The BP world energy statistics (available to download in excell form on the BP website) is regarded as one of the best sources of information about oil and gas production and reserves. You can show the current reserves of gas and oil and the current global production of both and show that one will outlast the other (with all the usual caveats about proved vs. probable vs. possible reserves; future economic considerations ignored in reserves estimates; assumptions about future demand and production; ignoring non conventional sources of oil and gas and so on).
If you want to you could convert gas to 'barrels of oil equivalent' using the SPE conversion which is based on the energy content of sales gas and typical crude oil:
1 million standard cubic feet (1MMscf) = 176.7barrels oil equivalent (boe)