This relies on fudging the distinction between "reserves" and "resources". The actual existing quantity of oil and gas in the earth is fixed, unknown but estimated. As it's surveyed and deemed economically recoverable it's moved into the "reserves" category.

This doesn't take into account "unburnable" fossil fuels which can be extracted at the cost of pushing us over the "catastrophic" IPCC predictions.

http://www.oilandgasinvestor.com/stephen/2010/02/19/energy-t...