This is a complex and very specialized are of real estate law and should not be undertaken until counsel is consulted.
Now, if you received a letter requesting your signature, you can almost bet that you'd be on the losing end of this deal. You must review it carefully and allow counsel to do the same. I've run into some pretty strange things in these leases and they will affect your ability to sell down the line if the leases are still in place.
If the money is enticing, play it safe and allow a competent oil/gas attorney to review it and offer feedback.