HEAVY OIL AND TAR SAND

crude oils below 20 API gravity are usually considered to be heavy. The lighter conventional crudes are often waterflooded to enhance recovery. The injection of water into the reservoir helps to maintain reservoir pressure and displace the oil toward the production wells. In general, waterflooding is most effective with light crude oils of API gravity 25 and higher and becomes progressively less effective with oils below 25 API. With crudes of 20 and lower, waterfloods are essentially ineffective and thermal recovery becomes necessary. Very few thermal projects are successful in recovering oil of less than 10 API gravity. Heavy crude oils have enough mobility that, given time, they will be producible through a well bore in response to thermal recovery methods. Tar sands contain immobile bitumen that will not flow into a well bore even under thermal stimulation. The recovery of these resources requires mining. Joseph P. Riva, Jr. Gordon I. Atwater Additional reading Current information concerning the characteristic properties of oil shales, their sources, and special problems of exploitation may be found in the proceedings of meetings, such as Oil Shale Symposium Proceedings (annual); material originating at symposia chaired by Paul B. Tarman, Synthetic Fuels from Oil Shale (1980), Synthetic Fuels from Oil Shale II (1982), and Synthetic Fuels from Oil Shale and Tar Sands (1983); and H.C. Stauffer (ed.), Oil Shale, Tar Sands, and Related Materials (1981), symposium papers, including essays on oil shale cracking and retorting. General works include Ken P. Chong and John Ward Smith (eds.), Mechanics of Oil Shale (1984), a collection of summary papers on the exploitation of oil shales; T.F. Yen and George V. Chilingarian (eds.), Oil Shale (1976), background essays on different aspects of oil shale technology and science; Paul L. Russell, History of Western Oil Shale (1980); and Perry Nowacki (ed.), Oil Shale Technical Data Handbook (1981). The transitional character of kerogen rocks and their limnological and stratigraphical properties are treated in Bernard Durand (ed.), Kerogen: Insoluble Organic Matter from Sedimentary Rocks (1980); Bartholomew Nagy and Umberto Colombo (eds.), Fundamental Aspects of Petroleum Geochemistry (1967); and A.I. Levorsen, Geology of Petroleum, 2nd rev. ed. (1967).Data on world distribution, exploitation, and technology of heavy oil are included in Ferdinand Mayer, Weltatlas Erdl und Erdgas, 2nd ed. (1976). Richard F. Meyer (ed.), Exploration for Heavy Crude Oil and Natural Bitumen (1987), contains information on the size and distribution of the world's largest accumulations of heavy crude and bitumen. Additional information is presented in Zeitschrift fr Angewandte Geologie (monthly); Erdl-Erdgas (monthly); and Oil and Gas Journal (weekly). Joseph P. Riva, Jr.