Abstract: In a dipping underground petroleum reservoir devoid of a natural gas cap, a mixture of gases soluble in petroleum is injected into a well positioned in an up dip location. Gases in the mixture are selected for their diverse solubility and diffusion characteristics. Gas injection is terminated followed by attic oil production wherein the gases accompanying the produced oil are separated and measured. The separated gases are then compared quantitatively with gases injected and the solubility capabilities of the produced oil. Results are used to estimate the size of the attic portion of the reservoir. Alternating cycles of gas injection and attic oil production are continued until the well is engulfed by the expanding artificial gas cap.