1. A method for modeling an area of the subsoil, comprising: constructing a training image on a first three-dimensional grid of cells, the training image giving, for each, cell of the first grid, a respective type of geological facies assigned to said cell, the types of facies including a first family of facies and a second family of fades that are less permeable than the facies of the first family; obtaining well data based on observations conducted in a plurality of wells drilled, in the area of the subsoil, the well data referring to a second three-dimensional grid of cells having a morphology is adapted to estimated geological horizons in the area of the subsoil, the well data including: types of geological facies respectively estimated from the observations for the cells of the second grid located along the wells; and inter-well connectivity data indicating at least one group of cells of the second grid located along multiple wells, with hydrodynamic communications having been observed between said cells; in an initialization image defined on the second grid, assigning the types of facies included in the well data to the respective cells of the second grid located along the wells; for each group of cells indicated by the inter-well connectivity data; selecting a thumbnail in the form of a set of connected cells of the first grid in which cells corresponding to the ceils of said group on the second grid exhibit the same types of facies in the training image as the respective cells of said group in the well data, and belong to a connected subset of cells for which the types of facies in the training image are in the first family, and in the initialization image, assigning the types of facies given by the training image for the cells of the selected thumbnail to the respective ceils of the second grid to which said ceils of the selected thumbnail correspond; and constructing, on the second grid, an image that models the area of the subsoil, by propagation from the initialization image by a multipoint geostatistical technique using the training image.