The clay that settles out of suspension in quiet waters is buried and transformed into shale. Organic matter trapped within is subjected to pressure that increases at slightly less than the geostatic pressure gradient, which is about 1 pound per square inch (psi) per foot of depth. The temperature increases gradually, both from compression and by heating from the earth's interior. (Below a thin zone that is affected by climate, the temperature rises about 1.5°F for every 100 feet of depth.)
At 120° to 150°F, certain chemical reactions that ordinarily proceed very slowly begin to occur much more quickly. The organic matter trapped within the rock begins to change. Long-chain molecules are broken into shorter chains; other molecules are reformed, gaining or losing hydrogen; and some short-chain hydrocarbons are combined into longer chains and rings. The net result is that solid hydrocarbons are converted into liquid and gas hydrocarbons. Thus the energy of the sun, converted to chemical energy by plants, redistributed among all the creatures of the food chain, and preserved by burial, is transformed into petroleum.
The petroleum window - the set of conditions under which petroleum will form - includes temperatures between the extremes of 100°F and 350°F. The higher the temperature, the greater is the proportion of gas. Above 350°F almost all of the hydrocarbon is changed into methane and graphite (pure carbon). Source beds (or reservoirs) deeper than about 20,000 feet usually produce only gas.
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