The origin of petroleum still has uncertainties despite the tremendous researches and studies devoted to it rather than any other natural substance.

1. Origin of Petroleum

2. Origin of Petroleum
 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;
Inorganic
and Organic origin.

3. Inorganic and Organic Theory
Inorganic theory of the origin of the petroleum states that hydrogen and carbon came
together under great temperature and pressure, far below the earth’s surface and formed oil
and gas.
The oil and gas then seeped through porous rocks to deposit in various natural underground
traps.
Organic theory is the one most widely accepted.
According to organic theory, the oil and gas are formed from remains of prehistoric plants and
animals.

4. Inorganic Theory

5. Inorganic Theory
Also Known as
Abiotic Theory
Abiogenic Theory

6. Though the current accepted theory of oil formation involves the slow transformation of
animal and plant matter into hydrocarbon (the biotic or biogenic theory), it is not the only
theory that has been put forth.
As early as the 16th century, one theory of the origin of oil claimed that it resulted from deep
carbon deposits that have been around far longer than life on this planet.
The theory, which came to be known as the abiotic oil formation (AOF) theory, was largely
forgotten until rather recently when a few people (some of them scientists) revived(recharged)
it.

7. The newest version of the AOF theory states that oil arises from inorganic processes that occur
deep within the core or lower mantle of the Earth.
Here, they say,
oil is formed
and then percolates up through cracks and porous rock to fill the reservoirs that humans tap to get oil.
If this claim is true, then oil may not be nearly as limited in quantity as
proponents(followers) of the biotic theory claim.
This would mean oil is more “renewable” than we have been led to believe.

8. The Deep Seated Terrestrial Theory:
Dmitri Mendele'ev (1877), a Russian Scientist and the father of the periodic table of elements,
reasoned that;
Metallic carbides deep within Earth reacted with water at high temperatures to form acetylene
(C2H2) which subsequently condensed to form heavier hydrocarbons.
This reaction is readily reproduced in the laboratory. Iron carbide which would react with
percolating water to form methane.
FeC2 + 2H2O = CH4 + FeO2
At some references the Dmitri Mendeleev's year is mentioned as 1902.

9. The Extraterrestrial Theory:
Another inorganic hypothesis was suggested by Sokoloff (1890) who proposed a cosmic origin.
He said;
Hydrocarbons precipitated as rain from original nebular matter from which the solar system
was formed and then ejected from earth's interior onto surface rocks.

10. Problems with Inorganic Theories
No field evidence that inorganic processes have occurred in nature.
Commercial accumulations are restricted to mainly sedimentary basins.
Accumulations are absent from igneous and metamorphic rocks.

11. Organic Theory

12. Introduction
The crude oil that has been extracted over the past century was formed millions of years ago.
Some deposits were formed over 500 million years ago with the newest deposit being
approximately formed over 50 million years ago.
Crude oil was formed from the remains of tiny sea animals and plants and is therefore a fossil
fuel.
Small marine animals and plants that died millions of years ago sunk to the seabed or ocean
floor where they decomposed and mixed with the sand and silt.

13. The small and microscopic organisms drifting or floating in
the sea or fresh water.

14. The remains of the animals and plants were buried under layers of sand and silt on the seabed.
Due to the lack of oxygen on the seabed the bacteria feeding on the remains could not
decompose the remains of the animals and plants completely.
The partially decomposed remains overtime formed into a large mass.
This over the course of millions of years was covered by multiple layers of sand, silt and mud.

16. As the depth of the sediment layers built up so did the weight of the sand and silt pressing
down on the layer of partially decomposed remains causing it to be compressed into a thinner
layer.

18. Finally, the depth of the layers on top of the buried partially decomposed layer
reaches thousands of meters
and the immense pressure exerted by the weight of these layers
along with the natural heat from the earth results in the formation of crude oil/petroleum.

20. Whilst the mud and silt layers become heavier the forces applied causes the bottom layers
above the crude oil to turn into shale.

22. The oil is forced out from its original area of formation and travels upwards through the cracks
and gaps in the shale formation until it reaches a new impermeable rock formation called
reservoir rock. The oil lays trapped here until it is discovered and extracted.

24. Organic material:
The basis for
hydrocarbon fuels

25. The [organic] chemical composition of
bacteria, phytoplankton, higher order plants (trees etc.), and the things that eat them,
can generally be grouped into proteins, carbohydrates, lipids, and lignin.
Proteins are amino acids;
they account for nitrogen compounds in organisms
and form materials such as muscle fibers, silk, and sponge
and can even behave as enzymes to catalyze biochemical reactions.
Phytoplankton are the autotrophic(self feeding) components of the plankton community and a key
part of oceans, seas and freshwater basin ecosystems.

26. Carbohydrates are sugar compounds such as cellulose and chitin.
They are sources of energy and form supporting tissues in plants and certain animals.
Lipids are substances that, due to their structure, are insoluble in water,
such as animal fats, vegetable oils, and waxes.
Fats form a portion of the energy budget in organisms, while waxes are primarily of value for
protection.
Lignin is an integral part of cell walls in plants and is one of the most slowly decomposing
components of dead plant matter.
It is highly prevalent in wood (25-30%) and is crucial in helping plants to transmit water in their stems.
It is used mainly in supportive skeletal structures (stems or trunks), which, of course, are not needed in
aquatic plankton.

27. Marine plankton are comprised mainly of
protein (50% or more),
with 5-25% lipids,
and not more than 40% carbohydrates.
Higher order plants, on the other hand, are
Largely cellulose (carbohydrate) (30-50%)
and lignin (15-25%),
and average less than 3% protein.

28. Therefore, we have a significant division between terrestrial and marine biomass.
The former is high in lignin and carbohydrate and is more aromatic and oxygen rich, exhibiting
a hydrogen- carbon ratio of 1.3 to 1.5.
The latter is rich in protein and lipids, reaching a hydrogen-carbon ratio of 1.7 to 1.9. These
differences incur different properties in the fuels that they will one day become, and alter the
way that they decompose.

30. Humins are a
class of organic
compounds that
are insoluble in
water at all pH's

31. Step 1: Diagenesis forms Kerogen
Diagenesis is a process of compaction under mild conditions of temperature and pressure.
When organic aquatic sediments (proteins, lipids, carbohydrates) are deposited,
they are very saturated with water and rich in minerals.
Through chemical reaction, compaction, and microbial action during burial, water is forced out
and proteins and carbohydrates break down to form new structures that comprise a waxy material
known as “kerogen” and a black tar like substance called “bitumen”.
All of this occurs within the first several hundred meters of burial.
The bitumen comprises the heaviest components of petroleum, but the kerogen will undergo
further change to make hydrocarbons and, yes, more bitumen…

32. Step 2: Catagenesis (or “cracking”) turns
kerogen into petroleum and natural gas
As temperatures and pressures increase (deeper burial) the process of catagenesis begins,
which is the thermal degradation of kerogen to form hydrocarbon chains.
Importantly, the process of catagenesis is catalyzed by the minerals that are deposited and
persist through marine diagenesis.
The conditions of catagenesis determine the product, such that higher temperature and
pressure lead to more complete “cracking” of the kerogen and progressively lighter and smaller
hydrocarbons.
Petroleum formation, then, requires a specific window of conditions;
too hot and the product will favor natural gas (small hydrocarbons),
but too cold and the plankton will remain trapped as kerogen.

33. Contd
This behavior is contrary to what is associated with coal formation.
In the case of terrestrial burial, the organic sediment is dominated by cellulose and lignin and
the fraction of minerals is much smaller.
Here, decomposition of the organic matter is restricted in a different way.
The organic matter is condensed to form peat and, if enough temperature (geothermal energy)
and pressure is supplied, it will condense and undergo catagenesis to form coal.
Higher temperatures and pressures, in general, lead to higher ranks of coal.