Shale oil, not to be confused with oil shale, refers to the liquid hydrocarbons occurring naturally as free or adsorption state in mudstone. It can also occur in shale formations that have reached the oil generation threshold and can therefore be directly exploited using such techniques as horizontal drilling and hydraulic fracturing. Tight oil shale formations are heterogeneous and vary widely over relatively short distances. Oil shale, an organic-rich sedimentary rock, is a combustible solid mineral resource with high ash and organic matter content from which unconventional oil resources such as natural oil can be refined through low-temperature carbonization. Oil shales are composed of solid hydrocarbon (kerogen), a decomposed organic matter still in a solid state, while tight oil is a light crude.