Oil companies cannot release royalty payments until you prove ownership in the county records. If there was a probate, you must file certified copies of the Will and probate order in the county courthouse. If there was no probate, you may need to file an Affidavit of Heirship signed by disinterested parties who knew the family.
Once deeds are recorded, contact the operator to sign a Division Order, which specifies your decimal interest and authorizes the release of funds. If your relative's royalties went unpaid for years, check state Unclaimed Property registries to recover escheated funds.