Coal seam gas

Within the Clarence‑Moreton bioregion, there are both conventional gas and unconventional gas (CSG) resources (e.g. Martin and Saxeby, 1982; Ingram and Robinson, 1996; Doig and Stanmore, 2012). However, potential impacts of conventional gas development are not considered by the Clarence‑Moreton Bioregional Assessment, and this product therefore focuses on unconventional (CSG) resources.

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

In some wells, for example Shannon-1 in the Casino Trough, strong gas shows have been reported from the Walloon Coal Measures suggesting that there is potential for gas and possibly also for oil and wet gas. However, Ingram and Robinson (1996) noted that the high vitrinite levels of the Walloon Coal Measures indicate that the overall potential for oil generation may not be very good. Total organic carbon ranges from less than 1% to more than 20% for the older Ipswich and Nymboida coal measures. Vitrinite reflectance from 0.95% to approximately 4% suggests a high level of maturity, which is likely to promote methane rather than wet gases.

The primary target for CSG exploration in the Clarence‑Moreton bioregion is the Walloon Coal Measures. In addition, ten exploration wells have targeted the Ipswich Coal Measures near the basin margins in Queensland (Figure 5). While there has been exploration for conventional gas and oil from the older coal measures in the Clarence-Moreton Basin in the past, no CSG exploration programme has to date targeted the Nymboida, Red Cliff or Evans Head coal measures and the exploration of the CSG potential of the Ipswich Coal Measures has targeted areas where they occur at shallow depths near the basin margins (Figure 4). This focus on the Walloon Coal Measures as the primary exploration target is for example related to the more continuous nature and greater thickness of the Walloon Coal Measures compared to the Triassic coal measures. In addition, the substantial depth at which these older coal measures occur throughout much of the Clarence-Moreton Basin (Figure 6) suggest that that there is no potential for coal seam gas extraction from these older coal measures.

Recent evaluation by the Australian Energy Market Operator (2013) estimated total CSG reserves and resources as at 31 December 2012 of 16,808 PJ in the Clarence‑Moreton Basin, of which 445 PJ are 2P (proved and probable). However, this estimate includes the western part of the Clarence‑Moreton Basin (part of the Northern Inland Catchments bioregion). Metgasco Limited reported CSG reserves and resources within the petroleum exploration licence PEL 13 and PEL 16 in NSW of 4768.7 PJ, of which 338 PJ are considered to be 2P (Metgasco, 2013). In 2012, Red Sky Energy Limited reported certified 2P resources of 17 PJ based on a two core hole programme. The Australian Energy Resource Assessment (Geoscience Australia and BREE, 2014) estimated that there are 100 PJ of total demonstrated conventional gas and 428 PJ of proved and probable CSG resources in the Clarence‑Moreton Basin.

Methane adsorption tests of coal samples from the Clarence‑Moreton bioregion in NSW demonstrated that the coals are fully saturated or close to full saturation (MHA, 2009, as cited in Doig and Stanmore, 2012), which indicates that there was no loss of gas from the coals (Doig and Stanmore, 2012). This was attributed to the low permeability and interbedded nature of the coals. In addition, they suggested that where the feldspathic Maclean Sandstone overlies the coal seams of the Walloon Coal Measures and where this sandstone is not compromised by faulting, it may act as a top seal which prevents the escape of gas and has an important role in CSG accumulation. The ‘fair to good’ seal potential (Stewart and Alder, 1995) of the Maclean Sandstone is attributed to the tight character of this sand- and siltstone and the clay content in the matrix, which reduces the permeability.

Exploration drilling by Arrow Energy has shown a very wide range of gas saturation levels within the coal seams of the Ipswich Coal Measures (from about 5.8 to 75%), but most intersected seams are undersaturated and only three seams had saturation levels greater than 50% (Oberhardt and Pinder, 2005).

The Clarence‑Moreton Basin formed over basement rocks that are intensively intruded by granite (Sommacal et al., 2008). Cenozoic intrusive and extrusive activity has been widespread throughout the Clarence‑Moreton bioregion. The granitic rocks may have potential as a geothermal energy source, and may also have influenced the maturity of overlying coal measures. In addition, the Cenozoic igneous activity could have been important locally for generating hydrocarbons, although it is possible that this magmatic activity may have been a general heating event in the Clarence‑Moreton Basin, which could have affected hydrocarbon generation on a regional scale (Martin and Saxby, 1982; Ingram and Robinson, 1996; Doig and Stanmore, 2012). A similar potentially beneficial influence of igneous intrusions on CSG potential has also been discussed for the Gunnedah Basin, part of the bioregional assessment for the Namoi subregion (e.g. Gurba and Weber, 2001; Northey et al., 2014).

Isotope data suggest that methane in coal seams in the adjacent Surat Basin is of biogenic origin (e.g. Papendick et al., 2011). In contrast, gas isotope analyses by Metgasco in its NSW tenements confirmed that the gas in the Walloon Coal Measures is of a thermogenic origin (Doig and Stanmore, 2012), and a thermogenic origin has also been reported for CSG in the Namoi subregion (Stewart and Alder, 1995). A gas isotope analysis by Arrow Energy (Pinder, 2007) collected from flowing gas at the surface at Kalbar-1 also suggested a thermogenic origin of the methane.

Last updated:
3 August 2016