Coal seam gas

Numerous conventional and unconventional hydrocarbon occurrences have been reported in the Gloucester Basin, with the marine influenced Durallie Road and Speldon formations (Figure 3 and Figure 4) reported to contain some oil occurrences (Hughes, 1995, p. 424). Gaseous hydrocarbons, mainly methane (CH4) with traces of carbon dioxide (CO2) and nitrogen (N2), have been identified, encouraging more recent exploration efforts by coal seam gas (CSG) operators.

The top 200 m of the Avon Subgroup and the basal 250 m of the Craven Subgroup are considered the best-known CSG resources in the Gloucester Basin (Gurba and Weber, 2001), with gas contents of less than 10 m3/t (dry, ash-free) for coals shallower than 200 m and 20 to 25 m3/t at around 600 m depth. The CSG comprises 95 to 100% CH4 with minor amounts of CO2 and N2 (Gurba and Weber (2001) as cited in Ward and Kelly (2013, p. 57)).

Gas exploration on Petroleum Exploration Licence tenement 285 (PEL 285), which was conducted by Lucas Energy Pty Limited in 2004, investigated 200 km2 of coal-bearing strata, targeting 11 major coal seams of greater than 2.5 m thick and an average total thickness of 30 to 60 m. Dry, ash-free gas contents for this investigation reported by Bilston (2008, p. 7) range between 12 and 25 m3/t; although Ward and Kelly (2013, p. 57) suggest that in situ gas contents will be somewhat lower as the coals are relatively high in ash. Initial reserves of 14.9 billion cubic feet (bcf) (1P, proved reserves), 170.2 bcf (2P, proved plus probable) and 359.2 bcf (3P, proved plus probable plus possible), with a contingent resource of 166.2 bcf, were reported by Bilston (2008, p. 7).

SRK Consulting (2010, p. 21) reported results from coal seam intrinsic permeability testing, conducted by Pacific Power in 1999, noting that permeability decreases sharply with increasing depth in the basin. Permeabilities at approximately 100 m depth are on average 100 millidarcy and drop significantly to between 7 and 27 millidarcy at 300 m depth, with a value of 0.56 millidarcy measured at 500 m. Porosity provides information about storage capacity of gas in the coal whereas permeability defines the transportability of that gas (Moore, 2012).

AGL Energy Limited recently reported 2P reserves of 454 PJ of gas for the Gloucester Basin CSG Project (PEL 285) (AGL Energy Limited, 2013). Gas samples from shallow monitoring wells were collected in 2011 for AGL’s Stage 1 Gas Field Development Area (see Figure 9, Section 1.2.4) and reported by Parsons Brinkerhoff (2012, p. 122). The gas at shallow depths consists of low concentrations of oxygen (O2), argon (Ar), nitrogen (N2), which are typical of air, and only slightly elevated CO2 concentrations. Methane concentrations range from 16 to 138 ppm (Parsons Brinckerhoff, 2012, p. 122).

Last updated:
5 January 2018