Patchawarra Formation coal

Patchawarra Formation coal seams average 2.1 m thick but can be 22 to 30 m, with 30% of seams exceeding 2 m thick (Alexander et al., 1998). The thickest laterally extensive coal seam in the Patchawarra Formation is known as the VC50 coal. It ranges in thickness from 13 to 23 m, and is thickest in the Nappamerri and Patchawarra troughs (in the western part of the basin, see Figure 5). Recently reported coal intersections in the Weena Trough includes a 45 m thick seam in the Patchawarra Formation, part of an 89 m net coal interval (Strike Energy Limited, 2014b). The Patchawarra Formation is sufficiently mature to generate gas from coal seams over much of the basin, and elevated gas readings are recorded when mature Patchawarra Formation coals are intersected in wells. Given that depths (from surface) to the coal-bearing Patchawarra Formation exceed 1000 m across the subregion, there is no potential for coal to be mined (Menpes et al., 2012).

Davenport-1/ST1, an unconventional gas exploration well, was drilled into the Milpera Trough, with the aim of testing the thickest, most thermally mature coals (Beach Energy, 2012a). This well encountered thick Permian coal seams, notably a 10 m thick seam from the Toolachee Formation, a 29 m thick seam in the Epsilon Formation, and 19.5 m thick Vm3 seam and 40 m thick Vu seam from the Patchawarra Formation. The thickest Toolachee Formation coal (1611 to 1621 m depth) is described as black, subvitreous to vitreous, silty, grading to carbonaceous siltstone, firm to moderately hard. The thickest Epsilon Formation coal (1697 to 1726 m depth) is described as black, subvitreous to vitreous, brittle to hard, with silty sections. The Patchawarra Vm3 seam (1905 to 1926 m depth) is described as black to brownish black, dull, with vitreous laminations, brittle to hard, and grading to silty in places. The Vu coal (1975 to 2012 m depth) is described as black, dull to subvitreous, soft to firm. The Vm3 and Vu coal seams were sampled, and analyses undertaken to determine gas characteristics. Proximate analyses of the Patchawarra coal seams had an average ash content of 3.66%, average moisture content of 3.33%, average volatile matter content of 29.23% and average fixed carbon content of 63.78% (Beach Energy, 2012a).

Coal seam gas (CSG) exploration well Forge-1 in the Weena Trough encountered 24 m total thickness of coal from the Permian section (Figure 6). This included a 16.5 m interval (intersected at 1313 to 1329.5 m depth) cored from the Patchawarra Formation, described as a black, dull, earthy coal. Proximate analyses of the Patchawarra Formation coal indicated average ash content of 4.86%, average moisture content of 12.03%, average volatile matter content of 35.78% and average fixed carbon content of 47.34% (Strike Energy Limited, 2010).

CSG appraisal wells in the Weena Trough at Le Chiffre-1, Klebb-1, Klebb-2 and Klebb-3 have intersected up to 147 m of total coal thickness (Strike Energy Limited, 2015), with up to 89 m of coal in the Patchawarra Formation. The main coal seams intersected in these wells are the Vm3 and Vu seams. The Vu seam is up to 45 m thick (Strike Energy Limited, 2014b). These are the thickest cumulative and individual coal thicknesses recorded in the Patchawarra Formation, as well as the wider Cooper Basin.

Figure 5

Figure 5 Cooper Basin structural elements shown over depth to basement image

Depth to basement image is the seismic z-horizon. This is the top of the Warburton Basin, interpreted from seismic data and well intersections.

Data: Draper (2002b), Geoscience Australia (Dataset 2), Gravestock and Jensen-Schmidt (1998), Ransley et al. (2012b), Department for Manufacturing, Innovation, Trade, Resources and Energy (DMITRE), South Australia (Dataset 3)

Figure 6

Figure 6 Example of Patchawarra Formation coal from Forge-1 coal seam gas exploration well

This core photograph shows the uniform black, earthy coal with silty interbeds from the Weena Trough.

Source: Strike Energy Limited (2010, p. 248, Appendix 5) © Strike Energy

Last updated:
5 January 2018