State energy group PetroVietnam (PVN) signed an agreement with the Vietnamese unit of ExxonMobil Corp. (NYSE: XOM) on Jan. 13 to develop Blue Whale, which is the country's biggest gas project, for power generation, the state firm said.

The Blue Whale project aims to produce the first gas for power plants by 2023. PVN said the project would contribute nearly $20 billion to the state budget, but it gave no time frame for that contribution.

Blue Whale is Vietnam's biggest gas project, with an estimated 150 billion cubic meters of reserves.

The signing of the agreement took place during the visit to Vietnam of U.S. Secretary of State John Kerry.

Vietnam is trying to switch to cleaner energy from coal-fired power plants. Annual power demand growth was projected at up to 20%.