Heavy oil resources, as a non-renewable energy resource, often require extra enhanced oil recovery techniques such as solvent-based processes. Many kinds of solvents including pure and mixed solvents have been tested in solvent-based applications. Compared with pure solvent, the solvent mixture has the advantage of relatively higher dew point pressure while maintaining desirable solubility in heavy oil. The characterization of foamy oil behavior in a pure solvent system is different from that of the solvent mixture system despite their similarities. Thus, an additional numerical simulation study is necessary for the solvent mixture system. This work conducted simulation studies to investigate foamy oil behavior in a heavy oil-mixture solvent (C1 + C3) system from pressure depletion tests. A better understanding of foamy oil characterization and mechanism in a heavy oil-mixture solvent system is obtained. A reliable non-equilibrium model is developed to perform simulation studies. Since previous experiments suggest the behavior of foamy oil in the solvent mixture system share similarities with the heavy oil-methane system, this investigation first conducted a simulation study with consideration of two reactions in the model and achieved good agreements between the simulated calculation results and experimentally measurement. Then, four reactions are considered in the model for simulation study and obtained better history match results. The simulation results suggest methane has more impact on the foamy oil behaviors than propane in the heavy oil-mixture solvent system. This work also discussed the effect of model parameters involved in the history matching process and conducted sensitivity analysis.