"The oil has already arrived at Prao Bay on Samet," said Chuchart Oncharoen, Tourism Authority of Thailand (TAT) director for Rayong province.
Samet Island is situated about 150 kms south-east of Bangkok.
The leak sprung on Saturday in PTT Global Chemical Public Limited Company's offshore pipeline, flooding the Gulf with about 50,000 litres of oil.
Much of the spill has been contained although portions evaded the booms, The Nation paper said.
PTT Global Chemical is a subsidiary of the PTT PLC, the national petroleum company.
