We developed a comprehensive numerical model for analyzing the effects of 35 years of CO2 injection and for evaluating various sequestration mechanisms at SACROC (Scurry Area Canyon Reef Operators Committee), a carbonate reef complex in the Permian Basin of west Texas. We evaluated previously estimated CO2 storage capacity at SACROC, the oldest CO2 enhanced oil recovery site in the United States. CO2-aided oil recovery operations began in 1972, and since that time more than 55 million tons of CO2 have been injected in SACROC. According to detailed well records for the northern area of SACROC (the “northern platform” area), 13 million tons of CO2 have been injected and 6 million tons of CO2 produced, suggesting a net of 7 million tons of CO2 sequestered in this part of the field. Using operator well records, we simulated the full production/injection history of the northern platform to understand the range of trapping mechanisms applicable. We used three-dimensional distributions of porosity and permeability that were estimated based on seismic and core data. Data from over 1000 water chemistry samples collected from diverse sources were analyzed to investigate effectiveness of seal integrity. Mineral assemblages were quantified by previous XRD analyses. Model results indicate that about 3.5 millions tons of injected CO2 is stored as separate CO2, 1.7 million tons as residual CO2, 1.7 millions tons as aqueous CO2, and less than 0.1 millions tons as minerals during last 35 years. We also evaluated CO2 storage capacity with including oil. Both hydrostratigraphic and residual trapping are dominant mechanisms in brine-saturated reservoirs. However, both oil and stratigraphic trapping are dominant mechanisms in oil-brine reservoirs.