The Fizzy discovery, a southern North Sea (UK) gas accumulation with ~50% natural CO2 content, provides an opportunity to study the long-term quantity of CO2-related mineral reaction as an analogue for engineered CO2 storage. The reservoir contains diagenetic dolomite typical of the formation; to identify and quantify any sequestration-related dolomite is challenging. To this end, CO2 was extracted by stepwise extraction from dolomite from both the Fizzy discovery and equivalent sandstones from a low-CO2 location. Between 0% and 22% of the dolomite in the Fizzy discovery precipitated due to the high CO2 concentration. This corresponds to 11% ± 8% of the recent high-CO2 charge sequestered as dolomite, a relatively low proportion after ~50 m.y. of potential CO2-water-rock interaction.