Oil shale

Oil shale is shale rock containing crude oil that does not flow easily out of the rock. As a petroleum resource, it is mined as a solid and either burned in that form or crushed and heated to extract the liquid oil. Oil shale should not be confused with shale oil, which is oil obtained from wells drilled in shale. ( Oil shale is taken from the ground as a solid, and shale oil is taken from the ground as a liquid.) Oil shale contains between 60 and 400 litres of oil per metric ton of rock. It has been used as a fuel since the late 19th century if not earlier. Limited but continuous use of it has been made in China since 1909 and in Russia since 1916.

The amount of oil shale in the world is huge, and it is widely distributed, but it is a difficult resource to exploit. The amount of in-place shale oil in the world is estimated at 3.2 x 1014 m&sup3; (2 x 1015 barrels). Only a small fraction of that is thought recoverable. World production in 2000 was 347,000 m&sup3; (2.2 million barrels), much of that used as a solid fuel in electrical generation.

The retorting process to get the oil out of the rock involves crushing the rock, then heating it to 330 to 440 degrees C. The process does not involve high pressures or complicated chemistry. However, disposal of the spent rock can be problemmatic. After the most common retorting method, it consists of a fine powder that is 80-85% of the original rock by weight, but about 112% by volume. If it is disposed of above ground it must be wetted to turn it into a pseudo-concrete so that it does not blow away, and to rinse salts out of it so that vegetation can grow on it. This uses about 3 volumes of water for each volume of oil produced.