If your mother-in-law’s mother owned minerals at the time of her death, and there was never a probate on the estate, then the people running title would have vested your mother-in-law with a child’s share of the estate per the law of intestate succession in the state of Oklahoma. Paperwork would not have been required for them to vest your mother-in-law. Paperwork may be required to be paid on any production from the well.
If there was a will but not a probate done, I would suggest that a probate be done on the estate.