How do I determine the basis for mineral rights I inherited in 1997?

In Sept. of 2018, I sold the mineral rights that I inherited in 1997. I am trying to file my income tax. After numerous tries, no one can tell me where to look for the information of how to determine the basis of these mineral rights inherited in 1997. At the time, there was no drilling so there was no royalty checks… Years later, they began drilling, and I started to receive the checks. Then, I sold the rights last year. Please advise where I can look. Thank you so much in advance…

Your basis is the value in 1997 as reported on the estate tax return. Do you have any of the estate papers and valuation? Values were quite low at that time because there was so little oil and gas activity.