Abstract: Method of determining the compositional evolution of fluids present in a porous medium as a result of biodegradation. A biodegradation compositional scheme is defined from twelve chemical classes allowing the hydrocarbons and the gases resulting from biodegradation to be described. The initial amount of each one of these chemical classes contained in the fluids before biodegradation is determined. A reaction scheme is then defined for biodegradation, wherein the chemical classes react with electron acceptors according to parallel reactions having different velocities, and each class reacts with these acceptors according to sequential reactions. The biodegradation reaction velocities are determined for each class. The composition of the fluids is then deduced by assessing the amount of each chemical class by applying the reaction scheme. The composition of the biodegraded hydrocarbons, as well as the amount of acid gas and of methane produced, are notably deduced.

Abstract: At least one type of carbon material susceptible to generate a gas G is selected. Then, a model is built from the carbon isotopic composition of a gas P generated by thermal degradation of this type of carbon material. The model is calibrated by calibrating the kinetic effect independently of the precursor effect. In order to achieve that, pyrolyses are performed on compounds modeling the type of carbon material, said compounds being synthesized and labeled with 13C. Using said model, a carbon isotopic composition of gas P generated is estimated. Finally, it is determined whether gas G has been generated from the selected type of carbon material, by comparing a measured isotopic composition of gas G to the estimated isotopic composition.

Abstract: Computer-implemented method of determining a composition of hydrocarbons present in a sedimentary basin as a result of biodegradation. The displacement rates of the hydrocarbons and the displacement rates of the water present in the porous medium are estimated, from a genesis time in a mother rock to a possible accumulation in a reservoir rock. The biodegradation in the entire porous medium is then modelled as a function of time, from the genesis in the mother rock, considering that the hydrocarbon composition varies proportionally to the displacement rates.