The Cantarell oil field, in the shallow waters of Campeche Bay, is regarded by Mexicans as their crown jewel. It is the second largest oil field in the world by production, behind Saudi Arabia’s mammoth Ghawar oil field, pumping 2.2m barrels a day, the same amount as all the Kuwaiti fields together.

For that reason, Mexicans were recently dismayed when Petróleos Mexicanos, the state oil company, said that the field’s production would decline this year, signalling a trend towards its depletion.

Cantarell’s difficulties are not unique. Other mature oil provinces outside the Organisation of Petroleum Exporting Countries, such as the North Sea and Alaska, are now suffering huge yearly declines, constraining the world’s supply of oil and helping to push up prices.

Last year’s surge in oil prices was driven by the biggest yearly increase in demand since 1976. But analysts say today’s high prices are the result of strong demand and a significant slowdown on oil supply growth from non-Opec countries. Barclays Capital estimates that non-Opec supply outside the former Soviet Union rose by 700,000 a year between 1990 and 2000. But since then growth had been roughly flat every year.

This slower increase in non-Opec supply is boosting demand for the cartel’s oil, reducing Opec’s already low spare capacity. The market takes the reduction of this cushion against unexpected shocks as a bullish signal, sending prices higher.

“We got a lot of new capacity additions, but the problem is when you net those with the declines in mature regions, you got a flat line,” said Paul Horsnell of Barclays Capital. Economists at PFC Energy add: “A number of large and long- anticipated supply projects are coming on stream in 2005. However, these projects will only offset . . . declining rates in mature regions and slowing Russia growth.”

Among the new projects are several deep-water platforms in Brazil, the BTC pipeline project in Azerbaijan, the Thunderhorse field in the Gulf of Mexico and the huge Kizomba field in Angola.

The International Energy Agency, the industrial countries’ energy watchdog, forecast non-Opec supply would grow this year by 900,000 barrels a day, but Russia and other countries of the former Soviet Union would account for about 60 per cent of the increase.

However, Russian production, which helped non-Opec supply to achieve growth in the last year, has itself slowed significantly.

Production elsewhere in Europe, Asia and North America will decline, with significant increases only in west Africa, Brazil and Ecuador. Even non-Opec countries in the Middle East, such as Oman, Syria and Yemen, would see production declining by nearly 100,000 barrels a day.

At the same time, world oil demand would jump by 1.8m barrels a day, increasing the dependence on Opec oil for the third year in a row.

Analysts said the trend would continue because oil companies were not investing enough and also lacked the opportunity to drill in promising regions such as Mexico whose constitution bars foreign oil companies.

Lehman Brothers and Citigroup, the investment banks, forecast an increase in worldwide exploration budgets of less than 6 per cent for 2005, a significant slowdown from last year’s 12 per cent.

“With increasingly depleted reserve bases, non-Opec declines are only expected to gather steam in the years to come,” Washington-based PFC Energy said in a report.

Reduced exploration spending translates into less oil being found.

IHS Energy, a leading consultancy advising oil majors on upstream operations, estimates that the world has been consuming oil faster than discovering it since 1986.

Analysts also warn that new non-Opec production, which in the past came from politically stable places such as Alaska and the North Sea, is now coming from more volatile countries. In addition, new discoveries are more expensive than those of the early 1990s, as a large proportion of oil fields are found in deep waters rather than onshore or the shallow continental shelf.

In 2003, about 70 per cent of the largest oil discoveries were in deep waters, with a large proportion of that in waters more than 1,000 metres deep. A decade before, only 16 per cent were in deep waters.

With specific reference to non-Opec countries, the IEA has warned that “a rising share of production will have to come from smaller oilfields, where the unit costs are higher”.

For this reason, the marginal cost of production in mature basins in non-Opec countries is rising. “This may well deter investment and capacity additions in the long term,” the IEA warned in its latest outlook for the oil market.