2.1.2 Geology


The Galilee subregion covers an area of about 248,000 km2 in central Queensland, and is defined by the known extent of the geological Galilee Basin which hosts important coal-bearing sequences that are in current demand for coal mining, coal seam gas production, and continued petroleum exploration. These coal seams are the focus of some 13 coal and 3 coal seam gas (CSG) projects that are targeting resources in the upper Permian coal measures. The Galilee Basin sequence is only exposed along its eastern margin in a tract of about 600 km long and up to 80 km wide, where the outcropping and near-surface sub-bituminous coal resources are present.

The geology of the Galilee subregion can stratigraphically be subdivided into three geological sequences, which from oldest to youngest are: Galilee Basin, Eromanga Basin, Cenozoic sediments and volcanics. Each sequence comprises a number of stratigraphic units.

The Galilee Basin, accumulated over a period from 323 to 238 million years ago (Upper Carboniferous to Middle Triassic), is largely buried by the Early Jurassic-Late Cretaceous Eromanga Basin and scattered Cenozoic deposits. The Galilee Basin surrounds three sides of the Maneroo Platform, which is comprised of rocks from part of geological basement for the Galilee subregion. The Galilee Basin has four main depocentres: the Koburra Trough, and Aramac, Lovelle and Powell depressions. Maximum thickness of the sequence is about 2800 m in the Koburra Trough.

The overlying Eromanga Basin sequence accumulated over the period 197 to 95 million years ago, from Early Jurassic to Late Cretaceous. Deposition initially took place in river, lake and swamp environments to produce an alternating series of sandstone aquifers and mudstone-siltstone aquitards. A marine incursion in the Early Cretaceous terminated this cyclicity with a very thick sequence of fine-grained marine sediments. With retreat of the sea, uplift and major erosion to the east, another very thick clastic sequence accumulated in estuarine and then continental conditions.

During the subsequent 60 million years to the present in the Cenozoic, subtle regional deformation created scattered small basins as well as extensive sheet-wash gravels from erosion of the uplifting Great Dividing Range.

A review of geological structures is included in this section to assist in understanding and interpreting features that are apparent in the geological model. Much of the existing information focuses on regional structures, there is much less information available on distribution and occurrence of smaller-scale (semi-regional to local) structures in the Galilee subregion.

Significant fault displacements in the Galilee-Eromanga sequence are present along the subregion margin against the Maneroo Platform, the Holberton-Cork-Wetherby structures in the Lovelle Depression and the Canaway Fault in the Powell Depression. These structures are in or adjoin deeply buried depocentres where CSG may be of interest, but do not impinge on proposed coal mining areas on the eastern basin margin.

There is considerable variation in the quality and distribution of publicly available drillhole data across the Galilee subregion. Approximately 8% of all bores located in the Galilee subregion lacked drillhole depth information. Furthermore, most bores did not have stratigraphic data that could be used for modelling purposes.

From available data, CSG contents for the upper Permian coal measures tend to peak at their highest levels between 900 and 1200 m depth. CSG contents appear to drop off below 1300 m. Available gas isotope data suggest that CSG down to around 1400 m depth is primarily derived through biogenic (microbial) processes.

In the upper Permian coal measures, high average gas contents tend to occur in the central parts of the northern Galilee Basin, in the vicinity of CSG project areas. A zone of higher gas contents forms a north-east trending CSG fairway. Several faults in upper Permian coal measures also trend in the same direction as the CSG fairway, which suggests some structural control (faults) are influencing the distribution of CSG. It is likely that structures that affect the distribution of CSG may also influence potential groundwater flow in the upper Permian coal measures.

The geological model developed for the bioregional assessment (BA) for the Galilee subregion consists of a series of upper surfaces of stratigraphic sequences (presented as structure contours) and thickness (isopach) maps for each stratigraphic sequence. These were compiled to form a composite geological model of stratigraphic units in the Eromanga and Galilee basins. For Eromanga Basin stratigraphic units, the model layers were derived from the Hydrogeological atlas of the Great Artesian Basin (GAB Atlas; Ransley et al., 2015). For the Galilee Basin, geological model layers were developed as part of the BA for the Galilee subregion. Galilee Basin model layers define the upper surface of each of the units which are, in descending order: Moolayember Formation, Clematis Group, Rewan Group, upper Permian coal measures, Joe Joe Group and base of the Galilee Basin. These units represent a significant update on previous sub-surface regional geological mapping for the Galilee Basin.

The variable extent and thickness of individual stratigraphic units within the combined Galilee-Eromanga basin sequence indicate potential interconnectivity of aquifers between these two basins along the north-eastern margin of the Maneroo Platform, in the deeper south-westward extent in the Lovelle Depression, and in parts of the southern Galilee Basin.

The various Galilee Basin units are not evenly distributed across the basin. The Joe Joe Group shows considerable variation in thickness, which is consistent with it being deposited in areas of active subsidence (e.g. the Koburra Trough) during the early formative phases of the Galilee Basin. The upper Permian coal measures blanket much of the Joe Joe Group and are generally less than 160 m in thickness. The thickest parts of the coal measures coincide with the areas of interest of the most advanced proposals for coal exploitation along the eastern margin of the Galilee subregion. Where the overlying Rewan Group is present, its average thickness is 148 m. Erosion of the overlying Rewan Group, Clematis Group and Moolayember Formation has taken place in areas such as Barcaldine Ridge and around the western margin of the Galilee subregion. This has enabled upper Permian coal measures and Clematis Group to be in direct contact in places with the overlying Hutton Sandstone of the Eromanga Basin. These erosional windows may provide potential pathways for hydraulic connectivity between Galilee and Eromanga basins.

Further work may include: improved quality assurance/quality control of drillhole databases and, where possible, attainment of missing drillhole stratigraphic data; refinements to the regional stratigraphic correlations for upper Permian coal measures; and access to coal company drillhole data and geological mapping. Collectively this work would help to refine the conceptual geological model in particular in the vicinity of coal resource development proposals, identify finer-scaled structures in the Galilee subregion and improve the resolution of extent and thickness of stratigraphic units across the Galilee Basin. More detailed assessments of available two-dimensional seismic reflection and well log data may improve understanding of the distribution and variations in lithology and porosity for different stratigraphic units as well as provide further detail on the nature and distribution of geological structures in the subregion. This information could be incorporated into future iterations of the geological model, which would in turn improve the understanding of hydrogeology of the subregion.

Last updated:
6 December 2018
Thumbnail of the Galilee subregion

Product Finalisation date