As I read the intelligent (and very useful) discussions on this sight, I have come to find the following two thoughts helpful in understanding our present predicament.
a) It is not constructive to approach our oil supply problem from the perspective of how much oil is left because it gets one into endless arguments with non-believers (e.g., most capped fields have 70% of original oil left in place because it can't be recovered).
b) It is more productive and more illuminating to approach the problem from the perspective that we have already reaped the "low-hanging fruit" so that it is becoming increasingly expensive to recover the remainder.