Washington ? Oil prices darted above $51 a barrel Tuesday as output in the Gulf of Mexico remains in shambles more than two weeks after Hurricane Ivan tore through the region.

Even as the advance in crude futures begins to appear unstoppable, however, some analysts are convinced a speculative bubble is forming. They say prices have become inflated as institutional investors disappointed in stock-market returns pile on bets in the energy markets.

Fishermen's boats, foreground, block the Marseille harbor in southern France and prevent access for a ferry from Algeria. Fishermen who are unhappy with the high price of oil used their vessels for the second day Tuesday to block ports on the south coast of France.

“Oil has become the only game in town,” said Fadel Gheit, senior vice president of oil and gas research at Oppenheimer & Co. in New York. “Every other investment vehicle has disappointed over the last 12 months.”

On Tuesday, light sweet crude for November delivery soared $1.29 to $51.20 a barrel on the New York Mercantile Exchange. The latest surge came as traders expressed fears about violence in oil-producing giants such as Nigeria and Iraq and concerns about the slow pace of recovery in Gulf of Mexico oil output.

Oil production in the region is more than 3 million barrels per week below average, putting U.S. crude inventories at historically tight levels and placing extra importance on imports at a time when the market is already nervous about possible supply disruptions.

“The market could keep going up in the near term until there is definite visibility that additional oil tanker loadings in the Middle East actually start to appear and rebuild U.S. crude stocks,” said George Gaspar, an oil analyst at R.W. Baird & Co. in Milwaukee.