With the expansion of offshore petroleum exploration and extraction activities, validated models are needed to simulate the behaviors of petroleum compounds released in deep (>100 m) waters. We developed a thermodynamic model of the gas-liquid-water partitioning, densities, and viscosities of the Deepwater Horizon petroleum mixture with varying composition, pressure, and temperature, based on the Peng-Robinson equation-of-state and the modified Henry’s law (Krychevsky-Kasarnovsky equation). We define pseudo-components based on comprehensive two-dimensional gas chromatography (GC×GC) measurements, which enable the modeling of aqueous partitioning for >n-C8 compound fractions not quantified individually. The resulting thermodynamic model was tested against available laboratory data on petroleum gas and liquid densities, gas/liquid volume fractions, and liquid viscosities. The model was applied to the Macondo reservoir fluid, represented with 279–280 components including 129–130 individual compounds. The model allows to predict the volume percent of gas and liquid at local conditions near the Macondo well (~150 atm and 4.4°C). These high pressure conditions dramatically increase the aqueous dissolution of petroleum hydrocarbons and also modifies the buoyancies of gas bubbles and liquid droplets. Our results also affect published flow rate estimates of dead oil from the broken Macondo well stub.