1. A method of underground reservoir development wherein a representation of the underground reservoir is constructed from curves describing an evolution of a saturation of a fluid as a function of at least one property relative to flow of the fluid within the reservoir, the reservoir is represented as a set of cells, and a lithology type is assigned to each cell of the set of cells, comprising: performing a principal component analysis of a data table constructed from the curves to extract principal factors explaining data in the data table; representing the curves in a representation space constructed from the principal factors; classifying the curves represented in the representation space into a set of classes; defining a curve representative of each class of the classes resulting from the classification; for each class of the set of classes, associating the class with the lithology type and assigning the representative curve of the class to the cells of the set of cells associated with the lithology type corresponding to the class; using a flow simulator to define a development scheme for the reservoir from the representation of the reservoir; and developing the reservoir according to the development scheme.