LONDON Oil at $100 a barrel should give exporters every incentive to pump more, but their difficulty in doing so shows the world is struggling to sustain production.
A growing number of leading industry figures -- the CEOs of Total (TOTF.PA) and ConocoPhillips (COP.N) among them -- now question mainstream forecasts for supply, suggesting the era of "plateau oil" is nearer than many in the business have admitted.
While global oil demand is projected to grow to more than 100 million barrels per day later this century, some argue it may not be possible to boost flows beyond the current rate of some 86 million bpd.
Supply still falls short even after so-called unconventional oils extracted from tar sands and converted from natural gas are taken into account, said Sadad al-Husseini, a former top official at state oil giant Saudi Aramco.
"Today's oil prices are high because there are limited new supplies," Husseini, who ran exploration and production at the Saudi state oil company from 1986-2002, told Reuters. "There's a history now. We're several years into level production."
In 1980, when crude first hit an inflation-adjusted high of $100, the pace of drilling by producing countries and major oil companies became fast and furious, leading to rising output and a price collapse in 1986.
It remains to be seen whether they will respond the same way this time, even after a six-year price rally that sent crude through $100 for the first time last week.
Conventional supply from outside OPEC has missed forecasts in recent years and appears for now to have hit an "effective plateau", according to the International Energy Agency (IEA), adviser to industrialized countries.
Non-OPEC countries pump about 60 percent of the world's oil and the 13 members of OPEC make up the balance. OPEC sets output limits for 12 of its 13 members.
Many countries within the Organization of the Petroleum Exporting Countries -- whether for reasons of war or sanctions, lack of investment or falling supply at ageing fields --- are unable to raise output.
"OPEC can do little," Shokri Ghanem, the top oil official for OPEC member Libya, told Reuters. "Most OPEC countries are producing at capacity."
Many analysts expect prices to rise further unless demand crumbles as a result of a recession -- a gain that believers in peak oil put down to supply constraints.
"Every place is more or less running flat out," said Colin Campbell, a former exploration geologist and self-described advocate of peak oil -- where output reaches a high point and then falls rapidly.
"They can't pump enough to meet demand, so the price is going up. Assuming no particular economic crash, the only direction is upwards."
The views on supply from Campbell and Husseini are far more conservative than most.
The IEA, whose forecasts are a benchmark for the oil industry, predicts world oil output will rise to 116 million bpd by 2030, from about 86 million bpd now.
Global proven oil reserves of 1.208 trillion barrels are enough to sustain current rates of production for 40 years, according to figures compiled by BP in its Statistical Review of World Energy.
But Campbell believes supply of oil, including unconventional oil, will peak in 2010.
Husseini says high prices will only sustain a production plateau of 85 million bpd for up to a decade. From then he sees the start of a long, gradual decline sinking the world's supply of oil and natural gas liquids to about 78 million bpd by 2030.
His forecast is nearly 30 million bpd below the IEA and the U.S. government.
"The industry's ability to deliver more oil, quickly, is no longer there," he said.
Peak oil, proposed by the late geologist M. King Hubbert in 1956, has its detractors who say technology can help extend the life of reserves.
Some senior industry executives see no need for concern.
"I am no subscriber to the theory that oil supplies have already peaked," Tony Hayward, chief executive of BP Plc (BP.L), said at a conference in November.
But more industry leaders are questioning the conventional wisdom on long-term supply.
Libya's Shokri Ghanem said in October it may not be possible to boost supply beyond 100 million bpd as output peaks in some countries and few large new oilfields are discovered.
Christophe de Margerie, head of France's Total, and his counterpart at U.S. ConocoPhillips also questioned whether production would exceed that rate.
The Total chief says the problem is not with the amount of oil in the ground, partly because advances in technology had made more sources of oil accessible.
Instead, the constraints are with the industry's ability to produce the oil quickly enough and with countries' willingness or ability to develop their reserves.
Whoever proves right, Husseini says there's no need for panic -- oil's decline will spur conservation and alternative fuels.
"In the long term, this crisis is probably the only way to get to a sustainable global economy," he said.
(Editing by Richard Mably and William Hardy)