(Also, "it would never all be used" is a given, what is the expected "peak gas" date ?)
[0]: https://en.wikipedia.org/wiki/Green_River_Formation#Oil_shal...
2.) shale oil is NOT being extracted - you're confusing with tight oil
3.) we don't know whether extracting shale oil even makes sense - for all we know, it might never become economically and energetically viable (IMHO, considering global warming, tight oil/gas & tar sands did not make sense in the first place either - especially since the whole tight oil industry hasn't even shown to be financially and therefore probably energetically profitable !)
4.) even if we did, the (E)ROI (and therefore, the pollution involved) is likely to be dismal
5.) a century ago, world oil production was a few million barrels per day, you can't just both assume business as usual and the consumption to stay flat when taking into account future consumption...
Estimates generally exceed the total known amount of in-ground crude oil.
Enough to supply North America for a century.
Enough that it would never all be used, the price of extraction guarantees other renewable sources of energy will be very competitive while petroleum products will remain available near the previous peak raw petroleum prices from a few years ago (prices collapsed when production of this newer source started to ramp up).
It has been a really good foreign policy lever against Russia and unfriendly middle-east countries, limiting their oil profits and always holding over their heads that we could subsidize the shit out of it and drive them into poverty and ruin.