I doubt that the offer to purchase is a scam. You should be extremely careful if anything is leased or sold.
You should run your ancestors relative’s name through these databases:
Mineral Owner Escrow
Unclaimed Property
These first two will tell you if unpaid monies have been turned over to the State of Oklahoma.
Land record search Links to all Counties with Online Capabilities
(some counties have limited databases or require subscriptions)
Oklahoma Courts to search for Probate: Click Here
If an Okalhoma Probate is needed this has options. Link.