Determining capital gains basis


I inherited a working interest in Glasscock/Reagan County in Texas in 2008. In 2018, I kept the existing wellbore and production in its Straberry formation but sold only the NONPRODUCING Wolfcamp formation underneath it. I now need to determine the basis of this working interest for capital gains purposes. How would one determine the value of a working interest in a non producing formation?. I sold the Wolfcamp formation for 15k an acre in 2018…so I would think the formation had some value in 2008. Would it be possible to see what other sales of Wolfcamp formations brought in 2008? Any other suggestions or comments. I have limited tax background. Thanks! MEB