Royal Dutch Shell said Tuesday that it has shut down the main leak at one of its North Sea rigs, but a smaller leak in a hard-to-reach part of the ocean floor is still seeping about 84 gallons of oil daily into the ocean off Scotland's east coast.
A company spokesman said a main leak in the flow line to the Gannet Alpha platform had been stemmed by closing the well and isolating the reservoir, but the oil found another pathway, and the second, smaller leak has proved more elusive to control.
"The residual small leak is in an awkward position to get to," said Glen Cayley, technical director of Shell's European exploration and production activities. "This is complex sub-sea infrastructure, and really getting into it among quite dense marine growth is proving a challenge.
"It's taken our diving crews some time to establish exactly and precisely where that leak is coming from," Cayley said.
While small when compared to the BP spill in the Gulf of Mexico last year -- which dumped 206 million gallons of oil into the Gulf -- this leak is still substantial for the U.K.'s continental shelf, the British government said. It backed up Shell's predictions that the oil would disperse naturally.
Royal Dutch Shell estimated the secondary spill is pumping about two barrels -- or 84 gallons -- into the cold water each day. The company estimated Monday that 54,600 gallons of oil had spilled in the past week into the North Sea from both leaks on the rig off Scotland's eastern coast.
In an interview with the BBC, Cayley said an investigation is under way to determine the cause of the spill, which was first noticed a week ago Wednesday. Shell said the first leak was stopped Thursday, but said "the oil found a second pathway to the sea."
Cayley said Shell informed government agencies of the spill immediately, but did not make it public until Friday. On Saturday it declared that the leak had been contained.
Shell said it believes the oil is now leaking from a relief valve close to the original leak. It said that once it is certain of the source, it will stop the spill.
Vicky Wyatt of Greenpeace said Shell had not been released information about the spill as quickly as it should have.
"The news that there's now a second leak from the Shell platform will only heighten concerns over how this episode is being handled," she said. "While oil has been flowing, timely information has not."
Cayley said at its largest, the oil sheen covered an area 19 miles wide by 2.7 miles long. He said most of it has now been dispersed by strong waves and it would not hit shore.
The Gannet Alpha oil rig, 112 miles east of the Scottish city of Aberdeen, is operated by Shell and co-owned by Shell and Esso, a subsidiary of the U.S. oil firm Exxon Mobil.
The British government has promised to investigate the spill, which is four times the amount leaked by all British rigs into the North Sea in 2009.
Britain has already beefed up its inspections of the 24 drilling rigs and 280 oil and gas installations in its part of the North Sea in the wake of the 2010 Gulf spill.