Published in Petroleum Transactions, AIME, Volume 219, 1960, pages 229–237. Abstract The mechanism involved in creating a miscible fluid displacement of typical reservoir fluids through porous media by light hydrocarbon mixtures usually involves a number of mixing and separation stages in the contact area. This mechanism has been illustrated on ternary diagrams for miscible displacement of reservoir fluid by rich hydrocarbon gases. A relatively simple method has been devised for calculating the approximate conditions for a miscible displacement of reservoir fluid by rich, light hydrocarbon gases or by LPG mixtures. Miscibility is favored by increased pressures, decreased temperatures, light reservoir fluids and light hydrocarbon-displacing fluids rich in the heavier components. The conditions for miscible displacements of five typical reservoir fluids by various light hydrocarbon mixtures were determined for a range of temperatures from 700 to 260F and pressures from 1,000 to 3,000 psia. A correlation of these conditions was obtained using the variables of temperature, pressure, C5+ molecular weight of the reservoir fluid, C2+ molecular weight of the displacing fluid, and mol per cent methane in the displacing fluid. This correlation has been applied to data appearing in the literature with very good success. A partial check of the correlation has been made using a step-wise procedure with a windowed PVT cell. Introduction The term, "miscible fluid displacement", may be defined as any oil-recovery displacement process where there is an absence of a phase boundary or interface between the displaced and displacing fluids. A water flood would not be a miscible fluid displacement, although cycling in a condensate reservoir would be. Another example is displacement of oil by gasoline. Not so obvious, however, are the conditions under which fluids (such as propane, mixtures of propane and methane, or other similar systems) will give a miscible fluid displacement of oil. There are four standard miscible-fluid-displacement processes being used for the recovery of oil from an underground reservoir-high-pressure dry-gas miscible displacement,rich-gas miscible displacement,miscible slug using a slug of LPG followed by a lean dry gas under miscible conditions, andgas driven-slug process using a slug of LPG followed by a lean dry gas under immiscible conditions.