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How is Oil Formed?

Oil is a vital energy source.
Narayani Karthik Jan 27, 2019
Much of the energy that is generated today, comes from organic matter. One of the major sources of energy happens to be oil, which is also a product of organic matter. Nature has its own ways of reserving energy resources.
Any organic matter like dead aquatic life, fossils of plants and animals, wood, etc., decays over a period of time, and it is sedimented with piles of mud and rocks. It is a known fact, that the Earth's mantle is in molten state, which melts this organic material into resources of oil.

Underground Formation

The answer to the formation of oil in nature, lies in the two most widely acclaimed theories - biogenic theory and abiogenic theory.

Biogenic Theory

Plants make use of solar energy to produce oxygen and carbohydrates from carbon dioxide and water, which they receive from the environment. This process, is known as photosynthesis. When plants die, or for that matter when any organic substance tends to decay, they get buried naturally and tend to get sedimented deeper and deeper into the layers of Earth.
Due to the heat of the Earth's mantle, the carbohydrates stored in these dead organic compounds get converted into hydrocarbons. This process takes place in fine grained rocks, also known as black shale.
Coal (one of the fossil fuels), which is one of the primary sources of fuel, is a solid hydrocarbon derived from land plants through this process. Also oil, which is a liquid hydrocarbon, is derived from the dead animals and aquatic life.
As per the biogenic theory, the hydrocarbons are already trapped inside Earth, when they are being formed. Once the oil formation occurs, the tectonic plates of rock, which are always in motion, keep moving, thereby exerting more and more pressure on the permeable layers of rock.
So during oil formation, the oil reserves settle in porous sedimentary rocks (like sandstone and limestone/black shale). Such oil reserve traps happen for various reasons as listed here.


This process occurs, when a reservoir rock, on one side of the fault is aligned against a impermeable rock on the other side. So there is a rock shifting movement upward or downward. If the reservoir rock, overlain by an impermeable layer of rocks, has undergone a folding process, the fault is known as a thrust fault.

Pinch out

If the reservoir rock loses the porosity or one could say, the rock has started thinning out, with a significant decrease in the grain size, the condition is known as a stratigraphic pinch out, which is one of the tangible causes for oil traps.


Some of the major prolific oil reservoirs are found in the porous coral reefs, built by corals and other creatures of the marine biome. These porous rocks are often sealed by cap rocks or impermeable rocks.


Rocks undergoing compression is a known fact. And that is how the compression folds of rocks result in anticlines and synclines, which mean hills and valleys, respectively. The crests of these anticlines are lined by the impermeable rock reservoirs of oil.

Salt Domes

Oceanic crust, like the continental, is nothing but a huge platter of moving tectonic plates. Also, ocean water is packed with salt. Many times, salt domes occur when the salt content at depth is forced outwards, by the weight of the surrounding rocks. More the salt deposits bulge out, crevices are formed in the upturned reservoir rocks, flanking the domes.
There is yet another dimension to this theory, where a few scientists, like Thomas Gold, claim that methane (one of the sources of oil), need not have a bio-organic origin and is already contained in the molten mantle of Earth, which was there ever since the time the planet was formed.
Some claims include the Earth, when it was being formed, was repeatedly hit by methane rich asteroids, which bear some correlation with the oil reserves today. However, there is much evidence illustrating the fact that organic matter is one of the most palpable sources of oil.
The constant pressure of the rocks and the intense heat generated by the mantle of Earth, cause the organic matter to transform into a dark and waxy substance known as kerogen. Kerogen molecules, gradually break into shorter and lighter molecules of carbon and hydrogen, ending up as oil or natural gas, depending on the liquid or gaseous state of the product.
The process of oil formation is not instantaneous. It happens over a long span of time. If Earth has substantial (let's say just enough!) oil sources, it is the result of transformation of organic dead matter into these sources, over a million year duration (which is quite long a time).
The crude oil, which is obtained by drilling techniques, like producing shock waves, using compression guns or sniffers, which can detect hydrocarbon compounds across the land and sea, is a smelly yellowish black liquid.
From the previously mentioned information, it is apparent as to why we call these sources as depletable resources. No wonder, today, man resorts to other alternative forms of energy to save the oil resources for future. Oil conservation is an important responsibility for this generation, to provide sufficient oil to the future generations.