A study of the temporal behaviour of gas composition in the Larderello-Castelnuovo area shows that this behaviour varies, depending on whether the wells are affected or not by natural recharge. Where no natural recharge exists, gas composition seems to be governed by chemical equilibria. The recharge water, and the steam it produces, mix with the fluid already existing in the reservoir. At Larderello (far from absorption areas) the gas composition resulting from this mixing does not undergo further changes by chemical reactions. This is due either to a lack of reactivity where there is no liquid phase or to an ineffective fluid-rock interaction. At Castelnuovo (close to absorption areas) the H2S and CH4 contents in the gas have gradually decreased with the increase in recharge effects. The decrease in H2S can be attributed to dissolution in liquid water and oxidation. Various hypotheses have been forwarded for the methane. The correlation existing between CH4 and Np concentrations, even when the methane decreases in the areas affected by recharge, suggests that CH4 may be governed by more than just the Fisher-Tropsch reaction.

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