2.1.2 Geology


A regional scale geological model for the Hunter subregion has been built. This model represents the Carboniferous to Triassic strata of the northern part of the Sydney geological basin.

The geological model is an interpretation of the subsurface geometry, composition and structure of part of the Sydney Basin. The interpretation is based on petroleum exploration wells, geological maps and geophysical datasets and takes into account their inherent uncertainties and resolution. The well data provide critical point source information to constrain the depth of the main stratigraphic horizons, and understand the main rock types and sedimentary facies. The uncertainty surrounding these data increases with the distance from the wells. Geological maps and geophysical data were used to constrain the modelled interpolation of the point source well data.

Isopach maps (i.e. stratigraphic thickness maps) were built and used to define the basin scale architecture. Each isopach map was calibrated against well picks at the formation scale and constrained by trends observed within each interval. Definition of a reference regional horizon and the stacking of the successive isopach maps resulted in an initial non-eroded geological model. Major folds and fault trends were reviewed and the model was corrected based on the main anticlinal and synclinal axes, as this reduced correlation errors between the initial geological model and the formation tops in wells. The geological model was then eroded to conform with the present-day topographic surface and each stratigraphic unit populated with stochastic facies calibrated from well data and regional stratigraphic columns.

While there is considerable opportunity to improve upon the Hunter regional scale geological model through making use of more of the available data, the current version is fit for the purposes of the regional-scale groundwater modelling. The groundwater model is used to simulate hydrological changes across a wide range of parameter values to reflect regional differences and account for the uncertainty in the geological model. The groundwater model emulators provide a means of incorporating better local-scale information, where it is available, to constrain the results from the regional groundwater model (see companion product 2.6.2 (groundwater numerical modelling) for the Hunter subregion (Herron et al., 2018)).

This section outlines the rationale and describes the methods for building a geological model of the Hunter subregion. For bioregional assessment (BA) purposes, a three-dimensional geological model of the Hunter subregion was needed to define regional-scale geological architecture for modelling the impacts of coal mining on groundwater, particularly in terms of connectivity to the surface water system and to the subregion’s water-dependent assets. The Hunter subregion covers a specific geological area of the Sydney Basin, including Newcastle Coalfield, Hunter Coalfield and parts of the Western Coalfield as well as minor parts of the Gunnedah and Werrie geological basins. Additional documentation about the geology of the Hunter subregion is summarised in companion product 1.1 for the Hunter subregion (McVicar et al., 2015), particularly information relevant to the development of a regional geological model.

A number of coal seam gas (CSG) models have been developed for this region. Many focus on CSG distribution and quality in the individual coalfields (e.g. Creech, 1994; Faiz et al., 2003; Faiz et al., 2007; Burra and Esterle, 2012; Pinetown, 2010, 2014; Thomson et al., 2008), although some are at the scale of the Sydney Basin (e.g. Burra et al., 2015). These models provide details of CSG maturity and geochemistry but do not provide a regional geological interpretation at the scale of the Hunter subregion.

Other existing models do focus on the geological framework of the Sydney Basin, but were developed for understanding the geological basement structure and have limited resolution of the Permo-Triassic stratigraphy. Coal measures occur within the Permian age units, which when not outcropping at the surface lie below Triassic units. It is the Permo-Triassic stratigraphy then that is of most relevance for a geological model to underpin the Hunter bioregional assessment. It is the basement structure and main structural trends that have been particularly well investigated in the pre-existing geological models – for example: in a study that integrated multidisciplinary datasets, such as deep reflection seismic, gravity and magnetic datasets (Blevin et al., 2007); in a synthesis of the Sydney Basin deep, regional structure by SRK Consulting (Woodfull et al., 2004); more recently, a geological model was built to refine the gravity interpretation (Danis et al., 2011); and NSW Trade and Investment have undertaken a study based on Chemical Abrasion Isotope Dilution Thermal Ionisation Mass Spectrometry (CA-IDTIMS) tuff dating and stratigraphic correlation at the scale of the geological Sydney-Gunnedah Basin (Oliveira et al., 2014). This last study provides a regional-scale interpretation of a few stratigraphic intervals within the sedimentary pile such as the Permo-Triassic limit, the upper coal measures interval, the Greta Coal Measures and the top of the Dalwood Group.

Following a review of existing geological datasets and models available, it was concluded that a BA-specific geological model for the Hunter subregion could provide better representation of the Permo-Triassic stratigraphy and provide an independent review and interpretation of the hard datasets (i.e. original datasets, with the minimum of interpretation) in the time frame of the Assessment (June 2014 to February 2015). Quaternary alluvium is not represented in the geological model, but is incorporated into the groundwater model where it is needed to represent alluvial aquifers and connections between groundwater and surface water.

Details of the observed datasets are provided in Section Note that the Hunter geological model is based on publicly available datasets. The methods used to generate derivative datasets and their use in producing the three-dimensional geological model are described in Section

Last updated:
18 January 2019
Thumbnail of the Hunter subregion

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