More than 15 weeks after the ruptured well began spewing enough oil into the sea to fill four Olympic-sized swimming pools every day, the US government has finally got a handle on what has happened to it all.

"At least 50 percent of the oil that was released is now completely gone from the system," Jane Lubchenco, head of the National Oceanic and Atmospheric Administration, told reporters at a White House briefing.

"And most of the remainder is degrading rapidly, or is being removed from the beaches."

A government report released Wednesday found that eight percent of the 4.9 million barrels of oil which gushed out of the well was burned or skimmed off the surface and an equal amount was broken down with chemical dispersants.

A further 17 percent was captured at the source and suctioned to the surface with a series of mile-long pipes.

Mother Nature took care of much of the rest.

Heat from the sun helped some of the chemicals in the crude evaporate. Waves and currents broke the slick up into smaller patches. Then the microbes which feed on natural oil seeps in the Gulf got to work.

"It's very difficult for us to find any oil anywhere on the surface," BP senior vice president Kent Wells told reporters, noting that the well has now been capped for 20 days.

But while the cleanup efforts are now being focused on the shorelines, recovery efforts are far from complete.

Some 644 miles (1,037 kilometers) of shoreline stretching from Louisiana to Florida is currently sullied and fresh tarballs are still washing up.

Meanwhile, nobody knows what the long-term impact will be from the massive amounts of oil which remain in the already stressed ecosystem.

Louisiana has been struggling for decades with coastal erosion caused by a series of flood control levees and canals which starve wetlands of the sediment that used to get washed in by the Mississippi River.

Agricultural runoff carried deep into the Gulf by the mighty river has also created a massive "dead zone" after algae blooms fed by the fertilizer suck all the oxygen out of the water.

"The fact that so much of the oil has been removed and in the process of being degraded is very significant and means that the impact will not be even worse than it might have been," Lubchenco said.

"But the oil that was released and has already impacted wildlife at the surface, young juvenile stages and eggs beneath the surface, will likely have very considerable impacts for years and possibly decades to come."

The problem, she explained, is that oil is still toxic even when it has been broken down into very small droplets.

And there was simply so very, very much of it.

At 4.9 million barrels, the disaster is by far the biggest maritime spill on record.

While BP captured approximately 800,000 barrels of oil with its various containment devices, enough oil gushed into the sea to fill 260 Olympic-sized pools.

The well was capped on July 15, and on Wednesday BP succeeded in forcing the oil back down into the reservoir by pumping heavy drilling fluids into the ruptured well in a procedure called a "static kill."

With tourists likely to avoid many Gulf beaches for years and oil industry jobs under threat from a moratorium on new deep sea drilling permits, the future remains bleak for many coastal communities.

Huge swaths of the Gulf remain closed to fishing, and even when fishermen are able to fill their nets they fear consumers might not believe the seafood is safe to eat.

"It's going to take at least five years for the Louisiana seafood brand to come back in consumers' eyes," said Ewell Smith of the Louisiana Seafood Board.

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