In India there are four main areas from which crude petroleum is being extracted. These include (I) the Brahmaputra Valley, (2) Gujarat coast, (3) Western coast offshore area, and (4) Eastern coast off shore area. About 65 percent of crude oil production of the country comes from offshore areas (Bombay High 65.3% and Tamil Nadu coast 1.2%).
On shore areas contributed million tons of crude oil in 2002-03 (about 33% of the country's production). Here bulk of production came from Bharuch, Mehsana and Ahmadabad districts- of Gujarat (17.60%), Dibrugarh and Sibsagar districts of Assam (14.10%), and Arunachal Pradesh (0.22%).
(1) The Brahmaputra Valley
The Brahmaputra Valley is the oldest oil producing region of the country. Here oil bearing belt extends for 1,300 km from the Dehing Basin in the extreme north-east of Assam along the outer flanks of the hill ranges up to the eastern corner of the Surma Valley. Main oil producing areas are located in the valleys of the Nova, Dirling and Buri Dirling rivers in Dibrugarh and Sibsagar districts. This is mostly shale oil extracted from sedimentary rocks of Tipam sandstone. The oil occurs in a large numberof separate pools of depths, varying from 3,000 to 4,000 m. It contains higher percentage (about 9%) of gasoline, paraffin and naphthalene but lower percentage (about 0.3%) of sulphur. It provides about 9% of gasoline, 38% of kerosene, 49% of lubricating oil and 4% of fuel oil.
(a) The Digboi oilfield occupies about 13 sq km (13 km x I km) of area in Dibrugarh district. There are 24 oil bearing strata at a depth of 1200 m. Oil is mostly accompanied with gas. Oil occurs in lenses of unconsolidated sands interbedded with clays of Eocene and Miocene periods. There are about 80 oil wells in the region. The most important centers are Digboi, Bappapang, Hassapang and Paintola. Most of the oil is sent to the oil refinery at Digboi. Since 1959, the Digboi oil-fields are worked by the Oil India Ltd. (OIL).
(b) The Nahorkatia oil field lies 35 km southwest of Digboi along the left bank of the Buri Dihing River. Here oil was discovered in 1953 and commercial production started in 1954. The oil is deriving from the Barail rocks which underlie Tipam rocks a depth of 3700-4400 m. More than 85 oil wells have been dug of which 60 have yielded crude oil and natural gas. The annual production is about 25 lakh tones. Crude oil is sent to the oil refineries Nonmeat (443 km) and Barauni (724 km) throu pipe-line.
(c) Moran-Hugrijan oil field is located 40 south-west of Nahorkatia. Here oil was discovert in 1953 from the Barail rocks at a depth of 4,300i (maximum depth 4,900 m). Twenty-two out of J wells have sofar yielded oil. Hugrijan is combatively a new area. The Nahorkatia-Moran-Hugrijaj oil area has estimated reserves of 45 million tons of crude oil and 7, 40,000 million cubic feet natural gas. This oil is also transported to Barauni for refining.
(d) Rudrasagar-Lakwa oil fields are locates about 40 km and 20 km south-west of Moran respectively in Sibsagar district. Here crude oil by ONGC and OIL in 1961 (at Rudrasagar) a 1965 (at Lakwa). Oil deposits are found in the Bara rocks at Rudrasagar and in the Tipam rocks Lakwa. The annual production is 10 lakh tonnes lakh tonnes at Rudrasagar and 6 lakh tonnes Lakwa).
(e) Surma Valley-here first oil well was at Badarpur in 1917. Badarpur, Masimpur and Pathai are important centers of production. Badarpur and Masimpur are old areas where oil is extracted from the anticlinal belt of the Barail rocks at an average depth of 1,800 m. There are 60 wells with annual production of about 20,000 tones which has declined in recent years.
(f) Other Areas-In Tripura the ONGC has discoverdoil at Mamunbhaga, Baramura-Deotamura and Sobhang anticlines. Oil has also been found in seepages near Mailu, Ampi Bazar, Amarpur-Dambura, etc. Some new oil finds have been located in Arunaclial Pradesh at Kumchai, Manabhum, Kharsang and Charali; and in Assam at Hapjan, Borbil and Diroi. In Nagaland also several oil seepages have been identified.