Oklahoma tax basis?

I'm in the process of selling some mineral/oil interests in Oklahoma and am looking at the Federal tax situation and having to pay capital gains. The oil was inherited but I have to come up with a TAX BASIS. The probate case (inheritance) was April, 2016 so I think that is the time that I will need to value the minerals. Has anyone else had to deal with this and can you suggest the best way to find out what the minerals were worth back then.

Thanks Tom, will do. It's less than 10 acres and both sections are same area.