Give a geographical explanation of the distribution of off-shore oil reserves of the world. How are they different from the on-shore occurrences of oil reserves?
1. What decides where oil forms
Oil forms in ancient sedimentary basins, wherever organic matter was buried under the right conditions of heat and pressure over geological time.
2. Why basins straddle the coastline
A continental shelf is geologically just a submerged continuation of the adjacent landmass. Basins that formed near ancient coastlines don’t stop at today’s shoreline, they continue offshore.
3. Geographic pattern: onshore concentration
Major onshore reserves cluster where large sedimentary basins sit entirely on land: the Middle East, Texas’s Permian Basin in the US, Siberia in Russia, and Venezuela’s Orinoco Belt.
4. Geographic pattern: offshore concentration
Major offshore reserves cluster along continental shelves and their extensions: the North Sea, the Gulf of Mexico, Brazil’s pre-salt basins, West Africa’s coast, and India’s Bombay High.
5. The world’s largest examples
Ghawar in Saudi Arabia, the world’s largest onshore field, alone supplies about 6% of global oil output. Safaniya, also off Saudi Arabia, is the world’s largest offshore field.
6. Difference 1: extraction method
Onshore wells use conventional land rigs on established roads. Offshore wells need specialised platforms, from fixed structures in shallow water to semi-submersible rigs and drillships in deepwater.
7. Difference 2: cost
Onshore extraction is comparatively cheap, since infrastructure already exists. Offshore extraction demands far higher capital investment, rising sharply with water depth.
8. Difference 3: risk
A spill onshore is usually easier to contain and clean up. An offshore spill, especially in deepwater, is remote and far harder to control, as the 2010 Deepwater Horizon disaster in the Gulf of Mexico showed.
9. Difference 4: depletion pressure
Mature onshore basins are depleting faster, pushing producers toward offshore and deepwater fields once considered too costly to develop. Offshore output now supplies a rising share of world production.
10. India’s own onshore-offshore split
India’s production mirrors this same pattern: onshore fields in Assam and Gujarat sit alongside the offshore Bombay High field, historically India’s single largest producing asset.
11. Answer structure
Explain the shared geological origin of offshore and onshore reserves through continental-shelf continuity, then contrast the two across extraction method, cost, risk, and depletion pressure, using named fields as evidence throughout.
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