CO2 is one of the hazardous greenhouse gases causing significant changes to the environment. The sequestering of CO2 in a suitable geological media can be a feasible method to avoid the negative effects of CO2 emissions into the atmosphere. A numerical model was developed regarding CO2 sequestration in a deep saline aquifer. A compositional numerical model using CMG software (GEM) was employed to study the ability of the selected aquifer to accept and retain large quantities of CO2 injected in a supercritical state for long periods of time (up to 260 years). Supercritical CO2 is a one-state fluid which exhibits both gas and liquid-like properties. In this study, supercritical CO2 was sequestered in three forms in a deep saline aquifer. It was assumed to be supplied in an isothermal condition during the injection and sequestration processes and we ignored porosity and permeability changes due to mineralization. Also, CO2 adsorption was not considered in our numerical model. Gas bubble formation, dissolution of CO2 in brine and precipitation of CO2 with calcite mineral in aquifers have been discussed. The CO2 gas bubble displaces the formation water with immiscible behaviour. During and after displacement, the gravitational effects cause the CO2 to rise and accumulate under the caprock. Both vertical and horizontal permeability ratios and initial pressure conditions were the most dominating parameters affecting CO2 saturation in the three layers, whereas the CO2 injection rate influenced CO2 saturation in layers two and three since CO2 was injected from layer three at the bottom of the reservoir.