Underground storage of carbon dioxide (CO2) is attracting considerable interest worldwide as a means of avoiding continued release of CO2 from anthropogenic sources. Here, a heterogeneous oil reservoir in Alberta, Canada, was chosen for evaluating the potential use of this site for storage of a mixture of 90% CO2 + 10% H2S produced from a nearby gas plant. This reservoir produces 34 degrees API light oil from a pinnacle reef, which is a carbonate reservoir with a depth of 4800 ft (1441 m). A fully compositional, three-dimensional (3-D) reservoir simulation model, CMG-GEM, was used to simulate various operational conditions, study the reservoir and fluid characteristics, and investigate the amount of CO2 stored and oil recovered. The results of this study show that a combination of two vertical injectors and one horizontal producer optimizes the incremental oil recovered and amount of CO2 stored. The procedure developed in this study, and the findings of this study, can be used as guidelines for designing and implementing any future CO2 injection and storage project in similar oil reservoirs.