The concern is widely shared. The steady rise in oil prices over the past years came to an abrupt end with the financial crisis last year. After oil peaked at $147 a barrel in New York in July 2008, prices collapsed within a few months, falling to a low of $34 a barrel by December.

Paolo Scaroni, the chief executive of Eni, said that he expected oil prices to hover around $65 to $75 a barrel over the next five to seven years, a level he described as “low.” Some analysts are forecasting that oil could once again rise above $100 a barrel in the next few years as a result of renewed economic growth from China and elsewhere.

“If prices rise above $65, we will be happy,” Mr. Scaroni said, “but we don’t count on it in our planning.”

Oil prices jumped above $75 a barrel last week, their highest level in a year.

Many factors explain this new reality. Much of the “cheap oil” that can be extracted and developed at low cost lies in Saudi Arabia, Iran, Iraq and along the rest of the Gulf, where it is mostly beyond the reach of international oil companies for various political or safety reasons.

Elsewhere, as in Venezuela or Russia, which both hold substantial reserves, stricter government policies have made it difficult for companies to invest in recent years. In Brazil, which traditionally had been open to investors, the government has recently decided to tighten access to its promising offshore region, granting preferential treatment to its domestic industry.

Even in the United States, companies say they are constricted. Royal Dutch Shell paid more than $2 billion to acquire offshore exploration leases in the Arctic region north of Alaska a few years ago but has yet to drill a well because of legal hurdles. The U.S. Congress is also considering rescinding tax credits and other favorable policies — moves that would essentially increase taxes on oil producers.

“Any new taxes will hurt,” said Lamar McKay, the chairman and president of BP America.

The picture is not entirely grim. Companies have made large discoveries this year. BP announced a major offshore find in the Gulf of Mexico last month, while Anadarko Petroleum and its partners said they had probably identified a major petroleum basin running 1,125 kilometers, or 700 miles, from Ghana to Sierra Leone.