"tar sands" are a geological phenomenon found especially in Canada, in which bitumen-saturated sands are found near or at the surface of the ground; the hydrocarbon can be separated from the sand by mechanical or thermal processes. Heavy oil sands are similar to tar sands. Outside Canada the term "oil sands" generally refers to buried sandstone acting as an oil reservoir., sedimentary deposits of oil-bearing sands. The oil may be separated by steam heating or solvent extraction, and reforming processes are used to recover normal oil products. The Athabasca Tar Sands are extensive deposits in Canada and are estimated to contain recoverable oil equivalent to three quarters of the present world crude reserves, but extraction is expensive and difficult.