I’m not a CPA but I do have a good idea of what you are taxed at. I suggest that you contact your CPA for more concrete answers.
If you are selling your minerals You will pay longterm capital gains tax which is anywhere from 15% to 20% depending on profit made from the sale. Long-Term Capital Gains Tax Rates in 2019
Production tax in Texas is 7.5 % for Natural Gas and 4.6% for gas condensate and oil, the operator will deduct this from your royalty checks. The Fed treats royalties as income which will be taxed at your income tax bracket. https://comptroller.texas.gov/taxes/natural-gas/