Additive for drilling fluids

Subject matter of the present invention is a drilling fluid additive based on sulfonated asphalt, which comprises a water soluble and anionic polymer component in quantities of 0.1 to 20 wt. %, with respect to the total additive in addition to the asphalt component. With this additive the water solubility of sulfonated asphalt can be markedly increased. This additive, whose polymer component is thermally highly stable, is utilized primarily in water- and/or oil-based drilling fluids and in particular in the exploration of subterraneous petroleum and natural gas deposits. Typical polymer components to consider are representatives of polycondensation products, such as phenol resins, ketone resins and sulfonated naphthalene, acetone and amino plasticizer resins (for example urea and melamine resins).