Method using Black Oil type modelling for predicting, as a function of time, the detailed composition of fluids produced by an underground reservoir under production, combined with a delumping stage allowing detailed thermodynamic representation of the reservoir fluids. The input data entered for the model are the thermodynamic parameters of the fluids such as viscosity, volume factor, density, gas-oil dissolution ratio, etc. (in form of charts, and/or by correlation, as a function of the pressure, of the temperature if it varies) and, if need be, an additional parameter keeping a memory of the composition of the gas such as, for example, the density of the gas), as well as data relative to the variations, as a function of the same abscissas, of the phase parameters required for delumping, without the latter being used during the Black Oil simulation of the flows. Application: predictive profiles of the detailed composition of hydrocarbons produced by a reservoir for example.