Method for gradually modifying lithologic facies proportions of a geological model

A method for gradually deforming a geological model representative of an underground reservoir including set of grid cells comprising information on lithologic facies, for fixed average proportions of the lithologic facies, the average proportions being calculated by averaging facies proportions over at least one geographic zone of the reservoir, comprising:

• defining the at least one geographic zone of the reservoir by selecting a set of grid cells of the geological model;
• grouping together the lithologic facies in a first group of facies;
• within the first group, grouping together the facies in a second group of facies;
• defining a transformation parameter by a ratio of an average proportion of the facies of the second group to an average proportion of the facies of the first group;
• transforming the lithologic facies proportions of the second group and complements thereof in the first group, at each cell of the geographic zone, to impose a fixed value for the ratio;
• and
• simulating a new geological model constrained by the transformed lithologic facies proportions.