Method of modeling the biodegradation of hydrocarbons trapped in an oil reservoir or trap through the action of the bacterial population in an underlying aquifer.From data relative to the reservoir studied, concerning the form and the height of the reservoir, the physical characteristics of the porous medium, the thickness of the transition zone between the hydrocarbons and the water, the composition of the hydrocarbons, of the flow of electron acceptors entering the reservoir and data relative to the bacterial population in the aquifer, modeling is performed by discretizing the reservoir by means of a grid wherein the height of each grid cell is the thickness of the transition zone, and the variation, over the height of the reservoir, of the proportion in heavy fractions of hydrocarbons under the effect of the biodegradation is determined by iterative adjustment in each grid cell of the bacterial population to the amount of hydrocarbons available, the pore space available, the amount of electron acceptors present in the reservoir and the degradation capacities of said population.Applications: determination of the composition of oils in a reservoir and notably of the location of the heaviest fractions.