Thermal Characterization of a Geothermal Reservoir in a Volcanic Zone in Central America

Abstract

This study involves the analysis and interpretation of a geothermal reservoir using petrography and fluid inclusion studies, as part of the final work in PREG (Geothermal training program in El Salvador) and LAGEO S.A de C.V. The methodology consisted in petrographic analysis of core samples in the reservoir zone of three wells located in one of the geothermal areas in El Salvador (identity of wells not mentioned due to confidentiality). Mineralogical assemblage of alteration facie and mineralogical temperature range (Tm) through petrography were estimated. Microthermometry of fluid inclusions in calcite and quartz was used to determine the homogenization temperature o minimum entrapment temperature (Th) and melting temperature (Tm) to calculate salinity content in the system (% NaCl). Estimated mineralogical temperature obtained in petrography and microthermometry were compared with stabilized temperature of wells (Ts). Wells located at southwestern part of the volcano, named A and B, have mineralogical facies of phyllic-propylitic with temperature from 220°C to 260°C increasing in depth to propylitic facies with temperature of more than 260°C. These mineralogical temperatures match with Th, with a temperature range from 201°C to 258°C and 0.2 to 0.599 %NaCl. However, Ts of well A is lower than the temperatures obtained using mineralogy and microthermometry, probably indicating a cooling process in depth, while in well B the three temperatures (Th, Tf, Ts) are within the same range, indicating thermal equilibrium. Well C located at the northeast has mineralogical facies of propylitic, with temperature over 260°C. Results of Th show two thermal events, one of them with temperatures between 232°C to 257°C, and other hotter from 282°C and 0.382 de %NaCl. However, Ts is lower than Th and Tm, which could indicate thermal inversion.