UPSC DAW 17th October 2025 Mains Answer Writing - 2026

UPSC DAW 17th October 2025 Mains Answer Writing - 2026

Question

Give a geographical explanation for the distribution of off-shore oil reserves of the world. How are they different from the on-shore occurrences of oil reserves? (250 Words, 15 Marks) 

Model Answer

Approach: Introduction:  

  • Describe off-shore oil reserves. 

 Body :  

  • Geographical distribution of off-shore oil reserves. 

  • Differences from on-shore oil reserves. 

 Conclusion :  

  • Importance of these resources. 

Introduction: Onshore oil reserves are those found on land, such as the Cambay Basin and Assam fields in India, while offshore oil reserves lie beneath the seabed, like the Mumbai High and KG-D6 basins. As onshore fields mature and deplete, offshore reserves are increasingly important for meeting global and national energy demands.  

Body: Petroleum reserves are found in sedimentary basins, where organic matter is trapped under pressure. Offshore reserves, discovered since the mid-20th century, now account for ~30% of global crude oil production. Their distribution is linked to continental shelf geology, passive margins, and deep-water basins. 

  • Geographical distribution of off-shore oil reserves: 

  1. Continental shelves and margins: Most off-shore oil reserves are concentrated along sedimentary basins near the edges of continents. These regions were ancient seas, rich in plankton and algae, whose remains got buried and transformed into oil through heat and pressure. 

  1. Major off-shore fields: 

  2. Persian Gulf: Home to supergiant fields like Safaniya and Marjan (Saudi Arabia, Iran, UAE), formed in thick marine sediments and structural traps. 

  3. North Sea: Off the coasts of UK and Norway; oil is contained in Jurassic and Cretaceous sandstones and chalk deposited in ancient marine deltas and shallow seas. 

  4. Gulf of Mexico: The USA and Mexico exploit huge off-shore reserves in deep-water sediments formed by river deltas and structural traps. 

  5. Brazil (Campos and Santos Basins):Located in deep Atlantic waters, these are among the largest off-shore oil provinces, formed in sediment-rich rift basins. 

  6. West Africa (Nigeria, Angola), Caspian Sea (Azerbaijan, Kazakhstan), South China Sea (Malaysia, Vietnam): All feature sizable off-shore oil reserves, typically along faulted continental margins and structural or stratigraphic traps. 

  7. India: Mumbai High and Krishna-Godavari Basin are key off-shore fields, formed in Tertiary period sedimentary traps. 

  • Difference Between Offshore and Onshore Oil Reserves: 

Aspect 

Offshore 

Onshore 

Locaton 

Beneath seabed (continental shelf, slope, deep sea). 

Beneath land surface (sedimentary basins, deserts, plains). 

Formation 

Marine sedimentary basins, deltas, passive margins. 

Ancient sedimentary basins, continental interiors. 

Extraction 

Needs rigs, subsea pipelines, floating platforms: costly & technologically advanced 

Easier drilling, cheaper infrastructure. 

Distribution 

Gulf of Mexico, North Sea, Brazil, West Africa, Persian Gulf. 

Middle East deserts (Ghawar in Saudi Arabia, Kirkuk in Iraq), Russia, USA (Texas), Iran. 

Risks 

Oil spills, marine ecosystem damage (eg: Deepwater Horizon 2010 in Gulf of Mexico). 

Land degradation, groundwater pollution. 

Contribution 

~30% of world’s oil 

~70% of world’s oil 

Conclusion: 

The distribution of offshore oil reserves underlines geography’s role in global energy supply. While these reserves are crucial for energy security, their exploitation must be balanced with ecological safeguards and a gradual shift towards offshore wind and other renewables for a sustainable energy future.