Oil shale is a class of sedimentary rock (not necessarily shale) that contains significant amounts of Kerogen, a waxy substance from which oil can be obtained through a process called pyrolysis. The worldwide oil reserves contained in oil shale are estimated to be in the region of three trillion barrels , but obtaining this oil is both economically and environmentally expensive - a problem that has limited its use to date.
- World Energy Council (February 2006). Annual Energy Outlook 2006 (PDF). Energy Information Administration. Retrieved on 2008-04-18.