This section presents information about coal seam gas (CSG) in the geological Sydney Basin (referred to throughout this section as the Sydney Basin), the Permian coal measures and the Southern and Western coalfields. The NSW Government has introduced CSG exclusion zones around existing and future residential areas within the bioregion (see Figure 17 in companion product 1.1 for the Sydney Basin bioregion (Herron et al., 2018)), which prohibit CSG development within a 2 km radius of these residential areas. Thus, many areas of identified coal resources cannot be developed.
1.2.1.2.1 Coal seam gas in the Sydney Basin
The coals of the Sydney Basin are considered favourable for the development of methane (O’Neill and Danis, 2013, p. 23). Methane is the dominant gas produced within the coals of the Sydney Basin, although carbon dioxide, nitrogen and other high molecular weight hydrocarbons such as ethane are present in smaller quantities (Faiz et al., 2007; Pinetown, 2013; Ward and Kelly, 2013). Sporadic occurrences of elevated concentrations of carbon dioxide and ethane have also been reported (Faiz et al., 2007; Pinetown, 2013; Ward and Kelly, 2013). The varying gaseous compositions are related to the combination of sources (thermogenic, biogenic and magmatic) contributing to the overall coal seam gas content (Scott and Hamilton, 2006; Faiz et al., 2007; Pinetown, 2013; Ward and Kelly, 2013).
Natural gas flow within the Sydney Basin was discovered as early as the 1800s in the Clyde Coal Measures and the Illawarra Coal Measures (O’Neill and Danis, 2013, p. 23). The dry and ash-free gas content of the Sydney Basin coals span a wide range from less than 1 to 21 m3/t (Scott and Hamilton, 2006; Faiz et al., 2007; Pinetown, 2013; Ward and Kelly, 2013). In most occurrences, the gas content of the coals increases with depth (Ward and Kelly, 2013). The vitrinite reflectance data of the coals in the Sydney Basin range from 0.7% in the north of the geological basin to 1.3% in the (north of the) Southern Coalfield (Ward and Kelly, 2013). The vitrinite reflectance data indicates the hydrocarbon (gas) production phase reached by the coals of interest; the values indicated here indicate the catagenesis stage (Figure 6). The permeability of the coals is thought to be low in general due to the mineralisation of cleats and the decrease of permeability with depth (Pinetown, 2013; Ward and Kelly, 2013). The maximum permeability of the Sydney Basin coals is less than 5 mD (Ward and Kelly, 2013). Much of the basin’s coal has reached thermogenic maturation and is producing gas (Scott and Hamilton, 2006; Ward and Kelly, 2013).
Thomson et al. (2014) undertook a study on the distribution of CSG within the northern Sydney Basin (Hunter subregion) and defined four zones. These zones are based on the dominant gas content (Thomson et al., 2014). Similar investigations have not been carried out for the southern portion of the basin (Figure 3).
Many reports on CSG distribution and characteristics are available in the DIGS (Digital Imaging of Geological Systems) website (NSW Trade and Investment, 2015b).
1.2.1.2.2 Permian coal measures
Permian coal deposits are numerous across the Sydney Basin (O’Neill and Danis, 2013, p. 23). These coal deposits have been identified as good potential reservoirs (O’Neill and Danis, 2013, p. 24). Furthermore, the total organic carbon (TOC) values and the analysis have indicated that the Illawarra Coal Measures are good quality potential gas source rocks (Cadman et al., 1998; O’Neill and Danis, 2013, p. 24). Exploration drilling has identified oil and gas shows throughout the basin, though gas is more common onshore (O’Neill and Danis, 2013, p. 24).
The lower Permian source rocks (Clyde Coal Measures) are thought to have reached hydrocarbon maturation (as per the CSG generation window; Figure 6) during the Triassic, reaching peak maturity in the late Jurassic (O’Neill and Danis, 2013, p. 24). The upper Permian source rocks (Illawarra Coal Measures) reached the initial hydrocarbon production window during the late Jurassic (O’Neill and Danis, 2013, p. 24). The Permian Illawarra Coal Measures of the central and eastern Sydney Basin are classed as being overmature and within the dry gas window, based on vitrinite reflectance data (Cadman et al., 1998; O’Neill and Danis, 2013, p. 24). The vitrinite reflectance data also point to a mature status (oil window) within the western portion of the basin, whereas both the southern and northern regions of the basin are classed as immature to mature (no production of hydrocarbons to production of oil) (Cadman et al., 1998; O’Neill and Danis, 2013, p. 24).
Figure 6 Gas generation window for coal seam gas
Coal seam gas is generally extracted down to about 1 km.
Source: after Thomson et al. (2014). Observations on the distribution of coal seam gas in the Sydney Basin and the development of a predictive model. Thomson S, Thomson D and Flood P. Australian Journal of Earth Sciences. Copyright © Geological Society of Australia, reprinted by permission of Taylor & Francis Ltd, www.tandfonline.com on behalf of Geological Society of Australia.
1.2.1.2.3 Southern Coalfield
The production of CSG from the Illawarra Coal Measures is currently taking place in the Camden-Campbelltown region as part of the AGL Energy Limited (AGL) operated Camden Gas Project in the Southern Coalfield (Sydney Catchment Authority, 2012, p. 3; O’Neill and Danis, 2013, p. 23). This is the Sydney Basin’s only CSG production site (Sydney Catchment Authority, 2012, p. 8). The Camden Gas Project is located 64 km south-west of Sydney (Mine Subsidence Engineering Consultants, 2007, p. 5). Gas has been produced from the Camden Gas Project since 2001 (Ward and Kelly, 2013). There are currently 95 gas production wells in operation and several non-producing and sealed wells (AGL Energy Ltd, 2015). However, there have been some further CSG exploration activities within the basin (Sydney Catchment Authority, 2012, p. 4).
Production of CSG at the Camden Gas Project comes from the Illawarra Coal Measures (Sydney Catchment Authority, 2012, p. 4). The Illawarra Coal Measures are older and less permeable than the currently highest gas-producing coals in Australia, the Walloon Coal Measures of the Surat Basin (Sydney Catchment Authority, 2012, p. 4). The Illawarra Coal Measures are also considered to be at a more mature stage of hydrocarbon development (Sydney Catchment Authority, 2012, p. 4). The Illawarra Coal Measures are approximately 200 m thick in the Camden Gas Project region (Camden-Campbelltown) (Sydney Catchment Authority, 2012, p. 42). The Camden Gas Project produces 6% of NSW domestic gas requirements (Sydney Catchment Authority, 2012, p. 4).
The Southern Coalfield is considered to have a high potential for CSG resources with the main phase of thermal gas generation having been reached (Scott and Hamilton, 2006; Ward and Kelly, 2013). Scott and Hamilton (2006) conducted an in-depth analysis of suitable CSG locations across the Sydney and Gunnedah basins. Of the suitable sites presented in the analysis, only two sites fall within the Sydney Basin bioregion. These are a north-south trending belt on the western side of the Cumberland Plain (along the Nepean River and Hawkesbury River valleys) and an area of the Illawarra escarpment which includes a number of Sydney’s water supply catchments and storages. The late Permian coal has a maximum net thickness of 10 m at the first of these sites and 25 m at the other, and the coal has reached the thermogenic gas generation window, with the possibility for biogenic maturation (Scott and Hamilton, 2006). Both sites have a high gas content, though carbon dioxide is present and permeability of the host rocks can be variable (Scott and Hamilton, 2006; Ward and Kelly, 2013). In the Southern Coalfield underground mining operations use underground CSG extraction techniques to reduce the risks associated with mining the gaseous coals (Hanes et al., 2009). Much of the extracted gas is used for power production at the Appin and the former Tower mine sites (Ward and Kelly, 2013, p. 40).
1.2.1.2.4 Western Coalfield
Scott and Hamilton (2006) also identified some prospective CSG regions bordering, but not contained within, the Western Coalfield (Ward and Kelly, 2013). The geological conditions of the Western Coalfield are not considered to be favourable for CSG development, as these coals are considered to be thermally immature.
Nevertheless, there may be potential for some CSG extraction in the Bylong Coal Project within the northern portion of the Western Coalfield (within the Hunter subregion) in coal intervals that are not mined for coal (Hansen Bailey, 2015, p. 31), although this is generally limited. Gas can be extracted in areas that have not been affected by igneous intrusions and where the gas contents range from 0.56 to 2.34 m3/t (Hansen Bailey, 2015, p. 31). This level of gas content is considered unusually high for the Western Coalfield; investigations by Thomson et al. (2014) found that the gas contents of the region were generally low at around 0.7 m3/t at a depth of 60 m. The gas is also mainly composed of carbon dioxide (Thomson et al., 2014). There are currently no producing CSG wells (NSW Trade and Investment, 2015a) and minimal CSG exploration in the Western Coalfield.
