Gas injection is a common practice in many carbonate oil fields; however, there is a lot of debate around the viability of economical enhanced oil recovery by miscible gas injection. For a correct simulation of miscible gas injection and monitoring the progress of the miscible front in the reservoir, a compositional reservoir simulation is needed. Fluid characterization is one of the most important parts of this simulation.In this paper, fluid characterization for such a mechanism is discussed and a systematic approach is presented which could be used in any other similar study. The dynamic reservoir simulation is also brought at the end for comparison. The carbonate reservoir of the field of interest, contains 900 million barrels of under-saturated, 34 API degrees oil, with initial reservoir pressure of 8200 psi. After building a PVT model and adjusting the Equation of State (EOS), the Minimum Miscibility Pressure (MMP) of four different injection gases (N 2 , CO 2 , associated gas and sales gas) were calculated with different methods. Swelling test and slim tube test were also conducted which were used to cross check the EOS tuning. Although, MMPs in all cases were much lower than initial reservoir pressure, their effects on recovery factor were different. A compositional reservoir model was built based on the tuned EOS and the effects of all injection gases in different scenarios were examined. The procedures as well as the main results are explained in this paper.