Chemical and Thermal Changes in the Aluto-Langano Geothermal Field, Ethiopia: from Fluid Inclusion Study

Abstract

The study of fluid inclusions at the Aluto-Langano geothermal field has demonstrated the existence of a complex evolution of the geothermal fluids from the early stages of geothermal activity up to the present day. Evidence of heating in the upflow zone (wells LA-3 and LA-6) of the system is provided by a comparison of the present in-hole temperature (320- 335?C) with the average fluid inclusion homogenization temperature (Th= 240-330?C). On the other hand, in the lateral outflow zone of the system, in-hole temperatures are found to be about 176?C lower than the average homogenization temperatures indicating the occurrence of a strong cooling process after fluid inclusion formation. Evidence for the change of compositions of fluids has been documented by the microthermometric data where the earlier fluids, responsible for the fluid inclusion formation, are more saline and less rich in CO2 content in comparison with the present reservoir fluid. The apparent salinities of the fluid inclusions, estimated from the ice-melting temperature (0.0-3.1 wt% NaCl eq.) are, in most cases, higher than the salinity of the present reservoir fluid (0.1-0.4 wt% NaCl eq.). Crushing experiments have shown that the dissolved CO2 content in the Aluto-langano inclusions is variable. Most of the crushed inclusions from wells LA-3 and LA-6 contain a CO2 amount (<0.05 m) lower than those present in the discharge fluid (0.11-0.21 m). On the other hand, the CO2 content of most of the inclusions from wells LA-4, LA-7 and LA-8 is found to be higher than 0.08 m; the absence of clathrate during freezing runs limits the maximum CO2 content in these inclusions to 0.85 m(Hedenquist and Henley, 1985). The Tmi - Th distribution suggests the occurrence of both boiling and dilution processes in the Aluto-Langano geothermal system.