Combination solvent-noncondensible gas injection method for recovering petroleum from viscous petroleum-containing formations including tar sand deposits

Petroleum may be recovered from viscous petroleum-containing formations including tar sand deposits by injecting into the formation a solvent which is liquid at formation conditions and simultaneously therewith injecting a substance which will remain totally gaseous at the pressure and temperature conditions existing within the reservoir. The presence of noncondensible gas in the formation into which solvent is being injected prevents formation of a impremeable bitumen bank which blocks the further flow of fluids through the formation. The gas should be essentially unreactive with the solvent and formation fluids in order to obtain the desired benefits. Examples of suitable gases for the practice of this invention include methane, ethane, nitrogen, carbon dioxide, and mixtures thereof.