Petroleum is the term which means ‘rock-oil’. This is one 0f the important ‘mineral fuels’, and is a complex mixture of hydrocarbon compounds with minor amounts of impurities, viz., nitrogen sulphur and oxygen. The liquid petroleum is called ‘crude oil’ petroleum gas is called ‘natural gas’ and the semi-solid to solid forms of petroleum are commonly known as asphalt, tar, pitch bitumen etc.
Origin:
A number of theories have been put forwarded for the origin of natural petroleum and depending upon the primary source materials the theories may be grouped as
(I) Inorganic theories.
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(II) Organic theories.
(I) Inorganic theories:
(a) Brethelot’s alkaline-carbide theory:
According to him C02 might react with alkaline metals contained in the interior of the earth at high temperature with the formation of alkaline carbides. These on contact with water liberate acetylene which through subsequent processes of polymerisation and condensation forms petroleum,
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(b) Mendeleef’s carbide theory:
It is believed that iron carbides within the earth on contact with percolating water form acetylene, which escapes through fissures to the overlying porous rocks and there condenses. This theory is based on laboratory experiment, but the presence of iron-carbide within the earth has not been established definitely.
(c) Moissau’s volcanic theory:
He suggests that volcanic explosions may be caused by the action of water on sub-terranean carbides, and may lead to the formation of petroleum.
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(d) Cosmic theory:
Taking into account the presence of small quantities of hydrocarbons occassionally in meteorites, Sokolov considers petroleum to be an original product resulting from the combination of carbon and hydrogen in the cosmic mass during consolidation of the earth.
(II) Organic theory:
(a) This has been put forwarded by Engler. His theory is based the fact that by destructive distillation of fish-blubber, a product similar to natural petroleum- could be obtained.
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According to him, petroleum is formed by a process of putrefation of animal remains. Nitrogen thus eliminated and residual fats get converted into petroleum by earth’s heat and pressure.
(b) Vegetable origin theory:
On the basis of certain facts as deposits of petroleum found in close association with sedimentary deposits containing diatoms, seaweed, peat, lignite, coal, oil-shale of known vegetable origin, this theory has been propounded. Besides the above, other facts also support the theory:
(i) Oil and coal appear to have close relationship indicating a vegetable origin.
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(ii) The large amount of methane in natural gas can be explained as produced by the decay of vegetable matter.
(iii) Oils closely resembling petroleum can be distilled from coal, lignite etc.
(iv) Microscopic vegetable remains have been noted in crude oil even though it is rare.
(c) Animal-origin theory:
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Since 95 percent of the oil-fields occur in marine sediments it is assumed that oil was formed from marine organisms buried in sediments. It has been suggested that bacterial action plays the most effective role in the conversion of organic material into oil. It is now commonly presumed that the primitive forms of life like diatoms, algae etc. which were mixed up and enclosed within the sediments in the sea-bed, were the primary source material for petroleum.
Mode of occurrence:
Four pre-requisites are necessary for petroleum to accumulate in commercial quantities in an area:
1. The oil originates in a source bed and marine shale once a black-mud rich in organic compound is thought to be a common source rock.
2. The oil then migrates to permeable reservoir rocks and to do this, it may travel for long distances both vertically or horizontally. The source beds tend to lack the permeability necessary for profitable extraction of the oil.
3. A non-permeable layer must occur above the reservoir bed.
4. A favourable structure must exist.
Migration:
Oil is found to occur in porous and permeable rocks like grits, sandstones, limestones and carbonate rocks which have served as reservoirs for oil and are called ‘reservoir rocks’.
The migration of oil is thought to be caused due to compaction of sediments during diagnesis due to which currents of water are set up in the source rocks and the oil is squeezed along with water and enters into the porous and permeable rocks in the immediate vicinity.
Since oil is lighter than water, the oil tends to float on top 0f the water. If the sandstone unit was formed under marine water it already contains salt water in its pore-spaces. The oil slowly moves up around the sand grains until it reaches the top of the sandstone unit. Gases that have been produced are lighter than the oil and tend to move to the top of the oil-accumulation. The greater the porosity, the greater the amount of oil that a reservoir rock can contain and the larger the pore space the greater the amount of oil.
Reservoirtrap:
It holds the oil and gas in place so that they do not escape until released by drilling. It is also known as ‘cop- rock’ which may be an impervious shale, clay, dense limestone, well cemented-fine grained or shaly-sandstones that are effective cap rocks and they seal the reservoir trap.
The occurrences of petroleum deposits are classified into two broad-categories:
(a) Surface occurrences
(b) Sub-surface occurrences.
(a) Sorface occurrence:
Petroleum occurs at the surface of the ground in the following ways :
(i) seepages, spring and bitumen,
(ii) mud flows and mud volcanoes,
(iii) oil shales or kerogene shales.
(b) Sub-surface occurrence:
Petroleum mostly occurs under the impermeable cap-rock of a reservoir. The barrier which helps in the accumulation of petroleum is called an oil-trap. Oil-traps are classified into the following three types :
(i) Structural traps.
(ii) Stratigraphic traps.
(iii) Combination of stratigraphic and structural traps.
(i) Structural traps:
These are caused by folding or rupture and displacement of the rock units. They include closed anticlines, domes, monoclines, terraces, synclines, faults, fissures, salt domes, igneous intrusion etc. The process of strata deformation may be compressional, gravitational, intrusional or rejuvenaud uplifting.
These are formed by conditions of which lateral and vertical variations in thickness of feds result. They include unconformities (angular unconformities are more effective), ancient shore-line sands, shoe-string sands, sandstone lenses and bars etc.
(iii) Combination stractural-stratigraphic traps:
Here are included these reservoirs where structural, stratigraphic and lithological features are significant in controlling the accumulation, migration and retention of oil and gas. They include both deformational well as erosional features, for example bald headed structure, traps with burried hills etc.
Distribution in India:
In India, deposits of petroleum and natural gas are associated with the belt of tertiary-rocks in Assam, Gujarat as well as in the offshore regions of Bombay High and in the Cauvery and Godavari deltaic areas.
Assam :
(i) Digboi oil-field:
Where the Tipam sandstones of Miopliocene age is the oil-bearing formation.
(ii) Nahorkatiya oil-field:
In the Brahmputra valley, where the oil-bearing formation is the Barail series of Oligocene age.
Besides the above, the other important oil-fields of Assam are ‘Moran oil-field’, ‘Rudrasagar oil-field’ and ‘Lakwa-oil field’.
Gujarat :
(i) Cambay-basin:
Where the main oil-bearing sand is of oligocene age. Here a majority of the wells are only gas producers.
The other oil-fields of importance are Kalol oil-field, Nawagam and Sanand oil-field.
(ii) Ankleshwar oil-field:
This is the most important oil-field discovered so far in Gujarat. The producing sands are of ‘Eocene- age’.
Bombay High:
About 115 miles off Bombay, in the Arabian sea, a huge oil-deposit has been struck in limestone rocks of miocene age. This has proved to be the richest oil-deposit in the country. The deposit is estimated to be around 4 billion-tonnes.
Andhra Pradesh:
In the off-shore regions of Tamil Nadu and Andhra Pradesh a number of oil-deposits have been discovered by ONGC, recently. Among them the deposits of ‘Nagapatinam’ is the most important one, from the commercial view-point.
Besides the above, a number of discoveries have already been made in the states of Tripura, Gujarat, and West Bengal etc.
Uses:
(i) The chief use of petroleum is as fuel particularly in transport operations.
(ii) The petro-chemical derivatives of petroleum have a wide range of uses in agricultural, industrial and medicinal industries.
(iii) It is also used for the purpose of generation of heat and power.