The government plans to open bids in March 2017 for expiring oil and gas contracts held by Chevron Corp and PTT Exploration and Production, an energy ministry official said on Thursday.
Chevron's Thai unit holds concessions to operate the Erawan gas field. PTTEP operates the Bongkot gas field. Contracts for the two offshore fields are due to expire in 2022 and 2023.
They have combined production of 2.2 billion cubic feet per day, or 76% of output in the Gulf of Thailand.
Authorities are drafting terms and criteria for the auction, which is expected to be completed by the end of this year, Veerasak Pungrassamee, director general of the ministry's Department of Mineral Fuels, told reporters.
The winners will be announced in September next year, he said, adding legislators will take three months to amend the energy law, one month longer than planned to make sure of continuity in production.
PTTEP, the country's largest oil and gas explorer, has said it will bid to operate the Bongkot field. Chevron said it was committed to its Thailand investments, despite job cuts that have led to rumours of its exit.
Mr Veerasak said the energy ministry also planned to hold a long-delayed auction for new oil concessions in late 2017.
The military government put off a bidding round of concessions for 29 onshore and offshore blocks in early 2015 due to criticism of contract terms from politicians and activists.
The round was originally planned for 2011 but was on hold after devastating floods that year and then a political crisis that began in late 2013 and culminated in a military coup in May 2014.