Margaret:
I am assuming that your mother is deceased (either now or in 1994). If that is the case, then the company will likely require a probate in order to begin paying royalties to the rightful heirs.
You may want to search her name with Oklahoma Unclaimed Property. Click Here and Mineral Owners Escrow Here
If the interest is small, an Affidavit of Heirship may be sufficient. If not a simple probate might do the trick.