Maarg Manthan · Topic 21.3

Petroleum – Formation and Distribution

Geography › Mineral and Energy Resources · Topic 21.3

Petroleum, or crude oil, is a fossil fuel formed from the remains of marine plankton buried in sedimentary rocks and altered by heat and pressure, then trapped in porous reservoir rocks under a cap rock, often in an anticline. West Asia holds nearly half of the world's reserves. India's oil comes from Mumbai High, Assam, Gujarat and Rajasthan, but it supplies only about one-eighth of the country's needs, with import dependence near 88 to 90 per cent.

5 min read · UPSC Prelims · Geography · Mineral and Energy Resources

At a Glance

Origin Marine organisms in sedimentary basins
Trap Reservoir rock under cap rock, often anticline
Largest reserves Venezuela, Saudi Arabia, Iran, Iraq, Canada
India Mumbai High, Assam, Gujarat, Rajasthan
Import dependence About 88 to 90 per cent
Exam link UPSC Prelims, Geography: Mineral and Energy Resources

Where Does It Fit?

This is the third topic of the macro on mineral and energy resources. The previous topic covered coal, and this one covers oil, the world’s leading energy source in transport. The next topic covers natural gas. Previous topic: Coal – Formation, Classification and Distribution. Next topic: Natural Gas and the Petroleum-Gas Value Chain.

How Is Petroleum Formed and Trapped?

Source: millions of years ago, tiny marine plants and animals died and settled on the sea floor, where they were buried by sand and mud in oxygen-poor conditions.
Change: heat and pressure converted the organic matter into oil and gas in the source rock.
Migration: oil and gas, being lighter than water, move upward through porous rocks such as sandstone and limestone.
Trap: they accumulate in a reservoir rock capped by impermeable rock such as shale. The common traps are anticlines, upfolds of the rock layers, fault traps and salt domes.

Because of this origin, oil is found in sedimentary basins, mostly in young rocks of the Tertiary, as in Dravidian and Aryan Rock Systems of India, and almost never in the crystalline Peninsular shield. Crude oil is refined into petrol, diesel, kerosene, aviation fuel, LPG, lubricants, bitumen and petrochemical raw materials.

Where Is Oil Found in the World?

Reserves: West Asia, which has close to half of the proven reserves, led by Saudi Arabia, Iran, Iraq, Kuwait and the United Arab Emirates; Venezuela, with the largest reserves of heavy oil in the Orinoco; and Canada, with oil sands in Alberta.
Producers: the United States is the largest producer, with shale oil from the Permian basin, followed by Saudi Arabia and Russia; Canada, Iraq, China, the UAE, Iran and Brazil follow.
Organisations: OPEC, formed in 1960, and the larger OPEC Plus group of allies manage output to influence prices.
Shocks: oil prices are sensitive to wars and chokepoints. A conflict involving Iran from early 2026 disrupted shipping in the Persian Gulf and the Strait of Hormuz, through which a fifth of the world’s oil passes, and pushed Brent crude above 100 dollars a barrel at times, so prices should be checked in current sources, as in Bays, Gulfs, Straits and Isthmuses.

Where Is Oil Found in India, and What Is the Energy Security Issue?

Region	Details
Assam	The oldest field: Digboi, where oil was struck in 1889; also Naharkatia, Moran and Rudrasagar; oil flows by pipeline to the Guwahati, Bongaigaon and Digboi refineries
Gujarat	Cambay basin: Ankleshwar, Kalol, Nawagam; Gujarat Plain
Mumbai High	Offshore in the Arabian Sea about 160 km from Mumbai, discovered in 1974; India’s largest producing field, with Bassein; run by ONGC
Rajasthan	Barmer–Sanchor basin: Mangala, Bhagyam and Aishwariya fields, among the largest onshore discoveries, started production in 2009
East coast	Krishna–Godavari and Cauvery basins, with deepwater gas and some oil
Other basins	Andaman, Mahanadi and the Himalayan foothills are being explored

India produces about 26 million tonnes of crude oil a year, down from a peak of about 36 million tonnes in 2011–12, while it consumes about 240 million tonnes, so it imports 88 to 90 per cent of its crude, mainly from Iraq, Saudi Arabia, the United Arab Emirates, Russia and the United States. About half of the imports passed through the Strait of Hormuz in early 2026, which made the supply chain vulnerable, and India’s import bill rose as prices increased. India is the fourth largest refiner in the world, with a capacity of over 250 million tonnes a year, led by Reliance’s Jamnagar complex, and exports petroleum products. The strategic petroleum reserves at Visakhapatnam, Mangaluru and Padur, of about 5.3 million tonnes, would provide cover for roughly nine days of consumption.

The policy response includes the Open Acreage Licensing Policy for exploration, a push to raise domestic output from deepwater and Andaman areas, blending ethanol with petrol, electric vehicles, and more diversified sources of imports, as in Energy and Biotic Resources of the Oceans. As an analytical point, oil is the single largest vulnerability in India’s external accounts, so energy security rests on diversification of suppliers and routes, storage, efficiency and a faster shift of transport to electricity and biofuels, as in Natural Gas and the Petroleum-Gas Value Chain.

Why Is This Topic Important?

Facts Fields and basins asked
Current affairs Hormuz and prices
Security Import dependence

What Came Next?

The next topic describes natural gas and the petroleum-gas value chain. Next topic: Natural Gas and the Petroleum-Gas Value Chain.

Key People and Terms

Anticline Upfold trapping oil
Cap rock Impermeable layer sealing a reservoir
Crude oil Unrefined petroleum
OPEC Organisation of the Petroleum Exporting Countries
Strategic reserve Emergency crude stock
OALP Open Acreage Licensing Policy

Exam Corner

Points to Remember

Oil is found in sedimentary basins.
Digboi: first oil, 1889.
Mumbai High: discovered 1974.
India imports about 88 to 90 per cent of crude.
Hormuz carries a fifth of world oil.

Do Not Confuse With

Reservoir rock and cap rock Porous holder versus impermeable seal
Crude oil and petroleum products Raw versus refined
Mumbai High and Bassein Oil field versus gas-rich neighbour

Memory Hook

Plankton, burial, heat, rise, trap: oil under a cap in an anticline; Digboi, Mumbai High, Barmer.

Mains Angle

Questions ask about distribution and energy security. These points add depth.

Origin Sedimentary basins
Gap Demand versus output
Response Diversification and transition

A Question You May Face

An original practice question, not a past paper question.

“Describe the formation and distribution of petroleum in India and discuss the challenges to India’s energy security.”

How to Answer

Formation Source to trap.
World West Asia and others.
India Fields and refining.
Security Imports, storage, shift.

GS Relevance

Prelims: formation, traps, oil basins and fields of India, reserves in the world, refineries, strategic reserves. GS Paper 1: resource geography; GS Paper 3: energy security.

Frequently Asked Questions

How is petroleum formed?

Petroleum is formed from the remains of tiny marine organisms that were buried in sediments on the sea floor millions of years ago. Heat and pressure changed them into oil and gas, which migrated through porous rocks and were trapped beneath impermeable rock layers in structures such as anticlines.

Where are the main oil fields of India?

India's main oil fields are Mumbai High and Bassein offshore in the Arabian Sea, the Digboi, Naharkatia and Moran fields in Assam, the Ankleshwar, Kalol and Cambay fields in Gujarat, the Mangala and Bhagyam fields in Barmer, Rajasthan, and gas-rich fields of the Krishna–Godavari basin.

Where was oil first discovered in India?

Oil was first struck in India at Digboi in Assam in 1889, and the Digboi refinery is one of the oldest in the world. Mumbai High, discovered offshore in 1974, became the largest oil-producing field of India, and Rajasthan's Barmer basin is a large onshore discovery.

Why does India import most of its crude oil?

India imports about 88 to 90 per cent of its crude oil because its domestic production of about 26 million tonnes a year is much lower than its consumption of about 240 million tonnes, and output has fallen since the peak a decade ago. The rest is made up by imports from West Asia, Russia and the Americas.

What is the importance of the Strait of Hormuz for oil?

The Strait of Hormuz, between the Persian Gulf and the Gulf of Oman, is the exit for the oil of Saudi Arabia, Iraq, Kuwait, Iran and the UAE. About a fifth of the world's oil trade passes through it, so any conflict that disrupts shipping there can raise global oil prices sharply.

PYQ Practice — Statement Analysis

1 Oil is usually found in sedimentary rock basins.

True

It needs a source, reservoir and cap rock.

2 Digboi in Assam is India's oldest oil field.