பெட்ரோலியம் எங்கிருந்து வருகிறது? பரிணாமவியலின் விடை

பெட்ரோலியம் எப்படி உருவானது, எப்படி பூமிக்குள் கிடைக்கிறது என்பதற்கு இரண்டு தேற்றங்கள் கூறப்படுகின்றன. ஒன்று பல கோடி ஆண்டுகளுக்கு முன்பு பூமியில் இருந்த உயிரினங்கள் பூமியின் அழுத்ததாலும் வெப்பத்தாலும் கரைந்து திரவமாக ஆகி இருக்கின்றன என்பது. மற்றொன்று, மீத்தேன் போன்ற வாயுக்கள் பாறைகளின் இடையே சிக்கி அழுத்தத்தால் திரவமாகியிருக்கின்றன என்பது. இரண்டுமே பல கோடி ஆண்டுகளாக அழுத்தத்தில் நடந்திருக்கின்றன என்றே கூறுகின்றன. இரண்டாவதான உயிரினம் சம்பந்தப்படாத பெட்ரோல் உருவாக்கத்தை யாரும் நம்புவதில்லை. ஏனெனில், அதன் மூலம் எங்கே பெட்ரோல் கிடைக்கும் என்பதை நிரூபிக்க முடிவதில்லை. ஆனால், முதலாவது உயிரின தேற்றத்தின் வழியே எங்கு பெட்ரோல் டெப்பாஸிட் இருக்கும் என்பதை யூகித்து கண்டறிந்து பல இடங்களில் பெட்ரோலை கண்டுபிடித்து அந்த தேற்றத்தை நிரூபித்துள்ளனர்.

ஆனால் யூத பழங்குடியினரது பழங்குடி கதைகளை நம்பும் கிறிஸ்துவர்களும் முஸ்லீம்களும் இந்த பல கோடி ஆண்டுகளாக உருவாகியிருக்கின்ற பெட்ரோலை போட்டு கார் ஓட்டிக்கொண்டே, உலகம் ஐந்தாயிரம் வருடத்துக்கு முன்னால் யாஹ்வே என்ற யூத தெய்வம் ஆறு நாட்களில் உருவாக்கியது என்று மனதறிந்தே பொய் பேசுகிறார்கள்.

Biogenic theory
Most geologists view crude oil and natural gas as the product of compression and heating of ancient organic materials over geological time. Oil is formed from the preserved remains of prehistoric zooplankton and algae which have been settled to the sea (or lake) bottom in large quantities under anoxic conditions. Terrestrial plants, on the other hand, tend to form coal. 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 organic matter to chemically change during diagenesis, first into a waxy material known as kerogen which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis.

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 where it was not formed (the Athabasca Oil Sands is one example).


Hydrocarbon trap.Because most hydrocarbons are lighter than rock or water, these often migrate upward through adjacent rock layers until they either reach the surface or become trapped beneath impermeable rocks, within porous rocks called reservoirs. However, the process is not straightforward since it is influenced by underground water flows, and oil may migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir. Concentration of hydrocarbons in a trap forms an oil field, from which the liquid can be extracted by drilling and pumping.

Three conditions must be present for 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) or other mechanism that prevents it from escaping to the surface. Within these reservoirs fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs.

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. Oil companies are looking for 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 shales are source rocks that have never been buried deep enough to convert their trapped kerogen into oil.

The reactions that produce oil and natural gas are often modeled as first order breakdown reactions, where kerogen is broken down to oil and natural gas by a set of parallel reactions, and oil eventually breaks down to natural gas by another set of reactions. The first set was originally patented in 1694 under British Crown Patent No. 330 covering,

"a way to extract and make great quantityes of pitch, tarr, and oyle out of a sort of stone."

The latter set is regularly used in petrochemical plants and oil refineries.


Abiogenic theory
Main article: Abiogenic petroleum origin
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 in the 1800s. Gold's hypothesis was that hydrocarbons of purely inorganic origin exist in the planet Earth. Since most petroleum hydrocarbons are less dense than aqueous pore fluids, Gold proposed that they migrate upward into oil reservoirs through deep fracture networks. Although biomarkers are found in petroleum that most petroleum geologists interpret as indicating biological origin, Gold proposed that Thermophilic, rock-dwelling microbial life-forms are responsible for their presence.

This hypothesis is accepted by only a small minority of geologists and petroleum engineers, and to date has not been particularly successful in predicting petroleum deposits on earth,[8] so it is considered a fringe theory. Methods of making hydrocarbons from inorganic material have been known for some time, however no substantial proof exists that this is happening on any significant scale in the earth's crust for any hydrocarbon other than methane (natural gas). Abundant liquid methane (though not any form of petroleum) has been inferred to be present on Titan, a moon of Saturn, by research data from NASA's Cassini probe. However, Titan has completely different geology from Earth, and is 1,200 gigametres (750,000,000 mi) or 8.0 AU away.[9]