Carbon Capture and Storage (CCS) is considered a viable option for reducing CO2 emissions into the atmosphere from point sources such as coal-fired power plants. Monitoring of CO2 storage sites is widely considered necessary for safety reasons and for verification of injected CO2 in the reservoir. The latter is crucially dependent on the ability to determine CO2 trapping mechanisms and to assess pore-space saturation of CO2. Thus far, attempts to determine CO(2)pore-space saturations at CO2 injection sites have had limited success. Here, we present data from the Pembina Cardium CO2 Monitoring Project in Alberta. Canada, that demonstrate that changes in the oxygen isotope ratios (delta O-18) of reservoir water can be indicative of the extent of pore-space saturation with CO2. The delta O-18 value of injected CO2 at the injection site was + 28.6%. (V-SMOW) and delta O-18 values of reservoir water at eight observation wells varied between -13.5 and -17.1%. (V-SMOW) before CO2 injection. After commencement of CO2 injection the delta O-18 values of reservoir water at three observation wells increased between 1.1 and 3.9% due to the presence of large quantities of injected CO2 and equilibrium isotope exchange between water and CO2. Our calculations revealed that reservoir water fully saturated with CO2 would only result in increases of delta O-18(H2O) values of 0.4%.. Hence the observed larger increases in delta O-18 values of reservoir water indicate free phase CO2 with estimated pore-space saturations ranging from 12% (corresponding to a delta O-18 increase of 1.1%0) to 64% (delta O-18 increase 3.9%.) of the non-oil saturated pore-space. Contributions to oxygen in the system from mineral dissolution were calculated to be less than 0.01% of total oxygen and therefore did not alter the delta O-18 value of the reservoir water significantly. Hence we conclude that changes in the delta O-18 values of reservoir water caused by the presence of injected CO2 can be used as a tracer for CO2 plume migration in the subsurface provided that the injected CO2 is isotopically distinct. Furthermore, we submit that the extent of the changes in the delta O-18 values of the reservoir water provides a quantitative assessment of CO2 stored in dissolved form (solubility trapping), assuming no density driven convective overturn, and as free-phase CO2 (structural, stratigraphic and hydrodynamic trapping) thereby elucidating the trapping mechanisms within the reservoir. (C) 2011 Elsevier B.V. All rights reserved.