The environmental pressure group's German branch said it had filmed large oil screens around oil installations north-east of Shetland.
The problem is caused by the discharge of what is known as produced water, pumped up with the oil, which can legally contain up to 40g of oil per cubic metre.
The UK offshore oil industry said yesterday the problem highlighted by Greenpeace was largely historical as the industry had invested 450 million to tackle the issue.
Dr Christian Bussau, a biologist working for Greenpeace in Hamburg, said the industry had discharged 13,000 tonnes of oil into the North Sea in 2005, almost ten times the amount in 1984.
In June, a Greenpeace team had surveyed 75 production and drilling rigs in the British, Norwegian and Danish sectors of the North Sea.
They said the Dunlin (operated by Fairfield Energy), Thistle (Lundin Petroleum), Ninian Central (Canadian Natural Resources) and Brent B (Shell) installations were among the worst polluters.
Dr Bussau said that more oil was discharged from older oilfields as the amount of water in the oil exploited increased.
Greenpeace is now calling on North Sea countries to bring in legislation to stop the legal discharge of oil into the sea.