Shale oil is extracted from reserves, sometimes described as "tight oil" reserves, held in shales and other rock formations from which it will not naturally flow freely. Shale oil has become more accessible due to advances in the techniques of horizontal drilling and hydraulic fracturing or "fracking".
A US investment boom in shale oil and shale gas extraction caused a boom in natural gas supplies during 2012 and a dramatic reduction in the price of gas which sank to a 10 year low. Oil prices remained above $100 a barrel, however.
Unlike traditional reservoirs, shale is a low-porosity rock from which oil and gas will not flow freely without additional stimulation from hydraulic fracturing or fracking.
Shale gas production was pioneered in the early 2000s by US independent companies such as Devon Energy and Chesapeake Energy.