320 vs 640 acre unit

I recently discovered a well has been drilled where I own the mineral rights. I am awaiting a division letter. The lease that was signed indicates I have 20 acres of a 320 acre parcel. I understand drilling parcels are in 640 acre parcels. Will the division of the property be based on 20 acres of 320, as the lease indicates, or 20 acres of 640?

If it is a 640 acre spacing unit, it will be 20 acres of the 640.