Geology-What are Oil and Gas?
Oil and gas are primarily mixtures of compounds of carbon and hydrogen, known as hydrocarbons. They are formed as part of a natural cycle which begins with deposits of plant and animal remains and fine sediment. Trapped over millions of years, often deep beneath the ocean, this organic matter is transformed by the combined effect of temperature and pressure into oil and natural gas.
The formation of oil and gas deposits, or reservoirs, occurs when these hydrocarbons migrate upward through the rock layers towards the surface. These hydrocarbons often escape to the surface where they may form natural oil seeps or, in the case of gas, simply dissipate. Any hydrocarbons remaining on the surface are soon oxidized by bacteria. Sometimes oil and gas are trapped in deep underground structures which prevent them reaching the surface. They may be trapped underneath curved layers of rock called anticlines, or by faults in the rock. Faults occur when layers of rock split and move, such as in an earthquake or during normal seismic events. The term reservoir can be misleading, giving people the impression of large subterranean lakes full of oil. In fact, oil and gas are trapped within porous sedimentary rocks such as sandstone or shale and ma occupy as little as five percent of the rock volume.
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