Geothermal exploration in El Salvador, which started in 1954, has identified ten
geothermal areas characterized by medium-to-high temperature (180-300 °C) fluids. The
geothermal energy of this country is currently provided by two distinct areas: i)
Ahuachapan-Chipilapa, located approximately 80 km West of San Salvador, and ii)
Berlin, 100 km East of the capital city and related to the Pleistocene Tecapa-Berlin
volcanic complex. These two geothermal fields produce about 161 MW (95 MW from
Ahuachapan-Chipilapa and 66 MW from Berlin), accounting for the 24 % of the total
electricity output of El Salvador. The evolution of physical-chemical conditions of
Ahuachapan-Chipilapa and Berlin geothermal reservoirs during exploitation has been
extensively described on the basis of the compositional features of gas and water
phases discharged by exploration and production wells (e.g. D’Amore and Mejia, 1999;
Jacobo, 2003). 
The present study is aimed to test in these well-known liquid-dominated systems
different geothermometric techniques by using compositional data of naturally
discharged fluids. Our results have shown that the chemistry of the fumarolic gas
vents is significantly affected by both dissolution-driven fractionation, due to the
interaction of geothermal fluids with shallow aquifers, and complex secondary
gas-water-rock chemical reactions. As a consequence, a correct application of the
classical inorganic gas geothermometers was not possible. On the contrary, the
relative abundances of the structurally homogeneous C2-C9 hydrocarbons during
geothermal fluid rising through natural fumarolic pathways appear to be only
regulated by the diffusion velocity of gases within the shallow aquifers, a parameter
that is function of the molecular volume of each gas species. Therefore, the chemical
equilibrium of reactions among the C3-C3 and the C4-C4 alkane-alkene pairs, which are
characterized by similar molecular dimensions and structure, seems to be not
depending on secondary interactions thus, representing a useful tool for reliable
estimations of geothermal reservoir temperatures and dominating redox conditions,
especially during the first stage of exploration in promising areas for geothermal
resources, before drilling any exploration wells.

References
D’Amore, F., and T.J. Mejia, 1999). Chemical and physical reservoir parameters at
initial conditions in Berlin geothermal field, El Salvador: A first assessment.
Geothermics, 28, 45-73.
Jacobo, P.E., 2003. Gas chemistry of the Ahuachapan and Berlin geothermal fields, El
Salvador. Report n° 12. In: Geothermal Training in Iceland, UNU-GTP, pp. 275-304,
Reykjavik, Iceland.

Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy