Death Certificate from Oklahoma

We have property in Blaine county and leased to Cimarex - and its getting a lot of play now It is section 8 15 N 12 West, Recently we were contacted by them and were told that there is a great deal of oil on our property but they needed the death certificate of my grandfather who left acreage to my mother who is now deceased as is my father. My grandfather was in Shawnee Oklahoma and died in 1931 in December but was buried in Oklahoma City in January of 1932. I found the mortuary and even an image of his stone at the place of his burial. I sent to OK for his death certificate and was told they could not find one. My grandfather owed a home in Shawnee and a business and he was in the 1931 census I found that information. He committed suicide as he was ruined in the Depression. My question is this if Cimarex already pays us royalties as we have signed leases with them as his heirs why do they say now they need his death certificate when they accepted our notarized copies as his heirs - and what happens if I cant get one from Oklahoma? Can anyone help?