Oil reservoirs containing significant amounts of CO2 usually translate into scenarios with high compositional complexity. This complexity is manifested not only in strong areal and vertical compositional changes, but also in behaviours that deviate from what is usually called "thermodynamic equilibrium". Using a compositional reservoir simulator, we analyse in this work a scenario already documented in previous studies, where a CO2 cap coexists with a heavy sub-saturated oil. We numerically show that the equilibrium state, for geological events time-lengths, does not correspond to a complete mixture of the components of the system, but to the exchange of the different components until a column of equilibrated densities is reached. For high density oils in equilibrium with an excess of CO2, this situation may corresponds to stable systems characterized by a strong sub-saturation. In this work we analyse the dependence of the final composition distribution on the initial composition and the equilibrium time. This development illustrates the methodology to be used to generate a fluid model compatible with an apparently non-equilibrium initial situation.