Petroleum, the mineral which is in the greatest demand in modern industry, supplies half the world’s energy requirements. It provides fuel for heat and lighting, lubricants for machinery and raw materials for a number of manufacturing industries.
In comparison with other fuels, such as coal, it has several advantages; it occurs in great abundance; it is easily obtained; it can be cheaply distributed; and above all, it has the widest range of domestic as well as industrial uses. It is often, therefore, referred to as ‘black gold’.
It was first used where seepages occurred at the surface. In ancient times, the Chinese, who encountered oil in drilling for salt in brine wells, used it as fuel to evaporate the brine.
The Egyptians used it for embalming the dead before burial. But, the first man to have really ‘struck oil’ was probably Samuel M. Kier, who in 1848 found it by chance in wells on the banks of the Allegheny River of Pennsylvania. He named it after the local Indians as Seneca oil.
The modern petroleum industry recognizes three principal grades of crude oil.
(i) Paraffin-Base Oil:
This contains a high percentage of the lighter hydrocarbons such as methane and yields the commercially more valued products, e.g. petrol, paraffin and high grade lubricating oils.
(ii) Asphalt-Base Oil:
This consists mainly of the heavier hydrocarbons with a viscous, asphaltic base. It is of less commercial significance because it yields little motor-oil during distillation. Much of its residue is in the form of asphalt or bitumen, almost in solid state.
(iii) Mixed-Base Oil:
This is an intermediate group with mixed properties of the lighter and heavier oils. It carries a high percentage of naphthene and is graded 20° on the Baume scale. It is used both for lubricants and fuel oils.
The origin of oil is still not definitely known, though it is generally presumed to be derived from organic material. Analysis of oil samples shows that it is formed from the decomposition by anaerobic bacteria of innumerable small marine plant and animal organisms trapped in sediments as they were deposited on the sea-bed.
Some scientists believe that when an accumulation of sedimentary rocks in the ocean depths is compacted, the pressure generates heat which transforms the decaying matter into tiny droplets of oil.
Others think that oil may have formed relatively rapidly after the organisms were trapped in the sediments and that heat or pressure was not necessary to the process. Oil formation in sedimentary rocks has been going on since the beginning of geological time and is probably continuing today, and unlike many other minerals which require special conditions for their occurrence, oil can be found in many different rocks of various ages and is very widely distributed. Wherever there are areas of marine sedimentary rocks such as mudstone, shale, sandstone or limestone oil may be found in some of the strata.
The oil is trapped in the pore-spaces of the rocks and thus, rocks such as sandstone with a high proportion of spaces are most likely to contain oil. When the rocks originally formed under the sea, not only oil was trapped but also sea-water.
This also remains in the pore spaces of the rock. Because oil is lighter than water it usually lies above it in the rock. Above the oil are the lightest hydrocarbons forming natural gas. Thus, oil is usually found in a water-oil-gas sequence, though sometimes only gas is found, and no oil exists.
While petroleum, like coal, can occur in rocks of any geological age from Cambrian to Pliocene, the most productive petroliferous strata in Asia are of Jurassic to Miocene Age. The oil is found at the summits of anticlines or dome-like folds and is obtained by drilling through the overlying rocks to the summit of the fold. The degree of porosity of reservoir rocks plays an important part in the underground storage of petroleum.
Mode of Occurance:
Petroleum occurs in the pores and minute interstices of sands and in crevices in limestone which are of shallow water, usually marine origin. Most of the oil reserves in India, are associated with anticlines and fault traps in the sedimentary rock formation of tertiary times, about 3 million years ago. Some recent sediment less than one million years also show evidence of incipient oil. Oil and gas do not occur normally at their original sites.
Being lighter than water they collect in the anticlines of fault traps above water surface. Gas usually occurs above oil in most structures and a gas seepage, therefore, is taken as an indication of the occurrence of oil, although in many cases oil does not occur at such places.
About 14.1 lakh sq. kms or about 42 per cent of the total area of the country is covered with sedimentary rocks, out of which about 10 lakh sq. kms form marine basins of Mesozoic and Tertiary times. India’s offshore areas have Mesozoic and Tertiary rocks of marine origin spread over 2.5 lakh km2 upto a water depth of 10 metres and 3.2 lakh km2 up to 200 metres,
the probability of oil occurrences is rated high in these rocks (divided into 10 basins), particularly the Cambay region where oil was discovered in Bombay High in 1974.
The entire oil production of India so far comes from the Assam-Arakan belt, the Gujarat, Cambay belt and and the Bombay High offshore Zone. The first one extends from the Dehang Basin in the extreme north-east of Assam along the outer flanks of the Hill ranges forming the eastern border of Bhitra and Surma valley to the Andaman Nicobar Islands in the Bay of Bengal.
The second belt extends from Mahesana (Gujarat) in the north to the continental shelf offcast Ratnagiri (Maharashtra) in the south and includes the several important oilfields in Gujarat and the Bombay High and other newly discovered oilfields offshore. Againt a 63 per cent supply of primary commercial energy through hydrocarbons in the world in the case of India, it is 44.9 per cent (36% oil and 8.9% natural gas).
Crude oil extracted in the country and imported from abroad is refined and processed into various products like light distillates, kerosene, diesel, lubricants, bitumen and heavy ends in oil refineries.
India witnessed spectacular growth in refining sector in the recent past. As of June 2011 there are a total of 21 refineries in the country comprising 17 in the public sector, 3 in the private sector and 1 as a joint venture of BPCL and Oman Oil Company. The total refining capacity in the country as on 1.06.2011 stands at 193.386 MMTAPA.
Indian Oil Corporation Limited and its subsidiaries Chennai Petroleum Corporation Limited and Bongaigaon Refinary anbd Petrochemicals limited (10), Bharat Petroelum Corporation Limited and its subsidiaries Kochi Refineries Limited and Numaligarh Limited (3), Hindustan Petroleum Corporation Limited (2), Mangalore Refinary and Petrochemicals Limited (1) and Reliance Petroleum Limited (1) Mini Refinary at Tatipaka (Andhra Pradesh) is owned by Oil and Natural Gas Corporation Limited.