Geological sequestration is a means of reducing anthropogenic atmospheric emissions Of CO2 that is immediately available and technologically feasible. Among various options, M can be sequestered in deep aquifers by dissolution in the formation water. The ultimate CO2 sequestration capacity in solution (UCSCS) of an aquifer is the difference between the total capacity for CO2 at saturation and the total inorganic carbon currently in solution in that aquifer, and depends on the pressure, temperature and salinity of the formation water. Assuming non-reactive aquifer conditions, the current carbon content is calculated using standard chemical analyses of the formation waters collected by the energy industry on the basis of the concentration of carbonate and bicarbonate ions. Formation water analyses performed at laboratory conditions are brought to in situ conditions using a geochemical speciation model to account for dissolved gasses that are lost from the water sample. To account for the decrease in CO2 solubility with increasing water salinity, the maximum CO2 content in formation water is calculated by applying an empirical correction to the CO2 content at saturation in pure water. The UCSCS in an aquifer is calculated by considering the effect of dissolved CO2 on the formation water density, the aquifer thickness and porosity to account for the volume of water in the aquifer pore space and for the mass Of CO2 dissolved in the water currently and at saturation. The methodology developed for estimating the ultimate CO2 sequestration capacity in solution in aquifers has been applied to the Viking aquifer in the Alberta basin in western Canada. Considering only the region where the injected CO2 would be a dense fluid, the capacity of the Viking aquifer to sequester CO2 in solution in the formation water is calculated to be 100 Gt. Simple estimates then indicate that the capacity of the Alberta basin to sequester CO2 dissolved in the formation waters at depths greater than 1000 m is on the order of 4000 Gt CO2. The results also show that using geochemical models to bring the analyses of the formation waters to in situ conditions is not warranted when the current total inorganic carbon (TIC) in the aquifer water is very small by comparison with. the CO2 solubility at saturation. Furthermore, in such cases, the current TIC may even be neglected. Crown Copyright (C) 2003 Published by Elsevier Ltd. All rights reserved.