Oil shale is prevalent in the western states of Colorado, Utah, and Wyoming. The resource potential of these shales is estimated to be the equivalent of 1.8 trillion barrels of oil in place. Retorted oil shale yields liquid hydrocarbons in the range of middle-distillate fuels, such as jet and diesel fuel. However, because oil shales have not proved to be economically recoverable, they are considered a contingent resource and not true reserves. It remains to be demonstrated whether an economically significant oil volume can be extracted under existing operating conditions. In comparison, Saudi Arabia reportedly holds proved reserves of 267 billion barrels.