Origin of petroleum

Millions of years ago, the earth was populated with plants and animals. Much of that life was located in or adjacent to ancient rivers, lakes, and sea. As plants and animals died, their remains settled to the bottom of these bodies of water. Ancient rivers carried mud and sand that buried the organic material. As layers of organic material continued to build up, the oxygen supply to the lower layers eventually was cut off and decomposition slowed down. The thickness of such layers grew to 100s of feet over the time.

Introduction 

Petroleum is a Latin word of (Petra ‘‘rock’’ + Oleum ‘’ oil ‘’), It is completely different than oil that comes from vegetable sources such as the olive, but modern research has traced its origin to the lipids (oils) of planktonic (free floating) plants and animals which live in brackish water such as blue-green algaes and foraminifera. The brackishness is essential because aerobic bacteria does not live in brackish water which in turn would decompose all of the organic matter. In brackish water the organic matter of the planktonic plants and animals sinks to the bottom and incorporated into clay sediments which ultimately become sedimentary rocks, as we called shale rock. Under high pressure and temperature the oil of clay shales can be squeezed out and into porous rock. In porous rock the oil can travel, until it reaches an impervious barrier such as a salt dome.

Petroleum is so important to ensure life sustainability as a source of energy which has a big impact on society from several aspects including: economy, politics and human basic needs. It is a strategic commodity that every country is seeking by developing new technologies which contribute to maximizing petroleum recovery from underground.

Origin of petroleum dilemma 

The origin of petroleum still has uncertainties despite the tremendous researches and studies devoted to it rather than any other natural substance. There are two different theories for the origin of petroleum; Organic and Inorganic origin. 

Inorganic or Abiotic origin

States that hydrogen and carbon came together under great temperature and pressure, far below the earth’s surface and formed oil and gas where chemical reactions have occurred. The javascript:void(0)oil and gas then seeped through porous rock to deposit in various natural underground traps.It has also excluded the hypothesis that petroleum is a finite substance. There are some different theories that describe the inorganic origin of petroleum which include: </p>

Metal carbide theory

Developed by a Russian chemist and states that the deposition of petroleum is controlled by tectonic activities that occurred during the life of sedimentary rock. To explain his observations, he has put forth "metal carbide theory". Metal carbides deep in Earth reacted with water at high pressure and temperature to form acetylene which condenses to heavier hydrocarbons.

Reaction equation is: Cac₂+H₂O= C₂H₂+Ca(OH)₂

Volcanic theory

​Involves outgassing of the mantle via volcanic activity or eruption.

Earthquake theory

​Involves outgassing deep Earth's mantle via tectonic activities such as faults, and this is still happening till now.

Serpentinization theory

States that hydrocarbon is a by-product that came from a metamorphic transformation of the green dark Olivine mineral ,which was found in Earth's mantle

Overwhelming evidences for inorganic origin of petroleum 

• Geographical location: most of hydrocarbon producing regions are located close to belts of tectonic activities.
• Stability with depth: Corresponding to what organic theory's supporters have admitted themselves; petroleum is a fossil fuel, and there has never been a real fossil found below 16000 feet. Nowadays, there is drilling for oil reservoirs at 28000 feet or 30000 feet where there is no a fossil remains. 

Organic origin  

It is the most widely accepted. The oil and gas are formed from remains of prehistoric plants and animals. Remains of plants have been transformed to coal and animals to oil and gas. These remains were settled into seas and accumulated at the ocean floor and buried under several kilometers of sediments. Over a few milion years, the layers of the organic material were compressed under the weight of the sediments above them. The increase in pressure and temperature with the absence of oxygen changed the mud, sand, slit or sediments into rock and organic matter into Kerogen. After further burial and heating, the kerogen transformed via cracking into petroleum and natural gas.

Overwhelming evidences for organic origin of petroleum 

• Presence of brine (sea water) with petroleum.
• Petroleum is found only in association with sedimentary rocks. There is no petroleum associated with igneous or metamorphic rocks.
• Polarized light passing through all petroleum resources undergoes a rotation that is similar to all organic oils.
• Molecules in hydrocarbons are thought to be similar to that of the organic matter. 
• The organic carbon found in plants is depleted into C13 due to photosynthesis process.In dead organic matter, it is further depleted due to radioactive decaying. The same depletion was found in petroleum and natural gas. 

Conclusions

The battle between organic and inorganic theories are still persistent till now. All the supporters from both sides were struggling to prove their theory. According to organic theory, petroleum is a finite substance formed from organisms decaying at several kilometers below the ground surface. On the other side, Inorganic origin supporters consider petroleum a self-regenerating substance produced by the Earth itself with the assistance of chemical interactions occurring deep within the Earth. In other words, it is assumed that petroleum is not a finite substance as oil and gas didn't not run out till now. The most widely accepted one is the organic theory which defines the substantial conditions for the formation of petroleum such as: saturated soil, absence of oxygen and high pressure and temperature conditions. All these conditions contribute to the decay of organic matters which then is transformed into kerogen forming a source rock.