Chromatographic separations that occur during two-phase flow in a porous medium are analyzed for one-dimensional, dispersion-free displacement of a liquid hydrocarbon mixture (oil) by a vapor phase mixture (gas). We show that displacement behavior is controlled by a set of key equilibrium tie lines, all of which are determined by geometric constructions in composition space : two of the key tie lines are those that extend through the initial oil and injection gas compositions, and the remainder are tie lines that are located at intersections of ruled surfaces of tie lines. Very efficient displacement of oil by gas results if any of the key tie lines is a critical tie line. That high displacement efficiency is the basis of so-called miscible gas injection processes for enhanced oil recovery.