"Given that unconventional sources are needed, which ones make the most sense to extract? It takes an unusually high amount of energy to get at them—whether it be tar sands, natural gas from hydraulic fracturing shale, or old oil deposits that can be flooded with steam to scour out more petroleum. To help compare fuel sources with a common metric, ecologist Charles A. S. Hall of the S.U.N.Y. College of Environmental Science and Forestry has created a measure called the “energy return on investment” (EROI). It indicates the energy that fuels provide per unit of energy spent—a ratio of energy obtained."
Scientific American (sub required)
click on image to enlarge