Systems and methods for creating high-pressure mixtures of steam and one or more noncondensable gas species, which may be injected into deep, high-pressured oil reservoirs to supply heat and aid oil recovery. These systems and methods may include generating these mixtures, also referred to as a combined stream, controlling the total pressure of the combined stream, controlling the partial pressure of steam within the combined stream, supplying the combined stream to a subterranean formation that includes hydrocarbons, such as viscous oil, reducing the viscosity of the oil, and/or producing oil from the subterranean formation. In some embodiments, the total pressure of the combined stream may approach or even exceed the critical pressure of water while still retaining significant amounts of latent heat for delivery. In some embodiments, the partial pressure of steam may be less than the critical pressure of water.