Various types of syncrude can be obtained by processing oil shales and tar sand. Tar sand, as the name suggests, is sand containing a tarry material which can also release oil when heated. The world's resources of oil shale and tar sand are significantly greater than the reserves of crude oil, and nearly half of these resources are in the United States. Another approach coal gasification, the conversion of coal into a flammable gas. Older processes used coal to produce water gas or blue gas which was composed primarily of hydrogen and carbon monoxide.
More modern processes convert coal into methane.
Though promising, all of these fuels present some problems. Currently they are more expensive to produce than oil or coal, and oil shale and tar sand pose a greater threat to the environment since they must be mined and processed with water, and the shale and sand must be disposed of after the oil has been removed.