Tar sands contain immobile oil, which will not flow into a well bore even if stimulated by enhanced oil recovery processes. The carbon dioxide can be removed by commercially proved processes and purified for enhanced oil recovery use. If the carbon dioxide method of enhanced oil recovery is to be extensively utilized, large quantities of the gas will be needed. The rising value of natural gas and petroleum gas have adversely affected the economics of enhanced oil recovery projects utilizing these now-expensive hydrocarbons. Possible less-expensive substitutes include nitrogen, boiler flue gas, and gas engine exhaust. The physical and chemical changes that occur in the reservoir during enhanced recovery operations are extremely complex and not well understood. Estimates of the total domestic oil that may be recovered by the use of enhanced recovery methods are usually derived by applying a recovery factor to the entire resource base.