Geochemical models of CO2 injection into reservoir sandstones often predict the growth of minerals that will permanently store the CO2 in solid form, and injection experiments record significant fluctuations in porewater chemistry on a short time scale. Yet the proportion of CO2 reaction may be small, even over geological time scales. A southern North Sea (UK) gas accumulation with a high natural CO2 content (c. 50%) forms a natural analogue to engineered storage, and provides a calibration point for geochemical models of CO2–rock reaction. In the analogue site, the carbonate mineral dawsonite has formed in only trace amounts (0.4 ± 0.3% solid volume) despite exposure to high levels of CO2 for 50 Myr or more. It is calculated that only 2.4 (± 0.9)% of the CO2 present within the structure is currently locked up as dawsonite, and a similar quantity in solution in the porewaters. Comparison of stable O and C isotopes with a neighboring field with low CO2 content gas suggests that up to 0.7 (± 2)% solid volume dolomite cement is associated with the CO2 charge, equivalent to 0–25% of the total CO2. The remaining 70–95% of the CO2 is present as a free phase, after tens of millions of years. Consequently, geological storage of anthropogenic CO2 in reservoirs similar to the Rotliegend Group must rely on physical containment and not mineral sequestration. The Rotliegend Group is still an excellent candidate for a CO2 storage reservoir, though using physical trapping mechanisms and not chemical ones.