This work provides a semi-analytical development of the pressure-mobility behavior of solution gas-drive reservoir systems producing below the bubble point pressure. Our primary result is the "characteristic" relation which relates normalized (or dimensionless) pressure and mobility functions. This formulation is proven with an exhaustive numerical simulation study consisting of over 900 different cases. We considered 9 different pressure-volume-temperature (PVT) sets, and 13 different relative permeability cases in the simulation study. We also utilized 7 different depletion scenarios. The secondary purpose of this work was to develop a correlation of the "characteristic parameter" as a function of rock and fluid properties evaluated at initial reservoir conditions such as: API density, GOR, formation volume factor, viscosity, reservoir pressure, reservoir temperature, oil saturation, relative permeability end points, corey exponents and oil mobility: We did successfully correlate the characteristic parameter as a function of these variables, which proves that we can uniquely represent the pressure-mobility path during depletion with specific reservoir and fluid property variables, taken as constant values for a particular case. The functional form of our correlation along with all relevant equations are shown on the body of this document.