Per mineral acre royality average

I have 40 plus acres in williams county north dakota,..

It is leased now to a oil company. And they will be drilling soon.

this is in the Bakken/sanish 3-forks. prime stuff i hear.

what would the average royality per acre be ?

Well if you already have a signed lease it really has no bearing on your minerals. Unleased minerals will go anywhere from 18% to 20% depending on what area they are in.