Sedimentary Basins of India

A sedimentary basin is a large, low-lying area of the Earth's crust where layers of sand, mud and dead plant and animal matter have piled up over millions of years and turned into rock. Oil and natural gas are found only in such basins. India has 26 sedimentary basins, covering about 3.36 million sq km on land and under the sea. The government groups them into categories based on how much we already know about their oil and gas.

Why do these basins matter?

Oil and gas form only under special conditions. Without a sedimentary basin, there is no oil or gas, no matter how deep you drill. So knowing which basins exist, and how well each is explored, tells the government and companies where to spend money. For India, which imports almost 89% of its crude oil, this map is the starting point of energy security.

How do oil and gas form?

Think of it like slow cooking over millions of years:

1. Source rock: Tiny sea plants and animals die and settle on the seabed with mud. They get buried under more layers.
2. Cooking: As they sink deeper, heat and pressure slowly turn this organic matter into oil and gas. Oil formed this way by heat is called thermogenic.
3. Reservoir rock: The oil and gas move upward into porous rocks like sandstone or limestone, which have tiny spaces that can hold fluids, like a sponge.
4. Seal (cap rock): A tight rock layer, like shale or salt, sits on top and stops them from escaping.
5. Trap: The rock layers are bent or broken in a way that collects the oil and gas in one place.

If all these parts are present and work together, geologists call it an active petroleum system.

How is the search carried out?

First, ships send sound waves into the seabed and record the echoes. This is a seismic survey, which creates pictures of the rock layers. Then an exploratory well is drilled to see if hydrocarbons are actually present. If they are found, appraisal wells are drilled to measure how big and good the find is. Only after this does the company decide whether it is worth developing for production.

Potential, reserves and production: three different things

• Resource potential (prognosticated resources): An estimate of how much oil and gas might exist, based on geology. India's total is estimated at about 41.87 billion tonnes of oil equivalent.
• Reserves: The part that has actually been discovered and can be produced profitably with current technology.
• Production: What is actually being pumped out each year.

A big potential number does not mean India has that much oil ready to use.

The three categories

Based on the 2017 Hydrocarbon Resource Assessment Study, the 26 basins are grouped as follows:

• Category I: producing basins (7 basins). These already have reserves and production: Krishna-Godavari, Mumbai Offshore, Assam Shelf, Rajasthan, Cauvery, Assam-Arakan Fold Belt and Cambay.
• Category II: basins with contingent resources (5 basins). Oil or gas has been found, but it is not yet being produced commercially: Saurashtra, Kutch, Vindhyan, Mahanadi and Andaman-Nicobar.
• Category III: prospective basins (14 basins). Only possible (prospective) resources, still to be discovered: Kerala-Konkan, Bengal-Purnea, Ganga-Punjab, Pranhita-Godavari, Satpura-South Rewa-Damodar, Himalayan Foreland, Chhattisgarh, Narmada, Spiti-Zanskar, Deccan Syneclise, Cuddapah, Karewa, Bhima-Kaladgi and Bastar.

Older textbooks use a four-category system (Category I to IV). The three-category list above is the current one used by the government.

Where are the basins?

Of the total sedimentary area, about 49% is on land, 12% is in shallow water and 39% is in deep water. Of the 26 basins, 16 are fully on land, 7 are partly on land and partly offshore, and 3 are fully offshore. Offshore waters are divided by depth: shallow water (up to 400 m), deepwater (roughly 400 to 1,500 m) and ultra-deepwater (beyond about 1,500 m).

Milestones and famous fields (useful for map questions)

• Digboi, Assam (Assam Shelf): Oil gushed out of Digboi Well No. 1 in 1889, the start of India's oil industry. Digboi refinery, set up in 1901, is India's oldest working refinery.
• Ankleshwar, Gujarat (Cambay basin): ONGC's first commercial oil discovery, made in 1960.
• Mumbai High (Mumbai Offshore): Discovered in February 1974 and put into production in May 1976. It is India's largest oil field, about 160 km off the Mumbai coast.
• Mangala, Barmer, Rajasthan: Discovered in 2004; India's largest onshore oil find; production started in 2009.
• KG-D6 (Krishna-Godavari): A major deepwater gas discovery made in 2002 off Andhra Pradesh.
• Andaman-Nicobar: Its resource was estimated at 371 MMTOE in 2017. Its geology is linked to petroleum systems in Myanmar and North Sumatra.

India's position

India's eastern and western offshore basins together are estimated to hold more than 5,600 MMTOE of hydrocarbon potential, in water depths up to 3,000 m. But most of the deep and ultra-deep areas are still poorly explored. Until 2022, large offshore areas were "No-Go" zones because of defence, space or strategic concerns. Opening them, and funding deepwater surveys, is now the main route to new discoveries.

Commonly confused concepts

• Sedimentary basin vs oil field: A basin is a huge geological region (thousands of sq km). An oil field is one specific spot inside it where oil is actually trapped.
• Exploratory well vs appraisal well: An exploratory well checks whether oil or gas exists. An appraisal well checks how much and how good a known find is.
• Krishna-Godavari basin vs Pranhita-Godavari basin: KG is a Category I producing basin on the east coast (onshore and offshore). Pranhita-Godavari is an inland Category III basin in central India.
• Mumbai High vs Mumbai Offshore: Mumbai Offshore is the basin. Mumbai High is the biggest field inside it.
• Exclusive Economic Zone (EEZ) vs continental shelf: The EEZ extends 200 nautical miles from the coast and gives a country rights over resources. The continental shelf is the underwater extension of the land, which can, in some cases, go beyond 200 nautical miles.

Issues, criticism and the way forward

• Under-explored basins: Many Category II and III basins have seen very little drilling, so their real potential is unknown.
• Natural decline: Old fields like Mumbai High produce less every year, so new finds are needed just to keep output steady.
• High cost of deepwater work: Deep and ultra-deep wells are expensive and technically difficult, and finds are often too far from shore to develop cheaply.
• Environmental concerns: Offshore drilling carries risks of oil spills and harm to marine life, especially in sensitive areas like the Andaman Islands.
• Way forward: Complete seismic coverage of all basins, share data openly through the NDR, build common pipelines and processing hubs, and use public funding to lower the risk of early-stage deepwater drilling.

Concepts to Know

• Porous rock: Rock with many tiny holes or spaces inside, which can store fluids like oil, gas or water.
• Seismic survey: A method of sending sound waves into the ground or seabed and recording the echoes to build a picture of rock layers below.
• Million tonnes of oil equivalent (MMTOE): A unit that adds oil and gas together by converting gas into the amount of oil that would give the same energy.
• Contingent resources: Oil or gas that has been found, but is not yet ready to be produced commercially.
• Prospective resources: Oil or gas that geologists think may exist, but which has not yet been discovered by drilling.