Give a geographical explanation of the distribution of off-shore oil reserves of the world. How are they different from the…

Off-shore oil occurs where thick marine or deltaic sediments rich in organic matter were buried and sealed beneath continental shelves and slopes. Its distribution is therefore controlled by sedimentary basins, not by the sea itself.

Geographical explanation of the distribution

• Passive continental margins with wide shelves: long-subsiding margins accumulate kilometres of sediment, e.g., the Gulf of Mexico, Brazil's Campos and Santos basins (pre-salt reservoirs) and the Gulf of Guinea off Nigeria and Angola, the mirror-image margins of the South Atlantic rift.
• Large deltas: river-borne sediment and organic matter provide source rocks and sand reservoirs, e.g., the Niger and Mississippi deltas and India's Krishna–Godavari basin (KG-D6).
• Failed rifts and inland seas: the North Sea (a Jurassic rift) and the Caspian Sea (Azeri–Chirag–Guneshli, Kashagan).
• Offshore extensions of onshore provinces: the Persian Gulf (Safaniya oilfield, South Pars/North Dome gas field) continues the Arabian platform; Mumbai High sits on a rifted shelf off western India.
• Deep-water frontiers: Guyana's Stabroek block, West Africa and the eastern Mediterranean (mainly gas).

How off-shore reserves differ from on-shore occurrences

• Geological setting: offshore reserves lie mostly in younger Mesozoic–Cenozoic marine and deltaic sequences on continental margins; onshore reserves occur in interior, foreland and platform basins and often include older Palaeozoic rocks (e.g., North Africa, Russia's Volga–Urals).
• Discovery: offshore exploration depends on marine seismic surveys and drilling from ships and rigs; onshore finds also used surface geology and seepages (Digboi was found through seepages).
• Extraction: offshore needs fixed platforms, FPSOs and subsea wells, with high capital cost and long lead times; onshore wells are cheaper and quicker but face land acquisition and settlement constraints.
• Risk: offshore operations face cyclones, blowouts and large spills (Deepwater Horizon, 2010); onshore risks are more local (groundwater contamination, land subsidence).
• Share: offshore fields supply roughly 30% of world crude output.

For India, Mumbai High and the KG basin provide a large share of domestic output, which is why offshore blocks under the Open Acreage Licensing Policy (OALP) matter for energy security.