ecent announcements from Repsol YPF, the big Spanish oil and gas company, indicate an ambitious expansion program, with projects planned for countries like Libya and Equatorial Guinea that are not for the risk-averse. But none has attracted as much attention as its gamble on Cuba.

Last month, Repsol hired a Norwegian drilling platform, the Eirik Raude, at a cost around $200,000 a day to search for oil in Cuban waters, in a narrow sector of the Gulf of Mexico off the northwestern coast. The venture, established with Cubapetróleo, the government-owned oil company, is being watched about as closely in Houston’s executive suites as any in the energy industry.

A significant find by Repsol would, of course, be a boon for Cuba, which imports most of its fuel, mainly from Venezuela, and often struggles to find the hard currency to pay the bills. More broadly, it could shake up the dynamics of oil production in the Gulf of Mexico, dominated for decades by the United States and Mexico.

And a big oil discovery could change the political debate in the United States over the decades-old sanctions against Cuba, which now prohibit most commerce with the country.

The last thing that American energy companies want is to be trapped on the sidelines by sanctions while European, Canadian and Latin American rivals are free to develop new oil resources on the doorstep of the United States.

Halliburton, the nation’s largest oil services company, is among the wary watchers. John Gibson, president of Halliburton’s energy services group, recently said in a speech to employees that he favored lifting economic sanctions against Cuba, as well as Libya and Iran.

“Sanctions are a very U.S.-centric thing, and I believe that free enterprise will establish better relationships,” Mr. Gibson said, according to The Associated Press. “There are foreign companies making money in those countries, and I think American companies should have a shot at those markets as well.”

That sentiment runs counter to the Bush administration’s Cuba policy, which has been to maintain and even strengthen sanctions in hopes of isolating and weakening the Communist country’s economy. The administration has recently imposed new curbs on travel to Cuba and on the amount of money and goods Cubans can receive from relatives in the United States.

Cuba’s former lifeline of oil collapsed when the Soviet Union did, and the severe fuel shortages that the country suffered have prompted officials in Havana to allow foreign companies to explore for oil in Cuban waters, starting in the mid-1990’s.

In the meantime, new technology to squeeze more oil from the small existing fields on the island’s north coast has increased output to about 75,000 barrels a day from about 10,000 in the early 90’s.

More than half of Cuba’s oil is now produced by a Canadian company, Sherritt International, which has been active in Cuba for a decade. “We had the advantage of getting in early and sticking with it,” said Ernie Lalonde, Sherritt’s director of investor relations. He said the company was considering an exploration project similar to Repsol’s in the Cuban portion of the Gulf of Mexico, which covers some 43,000 square miles.

Other foreign energy companies that have ventured into Cuba lately have not been as lucky as Sherritt. Brazil’s national oil company, Petróleo Brasiliero, or Petrobras, one of the most experienced offshore producers, came up empty-handed after spending $17 million drilling in Cuban waters in 2001.

Senior executives at Repsol acknowledge that their Cuba venture is far from a sure thing. “These are high-risk areas, but we are optimistic,” said Alfonso Cortina, Repsol’s chief executive, earlier this year.

Cuba still depends on imports for about half its oil, nearly all bought on preferential terms under an agreement with the leftist Venezuelan government of President Hugo Chávez. The cost strains Cuba’s limited hard-currency earnings, and the country is striving to reduce its dependence on imports.

When more than 100 representatives of American agricultural companies attended a trade conference in Havana this spring, senior energy and finance officials in the Cuban government used the opportunity to suggest Cuba as a destination for energy investments. “There is no reason U.S. companies shouldn’t take advantage and compete so close to home,” said Juan Fleites, a senior executive at Cubapetróleo.

No reason, that is, other than the American sanctions, which are not expected to be relaxed anytime soon. A significant hurdle to any plan to ease sanctions are the large claims outstanding against Cuba for assets and businesses seized when Fidel Castro came to power in the late 1950’s and for debts run up since then. International creditors have been discussing the claims with the Castro government for a decade without making much headway.

An oil discovery large enough to yield exports as well as satisfy Cuba’s own energy needs could change that, by significantly improving the economy and adding a major new source of hard currency to the modest amounts Cuba now earns primarily from tourism. With more resources to make payments, the Cubans would have an easier time settling the old claims.

“Put simply, oil would lift Cuba’s boat, especially if crude prices stay high,” said Tim Lynch, an expert on the Cuban economy and director of the Center for Economic Analysis at Florida State University. “Enough oil would also lessen Cuba’s dependence on Venezuela, redrawing the political and economic map of the Caribbean.”

Matthew Pickles, a senior manager in the Barbados office of Ernst & Young, recently told energy executives at a conference in Port of Spain, Trinidad, that an oil discovery in Cuba could also lead to the building of new refineries or the expansion of existing ones, potentially making Cuba a refining center for the eastern Caribbean, Mexico and Venezuela.

Such rosy predictions aside, geologists and industry analysts are eagerly waiting to see what Repsol’s drill rig turns up, if only to learn more about one of the few stretches of the Gulf of Mexico that is still untested.

A significant find might catapult Cuba into a category of small countries that are emerging as risky but alluring new targets for oil development – among them Mauritania, Senegal, Morocco and Guinea-Bissau, according to Robert W. Esser, director of global oil and gas resources at Cambridge Energy Research Associates. A dry hole, on the other hand, would reinforce the conventional wisdom that American and Mexican waters are the best parts of the gulf for oil.

“Cuba’s way out on the frontier of present-day wildcatting,” Mr. Esser said.