Uranium in fission reactions is limited as well. Methane Hydrates is a enormous deposit of resources, containing several times more Btus than all other fossil fuels combined. But after a several centuries it would be used up. Of course trying to predict energy usage centuries away is rather foolish. Try to use 1900 as basis and predict the energy needs of today. I suspect Fusion reactions will become more import in the far future, or more likely, a technology we are not aware of today.
Exactly. One of the things I learned when briefly studying geophysics is that when most people talk about reserves, they are only talking about the reserves that are economically mineable today. I almost want to smack people due to their stupidity when claiming that our uranium deposits are going to run out in 100 years. It is a big planet and it has barely been explored at all for uranium (in contrast with oil).