You will have to excuse my limited knowledge on the subjet Stephen as I am only the mechanic, and no longer on rigs in Australia.
From what I remember with some of the wells drilled, there is a very hard
rock formation, this formation was reached fairly quickly then drilling would slow to a near stop, at times only making one drill pipe connection in a day.
Originally they would not, or maybe that was could not, drill through this formation and a lot of the wells were given up as dusters. Later on some of these wells were re-drilled and oil
reserves were discovered. One of these wells was for IOR which as you probably know, refine and supply diesel to outback towns such as
Birdsville.
You mentioned PDC drill bits, these are probably what made it possible to drill through this hard formation as the standard Tri-cone bits wouldn't touch it.