CO2injection in oil reservoirs has the dual benefit of enhancing oil recovery from declining reservoirs as well as sequestering a greenhouse gas to combat climate change. CO2injected in carbonate reservoirs, such as those found in the Middle East, can react with ions present in the brine and the solid calcite in the carbonate rocks. These geochemical reactions impact the overall mole numbers and in some extreme cases, even the number of phases at equilibrium impacting oil recovery predictions obtained from compositional simulations. Hence, it is important to study the impact of geochemical reactions on a real reservoir fluid mixture during CO2injection. In this study, the Gibbs free energy function is utilized to integrate phase behavior computations and geochemical reactions to find equilibrium compositions. The Gibbs free energy minimization method using elemental balance constraint is used to obtain equilibrium compositions arising at phase and chemical equilibrium. The solid phase is assumed to be calcite, the hydrocarbon phases are characterized using the Peng-Robinson (PR) Equation of State (EOS), and the aqueous phase components are described using the Pitzer activity coefficient model. The binary interaction parameters for the EOS and the activity coefficient model are obtained using experimental data. The impact of the changes in phase behavior of a real reservoir fluid with 22 components is presented in this manuscript. We observe that the changes in phase behavior of the resulting reservoir fluid mixture in the presence of geochemical reactions depend on two factors: 1) the volume ratio (and hence molar ratio) of the aqueous phase to the hydrocarbon phase and 2) the salinity (and composition) of the brine. These changes represent a maximum impact of geochemical reactions since all reactions are assumed to be at equilibrium. This approach can be adapted to any reservoir brine and hydrocarbon as long as the initial formation water composition and their Gibbs free energy at standard states are known. The resultant model can be integrated into any reservoir simulator as integrated algorithms can be used for minimizing the Gibbs free energy function of the entire system.