The organic facies concept states that kerogen abundance and composition are relatable to depositional settings. This link has been extended, using open-system pyrolysis methods, to include major petroleum types whose gross compositions are inherently defined by source rock kerogens and, hence, to petroleum composition, which directly controls the physical response of fluids to changing pressure and temperature conditions during secondary migration. The possibility to model petroleum composition during hydrocarbon generation as well as the phase behavior of the fluids during migration is now available in modern basin modeling software. However, whereas pyrolysis methods can accurately reconstruct hydrocarbon gas:oil ratios and liquid compositions, they are inherently incapable of correctly reproducing the gas composition of natural fluids, and because the gas composition dominantly controls the phase behavior of petroleums, multicompound compositional kinetic predictions based on pyrolysis results alone are inappropriate for the prediction of phase behavior. The new PhaseKinetics approach described here combines open- and closed-system pyrolysis techniques to characterize the compositional evolution of the fluids generated as a function of increasing thermal stress, as well as correcting experimentally determined gas compositions and, therefore, overcomes these deficiencies. It has been used to develop compositional kinetic schemes capable of predicting natural petroleum phase behavior for different organofacies types. The methodology uses a compositional description of the generated fluids similar to that used in pressure-volume-temperature (PVT) simulation. Selected studies in the Norwegian North Sea, Brazil, and Mexico demonstrate the quality of the tuned compositional predictions for different organic facies types.