They bestrode the earth like giants. Few companies match the global reach of the oil majors, even as the new fields they find get smaller. Hence the excitement of BP’s new “giant” discovery in the Gulf of Mexico. Value the Tiber field’s perhaps 1bn barrels of recoverable oil at $5 apiece, and it could be worth $3.7bn to BP – although at 10km below the earth’s crust, and under 1,259m of sea, the challenges developing it will be huge.

Giant fields are the industry’s lifeblood. The world’s 20 largest – out of a 70,000 total – account for a quarter of world production. Most have been pumping for decades; the largest, Saudi Arabia’s Ghawar, was discovered in 1948 and is only halfway through its 140bn barrels of recoverable reserves. Yet the rate of discovery is falling. In the 1960s, it stood at 56bn barrels a year. By the 1990s, it had fallen to 13bn. Furthermore, most newly discovered fields are small or offshore. A decade ago, deep-water finds accounted for a quarter of all discoveries; now they account for half. This is significant as small or offshore fields tend to decline faster.