The study of natural analogues can inform the long-term performance security of engineered CO₂ storage. There are natural CO₂ reservoirs and CO₂ seeps in Italy. Here, we study nine reservoirs and establish which are sealed or are leaking CO₂ to surface. Their characteristics are compared to elucidate which conditions control CO₂ leakage. All of the case studies would fail current CO₂ storage site selection criteria, although only two leak CO₂ to surface. The factors found to systematically affect seal performance are overburden geopressure and proximity to modern extensional faults. Amongst our case...