Most of BP oil spill has gone, says US

(Chris Graythen/Getty Images)

Most the the oil that spilled from the giant BP gulf leak has gone, the US government said today.

White House energy adviser Carol Browner said a new assessment found that about three quarters has either been captured, burned off, evaporated or broken down chemically.

"It was captured. It was skimmed. It was burned. It was contained. Mother Nature did her part," she said.

Meanwhile BP claimed a key victory in the effort to plug the blown-out Gulf of Mexico well.

It reached what it called a significant milestone overnight when mud that was forced down the well held back the flow of crude.

In the gulf, workers stopped pumping mud in after about eight hours of their "static kill" procedure and were monitoring the well to ensure it remained stable, BP said.

"It's a milestone," said a company spokeswoman. "It's a step toward the killing of the well."

The next step would be deciding whether to cement the well.

The pressure in the well dropped quickly in the first 90 minutes of the static kill procedure yesterday, a sign that everything was going as planned.

The static kill - also known as bullheading - involved slowly pumping the mud from a ship down lines running to the top of the ruptured well a mile below. BP has said that may be enough by itself to seal it.

But the mud that was forced down the broken wellhead to permanently plug the gusher is only half the story. To call the mission a success, crews working on a flotilla of vessels need to seal off the well from two directions.

Video: BP 'static kill' a success

A 75-ton cap placed on the well in July has been keeping the oil bottled up inside over the past three weeks, but is considered only a temporary measure. BP and the Coast Guard want to plug up the hole with a column of heavy drilling mud and cement to seal it off more securely.

A previous, similar effort failed in May when the mud could not overcome the unstemmed flow of oil.

BP will not know for certain whether the static kill has succeeded until engineers can use the soon-to-be-completed relief well to check their work.

The task is becoming more urgent because peak hurricane season is just around the corner.