Publication Name: Eastern Australian Basins Symposium III (EABS 2008)
Authors: A. Saghaf
Date Published: September 2008
Number of Pages: 4
Abstract:
Large volumes of coal seam gas (CSG) occur in Australian coal seams, with the majority of reserves located within the coalfields of the Sydney and Bowen basins in the eastern states of Australia. A peculiarity of these coalfields is that mixed gas conditions commonly prevail in the coal seams. While the main gas component is generally methane (CH4), there are many instances where both CH4 and carbon dioxide (CO2) are present in equivalent volumes, and in some instances CO2 is the dominant or the only component of the CSG. Furthermore, the CSG composition can vary considerably over short distances within a single coal seam, from almost pure CH4 to almost pure CO2. The occurrence of CO2 in Australian coal seams is attributed to biogenic, thermogenic and magmatic processes. Biogenic degradation reactions have generated small amounts of CO2 in the Sydney and Bowen basin coals (Ahmed & Smith, 2001). However, the CO2 is primarily of magmatic origin and has been injected into the coal systems by igneous intrusions during several major intrusion events. These events occurred during the Late Permian and continued into the Jurassic and Cretaceous. Some major igneous intrusive events have also occurred during the Tertiary (Embleton et al. 1985; Facer & Carr, 1979). These events have partly displaced some of the initial thermogenic CH4 and replaced it with CO2. Studying this natural injection of CO2 into Australian coal seams can contribute to a better understanding of the physiochemical mechanisms in these coals, and assist in predicting what consequences of injecting anthropogenic CO2 into these seams might hold.