Also, it’s common for the mineral interests to not be on the tax books. I’ve seen loads of properties where when the surface and coal were divided, the coal was listed in the tax books as a mineral interest. (It is, technically, a mineral interest after all.) Then, when the oil and gas was severed it wasn’t listed, sometimes because the clerk thought the minerals were already being taxed, sometimes because someone just made a mistake. So the oil and gas was never taxed.
This isn’t going to hurt you. Most counties are just letting these untaxed interests remain off the books until they start producing gas. The taxes for unproducing minerals are just a few dollars a year most of the time, and the courthouse staff are so busy, that it’s just not worth their time.