1.1.3.4 Coal and hydrocarbons

1.1.3.4.1 Coal

Economic coal occurrences in the Clarence-Moreton Basin have been reported primarily from the Walloon Coal Measures, where large near-surface deposits of thermal coal areas are widespread. Many historic records exist from the Clarence-Moreton Basin with mining of the Walloon Coal Measures in both open pit and underground operations since the 1870s (e.g. Matheson, 1993). However, at present there are only two operating coal mines in the Clarence-Moreton Basin (Jeebropilly and New Oakleigh coal mines), and these occur west of Ipswich (approximately 60 km south-west of Brisbane). The coal of the Walloon Coal Measures in the Clarence-Moreton Basin has been described as slightly higher in rank than the coals of the Surat Basin,and it is highly volatile and sub-bituminous (Matheson, 1993; Ingram and Robinson, 1996). The maceral composition of the coal is dominated by vitrinite, whereas the disseminated organic material is composed of vitrinite, and lesser liptinite and inertinite (Ingram and Robinson, 1996). The thickness of coal within the Walloon Coal Measures decreases south of the Casino and Lismore troughs (Doig and Stanmore, 2012).

Coal is also present in other formations in the Clarence-Moreton Basin. For example, the coal of the Koukandowie Formation is compositionally similar to the Walloon Coal Measures. Coal also present in the Triassic sedimentary sequences of the Ipswich and Nymboida Coal Measures (Ingram and Robinson, 1996; Chern, 2004). Chern (2004) suggested that the Ipswich Coal Measures locally contain thick thermal coal with a moderate to high ash content, but that their lateral extent is discontinuous. Coal mining operations of the Nymboida Coal Measures have in the past been restricted to small areas near Nymboida (Ingram and Robinson, 1996) in the south-western Clarence-Moreton Basin in New South Wales.

Depth below surface

The Walloon Coal Measures form extensive areas of outcrop throughout the Laidley sub-basin in Queensland and along the periphery of the Logan sub-basin in New South Wales. They are covered by several hundred metres of volcanic rock (Main Range Volcanics and Lamington Volcanics) in the western Laidley and Logan sub-basins. In the centre of the Logan sub-basin in New South Wales, the Walloon Coal Measures are covered by the thick sedimentary sequences of Orara Formation and the Grafton Formation, and therefore mostly occur at depths of about 450 to 650 m. The Ipswich Coal Measures outcrop along the eastern part of the Ipswich Basin near Ipswich, and are buried underneath the thick sedimentary sequences of the Bundamba Group and Walloon Coal Measures throughout the remainder of the Laidley sub-basin in Queensland. The Nymboida Coal Measures outcrop at the south-western margin of the Clarence-Moreton Basin in New South Wales. Throughout the rest of the Clarence-Moreton Basin in New South Wales, the Late Triassic coal measures underlie rock of the Bundamba Group, possibly as restricted sediment packages within palaeovalleys as suggested by seismic interpretations (Ties et al., 1985b; Willis, 1994).

1.1.3.4.2 Conventional and unconventional hydrocarbon

The Clarence-Moreton Basin is considered under-explored for hydrocarbons (Ingram and Robinson, 1996; Doig and Stanmore, 2012). Nevertheless, hydrocarbon shows in the Clarence-Moreton Basin date to 1902, when flows of methane were reported from the Grafton-1 well in New South Wales. Since then, numerous oil and gas shows have been reported in the Clarence-Moreton Basin.

O’Brien et al. (1994b) indicated that the sedimentary sequences and especially the Walloon Coal Measures and Koukandowie Formation of the Clarence-Moreton Basin contain abundant oil prone organic material. In addition, Ingram and Robinson (1996) suggested that the Raceview Formation and Ipswich and Nymboida Coal Measures also have source rock potential.

Ingram and Robinson (1996) identified two primary reservoir targets in the Clarence-Moreton Basin in New South Wales, the Ripley Road Sandstone and the Heifer Creek Member. The Ripley Road Sandstone is the most widespread quartzose sandstone-dominated unit within the basin. In addition to the primary targets, Ingram and Robinson (1996) also identified five secondary reservoir targets, which include the Ipswich and Nymboida Coal Measures, Raceview Formation, Gatton Sandstone and the Walloon Coal Measures. Source rock potential has also been attributed to the Kangaroo Creek Sandstone (or Kangaroo Creek Sandstone Member, according to the proposed change in stratigraphy) (Arrow Energy, 2005).

Four different types of traps have been identified in the Clarence-Moreton Basin (Ties et al., 1985b; O’Brien and Wells, 1994; Ingram and Robinson, 1996): drape over sub-Clarence-Moreton Basin topography, reservoir pinch-outs against sub-Clarence-Moreton Basin topography, hanging wall anticlines on minor thrusts and stratigraphic traps.

Examples of these traps include reservoir pinch-outs of the Ripley Road Sandstone and the Raceview Formation near the flanks of base-Clarence-Moreton Basin features such as the Central Platform, the South Moreton Anticline and at the basin margin (O’Brien et al., 1994b). Other major structural traps are high angle reverse fault traps associated with Late Cretaceous to Early Cenozoic deformation. Stratigraphic traps are widespread throughout the Clarence-Moreton Basin, represented primarily by the fine-grained floodplain deposits that occur within most units (Ingram and Robinson, 1996).

Ingram and Robinson (1996) have identified nine primary play areas in NSW where geological conditions are particularly favourable for the potential accumulation of hydrocarbons, including the Casino Trough, South Moreton Anticline, Grafton Trough and the south-western basin margin.

The Clarence-Moreton Basin (as well as underlying infrabasins) formed over basement rocks that are intensively intruded by granite (Sommacal et al., 2008). These granitic rocks may have potential as a geothermal energy source, and may also have influenced the maturity of overlying coal measures. In addition, Cenozoic intrusive and extrusive igneous activity could have been important locally for generating hydrocarbons, although it is possible that this Miocene magmatic activity may have been a general heating event in the Clarence-Moreton Basin, which could have affected hydrocarbon generation on a regional scale (Ingram and Robinson, 1996).

Coal seam gas exploration within the Clarence-Moreton bioregion in Queensland and New South Wales has intensified during the last approximately 10 years, although the exploration occurs at a much smaller scale than for example in the western part of the Cecil Plains sub-basin (part of Northern Inlands Catchments bioregion, Section 1.1.3.1.1) or in the adjacent Surat Basin (less than 100 CSG exploration wells have been drilled within the Clarence-Moreton bioregion to date). The focus of recent CSG exploration activities in the Clarence-Moreton bioregion has been near Casino, primarily targeting the Walloon Coal Measures in the Casino Trough (Ward and Kelly, 2013). The exploration activities in New South Wales confirmed that there is a high gas content in the coals of the Walloon Coal Measures in New South Wales (Doig and Stanmore, 2012).