I would urge you to speak with a mineral rights attorney in your area and bring a copy of the contract of sale of the land.  In Texas, mineral rights are separate and apart from land surface property rights.  it is my understanding that they can be severed from surface rights - as they were in your purchase contract for the property - and do not expire or revert to the land surface owner ever.  They can be maintained for a lifetime and may be passed on to heirs and assigns.  Seek legal help.  Good luck.