The Cooper subregion incorporates the Cooper–Eromanga Basin hydrocarbon system. Gas is the main resource, with some oil production. Gas and dry gas are predominantly in the Cooper Basin sequence, whereas oil is mainly in the sandstone reservoirs of the Eromanga Basin (Radke, 2009). The Cooper Basin contains approximately 190 gas fields and 115 oil fields currently in production. These fields contain approximately 820 producing gas wells and more than 400 producing oil wells which feed into production facilities at Moomba in SA and Ballera in Queensland (Santos, 2014). Gas and oil fields, as well as oil and gas exploration and production tenements, and associated infrastructure are shown in Figure 20.
Figure 20 Oil and gas fields in the Cooper subregion, with tenements and infrastructure
The Cooper Basin also hosts significant unconventional hydrocarbon resources, including deep coal seam gas. Assessment of in-place shale gas and oil resources undertaken by the United States Energy Information Administration in 2013 estimated that the Roseneath Shale–Epsilon Formation–Murteree Shale section of the Cooper–Eromanga system contains shale gas in-place resources of 325 trillion cubic feet (Tcf), of which 93 Tcf is classed as technically recoverable. The same assessment estimated that the oil in-place for this section is 29 billion barrels, of which 1.6 billion barrels is deemed technically recoverable (U.S. Energy Information Administration, 2013). Other agencies have highlighted the potential for the Cooper subregion to host significant resources of other unconventional energy commodities, such as tight gas and deep coal seam gas (Goldstein et al., 2012; Gravestock et al., 1998; GSQ, 2012). For example, GSQ (2012) suggested that the Roseneath Shale–Epsilon Formation–Murteree Shale section and the Patchawarra and Toolachee formations are prospective for shale and tight gas, the Poolowanna, Birkhead and Toolebuc formations are prospective for shale gas, and the Winton Formation may be prospective for coal seam gas. Geoscience Australia and BREE (2014) report total production of 6.4 Tcf (converted from 6926 PJ) and remaining resources of 1.5 Tcf (converted from 1693 PJ) for conventional gas resources from the Cooper-Eromanga Basin system. The remaining resource figure is the Demonstrated Resource under the McKelvey resource classification scheme (Appendix C of Geoscience Australia and BREE, 2014).
The Eromanga Basin is predominantly oil-producing with minor gas. In contrast, the Cooper Basin is gas-dominant with a considerable light liquid component. Production in the Cooper Basin has been from eight formations in a thick sequence containing several coal-bearing units and carbonaceous siltstones, predominantly from the Gidgealpa Group (Radke, 2009). Thick, laterally extensive coal seams have been intersected in the Patchawarra and Toolachee formations. The distribution and cumulative thickness of coal in the Patchawarra Formation is shown in Figure 21, and for the Toolachee Formation in Figure 22 .
Patchawarra coal seams average 2.1 m thick but can be 22 to 30 m, with 30% of seams exceeding 2 m thick (Alexander et al., 1998). The thickest laterally extensive coal seam in the Patchawarra Formation is known as the VC50 coal. It ranges in thickness from 13 to 23 m, and is thickest in the Nappamerri and Patchawarra troughs (Figure 21), with coal thickness in the Weena Trough also significant (Simon, 2000). Recent drilling in the Weena Trough has encountered cumulative coal thicknesses of up to 145 m, with the thickest individual seam being 35 m (Strike Energy Limited, 2014). Drilling in the Windorah Trough in Queensland encountered cumulative thicknesses of coal in the Patchawarra Formation of 15 to 20 m (Real Energy Corporation Limited, 2014). The Patchawarra Formation is sufficiently mature to generate gas from coal seams over much of the basin, and high gas readings are recorded when mature Patchawarra Formation coals are intersected in wells (Deighton et al., 2003). Given that depths to the coal-bearing Patchawarra Formation exceeds 1000 m across the subregion, there is no potential for coal to be mined in the subregion (Menpes et al., 2012).
Thick, laterally extensive coal seams are also characteristic of the Toolachee Formation (Figure 22). The average coal seam thickness in the Toolachee Formation is 4.3 m, but individual seams can reach 22 m. Forty-two percent of coal seams are thicker than 2 m (Alexander et al., 1998). Drilling in the Windorah Trough in Queensland encountered cumulative thicknesses of coal in the Toolachee Formation of 10 to 15 m (Real Energy Corporation Limited, 2014). The Toolachee coals are sufficiently mature for thermogenic gas generation in the Nappamerri and Arrabury troughs, and parts of the Patchawarra Trough, high mud gas readings have been recorded during drilling through mature Toolachee coals (Menpes et al., 2012).
Coal seams are also present in the Epsilon Formation with cumulative thickness up to 15 m (equivalent to 13% of the total formation thickness). Coal is also present in the Daralingie Formation with total cumulate thickness of up to 10 m (Sun and Camac, 2004).
The principal coal seam gas play in the Eromanga Basin is the Winton Formation, comprising up to 1200 m of non-marine shale and siltstone with minor coal layers. Individual coal seams are thin (1 to 2 m) and not laterally extensive. The Winton Formation coals are not sufficiently mature to generate thermogenic gas (Goldstein et al., 2012).
Figure 21 Patchawarra Formation coal distribution and cumulative thickness
Data: Draper (2002b); Sun and Camac (2004)
Discontinuities at state boundaries are a result of using datasets compiled at different times by state agencies.
Figure 22 Distribution and cumulative thickness of Toolachee Formation coal in the Cooper subregion
Data: Draper (2002b); Sun and Camac (2004)
Discontinuities at state boundaries are a result of using datasets compiled at different times by state agencies.