Hundreds of tonnes of oil could still be inside an offshore pipeline which has been leaking for a week.
The estimate was revealed as Shell continues to try to stem the flow on the seabed near the Gannet Alpha platform, about 112 miles east of Aberdeen.
Since the leak started last Wednesday, more than 200 tonnes of oil has spilled into the North Sea, making it the worst single leak in the region for more than a decade.
The initial large leak was stopped the following day, but it later emerged that a smaller flow from the same source had been detected.
That leak was described as being in an "awkward" place surrounded by marine growth.
Shell technical director Glen Cayley said the company's pipeline maintenance programme had let it down, according to a report in the Press and Journal newspaper.
"We are talking about hundreds of tonnes of additional oil in the pipeline that we need to preserve and keep there," Mr Cayley told the paper.
"Until we have completely eliminated the leak and secured this pipeline, I would say there is still risk."
He added that work is continuing to figure out how to fix the breach.
Environmental groups have sharply criticised Shell for its handling of the spill, complaining about a lack of timely information.
Shell, in its last update yesterday, said the rate of leakage from the secondary breach had declined to one barrel a day.
The total spilled so far is estimated to be about 218 tonnes, which is equal to about 1,300 barrels. The overall amount of oil discharged into the North Sea in 2009 is 50.93 tonnes.
The oil sheen on the sea covers an area of about 16 square miles (41 sq km), estimated somewhere between one and 10 tonnes in volume.
The firm said the oil is not expected to reach land.
Dr Richard Dixon, of environmental charity WWF Scotland, said: "We are finally getting a fuller picture of the scale of this crisis as each bit of information is dragged out of Shell.
"What the company first dismissed as minor is now not only the biggest North Sea spill for a decade, it has the potential to go on for weeks or months, and there could be at least the same amount of oil to leak as the 200-plus tonnes that has already escaped."