The Bunter Sandstone formation in the UK’s southern North Sea has been identified as having the potential to store large volumes of CO2. Prior to injection, CO2 is captured with certain amounts of impurities, usually less than 5%vol. The dissolution of these impurities in formation water can cause chemical reactions between CO2, brine, and rock, which can affect the reservoir quality by altering properties such as permeability. In this study, we explored the effect of CO2 and impurities (NO2, SO2, H2S) on reservoir permeability by measuring changes in grain size distributions after a prolonged period of 9 months, simulating in situ experimental conditions. It was found that the effects of pure CO2 and CO2-H2S are relatively small, i.e., CO2 increased permeability by 5.5% and CO2-H2S decreased it by 5.5%. Also, CO2-SO2 slightly decreased permeability by 6.25%, while CO2-NO2 showed the most pronounced effect, reducing permeability by 41.6%. The decrease in permeability showed a correlation with decreasing pH of the formation water and this equally correlates with a decrease in geometric mean of the grain diameter. The findings from this study are aimed to be used in future modelling studies on reservoir performance during injection and storage, which also should account for the shifts in boundaries in the CO2 phase diagram, altering the reservoir properties and affecting the cost of storage.