As sedimentary organic matter is buried, it experiences progressively higher temperatures and pressures. Although most bacterial decomposition occurs quickly, in the upper meter or so, it may continue at a much slower pace almost indefinitely.
Indeed, bacteria have been found in subsurface rock at temperatures of up to 75ºC and depths of nearly 3 km.
As bacterial activity ceases, a number of new reactions begin as the organic matter attempts to come to equilibrium with higher temperature and pressures. These reactions, in which kerogen breaks down into a variety of hydrocarbons and a refractory residue, are collectively called catagenesis. As temperatures in the range of 100 to 150ºC are reached, a complex mixture of hydrocarbons, petroleum, is produced, along with less amounts of other bitumens, asphaltenes and resins. Collectively, this bitumen fraction is called oil or crude oil and is, of course, of great economic interest.
At temperatures above 150-175ºC, methane and graphite are the ultimate products, created in a process called metagenesis.
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