An aqueous drilling fluid which contains a diquaternary ammonium cation functionalized clay which provides a substantially constant rheological profile, with respect to salinity, when incorporated into well drilling fluids. A clay/organic chemical composition includes: (a) montmorillonite clay and (b) one or more diquaternary ammonium compounds. A further aspect of the invention is a well drilling process which employs a drilling fluid containing the diquaternary ammonium cation functionalized clay described hereafter.