Jickyjack is correct. Was talking to my mom, who owns some mineral rights. Perhaps it is different in Wheeler County, but most places, you don't pay property taxes on mineral rights, only on the surface if you own the land.
She would be paying income tax on any money she is receiving from leases on the mineral rights or any production. I don't see her paying property taxes on mineral rights severed from the surface, unless they were producing.
My gut feeling is that if a company is willing to pay her six figures for the rights, there is current production on the land at some percentage of their offered price. She should be getting royalty checks.
Mom also agreed with me that if she has mineral rights, they will be recorded in the county clerk's office. She says they will also be on file with the Texas Railroad Commission (whatever they are called now) All of the above are public record, you should be able to retrieve it, or possibly even look it up online.
Someone really needs to look into this. It is possible her cut of the royalty money is going into a holding account because the oil companies no longer have her address. Or if grandpa has passed, no one ever sent them a death certificate so they could legally pay her the royalties.
May I suggest you gather your thoughts and information, and post the situation (as clearly as you know it) on the Outdoors Board?
Lots more traffic there, lots of folks who know oil and gas.