CO2 injection in oil reservoirs offers the dual advantage of increasing oil recovery as well as storage to combat global warming. The injected CO2, however, may react with ions present in the brine as well as the rock. In this research, we present a model that quantifies the impact of these geochemical reactions on the phase behavior of hydrocarbon mixtures. A Gibbs free energy model is proposed that unifies different phase descriptions - the Peng-Robinson Equation of State (PR EOS) description for hydrocarbon phase components, the Pitzer activity coefficient model for aqueous components and an ideal solid phase. In particular, we illustrate the use of this model to determine the impact of geochemical reactions, introduced by the presence of an aqueous phase with ions and a solid calcite phase, on the phase envelope of CO2-nC(14)H(30) mixture with different phase descriptions. The presented Gibbs free energy model is adaptable to different reservoir brines and can be used to assess the maximum impact of geochemical reactions during CO2 injection in different hydrocarbon reservoirs. (C) 2015 Elsevier B.V. All rights reserved.