Formation of Crude Oil and Discovery in India

Categories: Deforestation


The economic world today is largely dependent on fossil fuels, particularly crude oil (often known as petroleum). It refers to naturally occurring oil that is produced from hydrocarbons and other substances present in small quantities. It can be found underneath the crust of the earth trapped between the layers of impervious rocks, along with natural gas and water. There have been numerous theories surrounding the formation of crude oil, of which the Organic Theory is most commonly believed and suggests that the formation was possible due to the thermal maturation of organic matter within the earth's surface over a very long period.

A large part of the oil reserves in India are found in the anticlines and fault traps in the sedimentary rock which covers a total area of around 3.2 lakh km2 formed during the tertiary times. The oil reserves of India comprises of 27 basins, both on shore and offshore. Dibgoi in Assam became the first place in India to initiate the process of systematic oil drilling in November, 1866 and we have only looked at a developing India ever since.

Crude Oil: A brief description

Crude oil is the naturally occurring unaltered from of oil which is made up of hydrocarbon deposits and some other substances. It is typically obtained via the process of drilling, where it is found along with other resources, such as the natural gas (which sits above it as it is lighter) and saline water (which is denser and sinks below the oil).It is generally referred to as the raw form of fossil fuels and is refined to generate a large number of usable products.

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It is prominently known as "dark gold" due to its high demand and ever decreasing sources in today’s world. Unrefined petroleum has varying consistency and shades from dark to yellow contingent upon its hydrocarbon composition and it comprises of 50% and 97% of hydrocarbons; between 6% and 10% of it are nitrogen, oxygen, and sulphur and less than 1% of the contents are metals such as copper, nickel, vanadium, and iron.[1]
After crude oil is dug out from the oil reserves, it is sent to a refinery where the distillation process is employed by which crude oil is isolated into various useable petroleum components including gasoline, diesel and various other distillates like warming oil, stream fuel, petrochemical feedstock, greasing up oils and asphalt.

Inorganic Theories:

These theories suggest an inorganic origin of crude oil across the globe. Dmitri Mendele'ev (1877), the father of the periodic table of elements stated that metallic carbides present inside the Earth reacted with water at high temperatures to form acetylene (C2H2) which further condensed to form heavier hydrocarbons.
Other hypotheses (Berthelot, 1860, Mendele'ev, 1902) which is a new version of the acetylene theory stated that the mantle contained iron carbide which would react with percolating water to form methane:
FeC2 + 2H2O CH4 + FeO2
These theories are referred to as the deep-seated terrestrial hypothesis.
Another inorganic hypothesis which was suggested by Sokoloff (1890) proposed a cosmic origin. His theory suggested that one of hydrocarbons precipitated as rain from original nebular matter by which the solar system was made ofand that hydrocarbon penetrated into the earth's interior onto surface rocks. This theory and related theories are referred to as the extra-terrestrial hypothesis.[2]

Drawbacks of the Inorganic Theories:

The problem with modified acetylene theory is that it stated the presence of iron carbide in the mantle but still there is lack of evidence for the existence of iron carbide in the mantle.
There is no field evidence that inorganic processes have occurred in nature but there is lot of evidence for an organic origin.

Organic Theories:

It states that most of the petroleum is formed by the thermal maturation of organic matter over a very long period of time inside the earth’s surface which concludes that an organic origin generated the vast reserves (pools) of oil and gas.

Factors supporting the Organic Theories:

Carbon-hydrogen-organic matter connection: A discovery made by Smith, 1952; Smith, 1954; Stevens, 1956; Hunt, 1957; Meinschein, 1959; Erdman, 1961; Kvenvolden, 1964; Silverman, 1965 stated that hydrocarbons and relatedcompounds present in many living organisms and that which are deposited in the sediments are almost similar with very little change.
Chemical characteristics of petroleum reservoirs: Nitrogen and porphyrin (chlorophyll derivatives in plants, blood derivatives in animals) are found in all organic matter and they are also found in many petroleum sources.
Physical characteristics: Nearly every petroleum source occurs in the sediments that are primarily of the marine origin.
Due to these favouring factors of the organic theory as well as the drawbacks of the inorganic theories currently the Organic Theory holds the highest reason for the origin of crude oil and petroleum products among all of the theories. \nFig4: Location of crude oil in earth’s crust

Formation of Crude Oil:

It takes a large amount of time with specific environmental conditions for the process of formation of petroleum to occur. Almost 70% to 75% of oil deposits existing today were formed in the Mesozoic age (252 to 66 million years ago), 15% – 20% were formed in the Cenozoic age (65 million years ago), and only 8% – 10% were formed in the Paleozoic age (541 to 252 million years ago). The reason for this is the tropical climate with large amount of plankton in the ocean during the Mesozoic age.[3]
The process of oil formation starts in warm, shallow oceans which were present millions of years ago. In these oceans, extremely small dead organic matter – classified as plankton – falls to the bottom of the ocean. The planktons are of two type:

  1. Zooplankton- Animals
  2. Phytoplankton- Plants

Fig.5: Planktons found in the ocean
The plankton settles on the bottom of the ocean and mixes with the inorganic materials that enter in the ocean through the river. This results in the formation of oil over millions of year and energy in the oil comes from the Sunlight that is trapped in chemical form by dead planktons.
The process that creates oil is generally the same in most areas, although the difference in the variety, type of plants and animals and environmental condition may occur from place to place.
Steps Occur during the formation of crude oil are:

  1. The Dead plankton including phytoplankton and zooplankton along with algae and bacteria sink to the bottom of very old oceans that were present millions of year ago and mix with inorganic, clay-like materials that comes from rivers. This promotes the formation of an organic-rich mud. This mud can only form in stagnant water environments. This step is depicted in Figure.
  2. This mud if exposed to too much oxygen can lead to decompose of organic matter by bacteria quickly therefore it is buried by more sediments creating organic shale to provide an anoxic environment. This step is depicted in Figure.
  3. The shale buried between 2 and 4 kilometres sees a temperature increases due to its location in the Earth’s interior core. This results in increment of pressure and temperature of the shale and transforms it into a waxy material known as kerogen. Shale having this material is known as oil shale.
  4. If temperatures of the kerogen are above than 90°C but lesser than 160°C, the kerogen is transformed into oil and natural gas. At temperatures higher than this, only natural gas or graphite is formed. This temperature range is known as the "oil window".
  5. Since Oil is lighter than water, so as it escapes from the source oil shale it rises through pores in rocks and the rock containing significant amounts of oil are known as reservoir rocks.
  6. To remain trapped inside the reservoir there must be a thick, impermeable layer of rock to seal the reservoir. If this seal exists, then oil, gas, and water are trapped and further drilling can be done in order to get crude oil and petroleum products.
  7. Geological changes like movement in tectonic plates and earth quakes in the Earth's crust bring these deposits of oils up to the surface. This step is depicted in Fig.

Fig.6: Process of oil and natural gas formation
Creation of image: internally by a member of the Energy Education team

Crude oil in India:

Most of the oil reserves in India are found in the anticlines and fault traps in the sedimentary rock formed during the tertiary times i.e. almost 3 million years ago. Some of the recent sediments which are even less than one million years old also seem to develop crude oil layers. The lower and middle Tertiary period provided favourable conditions for oil formation. Dense forests and sea organisms flourished in the gulfs, deltas and other lands surrounding them during this period and as mentioned earlier, the decomposition of organic matter in the sedimentary rocks led to the formation of oil.
In India, crude oil as well as natural gas occur in between sedimentary rocks. Sedimentary rocks cover about 14.1 lakh km2 or 42% of the total area of the country, out of which about 10 lakh km2 form the marine basins from the Mesozoic and Tertiary times. Besides, there are offshore areas having Mesozoic and Tertiary rocks originated from marine organisms which tend to cover up a total area of 3.2 lakh km2 upto a depth of about 0.1 to 0.2 km and make up a region of probable oil bearing rocks.[4]
The total sedimentary area upon combining both on-shore and offshore comprises of 27 basins. The geological and geophysical studies have been conducted in 14 basins while exploration and drilling has been carried out in 9 basins of which Mumbai High, the Khambhat Gulf and Assam are the most productive areas.

Fig.7: Formation of crude oil over the years Fig.8: Occurrence of Crude oil in India


In 1859, when USA drilled out the first commercial oil, seven years after in November, 1866 India also put its hands on. Dibgoi in Assam, 48kms from Calcutta (now called Kolkata) was the first place to start the systematic oil drilling process. It was first in entire Asia to do this revolutionary act and therefore is now renowned as the Oil City of Assam.[5]
It was when Italian engineers along with British officials were working to build Assam Railways in the midst of malaria affected and very dense forest they saw elephants approaching with oil stained feet. It was called black liquid gold then. There may be some variations with this elephant story but this one happens to be the most prominent one. The English guy continuously said to the labourers “dig boi dig” while they were digging the ground which apparently gave the name Digboi to the city.
The first refinery was set up under British rule with their people working for Assam Oil Company. Soon, when India got its independence the infrastructure was improved.

Fig.9: Digboi Refinery in early 1900s Fig.10: Digboi Refinery at present

Fig.11:Story of 1st oil discovery in India
This “Gangotri of the Indian Hydrocarbon sector” became the part of Indian Oil Corporation Limited (IOCL) in 1981.
It had a crude processing capacity of 0.65 million metric ton per annum as of 2003. The waxy crude obtained from nearby oil fields are processed here. From 2018, Digboi refinery also started processing Crude condensate.[6]
If we talk about the recent oil discoveries in India, ONGC will certainly take a large credit. It was stated that the company has discovered 230 million tonnes of oil reserve in the country in the past three years. The fact was verified by petroleum minister Dharmendra Pradhan.
India owns a crude oil reserve of about 600 million tonnes as of 31st March 2018. With discovery of ethanol fuel applications, local exploration and renewable energy advances, India aims to bring down oil imports to 67% from 82% by 2022.


In order to sum up the detailed study that has been carried out for the formation process of petroleum we can say that it formed from the remains of small planktons which were buried under the sea bed around a million of years ago. India is not particularly rich in the reserves of fossil fuels; rather most part of the crude oil is imported here but we do have many indigenous sources of crude oil and geologists are focused on continuing the exploration of new oil reserves. As the crude oil reserves are fast exhausting and non-renewable, India should focus on replacing it with the renewable energy resources such as solar power, wind energy, hydrothermal energy as it is the need of the hour.



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Formation of Crude Oil and Discovery in India. (2021, Oct 31). Retrieved from

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