Page 186 - Synthetic Fuels Handbook

172 CHAPTER SIX

Oil production potential from oil shale is measured by a laboratory pyrolysis method
called Fischer assay (Speight, 1994) and is reported in barrels (42 gal) per ton. Rich zones
can yield more than 40 gal/ton, while most shale falls in the range of 10 to 25 gal/ton. Shale
oil yields of more than 25 gal/ton are generally viewed as the most economically attractive,
and hence, the most favorable for initial development.
Despite the huge resources, oil shale is an under-utilized energy resource. In fact, one
of the issues that arise when dealing with fuels from oil shale is the start-stop-start episodic
nature of the various projects. The projects have varied in time and economic investment
and viability. The reasons comprise competition from cheaper energy sources, heavy front-
end investments and, of late, an unfavorable environment record. Oil shale has, though, a
definite potential for meeting energy demand in an environmentally acceptable manner
(Bartis et al., 2005; Andrews, 2006).
Just like the term oil sand (tar sand in the United States), the term oil shale is a misno-
mer since the mineral does not contain oil nor is it always shale. The organic material is
chiefly kerogen and the shale is usually a relatively hard rock, called marl. Properly pro-
cessed, kerogen can be converted into a substance somewhat similar to petroleum which is
often better than the lowest grade of oil produced from conventional oil reservoirs but of
lower quality than conventional light oil.
Shale oil, sometimes termed retort oil, is the liquid oil condensed from the effluent in
oil shale retorting and typically contains appreciable amounts of water and solids, as well
as having an irrepressible tendency to form sediments. However, shale oils are sufficiently
different from crude oil that processing shale oil presents some unusual problems.
Retorting is the process of heating oil shale in order to recover the organic material,
predominantly as a liquid. To achieve economically attractive recovery of product, tem-
peratures of 400 to 600°C (752–1112°F) are required. A retort is simply a vessel in which
the oil shale is heated from which the product gases and vapors can escape to a collector.

6.1 ORIGIN

In the creation of petroleum, source rocks are buried by natural geologic processes and,
over geologic time, converts the organic materials to liquids and gases that can migrate
through cracks and pores in the rocks until it reaches the surface or is trapped by a tight
overhead formation. The result is an oil and/or gas reservoir.
Oil shale was deposited in a wide variety of environments including freshwater to saline
ponds and lakes, epicontinental marine basins, and related subtidal shelves. They were also
deposited in shallow ponds or lakes associated with coal-forming peat in limnic and coastal
swamp depositional environments. It is not surprising, therefore, that oil shale exhibit a
wide range in organic and mineral composition. Most oil shale contain organic matter
derived from varied types of marine and lacustrine algae, with some debris of land plants,
depending upon the depositional environment and sediment sources.
Oil shale does not undergo that natural maturation process but produces the material that
has come to be known as kerogen (Scouten, 1990). In fact, there are indications that kero-
gen, being different to petroleum, may be a by-product of the maturation process. The kero-
gen residue that remains in oil shale is formed during maturation and is then rejected from
the organic matrix because of its insolubility and relative unreactivity under the maturation
conditions (Speight, 2007; Chap. 4). Furthermore, the fact that kerogen, under the condi-
tions imposed upon it in the laboratory by high-temperature pyrolysis, forms hydrocarbon
products does not guarantee that the kerogen of oil shale is a precursor to petroleum.
Oil shale ranging from Cambrian to Tertiary in age occurs in many parts of the world
(Table 6.2). Deposits range from small occurrences of little or no economic value to those