A Walailak University lab test found that a tar-like substance discovered on Nakhon Si Thammarat beaches and other parts of the south recently is most likely processed crude oil.
The gas chromatography chemical analysis test showed the oil underwent a process to make it dense and thus able to sink underwater.
The university's president Keerath Sanguansai urged the authorities to locate the source of the leakage after Chumpon, Surat Thani, Koh Samui and Nakhon Si Thammarat reported the tar-like substance had washed ashore earlier this month.
The university's test on a seawater sample didn't detect any abnormal change yet, Keerath said.
There are concerns the substance could impact negatively on marine life and coastal ecosystems.
Nakhon Si Thammarat reported 11 dolphin deaths that were likely related to the crude oil.