The changes to the environment caused by anthropogenic climate change pose major challenges for energy production in the next century. Carbon Capture and Storage (CCS) is a group of technologies that would permit the continued use of carbon-intense fuels such as coal for energy production while avoiding further impact on the global climate system. The mechanism most often proposed for storage is injection of CO₂ below the surface of the Earth in geological media, with the most promising option for CO₂ reservoirs being deep saline aquifers (DSA’s). Unlike oil and gas reservoirs, deep saline aquifers are poorly characterized and the variability in their properties is large enough to have a high impact on the overall physical and economic viability of CCS. Storage in saline aquifers is likely to be a very high-capacity resource, but its economic viability is almost unknown. We consider the impact of geological variability on the total viability of the CO ₂ storage system from several perspectives. First, we examine the theoretical range of costs of storage by coupling a physical and economic model of CO ₂ storage with a range of possible geological settings. With the relevant properties of rock extending over several orders of magnitude, it is not surprising that we find costs and storage potential ranging over several orders of magnitude. Second, we use georeferenced data to evaluate the spatial distribution of cost and capacity. When paired together to build a marginal abatement cost curve (MACC), this cost and capacity data indicates that low cost and high capacity are collocated; storage in these promising areas is likely to be quite viable but may not be available to all CO₂ sources. However, when we continue to explore the impact of geological variability on realistic, commercial-scale site sizes by invoking capacity and pressure management constraints, we find that the distribution costs and footprints of these sites may be prohibitively high. The combination of issues with onshore storage in geological media leads us to begin to evaluate offshore storage potential. By considering the temperature and pressure regimes at the seafloor, we locate and quantify marine strata that has “self-sealing” properties, a storage option that we find is plentiful off the coasts of the United States. We conclude that further research into transport optimization that takes into account the true variation in geological media is necessary to determine the distribution of costs for carbon capture and storage to permit the full evaluation of CCS as a mitigation option.