Crude petroleum is obtained from the sedimentary rocks of marine origin which date back to three million years ago.
In India it occurs in the anticlines and fault traps of rock formations belonging to the Tertiary period. Oil is found in the pores and minute interstices of sand and in crevices of limestone and is capped by impervious strata. Normally it does not occur at its place of formation. Being lighter than water crude oil overlain with gas gets accumulated in the anticlines or fault traps above the water surface.
That is why discovery of natural gas or its seepage is an indication of the occurrence of oil in a region. Geologists propound two theories regarding the origin of crude oil: inorganic and organic. According to former carbides of calcium, iron etc. reacts with hot water to form hydrocarbon. Second view has got greater approval by the scientists according to which living organism (fish etch) and vegetal matter get buried under accumulated sediments of mud, silt and sand etc. Due to pressure and heat same undergoes chemical changes so as to form crude petroleum after millions of years.
In India there is 17.3 lakh sq. km of sedimentary rocks belonging to the Mesozoic and Tertiary periods which have potentiality for crude petroleum. These form part of 27 basins of which 17 basins with total area of 14.1 lakh sq. km occupy land surface. Remaining 10 basins with total area of 3.2 lakh sq km lie in offshore region (2.5 lakh sq. km up to a depth of 100 m and 0.7 lakh sq. km between the water depth of 100 and 200 m).
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The geological studies have been conducted in 17 of the 27 basins, while exploratory drilling has taken place it 12. The presence of hydrocarbons has been proved in 9 of these areas.
These include : (1) Upper Assam basin of about 60,000 sq km of area, (2) West Bengal Basin of 60,000 sq km of area, (3) Western Himalayan Basin with area of 100,000 sq km, (4) Rajasthan- Saurashtra-Kachchh Basin with an area of about 93,000 sq km, (5) Northern Gujarat Basin with an area of 1,39,000 sq. km. (6) Ganga Valley Basin covering an area of over 3,84,000 sq km, (7) Coastal Areas of Tamil Nadu, Andhra Pradesh and Kerala with total area of 75,000 sq km, (8) Andaman- Nicobar Coastal Basin with an area of about 2,000 sq. km, and (9) Offshore portion of the Cambay basin, Bombay High and Bassein area with an area of about 2,000 sq. km.