Contact the operator’s Division Order department and ask for a copy of the lease. You need to notify them of your inheritance so that they can put the new heirs into pay status. Be ready with death certificates, probate documents, driver’s licenses, W-9s, etc. They will tell you what they need. Ask them for a copy of the actual lease, not the memorandum.
You can also hunt on www.TexasFile.com and hunt by name of the ancestor. You may only find a memorandum of lease which is not terribly helpful. You want an actual copy of the lease.