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Industrial scale carbon capture and storage technology relies on the secure long term storage of CO₂ in the subsurface. The engineering and safety of a geological storage site is critically dependent on how and where CO₂ will be stored over the lifetime of the site. Hence, there is a need to determine how injected CO₂ is stored and identify how injected CO₂ interacts with sub-surface fluids. Since July 2008 ∼1 Mt of CO₂ has been injected into the Cranfield enhanced oil recovery (EOR) field (MS, USA), sourced from a portion of the natural CO₂ produced from the nearby Jackson Dome CO₂ reservoir. Monitoring and tracking of the amount of recycled CO₂ shows that a portion of the injected CO₂ has been retained in the reservoir. Here, we show that the noble gases (²⁰Ne, ³⁶Ar, ⁸⁴Kr, ¹³²Xe) that are intrinsic to the injected CO₂ can be combined with CO₂/³He and δ¹³C_CO2 measurements to trace both the …