1.2.1 Available coal and coal seam gas resources

Summary

The coal and coal seam gas (CSG) resources of the Gwydir subregion have not been extensively explored, thus limited published information is available on the distribution and properties of the subregion’s resources. Currently no coal mining is taking place in the subregion, although coal seams are contained within the Bowen, Gunnedah and Surat geological basins in the subregion. Little is known about the coals hosted within the Kianga Formation of the Bowen Basin in this area. The main structural unit of the Gunnedah Basin within the subregion is the Bellata Trough, which hosts coal deposits of the Maules Creek Formation and the Black Jack Group. The limited drilling data available shows that Gunnedah Basin coals are of high volatile bituminous rank with vitrinite reflectance values between 0.55 and 0.69%. The production of thermogenic gas begins to evolve at a coal rank of 0.5 to 0.6% vitrinite reflectance. However, biogenic gas, derived from naturally occurring microbes, may be present below this rank. In the Surat Basin coals are present within the Bungil and Orallo formations, as well as the Walloon Coal Measures. Although the Walloon Coal Measures occur over a large area, their thickness, depth and lateral extent is not precisely known.

In the Gunnedah-Bowen Basin in the Gwydir subregion the Hoskissons Coal of the Late Permian Black Jack Group, the Early Permian Maules Creek Formation of the Bellata Group, and the Middle Triassic Moolayember Formation have been targeted to assess their CSG potential. The Hoskissons Coal was found to be less than 3 m in thickness with a gas content of 5.9 m³/t (composition more than 90% methane). The coals of the Bohena Seam of the Maules Creek Formation are thick (14.8 m) with a high gas content (14.2 m³/t), but a very high carbon dioxide content (more than 80%). Both coal seams have low reservoir permeability. Exploration of the Moolayember Formation indicated net coal seam thickness ranging from 3.3 to 8.5 m, low reservoir permeability, and a mean gas content of more than 4 m³/t (gas composition not reported). In the Surat Basin some exploration has occurred in the Middle Jurassic Walloon Coal Measures and the Early Cretaceous Bungil Formation of the Blythesdale Group. The Walloon Coal Measures in the Gwydir subregion are much thinner than further north in southern Queensland and commercial CSG production does not appear viable. The Bungil Formation was found to have undersaturated coals with mean gas contents of approximately 4 m³/t (methane content greater than 93%), net seam thickness of up to 10 m, and low reservoir permeability. The low reservoir permeability indicates that these coals are not likely to produce CSG at economic rates without hydraulic stimulation of vertical wells or drilling of horizontal wells with multiple fractures placed along the well, a technology that is yet to be proven in Australia.