The key is drilling efficiency. What we drilled in a good day in 1970 is now considered to be a mediocre hour's worth. COmputers have enabled the collection and documentation of massive amounts of data as well, and MWD/LWD tools have reduced the time to take a directional survey in a directional/horizontal hole to the time it takes to cycle the pumps on the rig. Add to that the well pad concept, cutting the time it takes to put a rig on the next wellhead and the 4 wells a year (15,000 ft.) in the Williston Basin for a rig have been reduced, time wise, to taking 4-6 weeks, and go deeper.
The Pad concept reduced rig moves (dismantling and trucking to a new site) by 3/4--which cut demand for personnel. That one rig can drill eight to ten times as far in the same time as conventional rigs, which reduces the number of rigs needed (and crews and service personnel to the tune of roughly 50 per rig) by as much as 80 percent. Fewer people getting more done.