[William Tell]

Telepathic Intruder said: "I don’t know if we’re burning it faster than it replenishes itself, though."

Perhaps more importantly in the short term, abiotic creation of hydrocarbons may mean that there is much still to be found.

[Telepathic Intruder]

Of course, it just depends on how far you dig. In that case however, the most efficient route is geothermal energy. Drill a hole 10 or 20 miles deep, pour water in and have an almost inexhustible supply of steam power.