Análisis geoquímico-isotópico de las especies carbónicas del fluido geotérmico de los Humeros, Puebla, México

Abstract

A geochemical analysis of discharged fluids from wells showed a mixing process in the Los Humeros geothermal field. lt was proposed that th ere are two different reservoirs: an upper liquid-dominated reservo ir and a steam-dominated reservoir containing aggressive fluids at greater depths. Shallow wells such as H-1 are not affected. Excess steam calculations for deep wells yield high values except for H-1 suggesting equilibrium conditions. Chemical data for well H- indicated that the carbonic species behaviour agrees with thermodynamic variations produced as a result of the reservoir boiling process. The activity of COz in the liquid phase decreases as calcite is precipitated and carbon dioxide is released to shallower levels . The spatial evolution ofthe fluid is related to isotopic behaviour since the lighter carbon isotopes are found in the shallower levels. The deep calcites show average values of o13C from- 0.36 to - 3.86 °/00 toward the top of the reservoir. lsotopic results for carbon dioxide in total discharge show the same variation with values for o13C of- 5.8 °/00 in the upper zone of the reservoir.Unaltered rock from the Cretaceous basement ofthe Tamaulipas Formation yields an average value for o13C of -0.3 °/00• Thus sorne isotopes may originate from the Mesozoic sedimentary basement of the Quaternary caldera which caldera originated the geothermal field.