The future of Utah's petroleum industry may be found in the extensive oil-shale and tar-sand reserves located in the eastern half of the state. Utah's oil-shale reserves are estimated at several trillion barrels, with the richest deposits located in the Uinta Basin, where 90 to 115 billion barrels are contained in deposits that have the potential to yield twenty-five or more gallons per ton. Tar-sand, or oil-impregnated rock, is located in more than fifty deposits stretching from the Uinta Basin south into San Juan and Garfield counties. Utah's tar-sand deposits contain an estimated 23 to 29 billion barrels. Beginning in 1970, the United States Department of the Interior started several programs to stimulate oil-shale and tar-sand development; however, commercial extraction of petroleum from oil-shale and tar-sand at this time remains a potential rather than a reality.