The United States will rely more on natural gas than either oil or coal by 2035 as cheap domestic supply boosts demand among industry and power generators, the IEA said.
Birol said he realised how optimistic the IEA forecasts were given that the shale oil boom was a relatively new phenomenon.
"Light, tight oil resources are poorly known ... If no new resources are discovered (after 2020) and plus, if the prices are not as high as today, then we may see Saudi Arabia coming back and being the first producer again," he said.
The IEA said it saw US oil production rising to 10 million barrels per day (bpd) by 2015 and 11.1 million bpd in 2020 before slipping to 9.2 million bpd by 2035.
Saudi Arabian oil output would be 10.9 million bpd by 2015, the IEA said, 10.6 million bpd in 2020 but would rise to 12.3 million bpd by 2035.