Shell detects second Gannet Alpha North Sea leak point
A second leak point has been found in the flow line beneath the Gannet Alpha oil platform, 113 miles (180km) off Aberdeen.
Shell has been dealing with the release of an estimated 216 tonnes - 1,300 barrels - from a leak near the platform discovered last week.
The oil company said it was working to tackle what was described as a "second pathway" of leakage.
Shell said the overall leak rate was declining.
Glen Cayley, technical director of Shell's exploration and production activities in Europe, said: "The leak source remains the same. The initial release path was stopped, however, the oil found a second pathway to the sea.
"Since then we have been working to find the source of the much smaller flow of hydrocarbons. It had proved difficult to find because we are dealing with a complex subsea infrastructure and the position of the small leak is in an awkward place surrounded by marine growth.
"So, it has taken our ROV inspections some time to establish exactly where the source is.
"We believe now that the flow is coming from a relief valve adjacent to the original leak and from the same source. Once we've confirmed this we will then develop a series of mitigation options to stop this leak.
"There is no new leak."
On Monday afternoon, Shell said that the sheen from the "light crude oil with a low wax content" was 0.5 square kilometres in size.
A DECC spokesman said: "Shell has informed us that the oil leak at its pipeline at the Gannet Alpha platform east of Aberdeen is under control and has now been greatly reduced.
"They are working to completely halt any further leakage.
"DECC's environmental inspectors will continue to monitor the situation and have been working closely with the company and counterparts from the Health and Safety Executive, Maritime and Coastguard Agency and Marine Scotland since the spill was reported last week."
The spokesman added: "Although small in comparison to the Macondo, Gulf of Mexico, incident, in the context of the UK Continental Shelf the spill is substantial.
"But it is not anticipated that oil will reach the shore and indeed it is expected that it will be dispersed naturally.
"Current estimates are that the spill could be several hundred tonnes."
Scottish Environment Secretary Richard Lochhead said: "We take any oil leak extremely seriously, as the first minister has made clear, and we are continuing to monitor this situation very closely.
"While there are inevitable difficulties verifying the extent and size of the leak, it's vital that Shell and DECC make information available on an open, transparent and regular basis."
RSPB Scotland also urged Shell to be open.
The Gannet oil field reportedly produced about 13,500 barrels of oil per day between January and April of this year.
The field is co-owned by Esso, a subsidiary of US oil firm Exxon, but is operated by Shell.