Oil Shale

Intro

Colorado has the world’s largest resources of oil shale, by far. More than half of the world’s known oil shale resources are located in the Eocene-age Green River Formation which is located in Colorado, Utah, and Wyoming. The Piceance Basin in western Colorado contains an estimated 1.525 trillion barrels of oil. Although Colorado contains a large amount of oil shale with potential hydrocarbon resources, it is currently difficult to produce oil from these rocks. Oil shale is different than shale oil. Shale oil (and gas) occurs in rocks where the oil is as a liquid between sediment grains. On the other hand, oil shale does not contain liquid petroleum and 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 a petroleum liquid because it was never buried deeply enough for nature to convert the kerogen to a liquid.

People have been trying to economically produce oil from this rock for more than a century. Indeed, the CGS issued a report in 1921 entitled, Oil Shales of Colorado. Thus far, despite another significant burst of activity in the 1980s, technological and economic conditions have not combined to support a sustained oil shale industry in the state.

Details

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.

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 the past, 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.”

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, much of the current oil production is from shale oil in the Niobrara Formation, primarily in eastern Colorado. In shale oil plays such as the DJ Basin in Colorado and 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 technically no reserves of oil shale in the United States because one must have demonstrated commercial production in order to book reserves of oil. In 2012, the USGS upped Colorado’s oil shale resources to ~1.525 trillion barrels of potentially recoverable oil in the Piceance Basin. Whereas this seems like a huge number (and it is!), a major question is how, and 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. Since 2007, there have been several research projects associated with extracting the oil from these rocks in the Piceance Basin of Colorado, however, there is currently no oil shale industry in the state. One of the research projects attempted mining the rock and heating to high temperatures  while other projects experimented with different types of in-situ processes; i.e., putting heat into the ground and converting the kerogen into oil in place.