Rising atmospheric carbon dioxide (CO₂) concentrations are fueling anthropogenic climate change. Geologic sequestration of anthropogenic CO₂ in depleted oil reservoirs is one option for reducing CO₂ emissions to the atmosphere while enhancing oil recovery. In order to evaluate the feasibility of using enhanced oil recovery (EOR) sites in the United States for permanent CO₂ storage, an active multi-stage miscible CO₂flooding project in the Permian Basin (North Ward Estes Field, near Wickett, Texas) was investigated. In addition, two major natural CO₂ reservoirs in the southeastern Paradox Basin (McElmo Dome and Doe Canyon) were also investigated as they provide CO₂ for EOR operations in the Permian Basin. Produced gas and water were collected from three different CO₂ flooding phases (with different start dates) within the North Ward Estes Field to evaluate possible CO₂ storage mechanisms and amounts of total CO₂retention. McElmo Dome and Doe Canyon were sampled for produced gas to determine the noble gas and stable isotope signature of the original injected EOR gas and to confirm the source of this naturally-occurring CO₂. As expected, the natural CO₂produced from McElmo Dome and Doe Canyon is a mix of mantle and crustal sources. When comparing CO₂ injection and production rates for the CO₂ floods in the North Ward Estes Field, it appears that CO₂ retention in the reservoir decreased over the course of the three injections, retaining 39%, 49% and 61% of the injected CO₂ for the 2008, 2010, and 2013 projects, respectively, characteristic of maturing CO₂ miscible flood projects. Noble gas isotopic composition of the injected and produced gas for the flood projects suggest no active fractionation, while δ¹³CCO₂ values suggest no active CO₂dissolution into formation water, or mineralization. CO₂ volumes capable of dissolving in residual formation fluids were also estimated along with the potential to store pure-phase supercritical CO₂. Using a combination of dissolution trapping and residual trapping, both volumes of CO₂ currently retained in the 2008 and 2013 projects could be justified, suggesting no major leakage is occurring. These subsurface reservoirs, jointly considered, have the capacity to store up to 9 years of CO₂ emissions from an average US powerplant.