The vast majority of wells are old vertical wells, not shale wells. For example, in the Permian basin there are around 18,000 horizontal shale wells but over 300,000 vertical conventional wells.
Great question. For the Williston Basin, the numbers are 31,905 horizontals vs. 28,330 verticals. It definitely speaks to the difference in when these basins were developed.
[1] https://www.npr.org/sections/krulwich/2013/01/16/169511949/a...