1. A method for constructing a facies map representative of a petroleum reservoir, from a grid of a set of cells and petrophysical property maps representative of the reservoir and associated with the grid, comprising: a) using a computer to define a visiting order for the cells of the grid; b) for each cell i, according to the visiting order and for facies values k using a computer to calculate a likelihood defined by a probability of obtaining in cell i petrophysical property values of the maps, such that a facies in cell i is facies k, and based on knowledge of facies values already identified in previously visited cells and petrophysical property values of the previously visited cells; c) using a computer to associate with each cell i of the grid a facies value maximizing the likelihood in cell i; and d) calculating for each cell of the grid a ratio between a probability of observing the petrophysical property maps if the facies of cell i is a most probable facies and a probability of observing the petrophysical property maps if the facies of cell i is a second most probable facies, with the visiting order being determined by the calculated ratio of the cells with a cell of the grid having the calculated ratio which is greatest being visited first followed by successively visiting remaining cells of the grid which have not been visited in an order determined by a descending magnitude of the calculated ratio of each of the remaining cells.