The term “oil shale” refers generally to fine-grained sedimentary rock that contains solid bituminous materials, called kerogen, which can be converted into liquid and gaseous hydrocarbons (petroleum liquids, natural gas liquids, and methane) when the rock is heated in the chemical process of destructive distillation known as pyrolysis. Oil shale is found in a variety of depositional environments, including fresh water to highly saline lakes, epicontinental marine basins and subtidal shelves, and in limnic and coastal swamps, commonly in association with deposits of coal. Unlike crude oil or even tar sands, oil shale has not been subject to high enough heat over a long enough period of time to break the complex solid hydrocarbons down into lighter, liquid and gaseous compounds. One of its defining characteristics, however, is that it contains enough oil to burn without further processing, giving rise to its Ute Indian name, “the rock that burns.”
According to Jeremy Boak, director of the Center for Oil Shale Technology and Research, “oil shale” is not a misnomer, as is commonly held. “If oil shale is not shale,” he said, “then neither is the Barnett, the Haynesville, the Niobrara, or the Eagle Ford.” The difference is that shale oil, as currently used for plays like the Bakken and Eagle Ford, refers to rock that contains liquid hydrocarbons, and oil shale refers to rock that yields hydrocarbons upon heating under appropriate conditions. Many names have been used for oil shale over the centuries, such as albertite, algal coal, alum shale, bituminite, boghead coal, cannel coal, gas coal, kerosene shale, kukersite, schistes bitumineux, stellarite, tasmanite, torbanite, and wollongongite. Some of these names are still in use.
The organic matter in oil shale—composed chiefly of carbon, hydrogen, oxygen, and small amounts of sufur and nitrogen—is predominantly kerogen, which by definition is an organic material that is typically insoluble in ordinary organic solvents. The mineral and elemental content of oil shale differs distinctly from coal. The ash (or mineral) content of coal is generally less than 40 weight percent (wt%), whereas oil shale commonly has an ash content greater than 60 wt%. The organic matter of oil shale typically has a higher hydrogen and lower oxygen content than that of lignite and bituminous coal.