Encyclopedic Entry

Oil shale is a lot like coal—they're both rocks that can be burned for energy.
Photograph by Emory Kristof
Some carbonate-rich shale deposits are also dinosaur-rich rock formations. The Irati Formation, in the state of Parana, Brazil, is a thin deposit of carbonate-rich shale. It also contains many fossils of mesosaurs, a type of aquatic dinosaur.
Silica Gems
In addition to oil shales, silica from ancient organisms created rock formations that are rich in minerals such as quartz, chert, and opal.
Oil shale is a type of sedimentary rock that is rich in kerogen. Kerogen is a part of rock that breaks down and releases hydrocarbons when heated. Hydrocarbons are substances made entirely of hydrogen and carbon.Petroleum and natural gas are probably the most familiar hydrocarbons. The hydrocarbons in oil shale can be used as an alternative to petroleum or natural gas.
Like traditional petroleum, natural gas, and coal, oil shale and kerogen are fossil fuels. Fossil fuels developed from the remains of algae, spores, plants, pollen, and a variety of other organisms that lived millions of years ago in ancient lakes, seas, and wetlands. 
When these organisms died and drifted to the seabed, they were buried under new layers of plants and sediment. They encountered intense pressure and heat, decomposed, and slowly transformed into the waxy substance known as kerogen. 
There is not a consistent chemical composition of kerogen, because it has a variety of origins. Kerogen that formed from land plants (called humic kerogen) usually has a higher oxygen content than kerogen formed from plankton (calledplanktonic kerogen). However, all types of kerogen consist mainly of hydrocarbons; smaller amounts of sulfur, oxygen, and nitrogen; and a variety of minerals.
Oil shale can be thought of as a precursor to oil and natural gas. With more pressure and over more geological time, kerogen would heat to its “oil window” or “gas window” (the temperature at which it would release crude oil or natural gas).
A sedimentary rock, oil shale is found all over the world, including China, Israel, and Russia. The United States, however, has the most shale resources.
Spanning the U.S. states of Colorado, Utah, and Wyoming, the Green River formation is an underground oil shale formation that contains as much as 1.8 trillion barrels of shale oil. Although not all of this can be extracted, it is more than three times the proven petroleum reserves of Saudi Arabia. 
Oil Shale, Shale Oil, and Oil-Bearing Shale
Oil shale, shale oil, and oil-bearing shale are three different substances. Oil shale is a sedimentary rock. As it reaches its oil window, oil shale releases a liquid known as shale oil. Oil shale is the rock from which shale oil is extracted.
Shale oil is similar to petroleum, and can be refined into many different substances, including diesel fuel, gasoline, and liquid petroleum gas (LPG). Companies can also refine shale oil to produce other commercial products, such as ammonia and sulfur. The spent rock can be used in cement. 
Oil-bearing shales are underground rock formations that contain trapped petroleum. The petroleum trapped within the rocks is known as “tight oil” and is difficult to extract. Companies extracting tight oil often use hydraulic fracturing (fracking), while companies extracting shale oil most often use heat.
The Bakken formation, for example, is made of oil-bearing shale. It is a series of layered shale rocks with a petroleum reservoir trapped between the layers. The Bakken formation stretches from the province of Saskatchewan, Canada, through the U.S. states of Montana and North Dakota. Improved drilling technologies have allowed companies to extract oil from the Bakken formation, creating an economic boom in the region.