Abstract: A computer-operated method and corresponding system for simulating a reservoir to provide inputs to a surface model simulation is disclosed. The method of operating the simulation begins with the dividing of the reservoir into volume cells, and assigning reservoir parameters such as porosity and permeability of the rock to the particular cells. Temperature and pressure values are also assigned to each of the volume cells, and an equation of state is solved for the cells to determine densities of each of the components of the contents of the reservoirs, examples of such components including methane, ethane, and the like. A black oil simulation of the reservoir is then performed over a time step, to determine the fluxes of the vapor and liquid phases, as reservoir barrel volumes, at each cell boundary. The assigned density values are then used to transform the volumes determined by the black oil simulation to molar quantities for each component.