I think eventually it will be natural gas, but not CNG because it’s too difficult to carry as much compressed gas as you’d like. I think our economy will be running on GTL (gas to liquid) fuel made from natural gas, in the long term. I do think there will be a need for gasoline long term, although you can get a diesel chainsaw, they are somewhat heavier than their gas counterparts, last time I checked. In the extreme long term, if a technological society still exists on earth in 1,000 years; I think solar power collected in space, where it is 1,000 times more powerful than what reaches the earth’s surface, converted to microwave, beamed to earth, collected in receiving stations in desolate regions and converted to electricity, may work. As usual, I think we’ll cross each bridge as we come to them.