I purchased residential property in 1991 and sold it in 2012. I kept the mineral rights but sold them in 2018. How do I determine the original cost of the mineral rights?

[Rick-J]

The property and mineral rights are in the state of Oklahoma.

[kigerlaw]

Your original purchase contract may have assigned value to the land and Mineral rights separately.  If not, refer to the public data on file with the tax assessor where the land is situated where historical values are archived.