In real terms over the last 40 years at today's prices, Deutsche Bank estimates that oil prices have averaged around $35 a barrel, a price it says is far too low for long-term comfort.

"The 'sweet spot' is between $60 and $80, probably the top of that range. That is the long-term fair value," Lewis said.

Simon Wardell, director of the energy markets group at Global Insight Ltd in London, sees broad agreement between OPEC and consuming countries that around $75 is about right for oil.

"That price gets you investment in new production, is high enough to encourage more efficient use of oil and is enough to maintain the budgets of the Middle Eastern countries," he said.

Washington-based consultancy PFC Energy estimates most Middle Eastern oil producers need oil prices this year between $40 and $60 to balance their external accounts.

"My concern is the price going too low," said Wardell. "Closer to $70-75 is a decent equilibrium price."

The International Energy Agency (IEA), which advises 28 industrialized countries on energy policy, says it wants oil prices high enough to foster sustained investment in new energy sources, including costly deep-sea drilling.

"It is very difficult to put an absolute level on what price is fair," said David Fyfe, head of the IEA's oil industry and markets division. "But there is a lot of high cost oil, be it in ultra deep-water, or Canadian oil sands or Arctic developments in northern Russia, which needs a relatively high price."

"We would see a danger if prices fall a lot lower -- that would exacerbate the chances of a medium-term supply crunch."