Oil pool

Oil pool is a subsurface oil accumulation. An oil field can consist of one or more oil pools or distinct reservoirs within a single large trap. The term "pool" can create the erroneous impression that oil fields are immense caverns filled with oil, instead of rock filled with small oil-filled pores.

Formation of Petroleum

Petroleum is formed under Earth's surface by the decomposition of marine organisms. The remains of tiny organisms that live in the sea—and, to a lesser extent, those of land organisms that are carried down to the sea in rivers and of plants that grow on the ocean bottoms-are enmeshed with the fine sands and silts that settle to the bottom in quiet sea basins. Such deposits, which are rich in organic materials, become the source rocks for the generation of crude oil. The process began many millions of years ago with the development of abundant life, and it continues to this day. The sediments grow thicker and sink into the seafloor under their own weight. As additional deposits pile up, the pressure on the ones below increases several thousand times, and the temperature rises by several hundred degrees. The mud and sand harden into shale and sandstone; carbonate precipitates and skeletal shells harden into limestone ; and the remains of the dead organisms are transformed into crude oil and natural gas.

Once the petroleum forms, it flows upward in Earth's crust because it has a lower density than the brines that saturate the interstices of the shales, sands, and carbonate rocks that constitute the crust of Earth. The crude oil and natural gas rise into the microscopic pores of the coarser sediments lying above. Frequently, the rising material encounters an impermeable shale or dense layer of rock that prevents further migration; the oil has become trapped, and a reservoir of petroleum is formed. A significant amount of the upward-migrating oil, however, does not encounter impermeable rock but instead flows out at the surface of Earth or onto the ocean floor. Surface deposits also include bituminous lakes and escaping natural gas. The Israel 's capital - Tel Aviv derives its name from such oil seepage on the surface

The hypothesis of abiogenic petroleum origin holds that petroleum was formed by primordial non-biological processes deep in the earth's crust and mantle. It contradicts the more widely-held view that petroleum is a fossil fuel produced from the buried remains of ancient living organisms. The French chemist Marcellin Berthelot and the Russian chemist Dmitri Mendeleev proposed it in the nineteenth century, and saw a revival in the last half of the twentieth century by Russian and Ukrainian scientists.

The modern scientific consensus on abiogenic origin petroleum is that while there is evidence for it, most modern geologists do not support this for the vast majority of petroleum deposits within the Earth.

The deep biogenic petroleum theory proposes, mostly after the work of Thomas Gold , that the ‘'deep hot biosphere'' may be the source of some petroleum alteration and for the observation of biomarkers in produced petroleum.

Other planets of the solar system or their moons have large amounts of methane and other hydrocarbons, presumably not of biological origin.

Deep sea vent biogeochemical cycle diagram

OIL LOCALE

Many of the world's oil and gas fields are discovered in deltaic deposits. Geologically occurrence of various sedimentary environments in a delta improves the quality and quantity of hydrocarbon source material and sandstone reservoirs. It is particularly significant in oil and gas enrichment where coexistence of sands, shaly and limy sediments in one sedimentary tectonic unit facilitates generation and accumulation of hydrocarbons. Most of the deltas are tectonically stable areas and maintain a certain amount of equilibrium. Normally organic material in the deltaic sediments will be converted to Petroleum soon after burial during a period of time of growth of a delta and oil and gas. Thus identification of vertical growth of delta in time is important to delineate oil and gas structures buried underneath. Chenier sands, bar sands and channel sands constitute main oil and gas reservoirs in subsurface. Marine shales (potential source rocks) ,which interfinger upslope with sandstones (potential reservoirs) in deltaic sequences (cyclically repeated) are significant in oil exploration.

Though oil occurrences either in small quantities or large quantities are known practically throughout Geological Time, yet certain periods in the Earth's history appear to have been more favorable for their formation than the rest. Though oil deposits have been recorded from Devonian upwards, yet broadly speaking, the Tertiary era had been the most important epoch for commercial deposits of petroleum; the most important periods in this era are the Eocene, Oligocene and Miocene.

MINERAL OIL

Mineral oil or liquid petrolatum is a by-product in the distillation of petroleum to produce gasoline . It is a chemically inert, transparent, colorless oil composed mainly of alkanes and cyclic paraffins , related to white petrolatum . Mineral oil is a substance of relatively low value, and it is produced in very large quantities. Mineral oil is available in light and heavy grades, and can often be found in drug stores.

NON-CONVENTIONAL OIL

Non-conventional oil is oil extracted using techniques other than the traditional oil well method. Currently, non-conventional oil production is less efficient and some types have a larger environmental impact relative to conventional oil production. Non-conventional types of production include: tar sands, oil shale, bitumen, biofuels, thermal depolymerization (TDP) of organic matter, and the conversion of coal or natural gas to liquid hydrocarbons through the Fischer-Tropsch process . These non-conventional sources of oil may be increasingly relied upon as fuel for transportation when conventional oil becomes "economically non-viable" due to depletion.

OILWELL

An oil well is a term for any perforation through the Earth's surface designed to find and release both petroleum oil and gas hydrocarbons .

INTRODUCTION

Petroleum – the resource that shapes the geopolitics and economic power of the nations

Petroleum is, essentially, a compound of carbon and hydrogen or rather a mixture of such compounds in a bewildering variety. Petroleum is thus really a mixture of various kinds of hydrocarbons with small amounts of oxygen, nitrogen and a little sulphur. Petroleum varies greatly in colour, some are colorless, many of them are of nice bluish green colour, while others are pitch-black in colour. Petroleum has assumed utmost significance in modern society. Petroleum and petroleum products are used for a variety of purposes such as motive power, lubrication and as raw material for manufacturing synthetic derivatives and chemicals required by other industries.

PETROLEUM IN INDIA

Total reserves of the hydrocarbons are estimated by GSI at 17 billion tonnes, of which 75 per cent have been established so far. For exploration of petroleum, wells were drilled in Upper Assam valley in 1866, just seven years after the discovery of petroleum in Pennsylvania in USA . Oil was discovered in 1890 in Digboi area. Only Assam produced petroleum in the country until 1959 when Khambhat (Cambay) Oil Field was discovered in 1958 and the first well (Vasudhara) was drilled in Ankaleshwar in 1960. First offshore drilling was started in 1970 at Aliabet in Gujarat . Later, the Bombay (Mumbai) High was discovered in 1975 and production from this area started in 1976. As a result of these efforts, production of petroleum in the country increased rapidly.

IMPORTANT SEDIMENTARY BASINS OF INDIA

The sedimentary basins of India , onland and offshore up to the 200m isobath, have an areal extent of about 1.79 million sq. km. So far, 26 basins have been recognized and they have been divided into four categories based on their degree of prospectivity as presently known.

In the deep waters beyond the 200m isobath, the sedimentary area has been estimated to be about 1.35 million sq. km. The total thus works out to 3.14 million sq. km. of which about 30% is unexplored. Although this appears to be a sizeable proportion, it is a marked improvement over the corresponding figure of 50% in 1995-96. Also, exploration has been initiated in about 33% of unexplored areas vis-a-vis the 18% in 1995-96. Thus, across eight years, there have been significant forward steps in exploring the hydrocarbon potential of the sedimentary basins of India .

DISTRIBUTION OF OIL FIELDS

At present, commercial exploitation of oil and gas is being carried out in following regions: (1) the northeastern region extending over the Upper Assam Valley , Arunachal Pradesh and Nagaland; (2) the Cambay basin in Gujarat region; (3) the Mumbai High offshore region ; ( 4) the East coastal region encompassing the Cauveri and Krishna-Godavari basins; and (5) the West Rajasthan Basin .

1. North-Eastern Region: The Assam basin is located in the alluvial covered foreland shelf zone (known as Upper Assam Valley ) and contains several oil and gas fields. A few oil and gas fields discovered here are located in the Naga

thrust sheets of Assam-Arakan fold belt. Both these units constitute part of Assam-Arakan Basin . Parts of Assam-Arakan Basin fall in north eastern India , eastern part of Bangladesh , and western part of

Myanmar . The NE part of India has been drawing attention of geologists since 1825 due to a large number of oil and gas seepage in the area. The well-known oil fields of this region are Digboi (oldest

field, 1866), Naharkatiya, Moran, Rudrasagar, Lakwa, Lakhmani, Galeki, Borholla-Changpan, Hugrijan etc. The major oil accumulations in the Upper Assam are known in Barail and Tipam Groups of

Oligocene and Miocene age respectively. Source rock is carbonaceous shale. The principal reservoirs in this oil basin are sandstone in Barail and Tipam groups. The Girujan clay is the main cap rock for

accumulation.

In Tirap district of Arunachal Pradesh, there is Nigru oil field. In the nearby Nagaland, oil fields are at Borholla, close to Nagaland-Assam border.

2. Cambay Basin , Gujarat Region: The Cambay basin is located on the western margin platform of the Indian craton. Stratigraphically, the Cambay basin is almost covered by the alluvium of Sabarmati, Mahisagar, Dadhar, Narmada and Tapti rivers. The outcrops in the vicinity of this basin are of Middle Jurassic to Late Cretaceous age. All the

hydrocarbon reservoirs are either sandstones or siltstones. The Tarapur shales of Upper Eocene-Oligocene age are the principal cap rocks in north Cambay region and in the south, the Telwa and Khanwa shales of Middle Eocene to Oligocene age and the shales intercalated with sandstones in

Dadhar Formation form the Principal cap rocks. Important oil fields of this region are Ankleshwar, Kalol, Nawagam, Kosamba, Kathana, Bakrol, Balol, Mehsana, Sabarmati, Sanand , Lunej etc. Oil has

also been found on the Aliabet island situated 45 km west of Bhavnagar in Saurashtra.

3. Bombay (Mumbai) Offshore Basin: The Mumbai Offshore basin is the central part of western continental margin of India . Stratigraphically

this basin has a tertiary sedimentary fill overlying Deccan Trap floor. The sedimentary sequence in Mumbai high can be broadly classified in to three lithological units- the Lower unit composed of basal sand, lignite and clay of Paleocene age, the Middle unit composed of thick limestone in the lower part

and limestone-shale alterations in the upper part (Late Oligocene to Middle Miocene) and the Upper unit composed of shale and claystone of post Middle Miocene age. Oil and gas accumulations are present in

three different groups- within sandstone reservoirs of Oligocene-Lower Miocene age, as gas accumulation as in North, Mid and South Tapti field; within Miocene Limestone reservoirs, as oil with gas cap; as in Bombay High and in Bassein Limestone of Middle Eocene to Oligocene, oil with oil and

gas cap as in Panna, Bassein, Neelam, Heera and Mukta field. Panna Formation represented by sandstone at the bottom overlain by a section of coal-shale alterations is the main source rock. In basin

limestone are the main reservoirs. The post-Middle Miocene shales are the principal cap rocks. The Bombay oils are paraffinic and light with high wax content.

Mumbai high oil field is the important field of this basin. It is Located in the Arabian Sea 176 km north-west of Mumbai, it is an offshore oil field. The oil operations are run by India's Oil and Natural Gas Corporation ( ONGC ). Mumbai High field was discovered by a Russian and Indian oil exploration team operating from the seismic exploration vessel Academic Arkhangelsky during mapping of the Gulf of Cambay in 1964-1967. The naming of the field is attributed to a team including Mr. M. Krishnamurthy, from a survey run in 1965 analysed in the Rashmi building in Peddar Road, Mumbai. At present it is the most important oil field in the country as it contributes about 65% of the indigenous crude oil production. The oil and natural gas are brought through submarine pipelines to Uran on the mainland. Another important oil field-Bassein has been discovered south of Mumbai High. The deposits of this area are considered to be richer than those of the Mumbai High.

4. East Coastal Region: It extends over the Krishna-Godavari and Cauveri basins. The Oil and Natural Gas Commission and the

Oil India Limited carried extensive prospecting and exploration work in 1980. The Krishna-Godavari basin is located on the east coast of India between Vishakhapatnam town in the north and Ongole town in the south. Greater part of this basin is covered by alluvium. The outcrops of Precambrian crystalline and

sedimentary rocks of Late Permian to Pliocene age are seen on north western margin of the basin. Oil and gas accumulations are present in rocks ranging in age from Permian to Pliocene age. The reservoir

rocks occur in arenaceous sequences of different formations belonging in age from Permo-Triassic to Pliocene. Local red clays occur as cap rocks. Important oil fields are Mandapeta, Kaikalur, Lingala,

Amlapuram, Ravva, Razole etc. Some other Oilfields have also been discovered recently in the Krishna-Godavari basin of Andhra Pradesh .

The Cauveri basin occupies most of the coastal plains of Tamilnadu and Pondicherry and extends offshore in to the Bay of Bengal . Oil and gas accumulations are known in various geological formations from Pre-cambrian fractured basement to Oligocene multi stacked sandstone reservoirs. In the offshore area of the Cauveri basin, important oilfields are Narimanam and Kovilappal.

5. West Rajasthan Basin : Rajasthan is endowed with favorable geotectonic settings that encourage hydrocarbon formation. It has more than 150,000 square kilometer of land area with a potential for hydrocarbon reserves. Of this, only 30,000 sq km has been under exploration till date. However, the State government plans to increase the

extent of exploration as well as exploitation.

The west Rajasthan basin forms the western part of shelf zone of the Indus basin ( Pakistan ) where several oil and gas have been found. This basin is divided in to Bikaner-Nagaur Basin in north, the Jaisalmer Basin and the Barmer Basin in the south. These basins are bounded in the east and southeast by the Aravalli ranges and in the northeast by the Delhi-Sargodha Ridge.

In Bikaner-Nagaur basin, the oil shows are reported from Early Cambrian Limestone and evaporites, Late Cambrian Limestone, and Mesozoic Limestone. Heavy oil has been discovered in Bilara and Jodhpur Formations of Early Cambrian age in Baghewala structure and in Tavriwala structure in same formations.

In Jaisalmer Basin , the oil shows are reported from cretaceous rocks and gas shows are reported from Jurassic sandstone. Gas discovery has been made in Early Cambrian, Jurassic, Cretaceous and Tertiary reservoirs. Some of the gas fields are Bakhri-Tibba, Ghotaru, Kharatar, Manhera Tibba, Tanot and Dandewala.

In one of the largest on-shore oil finds in the past two decades, oil and gas deposits were found in 12 oilfields in the Barmer-Sanchore basin. The basin is ranked among the top 100 oilfields in the world. This emerged after 100 oil wells were drilled and 350 million tonnes of reserves were created. Of these, the Mangla oilfield is the largest. Commercial production from four oilfields is expected after 2006. In the last quarter of 2007, which would mark the peak production phase, a production of about 1 ,50,000 barrels a day is expected. This would in turn generate revenue of Rs.2, 000 crores a year.

PRODUCTION OF CRUDE PETROLEUM

It was only 269 thousand tonnes in 1951, which rose to 1.08 million tonnes in 1962, and 34.52 million tonnes in 1995-96. But it declined to 32.89 million tonnes in 1998-99 and 32.43 million tonnes in 2000-01. Out of the total production, one-third (36.4 per cent) was produced from on-shore oilfields and nearly two-thirds from offshore oilfields. Oil and Natural Gas Corporation (ONGC) and Oil India Limited (OIL), the two national oil companies and a few private and joint venture companies are engaged in exploration and production of crude oil and natural gas in the country. In 2000-01, ONGC produced 77.27 per cent, joint venture companies 12.61 per cent and OIL only 10.11 per cent of the total crude oil production in the country. The total consumption of petroleum products in 2000-01 was about 99.6 million tonnes while indigenous production was only 32.43 million tonnes. India produces only one-third of its total requirements. The country depends heavily on imports. During 1998-99, 39.81 million tonnes of crude petroleum and 18.09 million tonnes of petroleum products were imported from Russia , Iran and other West Asian countries.

Category Basin Basinal Area (Sq. Km.) Total

 UPTO 200M ISOBATH On land Offshore  

 I Cambay 51,000 2,500 53,500   Assam Shelf 56,000 - - - - 56,000   Bombay offshore - - - - 116,000 116,000   Krishna Godavari 28,000 24,000 52,000   Cauvery 25,000 30,000 55,000   Assam-Arakan Fold Belt 60,000 - - - - 60,000   Rajasthan 126,000 - - - - 126,000

SUB. TOTAL 346,000 172,500 518,500

 II Kutch 35,000 13,000 48,000   Mahanadi-NEC 55,000 14,000 69,000  Andaman-Nicobar 6,000 41,000 47,000  SUB. TOTAL 96,000 68,000 164,000  III Himalayan Foreland 30,000 - - - - 30,000   Ganga 186,000 - - - - 186,000   Vindhyan 162,000 - - - - 162,000   Saurashtra 52,000 20,000 80,000   Kerala-Konkan-Lakshadweep - - - - 94,000 94,000   Bengal 57,000 32,000 89,000   SUB. TOTAL 542,000 168,000 710,000  IV Karewa 3,700 - - - - 3,700   Spiti-Zanskar 22,000 - - - - 22,000   Satpura-South Rewa-Damodar 46,000 - - - - 46,000   Narmada 17,000 - - - - 17,000   Deccan Syncline 273,000 - - - - 273,000  V Bhima-Kaladgi 8,500 - - - - 8,500   Cuddapah 39,000 - - - - 39,000   Pranhita-Godavari 15,000 - - - - 15,000   Bastar 5,000 - - - - 5,000   Chhattisgarh 32,000 - - - - 32,000   SUB. TOTAL 461,200 - - - - 461,200   TOTAL 1,390,200 394,500 1,784, 700 DEEP WATERS   Kori-Comorin        85° E - - - - - - - - 1,350,000   Narcodam        GRAND TOTAL - - - - - - - - 3,134, 700