Conventional methods of oil recovery, including primary and secondary recovery, achieve, on the average, about 35 percent recovery of the original oil in place, less if the oil is heavy or viscous. The volume of oil remaining in already-discovered reservoirs in the United States is on the order of 340 billion barrels. Conventional wisdom in the 1970s held that additional recovery would involve a physical or chemical change in the reservoir or its contained fluids to move oil that was immobile.