An improved model for calculating physical properties and phase behavior of confined fluids (oil and gas resources in shale reservoirs) by considering adsorption film theory was developed based on the square-well chain-like fluid with variable well-width range (SWCF-VR) equation of state. The accuracy of the improved model is greatly increased via comparing the experimental data of argon in cylindrical pores at 87.3 K. The physical properties of pure component hydrocarbons, mixture hydrocarbons, and real Bakken oils in nanopores were predicted and analyzed. The results show that the properties of confined fluids are very different from those of bulk-phase fluids, where confinement decreases the gas–liquid phase equilibrium constant (K-value), bubble point pressure, and interfacial tension of the fluid. The presence of the adsorption film further decreases the K-value and bubble point and increases the capillary pressure, and these properties change more significantly in pore radius with only a few nanometers. The results demonstrate the importance of improving the accuracy in calculating the properties and phase behaviors of confined fluids and also draw the necessity of considering the adsorption film theory.