A critical measure of the viability of extraction of shale oil lies in the ratio of the energy produced by the oil shale to the energy used in its mining and processing, a ratio known as "Energy Returned on Energy Invested" (EROEI). An EROEI of 2 (or 2:1 ratio) would mean that to produce 2 barrels of actual oil the equivalent in energy of 1 barrel of oil has to be burnt/consumed.