Abstract

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.