The use is disclosed of at least substantially water-insoluble ethers which are fluid and/or at least plastically deformable at working temperature and have flash points of at least 80° C., of mono- and/or polyfunctional alcohols of natural and/or synthetic origin or corresponding solutions of such ethers in ecologically acceptable water-insoluble oils as the dispersed oil phase of water-based O/W-emulsion drilling fluids which are suitable for the environmentally acceptable development of geological formations and which contain, if desired, insoluble, finely particulate weighting agents for the formation of water-based O/W-emulsion drilling muds and/or further additives, such as emulsifiers, fluid-loss additives, wetting agents, alkali reserves and/or auxiliary substances for the inhibition of drilled rock of high water-sensitivity.