Gas Geochemistry of the Dogger Geothermal Aquifer ( Paris Basin , France )

The low enthalpy program developped in the Paris Basin provides the opportunity for studying the gas geochemistry of the calcareous aquifer of the Dogger. Hydrocarbons and C02 are mainly biogenic, He displays high concentrations. He, Ar and N2 have multiple origins (radioactive decay, atmospheric migration, biochemical processes). The distribution of the gases in the zones of the basin varies in relation to the general chemistry, sedimentology and hydrodynamics. The gas geothermometers do not apply to this environment but useful estimations of the redox potential of the fluid can be derived from CO2/CH4 and Nz/NH$ ratios. H2 and H2S are involved in corrosion processes and scaling in the pipes.