Distribution of Juvenile CO_2 in the Sydney Basin, Australia: A Natural Analogue for CO_2 Storage in Bituminous Coals

摘要

Coal seams in the Sydney Basin contain up to 100% juvenile CO_2 in places that provides a natural analogue to gain insights into the long term fate of anthropogenic CO_2 sequestered in high rank bituminous coals. Most of the CO_2 was apparently introduced from deep magmatic sources, via faults and this has replaced pre-existing CH_4 in the coals: the process is analogous in some respects to the sequestration of anthropogenic CO_2. The present-day distribution of CO_2 in the coals is controlled by a combination of pressure, temperature, depth and geological structure. Under present day P-T conditions most of the CO_2 occurs in solution at depths greater than about 650 m. At shallower depths, due to rapidly decreasing solubility, increasing volumes of CO_2 occur in gaseous form and as adsorbed molecules in the coal. Carbon dioxide has apparently migrated up to structural highs and is concentrated in anticlines and in up-dip sections of monoclines and sealing faults. Anthropogenic CO_2 sequestered in coal measure sequences similar to those of the Sydney Basin may behave in the same way and, in the long term, equilibrate according to the prevailing P-T conditions. Comparisons of adsorption isotherm data with in-situ gas contents of Sydney Basin coals indicate that, at a given depth, the volumes of CO_2 stored by replacing pre-existing CH_4 do not exceed ～60% of the total CO_2 storage capacity which may have implications for the long-term storage of anthropogenic CO_2.