About 400,000 litres of crude oil leaked from an undersea pipeline near Map Ta Phut in Rayong province on Tuesday.
The pipeline belongs to Star Petroleum Refining Public Company Limited (SPRC).
The Pollution Control Department said a slick 180,000-200,000 litres in size was heading to the shoreline between Mae Ramphueng beach and the Khao Laem Ya–Mu Ko Samet Marine National Park.
Unless stopped, it would reach there on Friday around 3-5pm.
The Marine and Coastal Resources Department, the navy and other agencies are working to solve the problem.
SPRC's emergency team had taken steps in line with safety procedures. Activities in neaby areas had been stopped.
The leak had been sealed at 12.18am on Wednesday and dispersants sprayed on the oil spill. The company was continuing to assess the situation.
