Page 23 - Synthetic Fuels Handbook

FUEL SOURCES 11

grade of oil produced from conventional oil deposits, but of a lower quality than the upper
grades of conventional oil.
Oil shale occurs in many parts of the world ranging from deposits of little or no eco-
nomic value to those that occupy thousands of square miles and contain many billions
of barrels of potentially extractable shale oil. Total world resources of oil shale are con-
servatively estimated at 2.6 trillion barrels of oil equivalent. With the continuing decline
of petroleum supplies, accompanied by increasing costs of petroleum-based products, oil
shale presents opportunities for supplying some of the fossil energy needs of the world in
the years ahead.
In the United States there are two principal types of oil shale: (a) Green River shale
from the Green River Formation in Colorado, Utah, and Wyoming, and (b) the Devonian-
Mississippian black shale of the eastern and midwestern states. The Green River shale
is considerably richer in organic material, occurs in thicker seams, and therefore is more
likely to be exploited for synthetic fuel manufacture.
In the Green River oil shale, the kerogen is not bound to a particular type of rock such
as shale and the largest concentrations of kerogen are found in sedimentary nonreservoir
rocks such as marlstone (a mix of carbonates, silicates, and clay). In contrast the black
shale of the eastern and midwestern states is true shale, insofar as it is composed predominantly
of the illite.
The organic content of oil shale is much higher than those of normal and ordinary rocks,
and typically range from 1 to 5 percent w/w (lean shale) to 15 to 20 percent w/w (rich
shale). This natural resource is widely scattered in the entire world, and occurrences are
scientifically closely linked to the history and geologic evolution of the earth. Due to its
abundance and wide distribution throughout the world, its utilization has a long history,
both documented and undocumented. It is also obvious that the shale must have been
relatively easy sources for domestic energy requirements for the ancient world, mainly due
to the ease of handling and transportation; solid fuels were more convenient in the earlier
human history and the examples are plentiful, including wood and coal.
There are two conventional approaches to oil shale processing. In one, the shale is frac-
tured in situ and heated to obtain gases and liquids by wells. The second is by mining, trans-
porting, and heating the shale to about 450°C, adding hydrogen to the resulting product, and
disposing of and stabilizing the waste. Both processes use considerable amounts of water.
The total energy and water requirements together with environmental and monetary costs
(to produce shale oil in significant quantities) have so far made production uneconomic.
During and following the oil crisis of the 1970s, major oil companies, working on some
of the richest oil shale deposits in the world in western United States, spent several billion
dollars in various unsuccessful attempts to commercially extract shale oil.
The amount of shale oil that can be recovered from a given deposit depends upon many
factors. Some deposits or portions thereof, such as large areas of the Devonian black shale
in eastern United States, may be too deeply buried to economically mine in the foreseeable
future. Surface land uses may greatly restrict the availability of some oil shale deposits for
development, especially those in the industrial western countries. The bottom line in devel-
oping a large oil shale industry will be governed by the price of petroleum. When the price
of shale oil is comparable to that of crude oil because of diminishing resources of crude,
then shale oil may find a place in the world fossil energy mix.
In order to extract hydrocarbons (or, oil in “loose” terms), the oil shale is typically
subjected to a thermal treatment, scientifically categorized as “destructive distillation.” A
collective scientific term for hydrocarbons in oil shale is called kerogen, an ill-defined mac-
romolecule which, when heated, undergoes both physical and chemical change. Physical
changes involve phase changes, softening, expansion, and oozing through pores, while
chemical changes are typically involving bond cleavages mostly on carbon–carbon bonds
that result in smaller and simpler molecules. The chemical change is often termed as pyrolysis