Biologists are continuing to examine watercourses in the area of an oil refinery near Immingham following an explosion and an oil spillage.
The spill happened when an overland pipeline fractured
Two workers were injured in an explosion at the Lindsey Oil Refinery in South Killingholme on Tuesday.
The Health and Safety Executive is also investigating a crude oil spill into the Humber from the refinery which is threatening wildlife sites.
The cause of the leak is thought to be a fractured overland pipeline.
The oil had spilled into the South Killingholme Drain, an effluent drainage channel which feeds into the South Killingholme Haven area of the estuary.
Lindsey Oil Refinery, which is owned by petrochemicals giant Total, said all the oil had now been removed from the drain using specialist equipment and techniques.
A boom had been put in place across the drain's exit to the Haven and oil was still being recovered from there.
Neither the company nor the Environment Agency could say how much oil had been lost from the pipeline, which takes crude oil from the shoreside terminal to the refinery.
The South Killingholme Haven is surrounded by mudflats which are used as feeding and breeding grounds by many species of birds and other animals, including water voles.
An Environment Agency spokesman said: "There is concern that if it spreads, the mudflats could be contaminated.
"But predictions are that the oil will break up naturally and disperse."
Officials from the RSPB, English Nature and North Lincolnshire Council are at the site assessing the threat to wildlife.
An RSPB spokesman said: "Although we're very concerned about reports of oiled birds, we are optimistic that the worst has been averted because this spill was spotted relatively quickly.
"But we are very concerned that there was a large element of luck involved and we would want any inquiry to focus on how these pollution incidents are dealt with in future."
The Environment Agency said its investigation could take several weeks to complete and would seek to determine whether legal action could be taken and how to prevent any future spills.