June 16 (Bloomberg) -- ConocoPhillips, the third-largest U.S. oil company, said it may take a century for the nation to replace fossil fuels with alternative energy sources.

The country will need to develop its own oil and natural- gas deposits and continue importing petroleum while developing alternative supplies in the decades ahead, ConocoPhillips Chief Executive Officer Jim Mulva said today at the National Summit economic conference in Detroit. At the same time, he said, the nation will need to address climate change.

The U.S. needs policy that encourages investments in all types of energy and avoids hurting the economy by making the nation less competitive than countries with cheaper energy, Mulva said. Proposed climate legislation in Congress threatens to drive U.S. refiners out of business by imposing higher carbon costs on domestic fuel than on imports, he said.

“Government action is needed on energy and climate policy, and some form of legislation seems likely to pass,” Mulva said. “We hope that it is realistically based on the hard energy facts I’ve mentioned and not on politically popular hot air.”

Exxon Mobil Corp. and Chevron Corp. are the biggest U.S. oil companies. ConocoPhillips has the largest U.S. refining capacity among major oil and gas producers.

To contact the reporter on this story: Edward Klump in Detroit at eklump@bloomberg.net.

Last Updated: June 16, 2009 13:27 EDT