I have inherited oil and gas rights in oklahoma

I just got paperwork in the mail from the oil company saying my great granfather oil and mineral rights are mine basically but would they have gone to my grandfather first? He has since passed and then to my mother who has passed and then to me but on the paperwork it say it from my great grandfathers name. My mother never mentioned anything and when she died nothing was in the will, so who has been getting royalty payments since my greatgrandfather died?