First, what it isn’t; it isn’t oil and it isn’t shale. The rock is a marlstone rich in calcium carbonate that is intermediate between mudstone and limestone. The black stuff is kerogen, a precursor to oil.
Because kerogen is a mixture of organic material, rather than a specific chemical; it cannot be given a chemical formula. Indeed its chemical composition can vary distinctly from sample to sample. Kerogen from the Green River Formation oil shale deposit of Colorado contains elements in the proportions: Carbon 215: Hydrogen 330: Oxygen 12: Nitrogen 5: Sulfur 1.
In order to convert kerogen to oil it must be heated to high temperatures. Nature converts kerogen into oil by burying the rock to the point where the temperature and pressure is sufficient to do the job. Colorado’s oil shale was never buried deeply enough to reach the conditions of converting the kerogen to oil. Historically, humans have tried to do this artificially by bringing oil shale to the surface and roasting it in kilns. Today, instead of bringing the shale to heat, companies are using different techniques to take heat down to the oil shale.
Colorado has the world’s largest resources of oil shale, by far. Oil shale is actually the rock marlstone which contains kerogen, a precursor to oil. The kerogen must be heated to more than 750 degrees to convert it into oil because it was never buried deeply enough for nature to convert the kerogen to oil.
People have been trying to economically produce oil from this rock for more than a century. Indeed, the Colorado Geological Survey issued a report in 1921 entitled, Oil Shales of Colorado. Thus far, technological and economic conditions have not combined to support a sustained oil shale industry in Colorado.
Oil shale has been facetiously branded “the fuel of the future”, because up until the 21st century, folks would claim, “when the price of oil reaches $xx.xx per barrel (usually about 50% higher than whatever the price currently was), oil shale will be economic.” However, a sea change occurred when Shell announced that they felt their process for extracting oil from the strata would be economic at a lower price than the current price.
Oil shale has a long history of production in the world, starting in 1837 in France (Autun mines, which were closed in 1957); Scotland 1850-1963; Australia 1865-1952, 1998-2004; Brazil 1881-1900, 1941-1957, 1972-??; Estonia 1921-??; Sweden 1921-1965; Switzerland 1921-1935; Spain 1922-1966; China 1929-??; and South Africa 1935-1960.
In 2007 there were five RD&D (Research, Demonstration, and Development) projects in the Piceance Basin of Colorado that have been awarded a 160-acre tract of federal land upon which to demonstrate to the Bureau of Land Management (BLM) the commerciality of their particular process. If they can prove that their process is capable of commercial production, then they have the right to purchase, at fair market value, an additional 5,000 acres that is pre-selected. In 2010, three additional RD&D projects were approved, but they do not have the right to acquire the 5,000 additional acres.
Oil shale should not be confused with shale oil. In shale oil, the strata were buried deeply enough that the temperature was sufficiently high to naturally convert the kerogen into oil. Currently, a major exploration effort is being carried out in Colorado to produce oil from the Niobrara shales, primarily in eastern Colorado. In shale oil plays such as the Bakken in North Dakota and Montana, the objective is to find brittle layers in the shale, drill horizontal holes along those brittle layers, artificially fracture the rock, and produce the resulting oil.
Colorado’s oil shales are found in the Eocene-age Green River Formation. The formation is widespread throughout western Colorado, southwestern Wyoming, and eastern Utah. A wide variety of exquisitely preserved fossils are abundant in the strata.In the Green River formation, at least, there seem to be fundamentally two pathways that lead to formation of oil shale beds: 1) accumulation of abundant planktonic organic matter in the bottom sediments, or 2) build up of organic material through the growth of microbial mats. The former process led to formation of homogeneous oil shale beds, and the latter resulted in finely laminated oil shales.
The Green River Formation was deposited in two large lakes that periodically dried up. This caused evaporite (salt, nahcolite, and dawsonite) deposits to be laid down with the oil shale. The nahcolite and dawsonite also comprise important resources of soda and aluminum.
There are no reserves of oil shale in the United States because one must have demonstrated commercial production in order to book reserves of oil. In 2010, the USGS upped Colorado’s oil shale resources to ~1.53 trillion barrels of potentially recoverable oil in the Piceance Basin.Whereas this seems like a huge number (and it is), a major question is at what rate it can be produced.
Historically, most oil shale production used retorts of one type or another. The rock was mined and then roasted in high temperature kilns that converted the kerogen to oil. That process is being attempted in one of the nine RD&D leases from the BLM in Colorado. A diagram of their setup is below.
The other RD&D leases in Colorado experimented with different varieties of in-situ processes; i.e., putting heat into the ground and converting the kerogen into oil in place. Below is a diagram of Shell’s In-Situ Conversion Process wherein they insert electric heaters into the oil shale.