Coal seam gas

Targets for CSG in the Maranoa-Balonne-Condamine subregion are primarily the seams of the Walloon Coal Measures of the Surat Basin. However, in the most north-western part and the eastern part of the subregion no significant volumes of CSG are expected to be present in the Surat Basin. Due to the proximity to the basin margins the coals are very shallow and the unconfined pressure within the seams allows gas to desorb and be lost from the system (Queensland Government EHP, 2013, p. 32; Scott et al., 2007). This is supported by observations of Scott et al. (2007, p. 11), who found that loss of gas from the Walloon Coal Measures is apparent in the north-east Surat Basin as a result of a reduction in confining pressure through tectonic movement and/or water movement.

In the central-northern and central-eastern part of the Maranoa-Balonne-Condamine subregion, CSG recovery occurs from the Walloon Coal Measures generally at depths of 200 to 600 m below the surface (Papendick et al., 2011, p. 123). Production targets of the Walloon Coal Measures are the seams of the Taroom and the Juandah coal measures (Papendick et al., 2011, p. 123; DNRM, 2014a); three coal seams have been identified for the Taroom Coal Measures and seven for the Juandah Coal Measures (QGC, 2009). Mean net thickness has been reported as 30 m (the Taroom Coal Measures contributing 8 m and the Juandah Coal Measures 22 m) (QGC, 2009), though it varies locally. Studies of the Dalby and Roma fields by Papendick et al. (2011, p. 123) reported that an average Dalby well contains 20 to 30 m of net coal and an average Roma well contains about 15 m. Gas production may also occur from other facies interspersed between the coal seams, such as shale and tight gas reservoirs. The extent of this contribution is unknown and has not been further investigated.

Individual seams of the Walloon Coal Measures can be thin and commonly contain lateral discontinuities disrupting the seams. However, they typically have high seam permeabilities (DNRM, 2014a); in the Roma field permeabilities can be as high as 200 mD, whereas permeabilities for a well sampled in the Dalby field ranged from 0.07 to 42.38 mD. High permeabilities decrease the need for well stimulation (such as hydraulic fracturing) to enable gas production at economic quantities.

Gas contents in the Walloon Coal Measures range from 1 to 15 m3/t (daf), with a mean of about 5 m3/t (daf) (Scott et al., 2007, p. 10). Similar ranges were also reported by Hamilton et al. (2012, p. 26). Gas contents within the upper coal seams of the Juandah Coal Measures (Kogan, Macalister Upper and Macalister Lower) appear to be lower and more variable compared to the lower seams (Scott et al., 2007, p. 10). Coals of the Taroom Coal Measures have lower gas contents than those of the Juandah Coal Measures (Scott et al., 2007, p. 10). Hamilton et al. (2012, p. 26) observed varying trends for gas content and depth, where it either increases; increases then decreases; or decreases with depth. The mean gas saturation of the Walloon Coal Measures is estimated as 80% (QGC, 2009).

Hamilton et al. (2012, p. 29) reported Walloon Coal Measures CSG as consisting almost entirely of methane (>98%; air-free basis), with only minor carbon dioxide and nitrogen, and very low levels of ethane (<1%). This is in agreement with average coal properties reported by QGC Pty Limited who stated that the gas is composed of more than 97% methane with only small amounts of nitrogen and carbon dioxide (QGC, 2009). The methane is mainly of biogenic origin (Papendick et al., 2011, p. 124).

The Bandanna Formation of the Bowen Basin has been targeted for CSG in the north-western tip of the subregion. However, the coals were not found to have potential for CSG recovery (Blue Energy, 2010). The coal seams of the Clarence-Moreton Basin, in the eastern part of the Maranoa-Balonne-Condamine subregion, have not been explored for CSG as of yet (as of October 2014).

Total proved and probable (2P) CSG reserves as per the Petroleum Resource Management System (World Petroleum Council, 2011) in the Maranoa-Balonne-Condamine subregion are in excess of 28,000 petajoule (PJ) as of 31 December 2013 (DNRM, 2014b). In comparison, Queensland’s total 2P reserves for the same period are 41,124 PJ (DNRM, 2014b). This indicates that the majority of Queensland’s current CSG reserves are in the Maranoa-Balonne-Condamine subregion.

Last updated:
5 January 2018
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