Oil Shale

Oil shale is a fine-grained sedimentary rock containing a significant amount of kerogen, a solid mixture of organic compounds from which liquid hydrocarbons can be produced. Unlike conventional petroleum source rocks, the organic matter in oil shale has not been heated enough in nature to convert into free-flowing oil.

The rock is typically dark brown, grey or black, layered, and relatively soft, and it can smell faintly oily. When heated artificially (retorting), the kerogen breaks down to release shale oil and combustible gas.

Oil shale represents an enormous but technically and environmentally challenging fossil-fuel resource.

Oil shale forms in oxygen-poor aquatic environments such as stagnant lakes, lagoons and restricted marine basins, where abundant algae, plankton and plant debris accumulate with fine mud. The lack of oxygen prevents full decay, allowing the organic matter to be preserved and buried.

With burial the organic material is converted into kerogen, but the rock has not experienced the higher temperatures (the oil window) needed to generate liquid petroleum naturally. Major deposits include the Green River Formation in the western United States, plus large reserves in Estonia, China, Brazil, and Australia, recording ancient productive lake and sea floors.

Oil shale is recognized as a fine-grained, laminated rock of dark brown to black or olive color that is relatively soft and often light for its size due to its organic content. It may give off an oily or bituminous smell, especially when freshly broken or warmed.

A classic test is that it can be lit with a flame and will burn or smolder because of its kerogen, and it leaves a brown to black streak. It resembles ordinary black shale and coal; unlike coal it is rock-rich and crumbles into thin sheets, and unlike ordinary shale it is combustible and oily. Its layering, combustibility and oily odor are key clues.

Oil shale's chief significance is as a fossil-fuel resource: heating it in retorts produces shale oil that can be refined into fuels, and the process also yields combustible gas. Some countries, notably Estonia, burn oil shale directly to generate electricity.

Byproducts and spent shale are used in cement and construction materials, and oil shale has been a source of chemicals and fertilizers. However, extraction is energy-intensive, water-demanding and environmentally controversial, so much of the world's vast oil shale resource remains undeveloped. It has no ornamental or metaphysical use.