Inside nanopores, solid-fluid interactions are of the same order of magnitude as intermolecular interactions of fluid molecules. This fact strongly modifies the thermodynamic properties of confined fluids with respect to bulk phases. Tight oil and shale gas reservoirs, where the proportion of micro (below 2 nm) and mesopores (between 2 and 50 nm) can reach more than 20% of the volume distribution, represent an environment with such problems and industrial challenges to hydrocarbon fluid pressure/volume/temperature (PVT) modeling. This study provides a detailed understanding of the thermodynamic behavior of confined fluid and reference data of the thermodynamic properties of pure components (methane, ethane, n-pentane, n-decane) and mixtures (methane/ethane, ethane/n-pentane) confined in graphite slit pores. Furthermore, a detailed explanation of the different pressures considered in a porous medium with nano-pores is given. The Gibbs Ensemble Monte Carlo (GEMC) NVT simulation is used for pure components instead of the more traditional Grand Canonical Monte Carlo ensemble (GCMC) simulation to get more precise results of liquid and vapor confined pressure avoiding the phase change location determination problem. The evolution of critical temperature and pressure versus pore radius is compared to literature correlations and confined vapor and liquid densities are calculated. A new and robust method in the GEMC ensemble called GEMC NPT Bubble point Monte Carlo (BPMC) completed with GEMC NVT simulations has been developed to get thermodynamic properties including pressures at equilibrium of confined mixtures. Pressure versus density diagrams, pressure versus molar fraction isotherms and examples of pressure versus temperature diagram for a specific composition are built. The phase envelope of the confined fluid is shifted and closes with respect to phase envelope of bulk fluid. The critical temperature and pressure are shifted from the bulk value to a lower value and the bubble point pressure is decreased as the dew point pressure is increased. With regards to the selectivity of the confined system compared to the bulk fluid, for the methane/ethane and ethane/n-pentane mixtures, the heavier component is preferentially adsorbed in the vapor phase and the lighter component is preferentially adsorbed in the liquid phase. All these results for pure components and mixtures provide relevant information concerning the understanding of the phase behavior in confined systems such as shale gas and tight oil reservoirs, emphasizing the difference from the bulk fluid. Furthermore all these data may be used as references for pore radius dependent equation of state (EOS) calibration. (C) 2019 Elsevier B.V. All rights reserved.