A method for forming a coarse-scale three-dimensional geological model of sedimentary structures, the method being implemented by a computer , and comprising: - forming a fine-scale three dimensional model of the sedimentary structures, by implementing steps of: ∘ modeling a plurality of meshed sedimentary surfaces, the plurality of meshed sedimentary surfaces delimiting superposed layers of lithology, ∘ forming an unstructured grid comprising a plurality of cells, wherein each cell extends between at least two sedimentary surfaces, ∘ attributing petrophysical parameters to each cell of the grid, based on experimental data of the sedimentary structures, characterized in ∘ attributing , to at least some of the sedimentary surfaces, a transmissivity reduction coefficient comprised between 0 and 1, the transmissivity reduction coefficient being a parameter describing an imperviousness degree of the sedimentary surface to which it is attributed, a transmissivity reduction coefficient equal to 0 corresponding to an impervious surface, and - upscaling the fine-scale three dimensional model to obtain a coarse-scale three dimensional model comprising a plurality of cells, wherein each cell is associated to petrophysical parameters determined from the petrophysical parameters of the fine-scale model, and from the transmissivity reduction coefficient of the sedimentary surfaces. - forming a fine-scale three dimensional model of the sedimentary structures, by implementing steps of: ∘ modeling a plurality of meshed sedimentary surfaces, the plurality of meshed sedimentary surfaces delimiting superposed layers of lithology, ∘ forming an unstructured grid comprising a plurality of cells, wherein each cell extends between at least two sedimentary surfaces, ∘ attributing petrophysical parameters to each cell of the grid, based on experimental data of the sedimentary structures, characterized in ∘ attributing , to at least some of the sedimentary surfaces, a transmissivity reduction coefficient comprised between 0 and 1, the transmissivity reduction coefficient being a parameter describing an imperviousness degree of the sedimentary surface to which it is attributed, a transmissivity reduction coefficient equal to 0 corresponding to an impervious surface, and ∘ modeling a plurality of meshed sedimentary surfaces, the plurality of meshed sedimentary surfaces delimiting superposed layers of lithology, ∘ forming an unstructured grid comprising a plurality of cells, wherein each cell extends between at least two sedimentary surfaces, ∘ attributing petrophysical parameters to each cell of the grid, based on experimental data of the sedimentary structures, characterized in ∘ attributing , to at least some of the sedimentary surfaces, a transmissivity reduction coefficient comprised between 0 and 1, the transmissivity reduction coefficient being a parameter describing an imperviousness degree of the sedimentary surface to which it is attributed, a transmissivity reduction coefficient equal to 0 corresponding to an impervious surface, and - upscaling the fine-scale three dimensional model to obtain a coarse-scale three dimensional model comprising a plurality of cells, wherein each cell is associated to petrophysical parameters determined from the petrophysical parameters of the fine-scale model, and from the transmissivity reduction coefficient of the sedimentary surfaces. <EOS>