14 



UNITED STATES MINERAL RESOURCES 



Table 2. — Some estimates of U.S. crude-oil reserves and resources, in billion barrels 



[API, American Petroleum Institute; NPC, National Petroleum Council] 



In identified fields 



Probable-possible 



In undiscovered 



fields and structures 



(speculative) 



Recoverable at present rate' 



Additional at 42 percent recovery 

 Additional at 60 percent recovery _ 

 Oil originally in place 



Total oil originally in place 



Ultimate production" 



31 (API) 



47 (NPC) 



69 (NPC) 



388 (API)^ 



74 (NPC) 



22 (NPC) 



40 (NPC) 



227 (NPC) 



67 (NPC) 



21 (NPC) 



37 (NPC) 



209 (NPC) 



2200 (Hendricks and Schweinfurth, 1968). 



190 (Hubbert, 1969); 353 (Moore, 1966); 433 (Weeks) = ; 450 (Elliott 

 and Linden, 1968); 432-495 (NPC); 550 (Hendricks and Schwein- 

 furth, 1968). 



1 Average recovery is 30 percent of oil in place. 

 ^ Includes past production of 86 billion barrels. 

 ' See McKelvey (1968. Table II, p. 18). 



potential gas resources in ordinary reservoirs to be 

 in the range of 100 to 200 trillion cubic feet, but 

 pointeci out that gas in impermeable strata which 

 might be released by nuclear stimulation would be 

 several times that amount. Gas occluded in coal — 

 now only a menace in this country as a cause of 

 explosions — is already recovered in some European 

 mines and is also a potentially large resource. 



The uncertainties concerning potential coal re- 

 sources center not on their total magnitude, as they 

 do for oil and gas, but on the amounts available at 

 present prices. Because coal beds have great lateral 

 continuity, geologic mapping and stratigraphic stud- 

 ies make it possible to project them long distances 

 from their outcrops and to categorize them in terms 

 of thickness of beds, thickness of overburden, rank 

 of coal, and other features that affect cost. The Geo- 

 logical Survey has prepared such estimates, but the 

 cost of recovering coal in the various categories has 

 yet to be determined. Coal in beds more than 14 

 inches thick totals at least 3.3 trillion tons in the 

 United States, but estimates of the amounts min- 

 able at present prices have ranged from 20 to 220 

 billion tons. (See U.S. Office of Science and Tech- 

 nology, "Energy R and D and National Progress.") 

 Because 20 billion tons represents nearly a 40-year 

 supply at present rates of consumption, it is easy 

 to see why the studies needed to determine how 

 much would be available at various costs have not 

 been undertaken. The question is by no means only 

 of academic interest, for the nuclear power develop- 

 ment program was justified in part in its early years 

 on the assumption that reserves of low-cost coal 

 were extremely limited, and part of the continued 

 growth of the nuclear power industry is said to be 

 the result of the difficulty power companies are 

 having in acquiring low-cost reserves. 



QUANTIFYING THE UNDISCOVERED 



Considering potential resources in the degree-of- 

 certainty, cost-of-recovery framework brings out 



the joint role that geologists, engineers, mineral 

 technologists, and economists must play in estimat- 

 ing their magnitude. Having emphasized the im- 

 portance of the economic and technologic side of 

 the problem, I want now to turn to the geological 

 side and consider the problem of how to appraise 

 the extent of undiscovered reserves and resources. 



It is difficult enough to estimate the extent of un- 

 explored resources of the inferred or possible class. 

 In fact, it is even difficult to estimate measured or 

 proved reserves with a high degree of accuracy until 

 they have been largely mined out. Thus, estimates 

 of proved reserves prepared in advance of appre- 

 ciable production commonly have an error of about 

 25 percent, and the error in estimates of incom- 

 pletely explored deposits is usually much larger. 

 Generally the combination of the geologist's inherent 

 conservatism and the lack of information on the 

 geology of concealed areas leads to estimates that 

 err in being too low rather than too high. 



One eminent mining geologist reported that, hav- 

 ing recognized these effects, he once arbitrarily 

 tripled his calculations to arrive at an estimate of 

 the ore remaining in a producing district; twice the 

 amount of his inflated estimate, however, was found 

 and mined over the next 20 years, and more was in 

 prospect. To match many such stories are at least 

 a few prematurely deserted mills and mine instal- 

 lations built on the expectation of finding ore that 

 did not materialize. Both kinds of experiences em- 

 phasize the difficulty of appraising the extent of 

 mineral deposits even in partly explored areas. In 

 the light of such experiences one is justified in ask- 

 ing — as many well-informed people have — whether 

 estimates of the magnitude of undiscovered deposits 

 can have enough reliability to make them worth- 

 while. 



The fact that new districts are still being discov- 

 ered for nearly every commodity and that large 

 areas favorable for the occurrence of minerals of 

 all kinds are covered by alluvium, volcanics, glacial 



