ABSTRACT: Danube Basin represents one of the largest areas in the Slovak Republic for oil and gas prospecting as well as for conversion of aquifer structures into potential underground gas storage objects. One of the essential conditions for such a conversion is the existence of appropriate reservoirs, i.e. hydrogeological closed or semi-closed structures with corresponding convenient lithological composition. These structures were evaluated mainly on the basis of direct and indirect geochemical criteria of underground water and gas chemistry originating from hydrocarbon prospecting wells.