Oil Shale

Tight oil should not be confused with oil shale, which contains kerogen. Kerogen is a heavy mixture of organic chemical compounds, the soluble part of which is known as bitumen.
Oil shale is a primitive form of oil that has not yet been converted into crude, because the sediment has experienced great heat or high pressure. It is typically found in shallow deposits and is mined, instead of drilled, with a consequent impact on the environment