Borehole geology and hydrothermal alteration of well SV-5A, San Vicente geothermal field, El Salvador, C.A

Abstract

Well SV-5A is an exploration well located in El Salvador in the San Vicente geothermal field. It is a directional well, drilled to a depth of 1798.5 m. The drill cuttings of the well were analysed through a binocular microscope and through a petrographic microscope in thin sections. The lithology of Well SV-5A consists of andesite lava flows, basalt-andesite, dacite and three different types of tuffs: crystal tuff, lithic tuff and crystal-lithic tuff. The alteration minerals show that temperature increases with depth. Mineralogical assemblages reflect five zones of alteration: a smectite-zeolite zone at 386-750 m, a mixed layer clay zone at 750-920 m, a chlorite zone at 920-1200 m, a chlorite-epidote zone at 1200-1242 m, and an epidoteactinolite zone at 1500-1798.5 m. Four aquifers were identified in the production section of the well, based on the temperature profiles and circulation losses during drilling. When the first appearance of alteration minerals and the results of the analysis of fluid inclusions were compared, equilibrium was observed, reflecting a temperature of 280°C at 1500 m depth. The formation temperature estimated at that same depth in the well is 240°C. By comparing the formation temperature with fluid inclusions and the alteration mineralogy, a cooling of about 40°C has occurred in the geothermal system. According to the calculated Horner temperature, the well was still in a thermal recovery process during the last temperature log, which showed an increase of 5°C, which means that the formation temperature of the well could possibly reach 245°C.