In this study, techniques have been developed to determine multiphase boundaries of C3H8/CO2/water/heavy oil systems in both the pressure-temperature (P-T) and enthalpy-temperature (H-T) phase diagrams. Experimentally, the upper three-phase boundary pressures (i.e., saturation pressures) between aqueous/liquid/vapor (ALV) and AL phases are measured for three C3H8/CO2/water/heavy oil mixtures. Moreover, the fluid samples in both L and V phases are collected to conduct compositional analysis by using a gas chromatography method. Theoretically, the previously developed water-associated model, which considers only the presence of solvents with high solubility in water, together with the new alpha functions tailored for water and nonwater components is employed to predict the multiphase boundaries of the aforementioned systems. A newly developed enthalpy determination algorithm is applied to calculate the enthalpy of the Lloydminster heavy oil. The recently proposed water-associated model is capable of accurately predicting the upper three-phase boundary pressures of the three C3H8/CO2/water/heavy oil mixtures with a reasonable accuracy. As for the newly constructed H-T phase diagram, it is found that the three-phase ALV narrow-boiling region follows the trend of moving toward a higher temperature region with an increase of a specified pressure for a given feed. Finally, the water-associated model is proven to accurately reproduce the experimentally measured vapor and hydrocarbon-rich liquid phase compositions of the aforementioned systems.