A clean-up operation and investigation are under way after oil leaked into the River Thames in Grays on Thursday.
The Port of London Authority (PLA) sent a specialist oil response vessel to the river at Grays after a member of the public reported oil on the water.
A spokesman for the PLA said the oil had got into the river through a storm drain.
A boom has been placed around both the drain and the spill and efforts are being made to clear up the oil.
He described the incident as “comparatively small” and added: “Officers from the Environment Agency are investigating what caused the oil spill.”
The incident comes a day after a family of dolphins were spotted in the area.