Petroleum: Encyclopedia II - Petroleum - Origin

Petroleum - Origin

Petroleum - 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. 150m 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.

Petroleum - 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. 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) (see results [3]).

As with any petroleum, the idea goes, these hydrocarbons would migrate upwards with methane, sometimes bearing helium and nitrogen and heavy metals. Diamondoids are common 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 was verified in 1999, when an oil basin named "Eugene Island 330" off the coast of Louisiana went from being a relatively depleted reserve to suddenly refilling with pure oil, causing production to quickly raise back up to levels competing with when drilling began. Further evidence, such as the 2004 creation of methane in a laboratory by Henry Scott of the University of Indiana of South Bend and associates using inorganic elements and compounds, and the well-known astronomical fact that hydrocarbons exist on planetary bodies that have never had life, evidence this theory.

As well, the implications of this theory are what have been used to find most of the recent reserves world-wide. Traditional biotic theory only predicts oil in certain rock at a certain depth, but the abiotic theory allows for much more. Reserves in much of Saudi Arabia, Iran, Kazakhstan, the coast of Vietnam, and virtually all the oil in Russia were found either at depths or in source rock that are incompatible with traditional techniques. Recently, in Switzerland, Thomas Gold led an experimental drill for oil straight into pure granite, where he did indeed find enough to quickly produce one million barrels for testing.

Adapted from the Wikipedia article "Origin", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki