Herbert Smith Freehills is advising Thai state-run PTT Exploration and Production Pcl. on a $1 billion purchase of Hess Corp.’s energy assets in the country.

PTTEP is paying cash for a 15 percent interest in an offshore natural gas project in the Gulf of Thailand, as well as a 35 percent stake in an onshore gas field in the northeast of the country. The deal for the offshore assets, which comprise two blocks, closed on Tuesday, while the onshore transaction is expected to close next month.