Thaweep Saengkrachang, chief of the Tambon Tapong Administration Organisation, said on Wednesday that the beach in Muang district remains closed to tourists due to concerns about the slick.
About 60 soldiers joined officials to clean up one of the most popular beaches in Rayong province and Mr Thaweep expected it will be closed at least until Thursday.
The oil slick of at least four kilometres long was found on Tuesday.
Sunee Piyapunpong, deputy director-general of the Pollution Control Department, could not confirm if the slick was diesel or fuel oil. Officials from local organisations and the department were trying to find out where the slick came from.
Representatives of the Marine Conservation Centre and the Marine Department filed a complaint with Rayong police on Wednesday to find who polluted the beach.