Tar sands are found in unconsolidated sand, sandstone, limestone and other sedimentary rock containing oil. This "in-place oil" has a low mobility so no significant primary production can be achieved. The material must be dug out and crushed in giant opencast mines. This takes five to 10 times the energy, area and water to mine, process and upgrade the tars it does to process conventional oil. Developing more suitable technologies requires considerable time and field experimentation. Possible effects on environment may be considerable.
The Athabasca deposits in Alberta, Canada are the world's largest resource, with estimated reserves of 1.8 trillion barrels, of which about 280-300bn barrels may be recoverable. Production now accounts for about 20% of Canada's oil supply.