Oil shale and tar sands

Although oil shale and tar sands are not generally thought of as potential sources of petroleum products, these resources have been developed to possibly replace dwindling oil fields. Tremendous reserves exist worldwide, but the current costs and technology preclude the use of oil shale and tar sands on a full scale.

Oil Shale Reserves

Oil shales are typically fine-grained, stratified sedimentary rock. The term “oil shale” is actually a misnomer because the reservoir rock does not have to be shale. Organic matter is present in the pores of these rocks in the form of kerogen. Kerogen is produced over a long time as the original organic-rich sediments are transformed into complex hydrocarbons. Unlike oil and natural gas, which move relatively easily in the subsurface, hydrocarbons in oil shale migrate at an almost imperceptible rate or not at all, because the sedimentary parent rock often has a very low porosity and permeability, and the kerogen molecules form complex networks that adhere to the grains in the host rock. The richest known oil shales produce between 320 and 475 liters (2 to 3 barrels) of oil per ton of processed rock. Based on current technology, though, less than 7 percent of these reserves are recoverable.

When examined on a global basis, the United States has large oil shale reserves. Oil shales are located under more than 20 percent of the land area of the United States. Roughly 50 percent of the worldwide reserves are found in the Green River formation, a shale and sandstone unit that formed during the Eocene epoch about 50 million years ago. This formation, the only one in the western United States that has been extensively studied for oil shale potential, covers approximately 42,000 square kilometers of southwest and south central Wyoming, northeast Utah, and northwest Colorado. The thickest portions of the Green River formation are located in large structural basins that allowed large, shallow lakes to form in the topographically depressed areas. Subsequent deposition of rich organic sediments and later burial and thermal alteration led to the present deposits. Up to 540 billion barrels of oil exist in the rock units that have a thickness greater than 10 meters, a thickness that is sufficient to produce enough hydrocarbons to be cost-effective using present-day recovery methods. Estimates of the total oil shale reserves for the Green River formation range as high as 2,000 billion barrels of oil. Unfortunately, wide-scale development of these resources will probably not happen, because the extraction process requires large amounts of water, a commodity in small supply in the arid western United States. Approximately 3 liters of water is required to extract 1 liter of oil, so each barrel of oil requires that almost 480 liters of water be used to remove the oil from the reservoir rock. Major projects have been initiated in the Green River formation in western Colorado to set up entire cities to handle the processing of the reservoir rock and its eventual products. Almost all these projects, however, have been terminated or put on indefinite hold because of changes in the worldwide petroleum market.

Although the richest oil shale deposits are in the western United States, another 15 to 20 percent of worldwide reserves are found in Devonian and Ordovician rocks located from New York to Illinois and into southwestern Missouri. These deposits are not economically useful, however, because the cost to extract the small amount of oil present far exceeds the value of the oil produced. Several nations attempted using oil shale as a source for petroleum products during World War II, but postwar economic variations shut down most of them within a few years. A proposal to extract natural gas from these rocks by fracturing them underground—a process called “fracking”—has generated widespread controversy since 2011. Advocates point to the availability of a new source of energy, while opponents are concerned about the potential contamination of groundwater and other environmental consequences.

Reserves

Tar sands constitute another major potential source of unconventional oil reserves. These highly viscous deposits—sometimes referred to as tar, asphalt, and bitumen—probably formed as residues from petroleum reservoirs after the lighter, more hydrogen-rich crude oils migrated toward the surface. These porous sands contain asphaltic hydrocarbons, which are extremely viscous. Thus, the hydrocarbons are not bound up as tightly in the reservoir as they are in oil shales. G. Ronald Gray defines these heavy substances as one of the following: bitumen, an oil sand hydrocarbon that cannot be produced using conventional processes; extra-heavy oils; and heavy oils (see the bibliography). These three types are usually lumped together when discussing worldwide reserve estimates, which are set at about 5,000 billion barrels.

The seven largest tar sand fields have roughly the same amount of oil as do the three hundred largest conventional oil fields in the world. The largest tar sand deposits are located, in descending order, in northern Alberta, Canada; northeastern Siberia; and along the northern bank of the Orinoco River, Venezuela. The amount of heavy oil in place in the tar sands in the Athabasca deposit of Alberta is essentially equal to all the petroleum reserves found in the Middle East. Tar sand deposits are found in twenty-two states in the United States, the largest deposits being in Utah. Given the massive reserves of tar sands throughout the world, they could potentially supply modern society with as much oil as that obtained from conventional flowing wells. The major problem in the future is developing the technology to recover the usable hydrocarbon products in a cost-effective manner.

Mining Techniques

Once the hydrocarbons are detected in the reservoir rock, they must be extracted. One of two primary mining techniques is used to recover the oil. After the upper soil and rock layers overlying the oil shale are stripped away, the reservoir rock is removed, crushed, and transported to a large retort, where it is heated. This process involves raising the temperature of the rock and hydrocarbons to about 480 degrees Celsius, the temperature at which kerogen vaporizes into volatile hydrocarbons and leaves a carbonaceous residue. When oil shale is heated, the amount of organic matter that is converted to oil increases as the amount of hydrogen in the deposit increases. This vapor is then condensed to form a viscous oil. After the introduction of hydrogen, the mixture can be refined in a manner similar to that used to refine crude oil, which is drawn from the ground using conventional methods of drilling and pumping. One obvious problem with this method is that a considerable amount of heat (energy) must be expended in order to yield products that themselves are potential energy sources.

The second technique used to recover oil from oil shales involves in situ heating of the reservoir rock after it has been fractured with explosives or water under pressure. Researchers at the Sandia National Laboratories in New Mexico may have discovered a method to enhance the extraction of kerogen from the reservoir rocks. Underground fracturing of the rock by controlled explosions increases the number and size of passageways for air to pass through. Air is a necessary component to complete the chemically driven thermal reactions (essentially, combustion) that release the hydrocarbons from kerogen. Heating the rock by pumping in superheated water loosens the viscous hydrocarbons from the pores and cracks. Hydrocarbons are driven off by the heat, collected, and then pumped to the surface for further distillation and refining.

A key factor in the removal of hydrocarbon compounds from the ground is the carbon-to-hydrogen ratio. Carbon is twelve times as heavy as hydrogen, so carbon-rich heavy crude oils (those rich in aromatic hydrocarbons) are denser than conventional oil, which contains a higher percentage of hydrogen. Carbon-rich crude oil yields smaller amounts of the more desirable lighter fuels, such as kerosene and gasoline. Heavy crudes, such as those derived from oil shale and tar sands, can be chemically upgraded by removing carbon or adding hydrogen in the refining process. These procedures, however, are rather complex and certainly add to the already high cost of extraction and refining.

Progress has been made in the recovery rate of hydrocarbons from reservoir rock. Extraction technology has increased the amount recovered from about 12 percent in 1960 to more than 50 percent in some instances by the late 1980s. The recovery rate is dependent on the percentage of complex hydrocarbons present, which varies from formation to formation. One main problem impeding the all-out effort to continue full-scale production is the economics of oil shale mining. When the price of oil, especially imported oil, is low and the amounts abundant, it is not feasible to consider oil shale as a source. Rising oil prices make oil shale more attractive, but investors will invest in oil shale only if they are confident prices will remain high enough to keep the recovery of oil shale profitable.

Environmental Concerns

There are several major environmental problems with oil shale production. The refining process actually generates more waste than the amount of rock that is processed originally. The processed rock increases in volume by about 30 percent, because the hot water and steam used to extract the kerogen from the oil shale enter into the clay molecules present in the rock and cause them to expand. The problem then is what to do with the increased volume of material, as it will overfill the void produced by mining the rock. This expanded material also weathers very rapidly so that it will not remain in place and form a stable tailings pile. In situ retorting precludes the need to mine the shale and, thus, circumvents this disposal problem.

Another environmental concern is that of the air quality in the vicinity of the processing plant. Large amounts of dust being thrown into the atmosphere have an adverse effect on the air in the immediate area surrounding the plant. On a large scale, global air quality can be affected by increased oil shale production through increases in carbon dioxide. Research has shown that high-temperature retorting methods (those using temperatures exceeding 600 degrees Celsius) may produce more carbon dioxide from the carbonate rocks containing the oil than the actual burning of the oil produced. As levels of carbon dioxide increase, the worldwide greenhouse effect increases. Additional carbon dioxide is generated through the combustion of free carbon, which is present in the kerogen.

Tar sands are formed in environments different from those in which oil shales form. While oil shales appear to form in lake environments, especially those characterized by sandstone and limestone deposits, tar sands are often found in conjunction with deltaic or nonmarine settings. Most of the major deposits of tar sands are in rocks of Cretaceous age or younger, whereas oil shales are often associated with older sedimentary formations. The lack of a cement other than asphalt to hold the sand grains together results in high porosities and permeabilities, thus affording the viscous tars an opportunity to flow. Another geologic setting that enhances the formation of large tar deposits includes areas with an impermeable layer overlying the deposits. The impermeable layer acts as a barrier to prevent the upward movement of the hydrocarbons.

The mining of tar sands must usually be done at or near the deposits, as large amounts of the sands are handled. The actual production of oil from tar sands in the Athabasca field involves large amounts of material being processed. For each 50,000 barrels of oil, almost 33,000 cubic meters of overburden must be removed and about 100,000 tons of tar sand mined and discarded. Extraction of bitumen and heavy oils from tar sands is relatively difficult because of the high viscosity of the hydrocarbons. Bitumen at room temperature is heavier than water and does not flow. The viscosity of these substances can be changed dramatically by applying heat. If tar sands are heated to about 175 degrees Celsius, the bitumens present flow readily and are capable of floating on water. In some cases, the injection of steam into the reservoir sands increases the flow, thus allowing the material to be pumped out of the ground. Hot water can also be used, but this method requires vast volumes of water, something not available in many regions where these sands are presently found. Underground combustion techniques require burning the tar sands underground and allowing the resulting heat to warm the bitumens to the point at which they flow and can be pumped to the surface.

Prospects for Production

The existence of oil shales and tar sands has been known for several centuries. Deposits were used as a source of oil for lamps in Europe and colonial America. Native Americans used them to patch their canoes. When commercial production of petroleum expanded in the latter half of the nineteenth century, interest in developing and using oil shales and tar sands as a source of oil decreased. Interest increased again during World War II and in the mid-1970s as a result of shortages in petroleum imports. Once the short-term oil crises were over, though, both the federal government and private industry in the United States dropped most of the research and development associated with extracting oil from oil shale. The Canadian government has moved ahead with its development of extensive tar sand deposits and has been producing substantial amounts of oil for many years.

Undoubtedly, interest in developing oil shale reserves in the United States will increase as the national reserves of petroleum diminish and imports become more scarce and expensive. Widespread production from oil shales will probably not occur until oil reaches a price of $50 to $60 per barrel, or until a technological breakthrough occurs. Such a breakthrough might include sonic drilling or genetic engineering. Sonic drilling uses a vibrating, rotating drill, and it can be faster and cheaper than other methods. At the end of the twentieth century, however, its use was limited to a depth of about 200 meters. Genetic engineering could involve the use of the many bacteria with the ability to break down hydrocarbons. It may be possible to genetically engineer them to efficiently convert kerogen into liquid or gaseous hydrocarbons. The mining method would then involve drilling holes into the oil shale and fracturing it with explosives below ground. Next, a nutrient solution containing the bacteria would be pumped into the formation; a sufficient time later, oil and gas could be extracted. If the bacteria were made dependent upon the nutrient solution, they would perish when the solution was exhausted.

It must be recognized that even the worldwide reserves of oil shale and tar sand are finite, and that other energy sources must be developed in the future. As of 2023, the United States consumed more than 20 million barrels of oil per day.

Principal Terms

barrel: the standard unit of measure for oil and petroleum products, equal to 42 US gallons or approximately 159 liters

bitumen: a generic term for a very thick, natural semisolid; asphalt and tar are classified as bitumens

fracking: a controversial method of extracting natural gas from shale by fracturing it to liberate the gas

hydrocarbon: an organic compound consisting of hydrogen and carbon atoms linked together

kerogen: a waxy, insoluble organic hydrocarbon that has a very large molecular structure

oil shale: a sedimentary rock containing sufficient amounts of hydrocarbons that can be extracted by slow distillation to yield oil

reservoir rock or sand: the storage unit for various hydrocarbons; usually of sedimentary origin

retort: a vessel used for the distillation or decomposition of substances using heat

tar sand: a natural deposit that contains significant amounts of bitumen; also called oil sand

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