For supercritical fluids there is a wedge-shaped region called Widom region, where several physico-chemical quantities (e.g., compressibility, heat capacities, density, thermal expansivity, speed of sound) show anomalous behaviour. In this paper several Widom lines of supercritical CO 2 have been computed with the Wagner–Span reference equation of state. The locations of the Widom lines are compared with the P–T range of the Snøhvit, Sleipner, Nagaoka and Ketzin reservoirs, which are recently studied for their fitness for CO 2 sequestration, and two natural CO 2 storage analogues, Montmiral in France and Mihályi-Répcelak in Hungary. The potential consequences of leaking CO 2 crossing any of the Widom lines are discussed.