But an examination of the American tax code indicates that oil production is among the most heavily subsidized businesses, with tax breaks available at virtually every stage of the exploration and extraction process.

According to the most recent study by the Congressional Budget Office, released in 2005, capital investments like oil field leases and drilling equipment are taxed at an effective rate of 9 percent, significantly lower than the overall rate of 25 percent for businesses in general and lower than virtually any other industry.

Oil industry officials say that the tax breaks, which average about $4 billion a year according to various government reports, are a bargain for taxpayers. By helping producers weather market fluctuations and invest in technology, tax incentives are supporting an industry that the officials say provides 9.2 million jobs.

The American Petroleum Institute, an industry advocacy group, argues that even with subsidies, oil producers paid or incurred $280 billion in American income taxes from 2006 to 2008, and pay a higher percentage of their earnings in taxes than most other American corporations.

As oil continues to spread across the Gulf of Mexico, however, the industry is being forced to defend tax breaks that some say are being abused or are outdated.

The Senate Finance Committee on Wednesday announced that it was investigating whether Transocean had exploited tax laws by moving overseas to avoid paying taxes in the United States. Efforts to curtail the tax breaks are likely to face fierce opposition in Congress; the oil and natural gas industry has spent $340 million on lobbyists since 2008, according to the nonpartisan Center for Responsive Politics, which monitors political spending.