Gas mixing in the subsurface could have crucial implications on CO2 storage capacity and security. This study illustrates the impact of gas mixing in the “Captain X” CO2 storage site, an open saline aquifer and subset of the greater Captain aquifer, located in the Moray Firth, North Sea. The storage site hosts several abandoned hydrocarbon fields where injected CO2 could interact and mix with any remaining hydrocarbon gas left in the depleted structures. For this study, compositional simulation of CO2 injection into the Captain X storage site reservoir model was conducted to quantify the impact of mixing. Results show mixing of CO2 with the remaining trapped hydrocarbon gas makes the plume considerably less dense and more mobile. This increases the buoyancy forces acting on the plume, causing it to migrate faster towards the shallower storage boundaries and therefore, reduces the storage capacity of the site. Mixing also compromises the storage security as it mobilises the structurally trapped hydrocarbon gas from within the abandoned fields. Informed injector placement helps to manage and reduce the impact of mixing. Correct assessment of mixing is also considerably dependent on the volume and property of the trapped hydrocarbon gas. To provide a correct long term understanding of storage capacity and security, the impact of mixing, therefore, needs to be correctly considered in all large-scale CO2 storage operations.