A final member of the petroleum group of fuels is oil shale. Oil shales differ from other forms of petroleum in that the hydrocarbon occurs in the form of a solid, rather than as a viscous liquid. It also differs chemically from crude oil and tar-like oils, a fact which poses more difficult problems of refining.

The best-known, and among the largest of oil shale deposits are those of the Green River shale which occur in four separate localities in western

Colorado, northern Utah, and southwestern Wyoming. Of these, the larg­est and richest is in the Piceance Basin in Colorado.

In hydrocarbon content these shales vary in richness from about 100 gallons to less than 10 gallons of shale oil per ton of rock. The total amounts of oil represented by these shale deposits in the range of 10 to 25 gallons per ton have recently been estimated by Duncan and Swanson (1966, p. 13) as follows:

Basin Oil (1CP bbls)

Piceance Basin, Colorado………………………………….. 800

Uintah Basin, Utah……………………………………………. 230

Green River and Washakie Basins, Wyoming…. 400

Total ………………………………………………………….. 1,430

These figures tend to be misleading, however, since the same authors (Table 2, p. 2) list only 80 billion barrels as being “recoverable under present conditions.”

The same authors have also compiled a summary of major oil shale deposits throughout the world. They give an estimate of 2 x 1015 barrels for the total oil content of these shales, but only 190 billion barrels (in­cluding 80 for the Green River shales in the United States) are said to be recoverable under present conditions.

When the low oil content per ton of rock in the oil shales is consid­ered, it is evident that the mining problem required to extract significant quantities of this oil becomes formidable. Accordingly, it may well be con­sidered preferable to obtain liquid fuels from coal, or to produce them eventually synthetically using other sources of energy, rather than to de-

stroy large sections of scenic country during oil shale mining and extrac­tion.

CONCLUSIONS REGARDING THE FOSSIL FUELS

From this brief review, it is clear that although the fossil fuels have been in use for about 800 years, and may continue to be exploited for a comparable length of time in the future, these fuels can serve as major sources of energy for a period no longer than about three centuries. The brevity of this episode in a context of human history extending from 5,000 years in the past to 5,000 years into the future is shown in Figure 16. Nev­ertheless, this episode represents a unique event of the first order of im­portance not only in human history, but of geological history as well.

Power from Current Energy Flux

In addition to the large supplies of energy available from the fossil fuels, the various channels of the continuous energy flux through the earth’s surface environment are conventional sources of power.