there's enough fossil fuel under the ground to last us a lot longer than 50 years. Oil may run out in that time frame (although it may be nearer 80 years before the wells finally run dry), but there's enough gas to last about 250 years (give or take a few decades), and the coal reserves will keep us fuelled for over 400 years if we want to use them.
The problem is in predicting the rate of rise in consumption. At the moment, all the world's oil wells produce around 30 billion barrels a year. If China gets to equal the car ownership rate of say, the USA, and there is no increase in supply, the Chinese nation will consume the equivalent of entire world's production - there simply will not be enough to go round.
Make no mistake about it, future international disputes and wars will be fought over oil rights, and smaller nations (like us) will feel the pinch in no uncertain terms.
This is a time for forging big, powerful alliances.