Most geologists view crude oil, like coal and natural gas, as the product of compression and heating of ancient organic materials over geological time scales. According to this theory, it is formed from the decayed remains of prehistoric small marine animals and algae. (Terrestrial plants tend to form coal.) Over millennia 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. Because hydrocarbons are less dense than the surrounding rock, these migrate upward through adjacent rock layers until they become trapped beneath impermeable rocks, within porous rocks called reservoirs. Concentration of hydrocarbons in a trap forms an oil field, from which the liquid can be extracted by drilling and pumping.
Geologists also refer to the ?oil window?. This is the temperature range that oil forms in-below the minimum temperature oil does not form, and above the maximum temperature natural gas forms instead. Though this corresponds to different depths for different locations around the world, a ?typical? depth for the oil window might be 4 - 6 km. Note that oil may be trapped at much shallower depths, even if it is not formed there. Three conditions must be present for oil reservoirs to form: a rich source rock, a migration conduit, and a trap (seal) that concentrates the hydrocarbons.
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.
FORMATION OF OIL: Abiogenic theory
The idea of abiogenic petroleum origin was championed in the Western world by astronomer 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.
According to the following authors; V. A. Krayushkin, T. I. Tchebanenko, V. P. Klochko, Ye. S. Dvoryanin from the Institute of Geological Sciences, Kiev, Ukraine, the modern Russian-Ukrainian theory of deep, abiotic petroleum origins is by no means simply an academic proposition. After its first enunciation by N. A. Kudryavtsev in 1951, the modern theory was extensively debated and exhaustively tested. Significantly, the theory not only withstood all tests put to it, but it also settled many previously unresolved problems in petroleum science, such as that of the intrinsic component of optical activity observed in natural petroleum. It also demonstrated new patterns in petroleum, previously unrecognized, such as the paleonological and trace-element characteristics of reservoirs at different depths. Most importantly, the modern Russian-Ukrainian theory of deep, abiotic petroleum origins has played a central role in the transformation of Russia (then the U.S.S.R.) from being a ?petroleum poor? entity in 1951 to the largest petroleum producing and exporting nation on Earth, principally with the drilling and development of the oil and gas fields in the Dnieper-Donetsk Basin.
However, this theory is very much a minority opinion, especially amongst western geologists. It often pops up when scientists are not able to explain apparent oil inflows into certain oil reservoirs. However, most of these ?abiotic? fields are explained as being the result of geologic quirks. No western oil companies are currently known to explore for oil based on this theory.
ALTERNATIVE MEANS OF PRODUCING oil
As oil prices continue to escalate, other alternatives to producing oil have been gaining importance. The best known such methods involve extracting oil from sources such as oil shale or tar sands. These resources are known to exist in large quantities; extracting the oil at low cost and without too deleterious an impact on the environment remains a challenge.It is also possible to transform natural gas or coal into oil (or, more precisely, the various hydrocarbons found in oil).
The best-known such method is the Fischer-Tropsch process. It was a concept pioneered in Nazi Germany when imports of petroleum were restricted due to war and Germany found a method to extract oil from coal. It was known as Ersatz (?substitute? in German), and accounted for nearly half the total oil used in WWII by Germany. However, the process was used only as a last resort as naturally occurring oil was much cheaper. As crude oil prices increase, the cost of coal to oil conversion becomes comparatively cheaper.
The method involves converting high ash coal into synthetic oil in a multistage process. Ideally, a ton of coal produces nearly 200 liters (1.25 bbl, 52 US gallons) of crude, with by-products ranging from tar to rare chemicals.
Currently, two companies have commercialized their Fischer-Tropsch technology. Shell in Bintulu, Malaysia, uses natural gas as a feedstock, and produces primarily low-sulfur diesel fuels. Sasol in South Africa uses coal as a feedstock, and produces a variety of synthetic petroleum products. The process is today used in South Africa to produce most of the country?s diesel fuel from coal by the company Sasol. The process was used in South Africa to meet its energy needs during its isolation under Apartheid. This process has received renewed attention in the quest to produce low sulfur diesel fuel in order to minimize the environmental impact from the use of diesel engines.
An alternative method is the Karrick process, which converts coal into crude oil, pioneered in the 1930s in the United States.
More recently explored is thermal de-polymerization (TDP). In theory, TDP can convert any organic waste into petroleum.