2. ORIGIN OF OIL
Theories about the origin of oil are;
Raw materials consists of simple marine organisms, which lives
in warm shallow seas.
Organic matter should accumulate at the bottom- where seawater
is stagnant and deficient in oxygen; substance is neither destroyed
by oxidation nor eaten away by other animals.
The bacteria that thrive in the upper mud of the sea floor are
considered to change the organic matter into mother material of
oil by removing oxygen and nitrogen and bringing about other
changes.
3. With the deep burial of sediment material, the bacterial action
ceases and provides pressure and heat coverting the substances into
droplets of liquid oil and tiny bubbles of gas.
Gradual compaction of the enclosed sediments under the pressure
of the increase in weight reduces the space with in the rock
particles, and squeezes out the oil, gas, and water into nearby layers
of the sand / sandstone where open spaces are larger.
The oil not remain in the source rock in which it orginates. To give
rise to a rock rich in petroleum deposits there should be a source
rock, from petroleum is derived, a porous formation to transmit it
and a cap rock to hold it in a proper geologic structure.
5. SOURCE BEDS
Sedimentary deposits from which petroleum or natural gas has been generated.
Trask noted a definite relationship between organic content of sediments and
the texture.
Organic matter are usually preserved in fine grained sediments because they
are usually deposited in quite water, which contain less oxygen than agitated
water.
He estimated that at least 90% of the organic material is destroyed before it is
deposited.
After deposition it tend to decrease in quantity with the passage of time.
6. Small percentage of organic matter, which is deposited is further
reduced in amount by bacterial action.
Bacteria do not destroy much organic matter after it is buried.
Zobell studied the abundance of bacterial population in cores of recent
sediments up to 75 cm in length.
He noted a sharp decrease in the number of bacteria with core depth
and concludes that, the greatest bacterial activity is with in the top most
few mm of the sediments.
Trask esimated that by the time the sediments have been buried up to
depth of 30 cm, the quantity of organic matter seems to have reduced
about 15%.
Then, lithification take place over a period of time.
7. Average decrease in organic content will be about 40%.
He believed that anerobic bacteria are the only living thing which
can alter, organic matter in sediments after it has been buried to
considerable depth.
Favourable source bed are fine textured shales and limestones.
Drilling in source rock rarely produces petroleum.
The porosity of the rock is lowered from 90 to 35 %, when the
source rock is compacted into a rock, so large quantities of water is
squeezed out from it.
The water, oil and gas had the power of movement and travel
through it, until an impervious rock structure stop them.
9. BIO-CHEMICAL CHANGES
The reduction of nitrogen, oxygen, hydrogen, sulphur, phosphorous content
of organic matter occur when the petroleum is formed from plant and
animal matter.
Zobell and others state that bacteria reduce those substance in the recent
marine sediments.
Its upper most layer has a large population of bio-chemically active bacteria.
Many thousands of them have been recovered from recent sediments to a
depth of more than 20ft below sea level.
Anerobic bacteria act on plant and animal matter and produces substances
like petroleum by splitting oxygen, nitrogen, sulphur and phosphorous from
the organic compound and the end product will consist of carbon and
hydrogen.
10. GEO-CHEMICAL CHANGES
Chemical reaction occurs after the sediments are buried to a
considerable depth.
Factors that influenced the reaction include heat, pressure,
earth movements, geologic time and possibly other known
factors.
11. TEMPERATURE
Geological and chemical evidences suggest that the petroleum has
had a low temperature history.
There are no carbonaceous residues or other evidences of heat
decomposition in beds associated with petroleum deposits.
Barton believes that much of the crude oil of the Gulf coast could not
have been subjected to temperature much higher than 50°C.
He is of the opinon that the chemical composition of the mother
source material was about same for all oil and the present character
depends upon the transformation of oil progressively with time i.e.,
from naphthenic to parafinic oil.
12. PRESSURE
It is of little importance.
Pressure when exerted on shale at room temperature does not
generate oil from organic matter.
Increased pressure may cause a rise in temperature due to
fiction of compaction.
13. RADIOACTIVITY
Concentration of radioactive substance is widely distributed in water
and rock near the surface of earth but relative amounts are variable.
Fine sediments rich in organic matter are generally high in
radioactivity.
Laboratory experiments suggests that radioactivity may alter organic
substances.
Hydrocarbon gases have been converted into substances, which are
oily, by radioactive bombardments.
14. CATALYSTS
Certain organic and inorganic substances which commonly
occur in sea water may act as catalyst in the transformation of
organic matter into petroleum.
Zobell believed that bio-chemically active bacteria and alive
micro organisms may be principle catalyzers of chemical and
physio-chemical reaction in recently deposited sediments.
Certain metallic elements like lead, nickel, iron, copper, etc are
commonly found in petroleum and some of them act as
catalyst in its generation.
15. CONCLUSION
Oil is originated from the sorce rock, from where it is migrated
and get accumulated in the reservoir rock and finally it is
trapped by a cap rock.
For the transformation of organic matter to petroleum two
changes occur; bio-chemical changes and geo-chemical
changes.
Bacteria plays a major role in bio-chemical changes.
Geo-chemical changes are influenced by temperature,
pressure,radioactivity, catalysts etc.
