PTT Global Chemical PCL reported a spill of about 300 barrels of oil from a faulty pipe last month.
The company said the July 27 spill had been curbed by booms and absorbent pads, 99 percent of debris had been removed, and it was working with the Royal Thai Navy to get oil-stained debris removed and transferred to a refinery "for further examination."
The country's tourism industry has taken a hit from the spill, with cancellations reported from Koh Samet island, a popular vacation spot.
Greenpeace said last week it was questioning the extent of the spill and visited the site to determine the extent of environmental damage.
"It was like we were no longer on earth," it said Friday. "The bay was completely blackened; the ocean had turned to a sea of black thick oil."
The advocacy group said it had serious concerns about the long term impact on the environment. Ecologists told The Wall Street Journal last week there may be some residual damage to coral reefs.