The company began top kill Wednesday afternoon at 1 pm CDT, and according to Suttles, ceased the pumping of drilling fluids "just before midnight" to assess how the process was doing. It was "suspended overnight to monitor" and restock pumping material, he said during a press conference accessible via telephone.

"Over the last 16 hours or so," BP has been "assessing" top kill, Suttles said. With the overnight resumption, top kill "will likely continue for another 24 hours," he said.

There were no problems with the start-and-stop nature of the top kill process, Suttles said.

"Nothing has gone wrong or unanticipated," he said. "But we do this operation, it is injecting fluids at high rates. At times you have to pump in the fluid and monitor."

BP has "pumped a lot of mud out of the riser, but haven't completed the operation," Suttles said. It is also possible BP may add "bridging materials" for a so-called junk shot approach, he said.

"We've been at this for quite some time now," Suttles said. "It is quite a roller-coaster. The well continues to flow."

"I believe this can work, but until" BP can complete the job, "it is difficult to be optimistic or pessimistic. If it isn't successful, we already have equipment staged for the next phase," Suttles added.

'CLEARLY HAD SUPPRESSED'

The company has had on standby another containment system if top kill does not succeed.

Given that the pumping of drilling mud was suspended, reporters asked what was seen emanating Thursday from the well holes shown in a live streaming video on BP's website. Suttles replied it is a combination of drilling mud that was pumped into the blowout preventer and oil and gas still coming out of the well bore at the same rate it has since before top kill began.

During the initial pumping, "we clearly had suppressed the amount of oil and gas coming out," he said.

The initial pumping involved "somewhere less than 15,000 barrels" of drilling mud and at rates as high as 65-70 barrels/minute, which is "very high," Suttles said, adding that "we've also gone to low rates." The process began with 50,000 barrels and during the pumping suspension, BP looked to have restocked supply vessels with drilling mud to make sure it had a "full complement to the next phase," he said.

Asked if junk shot would be folded into the top kill pumping and if so, what materials would be included, Suttles said the "bridging agents" range from "fibrous" materials to "very large things like rubber balls." He added, "experts" would "determine in what order to pump these based on what is occurring in the well."

--Katharine Fraser, katharine_fraser@platts.com

--Gary Taylor, gary_taylor@platts.com