Non-renewable Energy

Coal extraction has a long and complex history in Colorado. It has been mined commercially in the state since 1864 when it was used for local residential heating. Smaller amounts of coal were mined as far back as 1859 when a local landowner excavated coal from his property and sold it to homesteaders for heating fuel. Coal production expanded in the 1860s-70s, as a result of the demands associated with the Colorado Gold Rush. Later, around the turn of the 20th Century, it was employed for brick and steel production. Most coal currently mined in Colorado is for power generation.

The estimated value of Colorado coal production in 2015 was $675 million, decreasing to $602 million in 2019. In 2020, the estimated value of statewide coal production was $441 million, giving Colorado the rank of thirteenth largest among coal producing states. The overall decreasing trend in coal production is due primarily to the increased nationwide use of natural gas and renewable energy. Coal from the state is mostly bituminous and sub-bituminous—with both underground and surface mines currently in operation on the Western Slope—and is characterized by its high heat, low sulfur, low to medium ash, and low mercury content.

More detailed reviews of Colorado coal regions, resources, and geology are presented in two CGS publications including a 2005 Rocktalk and a 2003 summary of coal resources in the state. Coal-bearing geological formations underlie ~28% of the state and range in age from the Lower Cretaceous to the Eocene. As of 2022, coal is mined in Gunnison, La Plata, Moffat, Rio Blanco, and Routt counties at six mines.

Most coal produced in Colorado is used for power generation but continues to decline primarily due to low natural gas prices, federal greenhouse gas regulations and taxes designed to cut carbon dioxide emissions, along with the declining cost and growing use of renewable energy sources. In 2010, Colorado passed the Clean Air, Clean Jobs Act which promotes the replacement of Front Range coal-fired power plants with natural gas plants. Since 2010, several Colorado coal-fired plants and units have been shut down or converted to natural gas. Several more coal-fired plants will either shut down or convert to natural gas in the near future. About 36% of the electricity generated in Colorado in 2020 came from coal-fired power plants. For comparison, in 2015, 60% of the electricity generated in Colorado came from coal indicating a rapid move away from energy production from coal.

Colorado coal production continues to decline from a high of ~39.8 million tons in 2004 to ~10.6 million tons in 2020. In response, coal mining employment in Colorado dropped from a high of ~2,279 in 2012 to ~901 people in 2020. The Colorado Division of Reclamation, Mining and Safety is responsible for implementing and enforcing the Colorado coal mining regulatory program including mine permitting, bonding, and inspection.

Coal is an organic clastic sedimentary rock composed mostly of ancient plant material. It is combustible because it contains carbonized, solid but brittle, ancient plant matter. Originally deposited as leaf-litter and plant remains in a fresh-water environment, the material first begins as peat, as in a peat bog. Over time it is compressed, dried, and modified by heat and pressure by sedimentary layers that cover it and by the proximity to the heat of the earth. This process is called ‘coalification’. The peat then undergoes diagenesis with depth of burial and tectonic forces. Coal, a black-to-brown rock, is the end product of this diagenetic process. The more complete the process, the higher the grade, or rank of the coal.

Texturally, coal is subdivided into four main classes: lignite, sub-bituminous, bituminous, and anthracite. This last class is the hardest coal and contains the most carbon. Lignite is the least dense coal with lower carbon content. Coal, a sedimentary rock, is composed mostly of carbon, hydrogen, oxygen, nitrogen (volatile hydrocarbons), and lesser amounts of ash, sulfur, and trace elements.

Bituminous and sub-bituminous coals make up the majority of Colorado’s coal resources, and are mined as clean low-sulfur but high-heat content coal products. Generally, the higher rank bituminous coals are found in the Cretaceous age Dakota and Mesaverde groups in western Colorado from Moffat County south to the state line. Anthracite occurs in Gunnison and Pitkin counties, and lignite is found in the Paleocene-age Denver Formation in the Denver Coal Region.

Coal in Colorado was formed in peat mires during the Cretaceous and Paleocene-Epoch of the Tertiary Period, between ~100 and 55 million years ago. Fresh-water bogs formed along the coastal plains adjacent to the shorelines of the Western Interior Seaway that existed in the mid-continent during this time. The climate was hot, the sea level was generally high, and the seaway shorelines fluctuated greatly through geologic time. Lignite deposits in the Denver Formation were deposited after the Western Interior Seaway receded and likely formed in fresh water swamps on the east side of the piedmont extending from the Front Range (Kirkham and Ladwig, 1979).

Coal forms during four critical stages as summarized below.

Stage 1 is the accumulation stage, where peat, or parent material of coal, accumulates in freshwater marshes, swamps, and rainforests. The accumulation of organic material must exceed the oxidation or biodegradation of the environment.

Stage 2 is called the burial and preservation stage. Conditions of subsidence and fresh-water preservation and burial must be critical for coal to form.

Stage 3 is the diagenesis and coalification stage where peat is transformed or metamorphosed to form coal. Heat and pressure geochemically alter the organic material into coal. Regional heat flow, such as in parts of Colorado, also alter the process in sedimentary basins to where the coal rank is upgraded, sometimes from bituminous coal to anthracite. This entire process is called “coalification. ”

During coalification, peat undergoes several changes as a result of bacterial decay, compaction, heat, and time. Peat deposits are quite varied and contain everything from pristine plant parts (roots, bark, spores, etc.) to decayed plants, decay products, and even charcoal if the peat caught fire during accumulation. Peat deposits typically form in a waterlogged environment where plant debris accumulated; peat bogs and peat swamps are examples. In such an environment, the accumulation of plant debris exceeds the rate of bacterial decay of the debris. The bacterial decay rate is reduced because the available oxygen in organic-rich water is completely used up by the decaying process. Anaerobic (without oxygen) decay is much slower.

For the peat to become coal, it must be buried by sediment. Burial compacts the peat and, consequently, much water is squeezed out during the first three stages of burial.

Stage 4 is the continued burial and the addition of heat and time causing the complex hydrocarbon compounds in the peat to break down and alter in a variety of ways. The gaseous alteration products (methane is one) are typically expelled from the deposit, and the deposit becomes more and more carbon-rich as the other elements disperse. The stages of this trend proceed from plant debris through peat, lignite, sub-bituminous coal, bituminous coal, anthracite coal, to graphite (a pure carbon mineral).

Because of the amount of squeezing and water loss that accompanies the compaction of peat after burial, it is estimated that it took 10 to 15 vertical feet (3 – 5 m) of original peat material to produce one vertical foot (0.3 m) of bituminous coal in Colorado. The peat to coal ratio is variable and dependent on the original type of peat, coal rank, length of time of burial, and the depth at which it was buried.

Metallurgical Coal

Coal is used to make coke for the iron and steel industry, foundries, and other industries. The presence of large domestic deposits of coking coal, or metallurgical coal, played an important role in the development of the U.S. iron and steel industry. Coke is used chiefly to smelt iron ore and other iron bearing materials in blast furnaces, acting both as a source of heat and as a chemical reducing agent, to produce pig iron, or hot metal. Coke, iron ore, and limestone are fed into the blast furnace, which runs continuously. Hot air blown into the furnace burns the coke, which serves as a source of heat and as an oxygen reducing agent to produce metallic iron. Limestone acts as a flux and also combines with impurities to form slag.

Foundries use coke as a source of heat for producing metal castings. Other industrial uses of coke include the smelting of phosphate rock to produce elemental phosphorus and the production of calcium carbide. Small sizes of coke, termed breeze, are used as fuel in sintering finely sized particles of iron ore and other iron bearing material to produce an agglomerate that can be used in a blast furnace.

Coke is made by baking a blend of selected bituminous coals (called metallurgical coal or coking coal) in special high temperature ovens without contact with air until almost all of the volatile matter is driven off. The resulting product, coke, consists principally of carbon. A short ton of coal yields about 1,400 pounds of coke and a variety of byproducts such as crude coal tar, light oils, and ammonia, which are refined to obtain various chemical products. About 1,100 pounds of coke are consumed for every short ton of pig iron produced.

Colorado coal regions

Coal mining was first reported in 1859 and the first recorded official coal production was in 1864. Coal was first mined from horizontal seams south of Boulder in the Marshall area, and in vertical seams in Golden. There are eight major coal regions in Colorado. Historically, coal has been mined from all of these coal regions. There have been over 1,700 coal mine operations in Colorado’s history. On the Western Slope are the Green River, Uinta, San Juan River coal regions. On the eastern slope are the Denver, Raton, and Canon City coal regions. In the central part of Colorado are the North Park and South Park coal regions. Most of the coal mined today comes from northwest Colorado in the Uinta and Green River coal regions.

Coal is a less glamorous commodity than gold and silver, but its economic importance was similar in 2020 with Colorado coal production valued at ~$441 million and gold production valued at $481 million. In addition to coal being mined from Golden to Boulder in the 19th century, Colorado Springs and Trinidad also saw extensive coal mining during that period. Some coal is shallow enough to the surface that it can be mined by open-pit methods, like those in Moffat County. In other places the layers extend beneath mountainous terrain, thus requiring deep underground mining, such as the mines in Gunnison and Delta counties.

Colorado Division of Reclamation, Mining and Safety — Coal mine production data and permitting.

Colorado Oil and Gas Conservation Commission (COGCC) – Data Downloads — Access to all available statewide oil and gas data.

U.S. Department of Energy – Energy Information Administration – Coal — Comprehensive information on national coal production and consumption.

U.S. Department of Energy – Energy Information Administration – Colorado Energy Overview — Comprehensive information on state-wide energy production and consumption.

U.S. Geological Survey – U.S. Coal Resources and Reserves Assessment — Information about coal in Colorado and throughout the U.S.

U.S. Geological Survey – Coal fields of the conterminous United States—National Coal Resource Assessment (updated version) — Map-based information on coal fields with associated GIS data package.

U.S. Geological Survey – Map Showing the Extent of Mining, Locations of Mine Shafts, Adits, Air Shafts, and Bedrock Faults, and Thickness of Overburden Above Abandoned Coal Mines in the Boulder-Weld Coal Field, Boulder, Weld, and Adams Counties, Colorado — An important compilation from 2001 made in collaboration with the CGS.

U.S. Office of Surface Mining Reclamation and Enforcement (OSMRE) — Regulates coal mines and abandoned mines on federal property.

Coalbed methane (CBM) is natural gas produced from subsurface coal deposits. Much of the technology uses standard well-drilling and production techniques. The primary difference is the total usage of water in the process. In most cases, water must be pumped out of the formation in order to allow the methane that is adsorbed onto the coal particles to escape and travel to the surface via extraction wells.

Typical natural gas wells reach a peak and then begin a volumetric decline in production relatively rapidly. Coalbed methane wells behave differently. They increase in production for a long time as more and more water is removed from the strata. Decline in production does not usually begin until late in the life of the well.

CBM production in Colorado reached its highest levels, 59%, of total natural gas production during 1998 and has continuously declined to ~11% of the total natural gas production in 2020. This decline is largely due to the increase of natural gas production of unconventional reservoirs by the utilization of horizontal drilling and hydraulic fracturing techniques. As of 2017, Colorado contained about 27% of U.S. CBM reserves.

Water production from coalbed methane (CBM) wells has the potential to affect groundwater movement to streams and springs where the CBM-producing formations outcrop and are near the land surface. The CGS participated in studies of the San Juan, Raton, Piceance, and Sand Wash basins that assessed the significance, if any, of these potential impacts. The studies were a joint effort by the Colorado Division of Water Resources (DWR), the Colorado Oil and Gas Conservation Commission, and the CGS. All these agencies were part of the Colorado Department of Natural Resources at the time of these studies.

The CGS defined the geological framework of each basin and CBM-producing intervals to identify the primary pathways of groundwater movement. A consultant, S. S. Papadopulos & Associates, used this information in their hydrogeological models to ascertain any current and future impacts to streams. Note, for the Sand Wash Basin, depletion modeling was not performed because findings indicated that CBM and water production had been very limited so far and that the potential for future development was limited under current economic and technological conditions. Furthermore, geologic complexity of the basin indicated that regional quantitative assessments as originally proposed would not be suitable in this basin. Finally, the DWR oil and gas, produced-water rule making process in early 2010 reduced the need for delineating basin-wide non-tributary areas.

The concern has been raised that the removal of significant volumes of water from aquifers that may be tributary to the surface stream system could be resulting in stream depletions or a reduction in spring flows and/or formation outflows (accretions) that are of a magnitude sufficient to cause injury to senior water rights holders on over-appropriated stream systems throughout Colorado. This water historically has been disposed by one or more methods, including re-injection into deep formations, discharge to the surface stream system, and ponding/evaporation. These studies seek to develop a reliable assessment as to the levels of depletion, definition of the areas where CBM water production is ongoing that might be classified as non-tributary, definition of any potential correlations of water quality, geology, aquifer geometry, or formation/well depth that could lead to general guidelines about the potential for stream depletion that would be useful in either prompting or avoiding more detailed studies, and development of recommendations for further data collection or investigations.

Colorado Oil and Gas Conservation Commission (COGCC) – Colorado Oil and Gas Information System — The COGCC regulates non-renewable energy production in Colorado including coalbed methane and issues reports on both production and stream depletion.

Global Methane Initiative (GMI) — An international public-private partnership focused on reducing barriers to the recovery and use of methane as a clean energy source.

United Nations Economic Commission for Europe – Group of Experts on Coal Mine Methane — The principal activity of the Group of Experts on CMM is development and dissemination of the Best Practice Guidance for Effective Methane Drainage and Recovery in Coal Mines.

U.S. Department of Energy – Energy Information Administration – Colorado coalbed methane production — A variety of statistics on production with links to other resources.

U.S. EPA – Coal Mine Methane Project Resources — Resources and tool to maximize profitable methane reductions.

U.S. EPA – Coalbed Methane Outreach Program (CMOP) — CMOP works cooperatively with the coal mining industry in the US to reduce coal mine methane emissions.

U.S. Geological Survey – The Energy Resources Program — Research advancing the understanding of US energy resources including coalbed methane.

Colorado contains abundant oil and natural gas resources and reserves as well as a plethora of oil and gas fields throughout the state. A summary of these deposits is included in two CGS RockTalks on oil and the Niobrara Formation. Natural deposits of oil, or petroleum, include a variety of liquid compounds (crude oil), comprised of varying proportions of carbon and hydrogen (hydrocarbons), and gaseous hydrocarbons (natural gas) commonly comprised of methane and other constituents (e.g. ethane, propane, and butane). Minor amounts of nitrogen, oxygen, sulfur, and other trace elements are also present in petroleum. These non-renewable fuels are likely the result of decomposition and burial of organic matter produced by either marine or terrestrial plants and/or animals that died long ago. The formation of oil and natural gas from organic material is a complicated process. Petroleum geologists refer to the geologic elements and processes that are essential for natural oil and gas formation as a “petroleum system.” Generally, the geologic elements of petroleum systems include several components: source rocks, migration pathways, and reservoir rocks, seals, and traps. Geologic processes create each of these elements – specific factors associated with these elements determine if and where a commercial deposit may exist (Magoon, 1988).

At the end of 2019, Colorado contains ~1.56 billion barrels of proven oil resources and ranks 8^th in the U.S. for proven oil reserves. In 2020, Colorado produced ~171.5 million barrels of oil. Currently, in 2020, 29 counties produced oil with most produced from Weld County (~138 million barrels) followed by Adams, Broomfield, Rio Blanco, Arapahoe, Garfield, and Jackson counties (produced a combined ~17.6 million barrels of oil in the same year).

Natural gas, in itself, might be considered an uninteresting gas—it is colorless, shapeless, and odorless in its pure form. Quite uninteresting—except that natural gas is combustible, abundant in the United States, and when burned gives off a great deal of energy and few emissions. Compared to other non-renewable fuels, natural gas is cleaner burning and emits lower levels of potentially harmful byproducts into the air. We require energy constantly, to heat our homes, cook our food, and generate our electricity. It is this need for energy that has elevated natural gas to such a level of importance in our society, and in our lives.

At the end of 2019, Colorado contains 24,115 billion cubic feet (Bcf) of proven natural gas resources and ranks 7th in the U.S. for proven natural gas reserves. In 2020, 36 counties produced natural gas producing ~2,064 Bcf of natural gas (including coalbed methane). Currently, in 2020, most natural gas in Colorado is produced from Weld County (~953.8 Bcf) followed by Garfield (~424.9 Bcf), La Plata (~220.8 Bcf), Rio Blanco (~100.0 Bcf), Las Animas (~41.7 Bcf), and Mesa counties (~40.5 Bcf).

Oil

For kerogen to be transformed into oil, it must be buried to a depth where the temperature and pressure are sufficiently high to convert the kerogen into oil. The place where the depth is sufficient to achieve this, is called the “oil window”. Oil shale which contains only kerogen was never buried deeply enough to generate oil. Therefore, humans must artificially convert the kerogen in oil shale into oil at high temperatures.

Oil shale is different from shale oil. People are increasingly developing shale oils such as the Bakken shale oil in North Dakota and Niobrara shale oil in Colorado. These strata were buried deeply enough to convert the kerogen into oil. However, substantial quantities of the generated oil remained in the source rock, rather than migrating out of the source rock into a more conventional reservoir rock. So, oil companies are successfully using fracking technology and horizontal drilling to extract the oil from these units of shale oil. As of 2020, most of the oil production in Colorado is from these unconventional resources such as the Niobrara Formation in Weld County.

Natural gas

Natural gas is a non-renewable fuel. Like oil and coal, this means that it is, essentially, the remains of plants and animals and microorganisms that lived millions and millions of years ago. But how do these once living organisms become an inanimate mixture of gases?

There are many different theories as to the origins of non-renewable fuels. The most widely accepted theory says that non-renewable fuels are formed when organic matter (such as the remains of a plant or animal) is compressed under the earth, at very high pressure for a very long time. This is referred to as thermogenic methane. Similar to the formation of oil, thermogenic methane is formed from organic particles that are covered in mud and other sediment. Over time, more and more sediment and mud and other debris are piled on top of the organic matter. This sediment and debris puts a great deal of pressure on the organic matter, which compresses it. This compression, combined with high temperatures found deep underneath the earth, breaks down the carbon bonds in the organic matter. As one gets deeper and deeper under the earth’s crust, the temperature gets higher and higher. At low temperatures (shallower deposits), more oil is produced relative to natural gas. At higher temperatures, however, more natural gas is created, as opposed to oil. That is why natural gas is usually associated with oil in deposits that are 1 to 2 miles below the earth’s crust. Deeper deposits, very far underground, usually contain primarily natural gas, and in many cases, pure methane.

Natural gas can also be formed through the transformation of organic matter by tiny microorganisms. This type of methane is referred to as biogenic methane. Methanogens, tiny methane-producing microorganisms, chemically break down organic matter to produce methane. These microorganisms are commonly found in areas near the surface of the earth that are void of oxygen. These microorganisms also live in the intestines of most animals, including humans. Formation of methane in this manner usually takes place close to the surface of the earth, and the methane produced is usually lost into the atmosphere. In certain circumstances, however, this methane can be trapped underground, recoverable as natural gas. An example of biogenic methane is landfill gas. Waste-containing landfills produce a relatively large amount of natural gas from the decomposition of the waste materials that they contain. New technologies are allowing this gas to be harvested and used to add to the supply of natural gas.

Natural Gas Constituents

Natural gas is a combustible mixture of hydrocarbon gases. While natural gas is formed primarily of methane, it can also include ethane, propane, butane and pentane. The composition of natural gas can vary widely, but below is a chart outlining the typical makeup of natural gas before it is refined.

In its purest form, such as the natural gas that is delivered to your home, it is almost 100% methane. Methane is a molecule made up of one carbon atom and four hydrogen atoms, and is referred to as CH₄. The distinctive smell that we often associate with natural gas is actually an odorant called mercaptan that is added to the gas before it is delivered to the end-user. Mercaptan aids in detecting any leaks.

Natural gas is found underground, although sometimes it leaks to the surface. Oil and gas are often found together in underground traps. Oil floats on water and natural gas floats on oil. Because of these density contrasts, oil and gas rise in a reservoir until they are trapped at the highest point. When the trap is a fold, it can create a commercial accumulation underground. In a case like this, it is possible to produce the oil and/or gas without producing any water, which is good for a variety of reasons. Since 1978, the industry has learned how to produce natural gas from coals. This is called coalbed methane production, or CBM. As of 2020, about 11% of our natural gas production in Colorado is from CBM.

For hundreds of years, natural gas has been known as a very useful substance. The Chinese discovered a very long time ago that the energy in natural gas could be harnessed and used to heat water. In the early days of the natural gas industry, the gas was mainly used to light street lamps, and the occasional house. However, with much improved distribution channels and technological advancements, natural gas is being used in ways never thought possible.

There are so many different applications for this non-renewable fuel that it is difficult to provide an exhaustive list of everything it is used for. And no doubt, new uses are being discovered all the time. Natural gas has many applications, commercially, in your home, in industry, and even in the transportation sector! While the uses described here are not exhaustive, they may help to show just how many things natural gas can do.

Natural gas is one of the most popular fuels for residential heating. In 2000, according to the American Gas Association (AGA), 51 percent of heated homes in the U.S. (or 49.1 million households), used natural gas heating. In addition to heating homes, natural gas can also be used to help cool houses, through natural gas powered air conditioning. Some examples of other natural gas appliances include space heaters, clothes dryers, pool and jacuzzi heaters, fireplaces, barbecues, garage heaters, and outdoor lights. Commercial uses of natural gas are very similar to residential uses. The commercial sector includes public and private enterprises, like office buildings, schools, churches, hotels, restaurants, and government buildings. The main uses of natural gas in this sector include space heating, water heating, and cooling. For restaurants and other establishments that require cooking facilities, natural gas is a popular choice to fulfill these needs.

Natural gas has a multitude of industrial uses, including providing the base ingredients for such varied products as plastic, fertilizer, anti-freeze, and fabrics. Natural gas is consumed primarily in the pulp and paper, metals, chemicals, petroleum refining, stone, clay and glass, plastic, and food processing industries. These businesses account for over 84 percent of all industrial natural gas use. Natural gas is also used for waste treatment and incineration, metals preheating (particularly for iron and steel), drying and dehumidification, glass melting, food processing, and fueling industrial boilers. Natural gas may also be used as a feedstock for the manufacturing of a number of chemicals and products. Gases such as butane, ethane, and propane may be extracted from natural gas to be used as a feedstock for such products as fertilizers and pharmaceutical products.

Natural gas has long been considered an alternative fuel for the transportation sector. In fact, natural gas has been used to fuel vehicles since the 1930’s! There are about 110,000 NGVs on U.S. roads today and more than 12 million worldwide. There are about 1,000 NGV fueling stations in the U.S. – and about half of them are open to the public. Most natural gas vehicles operate using compressed natural gas (CNG). This compressed gas is stored in a similar fashion to a car’s gasoline tank, attached to the rear, top, or undercarriage of the vehicle in a tube shaped storage tank. A CNG tank can be filled in a similar manner, and in a similar amount of time, to a gasoline tank.

Natural gas, because of its clean burning nature, has become a very popular fuel for the generation of electricity. In the 1970’s and 80’s, the choices for most electric utility generators were large coal or nuclear powered plants; but, due to economic, environmental, and technological changes, natural gas has become the fuel of choice for new power plants.

Colorado Oil and Gas Association — Industry trade association.

Colorado Oil and Gas Conservation Commission (COGCC) – Colorado Oil and Gas Information System — The COGCC regulates non-renewable energy production in Colorado including oil and natural gas and issues reports on both production and consumption.

U.S. Department of Energy – Energy Information Administration – Colorado Energy Overview — Comprehensive information on state-wide energy production and consumption.

U.S. Department of Energy – Energy Information Administration – Natural gas explained — Comprehensive background information on natural gas.

U.S. Department of Energy – Energy Information Administration – Oil and natural gas production — A variety of statistics on production with links to other resources.

U.S. Department of Energy – Energy Information Administration – Oil and petroleum products explained — Comprehensive background information on oil and petroleum products.

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.

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.