Environmental experts say they will have to continue to monitor a wildlife haven in North East Lincolnshire for several years.
There had been fears the oil could impact heavily on wildlife
It follows an explosion and oil spillage at the Lindsey Oil Refinery in South Killingholme last month.
Crude oil seeped into the Humber from a fractured overland pipeline.
An Environment Agency spokesman said: "We do not expect any long-term impact on water quality, but we do need to make sure lessons are learnt."
The oil had spilled into the South Killingholme Drain, an effluent drainage channel which feeds into the South Killingholme Haven area of the estuary.
There had been fears that the leak could impact heavily on the wildlife habitat.
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.