Origin of petroleum

The Petroleum was formed millions of years ago. According to the widely accepted organic theory, when microscopic plants and animals living in the ancient seas died and they felled to the bottom of the ocean. Gradually with the passage of time thick layers of organic ooze was formed and sediments buried this ooze deep within the earth creating an anaerobic environment. Eventually, pressure from overlying sediments. Heat from the depths and other forces transformed the organic matters into petroleum.
The oil was sparsely distributed in sediments underground the earth, pressure moved this widely dispersed oil and forced it upward through porous & permeable rocks which is its pint of origin. After this process the oil reached the surface to form an oil seep (or spring) and accumulated in rock formations underground.
A trap is an arrangement of rocks in which oil and gas hydrocarbons accumulated. In other words, a trap contains hydrocarbons in a porous and permeable rock bed. Just as a sponge has openings or pores, a porous rock also has openings. However, the pores in a rock are usually very small, even microscopic and if oil and gas occur they occur in these very small pores.
A porous and permeable rock also has microscopic passage ways which connects to the pores. Oil and gas move from pore to pore through the passage ways. In a trap an impermeable bed lies above the porous permeable bed. Since it has no passage ways for oil and gas to move through, this impermeable bed keeps hydrocarbons from moving out of the permeable bed.


Thus we can conclude that all the available evidence points to a recent catastrophic origin for the world’s vast oil deposits, from plant and other organic debris, consistent with the biblical account of earth history. Vast forests grew on land and water surfaces in the pre-Flood world, and the oceans teemed with diatoms and other tiny photosynthetic organisms. Then during the global Flood cataclysm, the forests were uprooted and swept away. Huge masses of plant debris were rapidly buried in what thus became coal beds, and organic matter generally was dispersed throughout the many catastrophically deposited sedimentary rock layers. The coal beds and fossiliferous sediment layers became deeply buried as the Flood progressed. As a result, the temperatures in them increased sufficiently to rapidly generate crude oils and natural gas from the organic matter in them. These subsequently migrated until they were trapped in reservoir rocks and structures, thus accumulating to form today’s oil and gas deposits.
Crude oil reservoirs:-
Three conditions must be present for oil reservoirs to form: a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil; a porous and permeable reservoir rock for it to accumulate in; and 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.
Because most hydrocarbons are lighter than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above. However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.
The reactions that produce oil and natural gas are often modeled as first order breakdown reactions, where hydrocarbons are 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 latter set is regularly used in petrochemical plants and oil refineries.
There are two theories on the origin of crude oil:-
A) The biogenic theory.
B) The abiogenic theory.
The two theories have been intensely debated since the 1860s, shortly after the discovery of widespread petroleum. There are several differences between the biogenic and abiogenic theories.

Raw material:
Biogenic: remnants of buried plant and animal life.

Abiogenic: deep carbon deposits from when the planet formed.
Events before conversion:
Biogenic: Large quantities of plant and animal life were buried. Sediments accumulating over the material slowly compressed it and covered it. At a depth of several hundred meters, it gets converted in to bitumens and kerogens (It is defined as the fraction of large chemical aggregates in sedimentary organic matter that is soluble in solvents).

Abiogenic: At depths of hundreds of kilometres, carbon deposits are a mixture of hydrocarbon molecules which leak upward through the crust. Much of the material becomes methane.

Evidence supporting abiogenic theory:-

In the late 19th century it was believed that the Earth was extremely hot, possibly completely molten, during its formation. One reason for this was that a cooling, shrinking, planet was necessary in order to explain geologic changes such as mountain formation. A hot planet would have caused methane and other hydrocarbons to be out gassed and oxidized into carbon dioxide and water, thus there would be no carbon remaining under the surface. Planetary science now recognizes that formation was a relatively cool process until radioactive materials accumulate together deep in the planet. Although this theory was support by a large minority of geologists in Russia, where it was intensively developed in the 1950s and 1960s, it has only recently begun to receive attention in the West, where the biogenic theory is still believed by the vast majority of petroleum geologists.
Evidence supporting biogenic theory:-
It was once argued that the abiogenic theory does not explain the detection of various biomarkers in petroleum. Microbial consumption does not yet explain some trace chemicals found in deposits. Materials which suggest certain biological processes include tetracyclic diterpane, sterane, hopane, and oleanane. Although extremophile microorganisms exist deep underground and some metabolize carbon, some of these biomarkers are only known so far to be created in surface plants. This shows that some petroleum deposits may have been in contact with ancient plant residues, though it does not show that either is the origin of the other.