Coal seam gas

In the Gwydir subregion the Surat Basin overlies the Gunnedah-Bowen Basin (see Figure 16 in companion product 1.1 for the Gwydir subregion (Welsh et al., 2014)) which have both been targets for CSG exploration and production outside the subregion. In the Gunnedah-Bowen Basin initial exploration for CSG investigated the potential of the Late Permian Black Jack Group (ESG, 2009a), the Early Permian Maules Creek Formation of the Bellata Group (ESG, 2009a), and the Middle Triassic Moolayember Formation (ESG, 2010a, 2010b, 2009c). For Gunnedah and Bowen Basin stratigraphies see Figure 19 in the companion product 1.1 for the Gwydir subregion (Welsh et al., 2014) which also presents the stratigraphy of the Surat Basin. In the Surat Basin some CSG exploration has occurred in the Middle Jurassic Walloon Coal Measures, as well as in the Early Cretaceous Bungil Formation of the Blythesdale Group (ESG, 2009a, 2009b).

The Walloon Coal Measures are the primary target for CSG production further north in southern Queensland (in the Maranoa-Balonne-Condamine subregion, see product 1.2 for the Maranoa-Balonne-Condamine subregion (Sander et al., 2014)). However, in the Gwydir subregion the Walloon Coal Measures are less prospective for CSG. The total thickness of the Walloon Coal Measures in the northern part of the Gwydir subregion is 200 m and the net coal thickness is less than 2 m with thin coal seams occurring in some of the thick shales in the upper half of the coal measures (Pangaea, 2010a). In comparison, about 200 km to the north of the subregion, the Walloon Coal Measures are more than 500 m in thickness and net coal thickness is greater than 30 m (Pangaea, 2010a). Exploration of the Walloon Coal Measures in the north-east of the Gwydir subregion did not show potential for commercial CSG recovery. The exploration results indicated very low net coal seam thickness (ranging from 0.6 m to 3.05 m), permeability (not quantified), and gas content (ranging from an average gas content of 0.5 m3/t to 0.8 m3/t on an air dried basis) in this area (Pangaea 2010a, 2010b, 2010c, 2010d). Depth to coal ranged from 460 to 665 m (Pangaea 2010a, 2010b, 2010c, 2010d).

The geology of the Bungil Formation is not well understood as it has not been well explored, because it is traditionally considered to be of no commercial interest for conventional petroleum production (ESG, 2009a). The formation consists of fine-grained sandstones and siltstones interbedded with numerous, thin, carbonaceous shales and coals, which are of lignite to sub‑bituminous rank (ESG, 2009a). For coals of such low rank, any measurable volumes of gas are a result of biogenic gas production as they are not mature enough to have generated thermogenic gas (Moore, 2012). The gas content of biogenically derived resources is generally not above 4 to 6 m3/t (Moore, 2012). However, on the upside, often the composition of the gas present in low rank coals is predominantly methane (CH4) (Moore, 2012). The Bungil Formation has been a target for CSG exploration in the western Gwydir subregion as well as in the Moree area in the centre of the subregion where a net coal thickness of up to 10 m from five to ten coal seams is typical and the coals are known to be gas-bearing (ESG, 2009a). Exploration indicated the coals were undersaturated with mean gas content of approximately 4 m3/t (dry ash free basis) (ESG, 2009a, 2009b). The gas composition is more than 93% CH4. Drill stem tests and log scanner data indicated low reservoir permeability, although the coal was buried at a comparatively shallow depth (on average 350 m depth to coal) (ESG, 2009a, 2009b). A gas content of 4 m3/t can be economic if other reservoir properties are favourable (as seen for example in the Powder River Basin in the USA where the coals were reported to have gas contents of less than 3.5 m3/t (US DOE, 2002)). However, when coals are undersaturated with respect to gas, water has to be produced first to depressurise the reservoir until a pressure is reached at which gas starts to desorb from the coal and flows to the production well. Furthermore, the low reservoir permeability implies that these coals are not likely to produce CSG at economic rates without hydraulic fracture stimulation of vertical wells or drilling of horizontal wells with multiple fractures placed along the well, a technology that is yet to be proven in Australia (Jeffrey, 20120). Jeffrey (2012) stated that target coal seams for fracture stimulation of vertical wells generally have permeabilities between 1 and 20 mD. The production of gas from coals below 0.1 mD is not economic with a hydraulically fractured vertical well (Jeffrey, 2012). However, coal seams that have permeabilities below 1 mD may be produced at economic rates using horizontal wells with multiple fractures along the well (Jeffrey, 2012).

South of the Moree area in the central Gwydir subregion, the Moolayember Formation has been a CSG exploration target while the Bungil Formation does not occur (ESG, 2010a). The mean gas content is more than 4 m3/t on a dry ash free basis (ESG, 2010a, 2010b, 2009c) with no gas composition reported. Net coal seam thickness ranges from 3.3 to 8.5 m with maximum individual seam thickness ranging from 0.75 to 1 m (ESG, 2010b, 2009c). Depth to coal varies greatly, ranging from 310 m in the Moree High to 810 m. Drill stem tests indicated low permeability (ESG, 2010a, 2009c).

In the south of the subregion, in the Bellata Trough region, the Hoskissons Coal of the Black Jack Group and the Maules Creek Formation (ESG, 2009a) have been targeted to explore their potential for CSG extraction. The Hoskissons Coal is widespread across the Gunnedah Basin and typically up to 10 m thick (ESG, 2009a). The coals are sub-bituminous to bituminous rank. The coals of the Maules Creek Formation are bituminous and individual seams can be up to 8 m thick, with a total net thickness of more than 35 m (ESG, 2009a). Initial exploration in the subregion has found the Hoskissons seam, encountered at 683 m depth, to be 2.3 m thick with a gas content of 5.9 m3/t (dry ash free basis, gas composition more than 90% CH4) (ESG, 2010c). The Bohena Seam of the Maules Creek Formation at 953 m depth is thick (14.8 m) with a high gas content (14. m3/t), but of a very high carbon dioxide (CO2) content (more than 80% CO2) (ESG, 2010c). Drill stem tests indicated low reservoir permeability for both coal seams (ESG, 2010c).

Last updated:
8 October 2018