Tar sands, also known as oil sands and bituminous sands, are sand deposits which are impregnated with dense, viscous petroleum. Tar sand deposits are estimated to contain more than 2,000 billion barrels of oil throughout the world. Despite its huge reserve, large scale commercial oil production from tar sand only began in the late sixties, and is currently restricted to surface mining ventures of deposits near the surface; however, most of the tar sand deposits lie below the strip mining limit and can only be produced through wellbores by in-situ processes. Because of its viscous nature, the oil is not recoverable by usual primary production methods.