The essential character of chemical fuels that can be used to produce useful energy is their reaction with oxygen - burning, or oxidation. The bulk of Earth's core is believed to be in a reduced state (primarily iron metal, with some mixture of lighter elements) so the planet certainly holds enough potential fuel to consume all the oxygen in the atmosphere. The limiting factor then is not fuel, but oxygen.
Earth's atmosphere has a total mass of 5.14x10^18 kg. 22% by weight of this consists of oxygen molecules, for a total oxygen mass of a little over 10^18 kg. Combining carbon (C) with oxygen (O2) to produce CO2 releases almost 400 kJ/mol(1). The oxygen molecule has a molar weight of 32 grams, so 10^18 kg of oxygen yields a bit over 12x10^24 J.
Primary energy use in the year 2000 was about 4x10^20 J, corresponding to a steady power consumption rate of about 13 TW. So the total fossil chemical potential for Earth, calculated by consuming all the oxygen in the atmosphere, amounts to 30,000 years at present rates.
30,000 years supply may seem substantial, but bear in mind (1) it involves consuming all the remaining oxygen in the atmosphere, and (2) it wouldn't last nearly that long with continued growth in energy use.
In fact at 2%/year growth, slightly less than the growth rate over the 20th century, 30,000 times year-2000 supply would be exhausted by the end of the 24th century. Whether or not growth continues at such high rates, this 30,000 figure is the ultimate limit for fossil energy on Earth.
With 10^16 kg of carbon, at 400 kJ/mol and 12 g/mol chemical mass we have a total of 3x10^23 J available from fossil fuels; this comes to about 750 nominal years of supply (and 75 years of supply from the world's total coal reserves). Fossil fuel supply for the world is clearly limited, and alternatives will be essential as world energy use continues to grow.
The total mass of living matter on Earth is estimated at 3.6x10^14 kg; this includes about 10^14 kg of combustible carbon, or 3x10^21 J of stored energy. This is about 8 years of present primary-energy demand.
More importantly, Earth's living matter processes incoming solar energy at about a 300 TW rate. Half of this is dissipated almost immediately in respiratory processes, and the remaining half goes to "net primary productivity", about 150 TW or roughly 10 times total year-2000 human primary energy use. At very high utilization rates biofuels could power human society through a few decades of present-day growth, but not much more.
So those are the limits for chemical energy on Earth. We'll continue with a look at nuclear energy from fission next.
(1) W. J. Moore, "Physical Chemistry", 4th edn., 1972, Prentice Hall, p56, at 298.15 K the standard enthalpy of formation of CO2 (g) is - 393.513 kJ/mol.