Method of constructing a geological for setting a depositional position of a stratigraphic unit. A depositional position of the stratigraphic unit is set with respect to a reference surface, to define a depositional space. The stratigraphic unit in depositional position is then calibrated to superimpose it on the initial stratigraphic unit. The wells are then positioned in this depositional space. A regular Cartesian grid is superimposed on the stratigraphic unit with a depositional position which is filled with property values characterizing the underground formation modelled by a geostatistical simulation. The values assigned to the cells of the grid are transferred to the cells of the stratigraphic mesh.