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Substantiation of the composition and properties of hydrocarbon (HC) reservoir fluids is one of the important stages in the calculation of reserves and has a significant impact on the results of field development planning and engineering. The article discusses the current methodological issues of identification of PVT properties of reservoir oil and free gas (gas cap) in a twophase deposits. The first part of the article is devoted to the problem of obtaining representative samples of reservoir oil in two-phase deposits and describes the methodical approach allowing to reproduce properties of the formation oil which is in thermodynamic equilibrium with the existing gas cap. The identification procedure is based on the complex use of the results of laboratory and theoretical studies, the basis of which is the mathematical modeling of the phase behavior of multicomponent hydrocarbon systems. The results are illustrated by the example of reservoir oil from the West Siberia field. It is confirmed that in the case of partially degassed (unrepresentative) deep oil samples, the saturated hydrocarbon fluid can be restored by modeling the process, reverse degassing in reservoir conditions. In the second part of the article, a scientific and methodological approach to the restoration of the composition and reproduction of the PVT properties of reservoir oil of two-phase deposit is proposed in the case of limited input information. On the basis of the identified composition of the reservoir oil, its thermodynamic model is created, which adequately reproduces the results of laboratory and field studies. The composition of the gas cap is determined using the fundamental physical foundations of the phase equilibrium in multicomponent systems and free gas thermodynamic properties are estimated on the basis of PVT-modeling. PVT models of reservoir oil and gas-condensate mixture of the gas cap can be used in the design and monitoring of the development using modern hydrodynamic simulators based on both Black Oil and compositional models. The application of the method is illustrated by the example of a real deposit. The suggested methodological approaches are recommended to be used in the estimation of reserves and design of fields development.