One out of every two cars will be either hybrids or some other alternative-fuel vehicle by 2040, Exxon Mobil predicted Thursday.

Hybrids, which rely on both gas and electricity for power, currently account for less than 1 percent of all vehicles on the world’s roads. They should move into the mainstream as governments boost fuel-efficiency requirements, Exxon said.

Power for those hybrids, along with other vehicles and a growing number of households around the world, will increasingly come from natural gas, nuclear power, and renewable energy sources like wind, Exxon said in its closely watched annual long-term energy outlook.

But the largest publicly traded oil company also makes clear that oil will remain king of the energy world for many years.