In most cases, CO2 captured from power plants or large industrial sources contains impurities. As purification of the stream is energy and cost intensive it is necessary to allow a certain level of impurities. The effects of impurities on (short- and long-term) geological storage are, however, uncertain. In this work, geochemical modelling with PHREEQC is performed to describe such effects on a sandstone reservoir (depleted gas field). The impact of two possible CO2 streams, originating from pre-combustion and oxyfuel capture technology is investigated. The streams contain O2, H2, CO, H2S, SO2, and/or NO as potential chemically reactive components. H2S, SO2 and NO are computed to oxidize, thereby forming sulfuric or nitric acid, and decrease the pH of the formation water. A low pH of the brine may be the result of extensive dissolution and dissociation, and therefore accumulation in the brine phase, especially close to the injection well. The impact of impurities on fast reacting minerals (short-term effects) is predicted to be relatively insignificant, due to the low amount of brine generally present in a gas field. On the long-term (equilibrium stage), impurities cause a slightly different mineralogy compared to pure CO2 injection. For the latter case a final increase in porosity of 3.5% is predicted whilst impurities (especially oxygen) could mitigate the porosity increase to zero due to the precipitation of minerals with higher molar volumes, like alunite and nontronite. Overall, the impurities do not seem to have a significant impact on the reservoir, even if accumulation in the brine takes place. The possible limiting effect of diffusion of impurities within the supercritical CO2 towards the brine has not been taken into account, even though the effect could be relevant. It could delay the effect of the impurities due to retarded dissolution. Further research should focus on this issue. Also the spatial effects and effects on different reservoir types, cap rock and well cement need to be investigated. © 2010 Elsevier Ltd. © 2011 Published by Elsevier Ltd.