Kind of a vague question, don't you think.
I would think that the "ultimate" energy source is still the sun, as it keeps the Earth at a reasonable temperature for plants and animals, including us, to be able to live on it.
ESL. How we are harvesting, converting and using energy, oil is most likely not being used as a energy source in the future. Of course thermodynamics physics is involved. To clear it up; the sun. wind, waves or geo. Or some other energy source which is to cone?