Region 4, comprising the northern Rocky Mountains, western Nebraska, and eastern Colorado, covers 483,000 sq mi (1,251,000 sq km), of which 368,000 sq mi (953,120 sq km), or 76 percent, is considered favorable for oil and gas occurrence. A total sedimentary-rock volume of approximately 596,000 cu mi (2,483,500 cu km) has afforded discovery of 23.5 billion bbl of oil in place and 13 trillion cu ft of recoverable gas. It is estimated as of December 31, 1968, that 25.7 billion bbl of oil remains to be discovered in "probable" and "possible" categories, along with 30.3 trillion cu ft of recoverable gas in place. If undiscovered oil in place is added to remaining oil in place as of December 31, 1968, the total reserve is 44.5 billion bbl. Addition of undiscovered recoverable g s in place to remaining recoverable gas as of December 31, 1968, gives 37.2 trillion cu ft. The finding of new oil and gas reserves will be difficult, expensive, and slow. Economic rather than geologic aspects will control rates for the future.