Petroleum often referred to also as crude oil is from the Latin words petra and oleum. Petrais rock and oleumis oil which literally means that petroleum is rock oil. This substance is considered to be the most demanded and consumed substance in the modern day.
Its demand is driven by its value and usefulness in the provision of energy for transportation, industrial activities, heating and in the synthesis/production of several useful products such as plastics, detergents and other petrochemicals.
It is often referred to as “black gold” because it is brown to black flammable liquid or semi-solid material. Petroleum and natural gas supply more than 50% of the energy demand of the world at large (BP Statistical Review of World Energy 2018).
Being such a valuable substance, it is very essential we learn about its origin. This section of the course is therefore dedicated to teach the theories of the origin of petroleum.
Petroleum sold by different sourced countries has not been of the same quality and contents which suggest that composition of the petroleum are not the same for all the sources.
This variation in the composition suggests that the various crude oils are from different sources and their composition must be source dependent.
There are two theories about the origin/source of carbon fossil fuels including crude oil: the biogenictheory and the abiogenic theory (Kenney et al., 2001, 2002). The two theories have been subject of debate for hundreds of years but the biogenic origin’s theory seems to be more accepted.
1. Biogenic Origin’s Theory
In the biogenic origin’s theory, scientists view the production process of crude oil and natural gas as the compression and heating of ancient vegetation buried over geological time scales (millions of years).
The theory postulated that fossil fuels are formed from the decay of the remains of prehistoric marine animals and terrestrial plants. These organic matters were mixed with mud and buried under thick sedimentary rock often referred to as source rock.
2. Abiogenic Origin’sTheory
In contrast to the biogenic origin’s theory discussed above, the early postulates about the origin of petroleum were that petroleum started with inorganic substances as source material.
In 1866, Berthelot postulated that acetylene (C2H2) 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 will liberate acetylene.
Another postulate, by Mendelejeff, was that the reaction of dilute acids or hot water with manganese and iron carbides produces a mixture of hydrocarbons of which acetylene is considered to be the basic and prominent material from which petroleum evolved.
These and many other theories supporting production of petroleum from abiogenic origins are based on the assumption that large amounts of carbon exist naturally in the planet, some of which are in the form of hydrocarbons.
These hydrocarbons are less dense than aqueous pore fluids in rock systems; therefore they migrate upward through deep fracture networks. This theory postulates that the biomarkers found in petroleum are as a result of microbial life dwelling the rocks in which petroleum is formed.
The roles of the microbial life in the formation, alteration, component determination and/or contamination of the various hydrocarbon deposits are not yet fully explained by the theory.
A significant point at which this theory breaks down is that experimental studies and thermodynamic calculations have shown that at typical pressures of sedimentary rocks in which crude oil are generated (unless it is below 200 km), n-alkanes (common petroleum components) cannot be spontaneously evolve from methane.
Moreover, empirical investigations (both chemical and geological) do not support the abiogenic origin concept. Thus, it is more generally accepted, though without conclusive proofs, that petroleum formation is predominantly from the decay of buried organic matters in the earth under pressure.
Despites the merits and demerits of the theories, until conclusive proofs are established that petroleum formation is due a specific geochemistry, alternative theories cannot be dismissed on origin of petroleum.
Raw Materials and Source Rock
Petroleum is postulated under this theory to be the product from the decay of plantsandanimals’debriswhich were incorporated into sediments at the time of deposition.
The details of this decays and the mechanism of formation of petroleum from the source sediment and its accumulation process into the reservoir rock are still not fully understood.
These notwithstanding, the constituents of petroleum are very much depended on the nature of the raw material (precursors) from which petroleum is formed which are the local flora and fauna that occur in the source material.
It is also dependent of the geochemistry of the source rock. While exactly how crude oils originated is not established, it is generally agreed that crude oils is derived from these marine animal and plant debris being subjected to high temperatures and pressures.
It is also suspected that the transformation may have been catalysed by the source rock’s constituents. Therefore, petroleum composition varies with locations and age of the field/source rocks beside other variables. It is also very possible for two petroleum wells adjacent to each other to contain petroleum with very different characteristics.
The generally accepted view is that marine plankton is the prominent source material for petroleum.
However, accumulation of other organisms, such as marine algae, the remains of larger marine animals or even material from terrestrial sources are also considered to contribute significantly to the raw materials transformed to produce crude oil.
This difference in the raw materials is responsible for the varying composition of crude oil. High-wax crude oils are from source materials containing high quantity of lipids from terrestrial higher plants and of microbial organisms.
High-sulphur crude oils are frequently found in carbonate-type source rocks. Thus, the source rocks are equally responsible for variations and alteration of the petroleum’s composition.
The source rocks, in the biogenic theory, must possess good permeability (fluid-transmitting capacity). The pores between the grains of the rocks are made of cavities which usually account for the bulk of the storage space.
The pores for oil raw material storage and formation are from continuously connected openings which provide the property of permeability.