A method of modifying a geological model representative of an underground reservoir is disclosed which respects average proportions of the lithologic facies imposed by a production data calibration process which has application to petroleum reservoir development. A geographical zone Z is defined within the geological model and an average proportion in zone Z allowing the production data to be calibrated is determined for k facies, with an optimization process. The proportions of these facies are modified using a block indicator cokriging method constrained by the average proportions to be respected. A new geological model constrained by the modified facies proportions is simulated and the development of the underground medium is optimized by the simulated model.