Fossil Fuels
According to the biogenic theory, oil is formed from the preserved remains of prehistoric zooplankton and algae which have been settled to the sea (or lake) bottom in large quantities under anoxic conditions. Over geological time this organic matter, mixed with mud, is buried under heavy layers of sediment. The resulting high levels of heat and pressure cause the organic matter to chemically change during diagenesis, first into a waxy material known as kerogen which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis. Terrestrial plants, on the other hand, tend to form coal. Many of the coal fields date to the carboniferous period.
Fossil fuels are of great importance because they can be burned (oxidized to carbon dioxide and water), producing significant amounts of energy. The use of coal as a fuel predates recorded history. Semisolid hydrocarbons from seeps were also burned in ancient times, but these materials were mostly used for waterproofing and embalming. Commercial exploitation of petroleum, largely as a replacement for oils from animal sources (notably whale oil) for use in oil lamps began in the nineteenth century. Natural gas, once flared-off as an un-needed byproduct of petroleum production, is now considered a very valuable resource. Heavy crude oil, which is very much more viscous than conventional crude oil, and tar sands, where bitumen is found mixed with sand and clay, are becoming more important as sources of fossil fuel. Oil shale and similar materials are sedimentary rocks containing kerogen, a complex mixture of high-molecular weight organic compounds which yields synthetic crude oil when heated (pyrolyzed), have not yet been exploited commercially.
