1.2.1 Available coal and coal seam gas resources

Summary

Significant deposits of black coal were first identified in the geological Galilee Basin in the 1970s, although no commercial mines have yet been established due to the basin’s relative remoteness and absence of mine-enabling infrastructure. However, a widespread exploration boom since the mid-2000s has helped transform the understanding of the basin’s coal and coal seam gas (CSG) resources, and there are currently six high-value greenfield mining ventures slated for initial development prior to 2020. These large scale open-cut and underground mining operations propose to target the laterally extensive coal seams hosted in the Late Permian aged strata of the Betts Creek beds (and their stratigraphic equivalents the Colinlea Sandstone and Bandanna Formation in the central-eastern part of the Galilee Basin).

The Betts Creek beds (and equivalents) are a mixed sedimentary sequence predominantly comprising sandstone, conglomerate, shale and coal. The units occur across most of the Galilee Basin, although it is only near the eastern basin margin that the coal seams are close enough to surface to permit economic extraction. There are six main coal seams recognised in the formation (seams A through F), and in places such as the Koburra Trough these occur as well-defined seams of uniform thickness that dip regionally to the west at low angles (1 to 5°). Most of the basin’s Late Permian coals (originally deposited from 262 to 252 million years ago) are low rank (sub-bituminous) with low sulfur and relatively high ash content. Moisture levels are typically 7 to 9%. They are non-coking coals, suitable mainly for thermal uses such as power generation.

Coal also occurs in Early Permian strata in the Galilee Basin, particularly the Aramac Coal Measures. The Aramac Coal Measures are not as spatially extensive as the Late Permian coal‑bearing formations and are mainly restricted to the central and western parts of the basin, such as the Lovelle Depression and Aramac Trough. The Aramac Coal Measures typically comprise about 10% coal, with the remainder of the unit consisting of interbedded sandstone, siltstone and shale. Coals in the Aramac Coal Measures are high volatile bituminous coals that are typically more thermally mature than those in the Betts Creek beds. The Aramac Coal Measures are buried too deeply throughout the basin for them to be mined economically, although they are a secondary CSG target.

Within the Galilee subregion, large tonnages of low rank coal (ranging from lignite to sub‑bituminous) occur in Cretaceous strata of the Eromanga Basin. These coals are about 150 million years younger than those in the Late Permian formations, and are mainly concentrated in southern parts of the subregion, south of Blackall. There are several major deposits with over 1 billion tonnes of identified coal resources buried at relatively shallow depths, although the much higher moisture levels of these coals mean that they are less profitable as an export product.

Exploration for CSG resources in the Galilee Basin has mainly focused on the Betts Creek beds, although the deeper Aramac Coal Measures have also been intersected in many CSG exploration wells. Gas sourced from both of these coal-bearing units has similar properties, and is predominantly methane with gas content typically less than 5 m³/t (dry ash free, DAF). Most coals in the Galilee Basin have gas saturation levels of around 30% (undersaturated), although there is a considerable range (5 to 75%). Isotopic evidence suggests that most CSG in the Betts Creek beds and Aramac Coal Measures is of biogenic origin, and derived mainly from bacterial reduction of carbon dioxide.