"We secured the stake in late May, but since the oil field is in the very initial stages of exploration, we don't have any details on estimated or proven reserves," said SK Energy's spokeswoman.
Shares of SK Energy rose 1.75 percent, trading at 116,500 won as of 0131 GMT, outperforming the wider market's 0.36 percent loss.
Australia's Santos Ltd (STO.AX: Quote, Profile, Research) owns the majority 50 percent stake in the 123 oil field on the east coast of Vietnam, and the remaining 30 percent is owned by state-run Petrovietnam, SK Energy said.
SK Energy is the country's top refiner with a capacity of running 1.115 million barrels per day (bpd) of crude. (Reporting by Angela Moon; Editing by Jonathan Hopfner)