Do you know if somebody is paying taxes on this? You can go here and select Tax and see if you find any of the names you have seen on the affidavits. Is there any production royalty that you or any of the relatives you know have been getting?
The title work might not be anywhere near that expensive. In some cases the affidavits of heirship might be a good first step. Once you know about who, if anybody, is paying the taxes you can start to sort things out.
When you were looking at the affidavits, did you see the place to see the older records, the "archives'" button? The index is a bit tricky to use but it can be very valuable information. Do you have a copy of the original lease? If not you can find it in the archives part.
Let me know if you want a recommendation for a good local attorney to help with the lease modification. It is best to sign, but with the help of a lawyer, and the more of your family who can go in together the better.