My favourite 'back of the envelope' calculation is to estimate how much oil we should use if we are to consider it a 'renewable resource'.

If you assume the world supply regenerates every 100 million years, and there are 2 trillion barrels available and 7 billion people on earth, then an equitable, renewable rate of consumption is about half a cup per person per lifetime

Of course, it might be as little as a thimbleful or as much as a pint...

RW,

I guess the point your making is it's not very much, whether a thimble or a pint. Even a barrel over our life time would would not help much.

Well, if we made something useful from it that we could reuse or recycle when we were done instead of just burning it there'd probably be quite a lot to go round.