Millions of years ago, oil shale was formed from the deposition of silt and organic matter found on the seabed and lakebeds. Over long periods of time, heat and pressure transformed this material into oil shale in a process similar to the formation of petroleum, although the heat and pressure were not as intense. Oil shale is a fine-grained sedimentary rock rich in organic matter. It contains kerogen (a solid mixture of organic chemical compounds) from which liquid hydrocarbons can be produced.