Valuation from 2001

I have sold mineral rights in Freestone County recently. I inherited them in 2001. For tax purposes how can I form a valuation dating back to 2001?

Were there active wells on the property at the time you inherited the acreage and did you collect revenue/file it on your taxes? If no wells were producing, then youd have to look at surrounding mineral sales during that time period that had no producing wells and vice versa if there were producing wells to establish what the minerals were worth.