method of evaluating the biodegradation , through the action of a bacterial population , of hydrocarbons trapped in a geologic structure . the mass of hydrocarbons present in said structure is determined without taking account of the biodegradation , from data relative to the physical and geometrical characteristics of the structure studied . then the mass of hydrocarbons consumed by biodegradation is calculated after evaluating the number of bacteria , their hydrocarbon consumption and the structure filling time . the mass percentage of oil disappeared through the bacterial action is deduced from these two masses . application : determination of the composition of oils in a reservoir and notably location of heavy oils . the method can notably be applied to the evaluation of the economic risk linked with the presence of biodegraded petroleum reservoirs .