Digboi oilfield

Geography glossary

Also called: Digboi oil field · Topic: Energy Resources of India: Conventional and Non-conventional · NCERT: Class 10, Ch 5 "Minerals and Energy Resources"; Class 12, Ch 5 "Mineral and Energy Resources"

Meaning

Digboi oilfield is India's oldest oilfield, in the upper Brahmaputra valley of Assam (about 27.4°N, 95.6°E). Oil was struck here in 1889, and it was India's only producing oil area until 1956.

It matters because India's oil story began here. It is also home to the Digboi refinery (1901), Asia's oldest refinery still working and the standard example of a field-based refinery.

Explanation

How oil formed and was trapped at Digboi

Rock type: Digboi's oil lies in Tertiary sedimentary rocks.
These are rock layers laid down in old seas and basins, roughly 66 to 2.6 million years ago.
The upper Brahmaputra valley is one of the Tertiary basins on India's edges.
What petroleum is: a mix of liquid and gaseous hydrocarbons (compounds of hydrogen and carbon).
How it formed:
Tiny sea plants and animals died and were buried under mud.
Heat and pressure acted on them for millions of years.
Over time they turned into oil and gas.
How it was trapped: in Indian oil basins, oil collects in two kinds of rock structure.
Anticline trap. An anticline is an upfold, an arch-shaped bend in rock layers. Oil and gas are lighter than water, so they rise through porous rock (rock with tiny holes). A non-porous layer (cap rock) on top stops them at the crest of the arch. Gas sits on top, oil in the middle and water at the bottom.
Fault trap. A fault is a crack in the rock along which the blocks have moved. Here a porous layer ends up against non-porous rock, so the rising oil is blocked and collects along the fault.

A short history

1889: oil was struck at Digboi.
1901: the Digboi refinery started. It is Asia's oldest refinery still working.
1889–1956: Digboi was India's only producing oil area.
1953: Naharkatiya (Dibrugarh district) was found. It was the first big find after Digboi.
1956: the Oil and Natural Gas Commission (ONGC) was set up to search for and produce oil and gas. After that, new fields were found in Assam, Gujarat and offshore. ONGC became a corporation in 1994.

Digboi as a field-based refinery

Field-based refinery: a refinery built near the oilfield. It uses local crude.
Advantage: it saves the cost of moving crude oil over long distances.
Digboi is the textbook example.
Assam's other refineries (Guwahati, Bongaigaon, Numaligarh) also stand close to the region's fields.
Why a refinery matters: refining is a nodal industry, meaning many other industries depend on it.
The refinery splits crude into petrol, diesel, kerosene, naphtha and other products.
Petrochemical, fertiliser, plastics and textile units then use these products as raw material.

Where it occurs

Location: upper Assam, in the upper Brahmaputra valley in north-east India (about 27.4°N, 95.6°E).
Other fields in the same Assam belt:
Naharkatiya (Dibrugarh district), found in 1953.
Moran-Hugrijan.
Refinery: the Digboi refinery (1901) sits at the field itself.
Compare with India's other oil regions:

Region	Basin / setting	Key fields	Key date
Assam	Upper Brahmaputra valley	Digboi, Naharkatiya, Moran-Hugrijan	1889; 1953
Gujarat	Cambay basin	Ankleshwar, Kalol, Mehsana, Nawagam, Kosamba, Lunej	1960 (Ankleshwar)
Offshore west	Arabian Sea shelf	Mumbai High	Found 1974, producing 1976
Rajasthan	Barmer basin	Mangala	2004 (Cairn)
East coast	KG and Kaveri basins	Onshore and offshore Andhra Pradesh and Tamil Nadu	—

Don't confuse with

Naharkatiya oilfield: also in Assam (Dibrugarh district), but found in 1953, the first big find after Digboi. Digboi is older (1889).
Ankleshwar oilfield: the most important field in Gujarat (Cambay basin), found in 1960, after ONGC was set up. Digboi was found long before ONGC existed.
Mumbai High: India's largest offshore oilfield, about 160 km north-west of Mumbai in the Arabian Sea. Digboi is an onshore field in the oldest oil region.
Market-based refineries (Barauni, Mathura, Panipat): these are built near buyers and get crude by pipeline. Digboi is field-based, so its crude comes from the field itself.

Prelims Hooks

Digboi (Assam): oil struck in 1889. It is India's oldest oilfield.
Digboi refinery started in 1901. It is Asia's oldest refinery still working.
Digboi was India's only producing oil area until ONGC was set up in 1956.
Order of discovery: Digboi (1889) → Naharkatiya (1953) → Ankleshwar (1960) → Mumbai High (1974; NCERT: 1973) → Mangala (2004).
State-matching trap: Digboi, Naharkatiya and Moran-Hugrijan are in Assam. Kalol, Mehsana, Nawagam, Kosamba and Lunej are in Gujarat.
Refinery types: Digboi is field-based. Barauni, Mathura and Panipat are market-based and fed by pipeline.

Mains Points

Oil lies on India's edges, not across the whole country: Digboi sits in a Tertiary sedimentary basin in the north-east, like the other oil basins (Gujarat, the western offshore, Barmer, the KG and Kaveri basins).
The old, hard rocks of the Peninsular plateau hold almost no oil.
So refineries far from the fields need pipelines (for example, Barauni was first fed with Assam crude by pipeline), and coastal refineries rely on imports.
Where to build a refinery (GS-I, industrial location): Digboi shows the field-based logic: refine where the crude is found and save on moving it. Barauni, Mathura and Panipat show the market-based logic: refine near buyers and save on moving many finished products.
Numaligarh in Assam shows refineries also being placed to help develop a region.
Energy security (GS-III): a single old field like Digboi could never meet the country's needs.
After 1956, ONGC's search found Ankleshwar, Mumbai High and other fields. Later, NELP (1999), HELP (2016) and OALP (2017) opened exploration to more companies [3][4].
Even so, India still imports about 88% of its crude oil [2]. This is why more exploration and strategic reserves remain important.

Related concepts

Petroleum: occurrence, oilfields, refineries and reserves

Sources

1Class 10, Ch 5 "Minerals and Energy Resources"; Class 12, Ch 5 "Mineral and Energy Resources" (primary)
2Energy Supplies Remain Secure (PIB)pib.gov.in · tier 1
3Exploration and Production – HELP (MoPNG)mopng.gov.in · tier 1
4Exploration and Production – Block Award (MoPNG)mopng.gov.in · tier 1