A graft polymer mixture comprising a grafting base based on brown coal and/or polyphenol is proposed, which has, as a graft component, a copolymer consisting of one or more ethylenically unsaturated monomers different from one another and one or more polyamides different from one another. Typical grafting bases are brown coal, brown coal coke, lignite and brown coal derivatives and tannins. Suitable graft components are in particular vinyl-containing components and styrenes, which may also be present in sulphonated form. Natural polyamides, such as, for example, casein, gelatin and collagen, are suitable polyamide components. These graft polymers having a preferred molar mass Mn>5000 g/mol are suitable as a mixture, in particular in construction chemistry applications, and in the development, exploitation and completion of underground mineral oil and natural gas deposits, and in deep wells, since they have excellent salt and temperature stabilities and are simultaneously water-soluble and/or biodegradable.