The ability of mudrock seals to prevent CO2 leakage is a major concern for geological storage of anthropogenic CO2. The long-term performance of a mudrock seal, which provides a natural analogue, in the North Sea Miller oil field has been evaluated. This mudrock seal is immediately above a natural CO2-rich reservoir. The paper reports the stable isotopes of carbon from carbonate minerals in the mudrock that have precipitated in contact with CO2 during 4 km of burial. A well-defined linear trend of upward-decreasing δ13C traces the progressive penetration of free-phase CO2 causing dissolution and reprecipitation of carbonate minerals. The CO2 was emplaced ca. 70–80 Ma, and has only penetrated 12 m vertically in this case. The infiltration rate is estimated as ~9.8 × 10−7g cm−2 yr−1. Engineered CO2 storage under this type of mudrock seal will have a considerable safety margin.