Origin of Crude Oil

Origin of crude oil has been a topic of debate by chemists and geologists for a long time. The most common theory is that the bodies of prehistoric plants and sea animals were trapped in sediments, which after many years due to heat and pressure were changed into crude oil and natural gas. 

Now let’s look at different theory that explains origin of crude oil in details:

Biogenic theory origin of crude oil

Most geologists view crude oil, like coal and natural gas, as the product of compression and heating of ancient organic materials over geological time. According to this theory, oil is formed from the preserved remains of prehistoric zooplankton and algae which have been settled to the sea bottom in large quantities under anoxic conditions. (Terrestrial plants tend to form coal, and very few dinosaurs have been converted into oil.) Over geological time this organic matter, mixed with mud, is buried under heavy layers of sediment. The resulting high levels of heat and pressure cause the remains to metamorphose, first into a waxy material known as kerogen which is found in various oil shale around the world, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis. 

Because most hydrocarbons are lighter than rock or water, these sometimes 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 often refer to an "oil window" which is the temperature range that oil forms in—below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Though this happens at different depths in different locations around the world, a 'typical' depth for the oil window might be 4–6 km. Note that even if oil is formed at extreme depths, it may be trapped at much shallower depths, even if it is not formed there. (In the case of the Athabasca Oil Sands, it is found right at the surface.)

Abiogenic theory origin of crude oil

The idea of abiogenic crude oil 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.

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 the result of geologic quirks. No western oil companies are currently known to explore for oil based on this theory.

Origin of crude oil reservoir

Three conditions must be present for crude oil reservoirs to form: first, a source rock rich in organic material buried deep enough for subterranean heat to cook it into oil; second, a porous and permeable reservoir rock for it to accumulate in; and last a cap rock (seal) that prevents it from escaping to the surface.

The vast majority of oil that has been produced by the earth has long ago escaped to the surface and been biodegraded by oil-eating bacteria. What oil companies are looking for is the small fraction that has been trapped by this rare combination of circumstances. Oil sands are reservoirs of partially biodegraded oil still in the process of escaping, but contain so much migrating oil that, although most of it has escaped, vast amounts are still present - more than can be found in conventional oil reservoirs. On the other hand, oil shale is source rocks that have never been buried deep enough to convert their trapped kerogen into oil.

Conclusion

Looking at origin of crude oil from most acceptable perspective, crude oil and gas form as a consequence of environmental conditions occurring in a sequence; the presence of organic material; organic remains are trapped and preserved in sediments, the material is buried deeply, then it is heated by increased temperature and pressure.