http://www.aspo-usa.com/proceedings/powerpoint/The%20Peak%20Oil%20Contex...

* Global annual decline rate: 4% - 6% / year?

So 4% is actually on the optimistic side.
Calculation is ln(.50)/ln(.96) =~ 17 for 6% ln(.50)/ln(.94) = 11.2 years. This is assuming we are past peak of course.

My, you are good, aren't you. Did somebody teach you about logarithms recently? Since quantities aren't logarithmic, the calculation you provided is nonsense.

Since quantities aren't logarithmic, the calculation you provided is nonsense.

Your objection makes no sense, a fact which is easy to see if you just plug his numbers back in:

4% decline for 17 years = (0.96)^17 = 0.499587 = 50% less

Tag him for basing the calculation on a wild-ass guess, sure, but the calculation itself is correct.