A graft polymer mixture with a graft base based on brown coal and/or polyphenol is proposed, which, as a grafting component, comprises a copolymer consisting of one or more different ethylenically unsaturated monomers and one or more different polyamides. Typical graft bases are brown coal, brown coal coke, lignite and brown coal derivatives, and also tannins. Useful grafting components include especially vinyl-containing compounds and styrenes, which may also be present in sulfonated form. Natural polyamides, for example casein, gelatin and collagen, are suitable polyamide components. These graft polymers with a preferred molar mass nM > 5000 g/mol are suitable, as a mixture, especially in construction chemical applications, and in the discovery, exploitation and completion of underground mineral oil and natural gas deposits, and in deep bores, since they have outstanding salt and thermal stabilities and are simultaneously water-soluble and/or biodegradable.