138 



UNITED STATES MINERAL RESOURCES 



and the need to protect oil and gas wells and fields. 

 From the long-term national point of view, average 

 recoverability is probably about 50 percent. How- 

 ever, it is not desirable to report coal resource data 

 on an arbitrary recoverable basis, because experi- 

 ence with most commodities has shown very signifi- 

 cant long-term changes in what is regarded as 

 economically recoverable. Coal in the ground is a 

 more certain value that can be modified now or in 

 the future by any recoverability factor deemed 

 appropriate. 



IDENTIFIED RESOURCES 



DISTRIBUTION BY SELECTED CATEGORIES 



In addition to the distribution by rank of the 

 identified resources of 1,581 billion tons as pre- 

 sented in table 27, about 60 percent of this total 

 has been classified into additional categories accord- 

 ing to the thickness of overburden, degree of relia- 

 bility of estimates, and thickness of beds as shown 

 in figure 16. This classified tonnage is fairly large 

 and is widely distributed in 21 States; it is likely 

 to be reasonably representative of the total identi- 

 fied tonnage. 



Overburden. — Figure 16 clearly shows the pro- 



nounced concentration of identified resources in the 

 0-1,000-foot overburden category. This concentra- 

 tion results in part from the fact that coal-bearing 

 rocks are near the surface in most parts of the 

 United States, and in part from the fact that pro- 

 gressively less information is available for the more 

 deeply buried beds. Much of the tonnage classed as 

 hypothetical in figure 17 is in the 1,000-2,000-foot 

 and the 2,000-3,000-foot overburden categories. As 

 exploration and development are carried to greater 

 depth it is certain that the identified resources will 

 be considerably increased by addition of tonnage in 

 the deeper overburden categories. 



Degree of reliability of estimates. — Figure 16 also 

 shows the progressive increase in tonnage from the 

 measured to the inferred categories. In the 0-1,000- 

 foot overburden category, for example, 8 percent of 

 the tonnage is classified as measured, 23 percent 

 as indicated, and 58 percent as inferred. The same 

 relation can be observed in the deeper overburden 

 categories. The large percentage of inferred coal 

 reflects merely distance from points of known infor- 

 mation. Resources classified as "inferred" obviously 

 exist, but the locations of such tonnage may differ 

 slightly from those assumed to make the calcula- 

 tions. As mapping and exploration continue, the 



Figure 16. — Approximate percentage distribution of original identified U.S. coal resources by major resource categories. 



