Mechanisms of Petroleum Migration and Petroleum Occurrence
The crude oil production process involves the transformation of the organic matter to crude oil over millions of years in a sedimentary source rock after which the crude oil migrates into an impermeable reservoir rock where the oil is kept for exploration.
This migration can be by primary migration mechanism or by secondary migration mechanism. Primary migration involves the movement of hydrocarbons (crude oil and natural gas) from mature, organic-rich source rocks to a point where the oil and gas can collect as droplets or as a continuous phase of liquid hydrocarbon.
The secondary migration on the other hand is attributed to various forces of buoyancy and hydrodynamics which cause the movement of the hydrocarbons as a single, continuous fluid phase through water saturated rocks, fractures, faults or conduits leading to accumulation of the oil and gas in traps (reservoirs) from which further migration is prevented.
There are a number of complimentary mechanisms of the migration of the crude oil that have been proposed based on geological event and the corresponding migration effects.
In some cases the source and the reservoir rocks are in close proximity and migration is considered negligible. It is however not always the case that source beds coincides with reservoir rocks.
The common belief is that migration of petroleum before it comes to rest in a trap is through large distances which can be in different direction including vertical migration depend on the prevailing force.
Petroleum Occurrence
There are a number of crude oil fields in different countries. An oil field is made up of a number of crude oil wells. Large portion of the crude oil currently explored are from underground reservoirs wells.
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Surface seepages of crude oil and natural gas occur in many regions and was particular the starting point for the usage of the high boiling material before underground exploration. The occurrence of majority of crude oil reserves recently identified is located in a relatively small number of very large fields.
Almost 75% of the global crude oil supply is from about three hundred large oil fields. These fields are primarily concentrated in few countries like Saudi Arabia, Russia, Nigeria, United States, Iran, Norway, Mexico, Venezuela, China, Iraq, Great Britain, the United Arab Emirates, and Kuwait.
The Middle East has the largest known reserves. These countries determine the global crude oil supply, price and economics.
In conclusion, there are two popular theories about the sources of crude oil: biogenic and abiogenic theories. The biogenic theory postulates that crude oil is from remnants of plant and animal life buried in marine sediments under pressure in source rock over millions of years.
The remnants were transformed through three stages: diagenesis, catagenesis, and metagenesis into maturity after which it migrates into a reservoir. Contrary to the postulate of the biogenic origin’s theory, the abiogenic theory postulates about the origin of petroleum were that petroleum started with inorganic substances as source material.
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It postulated that deep carbon, acetylene (C2H2), that was deposited in the rocks when the planet was formed was the basic raw material for the production of crude oil constituents.
The production process of acetylene was from inorganic carbides which were formed by the action of alkali metals on carbonates after which the reaction of the carbides with water willliberate acetylene. The more commonly accepted theory is the biogenic theory.
