what is oil

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There are three essentials in the creation of a crude oil field: First, a "source rock" whose geologic history allowed the formation of crude oil. This usually is a fine-grained shale rich in organic matter. Second, migration of the oil from the source rock to a "reservoir rock," usually a sandstone or limestone that's thick and porous enough to hold a sizable accumulation of oil. A reservoir rock that's only a few feet thick may …

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…oil-bearing rocks date back more than 600 million years; the youngest, about 1 million. However, most oil fields have been found in rocks between 10 million and 270 million years old. Subsurface temperature, which increases with depth, is a critical factor in the creation of oil. Petroleum hydrocarbons rarely are formed at temperatures less than 150 degrees Fahrenheit and generally are carbonized and destroyed at temperatures greater than 500 degrees. Most hydrocarbons are found at "moderate" temperatures ranging from 225 to 350 degrees.