Scrubbing process and chemical equilibria controlling the composition of light hydrocarbons in natural gas discharges: An example from the geothermal fields of El Salvador

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2007-05, Vol.8 (5), p.np-n/a
• Main Authors: Tassi, F., Vaselli, O., Capaccioni, B., Montegrossi, G., Barahona, F., Caprai, A.
• Format: Article
• Language: English
• Subjects: gas geothermometry; gas scrubbing; geothermal fluids; light hydrocarbons
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2006GC001487

The compositional features of fluids from both fumarolic discharges and productive geothermal wells of Ahuachapan‐Chipilapa, Berlin‐Chinameca, and San Vicente geothermal systems (El Salvador) are described and discussed in order to investigate the complex geochemical interactions involving geothermal fluids within the shallowest part of the hydrothermal circulation pathways. Our results highlight that secondary processes are able to strongly affect and modify the chemical characteristics of geothermal gases once they discharge to the surface as natural manifestations, mainly in relation to the chemical‐physical properties of each gas species. The effects of both gas dissolution in shallow aquifers and gas‐water‐rock chemical interactions on gas discharge composition make it difficult to get a correct evaluation of the thermodynamic conditions that characterize the geothermal reservoirs by applying the common geoindicators based on the chemical equilibria of the H2O‐CO2‐H2‐CH4‐CO system. Differently, the composition of the C1‐C2‐C3 alkanes and the C3 and C4 alkane‐alkene pair, established within the geothermal reservoirs under the control of chemical reactions, remains stable in samples collected from discharging gas vents. These results suggest that the relative abundances of hydrocarbons characterized by similar structure and molecular size seem to be mainly regulated by the diffusion velocity of gases through the liquid‐dominated system. Therefore the chemical features of the light organic gas fraction of naturally discharging fluids can be successfully utilized for the evaluation of geothermal reservoir temperatures and redox conditions, providing useful indications in terms of geothermal exploration and exploitation. On this basis, the distribution, speciation, and relative abundances of light hydrocarbons can also be considered highly promising in geochemical monitoring of active volcanic systems.