Carbonate cements in Miller field of the UK North Sea: a natural analog for mineral trapping in CO₂ geological storage.

Miller field of the North Sea has had high concentrations of natural CO<sub>2</sub> for ~70 Ma. It is an ideal analog for the long-term fate of CO<sub>2</sub> during engineered storage, particularly for formation of carbonate minerals that permanently lock up CO<sub>2</sub> in solid form. The Brae Formation reservoir sandstone contains an unusually high quantity of calcite concretions; however, C and O stable isotopic signatures suggest that these are not related to the present-day CO<sub>2</sub> charge. Margins of the concretions are corroded, probably because of reduced pH due to CO<sub>2</sub> influx. Dispersed calcite cements are also present, some of which postdate the CO<sub>2</sub> charge and, therefore, are the products of mineral trapping. It is calculated that only a minority of the reservoired CO<sub>2</sub> in Miller (6–24%) has been sequestrated in carbonates, even after 70 Ma of CO<sub>2</sub> emplacement. Most of the CO<sub>2</sub> accumulation is dissolved in pore fluids. Therefore, in a reservoir similar to the Brae Formation, engineered CO<sub>2</sub> storage must rely on physical retention mechanisms because mineral trapping is both incomplete and slow