The Bunter Sandstone Formation in the UK Southern North Sea has the potential to become an important CO2 storage unit if carbon dioxide capture and storage becomes a widely deployed option for the mitigation of greenhouse gases. A detailed geological model of a region of the Bunter Sandstone consisting of four domed structural closures was created using existing seismic, well log and core data. Compositional simulation of CO2 injection was performed to estimate the storage capacity of domes within the system. The injection was constrained by both pressure and CO2 migration criteria, and the storage efficiencies of the domes (volume of stored CO2 divided by the pore volume of the dome) were calculated when injection ceased. A sensitivity study evaluated the effect of varying the total aquifer volume, reservoir heterogeneity and injection well location. A wide range of storage efficiency values were obtained across the different simulation cases, ranging from 4% (closed dome) to 33% (homogeneous model). Intra-reservoir heterogeneity, specifically in the form of continuous low permeability layers has an important effect on storage capacity in dome-like structures, because it increases the tendency for CO2 to migrate laterally from the storage complex via structural spill points.