Gunnedah Basin

The area of the Gunnedah Basin within the Gwydir subregion includes the northernmost extent of the Mullaley sub-basin, in particular, the Bellata Trough which is situated in the south of the subregion against the western flank of the Boggabri Ridge (refer to Figure 17 of companion product 1.1.3 for the Gwydir subregion (Welsh et al., 2014)). The major coal deposits of the Gunnedah Basin (which occur south of the Gwydir subregion) are hosted within the Maules Creek Formation and the Black Jack Group (Tadros, 1993, 1995) (refer to Figure 19 of companion product 1.1.3 for the Gwydir subregion (Welsh et al., 2014)). However, as little exploration has been carried out in this area, little is known about coal distribution in these formations within the Bellata Trough. Although coal occurs in the Maules Creek Formation and the Hoskissons Coal of the Black Jack Group within the Gunnedah Basin, around Narrabri, south of the Gwydir subregion (Tadros et al., 1986, p. 16), it is not clear whether the Hoskissons Coal is present in the two Gwydir coal boreholes drilled as coal seams intersected are not specified in reports available (NSW DTI, 2014b).

Gurba et al. (2009, p. 96) reported that the Bellata Trough is largely unexplored with only two wells drilled, and most of the Black Jack Group and Watermark and Porcupine formations have been eroded from the sequence in this area (Othman and Ward, 2002, p. 155; Othman, 2003). Vitrinite reflectance (which is a measure of thermal maturity and capacity to produce oil or gas) data from one of these wells show a steady increase with increasing depth (Othman and Ward, 2002, p. 155; Othman, 2003), with coals of high volatile bituminous rank and reflectance values between 0.55 and 0.69% (Gurba et al., 2009, p. 96). The production of thermogenic coal seam gas begins to evolve at a coal rank of 0.5 to 0.6% vitrinite reflectance (Clayton, 1998). However, biogenic gas, derived from microbes, may be present below this rank and mixtures of biogenic and thermogenic gas will exist during the transition from one gas type to the other (Moore, 2012). In the area north and north-east of the Bellata Trough a number of petroleum wells have been drilled. Studies by Sherwood et al. (1995) and Othman and Ward (2002) using data from these wells show that the vitrinite reflectance values for the Surat-Bowen sequence in northern NSW range from 0.3 to 5.5%. The coals with values above 0.85% are reported to represent rocks affected by igneous intrusions, which has elevated their thermal maturity and increased their potential for producing gas. Information on the distribution and properties of coal resources is very limited.

Last updated:
8 October 2018