IIRC, coal reserves are approx 600 years at current usage, and coal represents 40-45% of the fossil fuels used.
If oil and gas became hugely expensive in the next 10 years, coal could replace them for 100-200 years, depending on demand growth.

The coal to oil/gas process has been used in several places. Town gas in the UK, oil in Aparthid era SA etc...

Bunnahabhain.