How much oil and gas are left?

It is interesting to speculate on how much oil and gas we have left.

Whilst there are always stories that these resources are dwindling, and no doubt they are, there seem to be quite a few large finds of late, as prospecting methods get better.

And also there are better and better technologies for extracting the parts that were harder to get at previously, and also to use slightly less perfect sources of these fuels.

But they certainly won't last too much longer - perhaps another 30 years or so? How long do you think oil and gas reserves will last...