Oil shales

A high proportion of organic material in mudrocks can be driven off as a liquid or gas by heating. These are called oil shales. The organic materials usually comes from dead algae that have broken down to form kerogen during diagenesis. Kerogen is a long chain hydrocarbon that form petroleum when they are heated.

The environments in which they formed must be anaerobic (lacks of oxygen) to prevent oxidisation of the organic material. This condition can only be found in lakes and shallow marine environment. Oil shales are black and the presence of hydrocarbons may be detected by the smell of the rock. It will make a brown, oily stain on other materials.

Largest oil shale reserves

Sources:   Gary Nichols. Sedimentology and Stratigraphy Second Edition

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