Two German chemists, Franz Fischer and Hans Tropsch, developed
early methods of extracting liquids from coal. The technology involves
gasifying coal at high temperature and pressure, and applying catalysts, but
there are various different procedures. The process was developed in Germany
during the Second World War, and later used in South Africa, when it was
subject to an oil embargo. It was sufficient to provide about 30% of that
country’s needs.
The so-called Oil Shales are immature normal source rocks that have
not been heated enough in Nature to give up their hydrocarbons. Strictly
speaking they are not shales at all in a geological sense, their scientific
name being kukersite. They were first exploited in Scotland around 1860,
which led to one of the earliest refineries to extract lamp-oil. Another early
development was in Estonia where they are still used as a fuel for power
stations.
Interest in the development of Oil Shale grew rapidly in the
aftermath of the Oil Shock of 1980, especially in the United States, which has
large deposits in the Green River Basin of Colorado and neighboring States. The
traditional method of extraction was simply to excavate the material, and then
place it in retorts at 350–1,000 °C: the higher the temperature, the lighter
the product. One drawback was the large amount of fi ne-grained toxic ash
produced in the process, whose disposal posed environmental problems. Attempts
have also been made at in situ retorting with the help of underground
combustion, and the injection of hot natural gas. A recent project, developed
by Shell in Colorado, involved inserting electric elements, using electricity
from a dedicated coal- fi red power station, and cooking the deposit for
several years, after which it is expected to deliver production to conventional
wells. In addition, it has been necessary to surround the workings with a
refrigerated underground ice-wall to prevent the escape of the liquids.
It sounds as if it will be subject to an extremely low, if not negative, net
energy yield.
There are many other large deposits around the world, especially in
Russia, China, Australia, Morocco, Zaire, South Africa, Egypt, Argentina,
Chile, Uruguay and Brazil. The resource is enormous, perhaps capable of
providing as much as three trillion barrels of oil, but so far none has proved
commercially viable, despite in some cases Government subsidy.
Of growing importance are the so-called tight reservoirs, known as Shale
Oil and Shale Gas. In essence they consist of beds of sandstone,
siltstone or dolomite with very low porosity and permeability lying within
source-rock sequences, commonly at relatively shallow depth. Highly deviated
boreholes are drilled into them to run parallel with the formation and thereby
be in contact with more of the oil-bearing rock.
Liquids, charged with various chemicals, are then injected under
high pressure to fracture the rock and cause artificial permeability, which
allows the flow of oil and gas from the adjoining source-rocks. The wells are
then placed on production and can produce pro fi table amounts of oil and gas
in the current high price environment, although subject to relatively rapid
depletion. There are some environmental hazards where the reservoirs are at
shallow depth, as the liquids used in the fracturing process may poison the
overlying aquifers or cause minor earth tremors. Interest in this new source of
oil and gas has expanded rapidly in recent years, especially in the United
States, where, the Barnet and Bakken Shales are of particular interest. The
environmental hazards have raised objections to developments in several other
countries. Obviously, there is a wide range of geological circumstances with
the more favorable being exploited first.
Oil can be produced from
some organic-rich fine-grained rocks that are normally referred to as “shales”
even if the host rock is not strictly a shale by a geologic definition.
Production from such sources has a long history. Production of oil from shales
for illumination preceded the discovery of conventional petroleum resources in
the mid-nineteenth century but for most of the twentieth century production of such
oil was a very minor component of global petroleum production. However, in the
first decade of the twenty-first century there has been a resurgence of
interest in these resources because of advances in technology and rising energy
prices.
“Oil shales” are rocks that contain significant amounts of solid
organic chemical compounds (kerogen) that have not been buried deeply enough to
allow for oil maturation. Production is generally done by mining the rock and
heating it in a retort in a processing plant close to the mine where the oil
and associated gases can be captured. The oil may also be extracted using in
situ methods which require heating the subsurface rock by injection of hot
fluids, gases, or steam, or by the use of heating elements. As the oil is expelled
from the kerogen it can then be induced to flow toward conventional oil wells
for extraction.
The leading producer of oil shale in the world is Estonia, where
90% of the power is generated from that source. By far the largest
accumulations of oil shale, however, are in the United States, particularly in
the Green River Formation of Colorado, Utah, and Wyoming, that were deposited
within ancient saline lake systems some 40–50 million years ago. There are also
major accumulations in Devonian–Mississippian black shales in the eastern
United States that were deposited in marine environments over 350 million years
ago. With the high oil prices of the late 1970s a number of pilot projects
produced oil from the Green River Formation in Colorado but plans for major
commercial exploitation were abandoned when prices fell in the early 1980s.
Interest in the potential for production has been rekindled with the high oil
prices in recent years. Other countries with significant
oil shale accumulations include Australia, Brazil, the Democratic Republic of
the Congo, and Russia.
“Shale oil” is oil that is trapped within a
fine-grained rock. Extraction of the oil does not require heating but the low
permeability of the rocks requires that the rock be artificially fractured in
situ to allow flow toward a well. Once regarded only of scientific interest,
recent advances in hydraulic fracturing have made shale oil economically viable
in some areas. The best-known shale oil accumulation is the Bakken Formation of
the Williston Basin of Montana and North Dakota and adjoining parts of Canada.
From a geologic perspective, the Bakken is a petroleum source rock that reached
maturation but, unlike most mature source rocks, the oil was never expelled to
migrate to conventional traps.
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