The well, which spewed more than 4.9 million barrels of oil into the gulf, fouling the coastline, killing marine organisms and affecting lives throughout the region, had not leaked since mid-July, after valves were closed on a new cap at its top. But BP and government officials said they would not consider the well killed until the first relief well did its job.

BP paid the federal government $34 million in 2008 to lease the Mississippi Canyon Block, and then it sold about one-third of its interest to other companies. One option for the company would be to sell its remaining stake, subject to government approval; in such a case, having proved that there is oil and gas there would probably make the stake more valuable.

“They could probably sell it to another operator,” said Dr. Patzek, who offered a quick analysis. “You have three and a half billion dollars of oil there. If you sell it for a billion and a half, someone will gladly take it.”

Experts said that if BP or another company decided to drill again, it most likely would not be through the original well bore, although technically that might be possible.

“I don’t think BP would reuse the original well,” Julius Langlinais, a retired professor of petroleum engineering at Louisiana State University, said in an e-mail response to questions. “If anything at all went wrong with the reused well bore, the press and P.R. would be terrible. And mechanically we’re not sure of anything, so it’s better to start over.”

The relief wells, however, which were being drilled at a cost of about $100 million each, would be an obvious choice for any future project.

The first intercepted the stricken well just above the reservoir, about 13,000 feet below the seabed. The second, meant as a backup, was halted about 10,000 feet down. Either well could be redirected and used to tap into the reservoir.