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[CENTER]Biogenic theory
Most geologists view crude oil, like coal and natural gas, as the product of compression and heating of ancient vegetation over geological time scales. According to this theory, it is formed from the decayed remains of prehistoric marine animals and terrestrial plants. Over many centuries this organic matter, mixed with mud, is buried under thick sedimentary layers of material. The resulting high levels of heat and pressure cause the remains to metamorphose, first into a waxy material known as kerogen, and then into liquid and gaseous hydrocarbons in a process known as catagenesis. These then migrate through adjacent rock layers until they become trapped underground in porous rocks called reservoirs, forming an oil field, from which the liquid can be extracted by drilling and pumping.

These reactions are thought to be very temperature sensitive: reactions that produce recognizable oil start to occur at about 130 °C, and those that continue the breakdown of oil to natural gas start to occur at about 180 °C. The range of 130 - 150 °C is generally considered the "oil window". Though this corresponds to different depths for different locations around the world, a 'typical' depth for an oil window might be 4 - 5 km. Three conditions must be present for oil reservoirs to form: a rich source rock, a migration conduit, and a trap (seal) that forms the reservoir.

The reactions that produce oil and natural gas are often modeled as first order breakdown reactions, where kerogen breaks down to oil and natural gas by a large set of parallel reactions, and oil eventually breaks down to natural gas by another set of reactions.

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Abiogenic Theory
The idea of abiogenic petroleum origin was championed in the Western world by Thomas Gold based on thoughts from Russia, mainly on studies of Nikolai Kudryavtsev. The idea proposes that large amounts of carbon exist naturally in the planet, some in the form of hydrocarbons. As with any petroleum, the idea goes, these would migrate upwards with methane, sometimes bearing helium and nitrogen and frequently heavy metals such as Nickel, Vanadium, Arsenic, Lead, Cadmium, Copper, Zinc, Mercury and others. Diamondoids are commom in oil and gas and its nature probably is related to natural diamonds that come from earth's mantle.The proponents of abiogenic petroleum claim that reserves are never exhausted because they are filled from below. This idea has not been supported by any critically reviewed research. It has been widely discredited by scientists and geologists alike. Also, even if oil fields can be replenished from abiotic deposits that exist deeper within the earth, it would be very near impossible that they could be replenished at current rates of depletion, never mind future rates. It would certainly take many thousands if not millions of years for oil fields to regain original levels. Thomas Gold was the most widely known Western proponent of the Russian-Ukrainian theory of abiogenic petroleum origin. This theory suggests that large amounts of carbon exist naturally in the planet, some in the form of hydrocarbons. Hydrocarbons are less dense than aqueous pore fluids, and migrate upward through deep fracture networks. Thermophilic, rock-dwelling microbial life-forms are in part responsible for the biomarkers found in petroleum. However, their role in the formation, alteration, or contamination of the various hydrocarbon deposits is not yet understood[1]. Thermodynamic calculations[2] and experimental studies confirm that n-alkanes (common petroleum components) do not spontaneously evolve from methane at pressures typically found in sedimentary basins, and so the theory of an abiogenic origin of hydrocarbons suggests deep generation (below 200 km)[/CENTER]


http://en.wikipedia.org/wiki/Petroleum