Oliver County, ND- Coal mineral rights

Mother passed away quite awhile back. Mineral Rights…may not have been mention in will; however filed a Claim of Mineral Rights by deceased mother was done in 1985. Forty (40) coal mineral rights located in Oliver County ND, were granted to our mother. I don’t think the mineral rights were mentioned in her will. There were 6 of us children…only 4 still survive. Not sure if the 2 deceased have a right to the mineral rights still? Not sure of the cost of placing the 40 acres into the names of the living heirs or a trust or if it’s even worth it. Plus I read that after 20 years or so, the owner of the land can do a quiet claim to own the mineral rights which may only be cattle grazing land anyway. Anybody have any thoughts…thanks John Lynch