The United States Department of Interior's Bureau of Land Management recently announced that BLM will "take a fresh look at commercial oil shale and tar sand plans issued under the prior administration." For some people, this will raise two questions. What is "oil shale"? What are "tar sands"?
The term "oil shale" in particular might confuse some people. Oil shale is something different than the shale formations that have been receiving so much attention recently in connection with hydraulic fracturing. Those shale formations contain natural gas, or sometimes oil, but traditionally it has not been economical to produce that oil or gas. In recent years, however, advances in hydraulic fracturing and horizontal drilling has made such production economically feasible (see my 3/28/11 post).
Oil shale is something different. Oil shale is a type of sedimentary rock that contains a solid hydrocarbon called "kerogen." Oil shale is formed in a process similar to the processes which are believed to lead to the formation of natural gas or oil. Organic material is deposited along with silt in sea bottoms. Over time, the organic material is buried by more and more silt, and is subjected to heat and pressure. Depending on the amount of heat and pressure, the organic material can be converted to natural gas, oil, or oil shale.
Oil shale can be mined like any other solid material. If the oil shale is then heated in a surface container to a temperature between 750 and 930 °F, it undergoes a chemical change in which an oily liquid is formed. This process is called "retorting." The oily liquid, sometimes called "shale oil," can be upgraded and then sent to a refinery. Because a chemical change takes place during retorting, the process is different than the process of melting a solid or the process of heating a viscous liquid to make the liquid flow more easily.
Retorting generally is performed in a plant, but companies sometimes perform retorting while the oil shale still is in the ground (called "in situ retorting") by using vertical heaters placed in holes to raise the temperature to about 650 to 700 °F. This saves the step of transporting the solid oil shale.
Some people criticize the use of oil shale on environmental grounds, objecting to the mining process, the energy needed in retorting, and the need to do something with the solid material left after retorting.
The largest deposits of oil shale in the world are found in the United States in the Green River Formation that covers parts of Colorado, Utah, and Wyoming. The Green River Formation is estimated to contain the equivalent of 1.2 to 1.8 trillion barrels of oil, with the recoverable portion of that being the equivalent of perhaps 800 billion barrels of oil. At present rates of oil consumption, 800 billion barrels of oil could supply the entirety of U.S. oil demand for about 100 years.
In a later post, I'll discuss tar sands.