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?

Where offshore oil sits

The world’s great offshore provinces follow drowned continental margins. The Persian Gulf is a flooded foreland with huge reserves under shallow water (Saudi, Iran, UAE, Qatar). The North Sea (UK, Norway) is a rift basin that became a late-twentieth-century province. The Gulf of Mexico holds US and Mexican fields, including deep water. West Africa (Nigeria, Angola, Ghana) and Brazil’s pre-salt under thick ocean are Atlantic margin stories. The South China Sea mixes geology with a sovereignty dispute. Arctic shelves (Russia, Norway, US) are ice-limited frontiers. India’s Mumbai High, the Krishna–Godavari offshore, and Cauvery waters are the domestic map.

These waters sit on continental shelves where Mesozoic or Cenozoic source rocks cooked, and where anticlines, faults or salt trapped the oil. Deep water is simply that geology further down the slope, needing floating rigs.

How onshore differs

Onshore giants — Ghawar in Saudi Arabia, the Texas Permian, West Siberian fields, Iraq’s land provinces, India’s Assam and Barmer — are cheaper to drill when the desert or plain is stable. A truck can reach the well. Offshore needs platforms, subsea pipes, helicopters and a navy’s worth of safety. A spill at sea (Deepwater Horizon) travels with the current; a land spill is ugly but more containable. National control of land fields is simpler than of fields under disputed water.

• Onshore can still be brutal: Niger Delta pollution, Amazon drilling, and war on land. Offshore is not cleaner by nature. It is technically heavier and capital heavier. When oil prices fall, deep water is the first to freeze.

Geographically, offshore oil is the drowned twin of onshore basins. Politically, it is a floating fortress. That is the difference the question wants.