Fresh sources of oil equivalent to the output of four Saudi Arabias will have to be found simply to maintain present levels of supply by 2030, one of the world's leading energy experts has said.

Fatih Birol, chief economist of the International Energy Agency (IEA), the developed world's energy watchdog, told The Times that the depletion of existing oilfields meant that vast new investments would be required to satisfy the demand for oil.

Global oil production stands at about 85million barrels per day. Saudi Arabia is the world's largest producer: it pumped an estimated 9.4million barrels per day during October.

Dr Birol's warning of a looming supply crunch emerged before the publication today of the IEA's 2008 World Energy Outlook, which for the first time includes details of a comprehensive study of depletion rates in the world's largest oilfields.

Dr Birol, who has been leading the analysis for the Paris-based IEA, which advises the Organisation of Economic Co-operation and Development (OECD) on energy matters, said that the decline rates varied by field and by region.

He said that in non-Opec countries, such as the United States, Britain and Mexico, depletion rates averaged 10 to 11 per cent a year. The average across the 13 member countries of the Opec cartel, which produces 40 per cent of the world's oil, was lower, at about 2 to 3 per cent.

Dr Birol, a 50-year-old Turk who worked for Opec in Vienna before joining the IEA in 1995, said that decline rates were faster in the smaller fields than in so-called “supergiants”, such as Ghawar in Saudi Arabia.

Ghawar is the world's largest oilfield and pumps about five million barrels per day, or roughly 6 per cent of global supplies.

“In Russia and the North Sea we are seeing significant declines,” Dr Birol said, adding that the supergiants were still showing low rates of decline.

Dr Birol said he believed that there was enough oil in the ground to meet increased demand but that it would be “a huge challenge” because of the scale of the investment required to develop new fields in remote and inhospitable places, such as the Arctic or the deep ocean off Brazil.

He said that the challenge was particularly acute because, in addition to the replacement 45million daily barrels needed simply to stand still, an additional 20million would be needed to keep pace with surging demand, mainly from developing countries. “It is possible, but it [will require] a major structural change,” Dr Birol said.

Almost all the growth in production will need to come from the national oil companies of Opec states, as this was where the bulk of the remaining reserves were to be found, he said.

In today's report, the IEA predicts that global oil production will increase from 85million barrels per day to 106million by 2030.

However, the bulk of this increase would need to come from costly unconventional fuels, such as liquefied natural gas, and the processing of bitumen-rich oil sands from northern Canada into synthetic crude.

Dr Birol also emphasised that the twin challenges of meeting surging global demand for energy while dealing with the threat of catastrophic climate change would require “a global energy revolution” over the next few years.

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