies have PELs in place or applications lodged to explore for this potential new energy resource. Significant investments in exploration drilling, seismic and geoscientific analyses (including vital coal desorption analyses) are included in current licence and licence application work programs.
 

Major Coal-bearing Sedimentary Basins & Coal Deposits SA Coal Bearing Basins
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