Two types of observed data were used to develop and build a geological model: a deep wells dataset (NSW Trade and Investment, Dataset 1) and geophysical datasets (Bioregional Assessment Programme, Dataset 2; AGL, Dataset 3; Geoscience Australia, Dataset 4). The deep wells dataset consists of stratigraphic and lithological data extracted from well completion reports (see listing of well completion reports within the list of references following Section 220.127.116.11). The geophysical dataset comprises the digital elevation model (DEM) and published interpretation of seismic reflection data. The 1:100,000 scale Dungog geological map (Roberts et al., 1991) has also been used as a guide to determine the geological limits and the major structural trends in the subregion (see Section 1.1.3 in companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)). The cross-sections associated with the geological maps are interpretative and thus not used in this model as we are producing a new interpretation of the hard datasets.
18.104.22.168.1 Deep wells dataset
The deep wells data (NSW Trade and Investment, Dataset 1) provide information about the rock types, stratigraphic units and geological structure of the Gloucester subregion from geophysical logs, core and cutting analyses and test data. In order to make the geological model in the timeframe of the project, a set of wells crossing all the geological layers with homogenous interpretation have been selected among the petroleum wells accessible in the public domain. Groundwater boreholes usually do not reach deep structures and are not used in this modelling step.
A first series of stratigraphic wells, named PGSD or Stratford 1 to 9, were drilled by Pacific Power in the Gloucester Gas Project from 1993 to 2001. From 2005 to 2008, Lucas Energy Pty Ltd. and Molopo Australia Ltd (until 2007) continued the exploration in the Gloucester Gas Project (wells APW01, LMG01 to 03, Stratford 4, 5A, 8, 9). They extended exploration in Petroleum Exploration Licence (PEL) 285 bloc: toward north of the Gloucester Gas Project (LMGW01, Waukivory 1), south of the Gloucester Gas Project (wells LMGC01, Craven 1 and 3a), the western part of the Gloucester subregion (Faulkland 1 and 3) and the southern limit of the Gloucester subregion (Weismantel 1 to 3). In 2008, AGL obtained the PEL 285 exploration licence, increased the depth of Stratford 7, drilled a new well in the prospect (Stratford 10), and extended the exploration zone toward the north (Waukivory 3 and 4) and in the Craven/Ward River central zone (Ward River 1 to 5, Craven 6).
Twenty-four recent deep wells (drilled in the last ten years and deeper than 650 m), with coherent, informative data (especially the geophysical logs) and the best spatial coverage were selected from publicly available reports (see Section 22.214.171.124.3). Data collection for the Gloucester subregion geological model was completed in November 2013. Table 3 shows a list of selected wells.
Table 3 List of selected wells for the Gloucester subregion geological model
aThe depths are measured depth (MD) relative to the kelly bushing reference height (KB). The coordinates are expressed in the coordinate system used in the well completion reports. The well abbreviations shown here are used to identify well locations in maps used later in this section.
Source: see the listing of well completion reports within the list of references following Section 126.96.36.199.
Well distribution varies within the Gloucester geological basin (inset (a) of Figure 6). The highest concentration is near the Gloucester Gas Project, with almost one-third of the wells concentrated within 7 km2, whereas the major part of the subregion is much less explored. The depths of these wells vary between 667.72 m MD (well Stratford 5A) and 1374.27 m measured depth (MD) relative to the kelly bushing (KB) (well Faulkland 1A). A kelly drive refers to a type of well drilling device on an oil or gas drilling rig. The kelly is the polygonal tubing and the kelly bushing is the mechanical device that turns the kelly when rotated by the rotary table. Together they are referred to as a kelly drive.
Figure 6 Deep well selection spatial distribution in the Gloucester subregion
See Table 3 for well name abbreviations.
No major lithological variations were recognised from one formation to another. Most of the formations were deposited in a fluvial to deltaic environment and the main rock types are sandstone, siltstone, mudstone and coal. Consequently, the highest level of facies variation is in a horizontal direction within each formation. The correlation of coal seams between wells remains ambiguous given the high probability of discontinuity/pinch out of seams or lateral splitting. Over-correlation of seams could likely lead to local over-interpretation of faults (Section 188.8.131.52.4), however, based on the available data, it is difficult to estimate the amount of stratigraphic induced error (i.e. coal splitting) and the number and location of real faults.
184.108.40.206.2 Geophysical dataset
The surface topography has been extracted from a DEM model described in Section 2.1.1 Geography of this product (Bioregional Assessment Programme, Dataset 2; Geoscience Australia, Dataset 4).
A review was conducted on Grieves and Saunders’ (2003) interpretation of three-dimensional seismic reflection data in the Gloucester Gas Project (AGL, Dataset 3). The interpretation consisted of reprocessed seismic data (acquired by Esso Australia’s Coal Exploration division in 1979 to 1983) with new borehole seam correlations (nine deep wells drilled by Pacific Power in 1993 to 1999). Grieves and Saunders (2003) highlighted a high degree of uncertainty in both structural mapping and the location and orientation of the mapped faults. This uncertainty is due to the sparse and incomplete seismic coverage and difficulties with the seismic interpretation. The interpretation by Grieves and Saunders (2003) was used to define a fault size population and constrain the structural interpretation of the Gloucester subregion (Section 220.127.116.11.4).
Product Finalisation date
- 2.1.1 Geography
- 2.1.2 Geology
- 2.1.3 Hydrogeology and groundwater quality
- 2.1.4 Surface water hydrology and water quality
- 2.1.5 Surface water – groundwater interactions
- 2.1.6 Water management for coal resource developments
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product