The invention relates to a method of developing an underground reservoir, notably a petroleum reservoir, traversed by at least a first well from which a fluid is produced, wherein a position of at least a second well to be drilled is determined by means of a production indicator map comprising a set of cells, each cell being associated with a production indicator defining an impact, on the fluid production, of a well addition in this cell. The method comprises constructing the map by: a) determining production indicators on a first group of cells of the map; b) determining production indicators on a second group of cells of the map; c) interpolating said production indicators on all the cells of the map; and defining the position of the second well by the cell where the production indicator is maximal.