Hydrothermal Alteration Mineralogy of New Wells Drilled for the Rehabilitation and Stabilization Program in Ahuachapan Geothermal Field

Abstract

Thousands of rock cutting samples were analyzed in Ahuachapan wells using XRD technique and petrography to determine the hydrothermal minerals and relate their occurrences to the geothermal system of the area. Description of the hydrothermal alteration minerals focuses on new wells drilled under the Stabilization Program of the Ahuachapan Geothermal Field.

The rock samples show a wide range of hydrothermal minerals similar to the mineral assemblage of other geothermal fields. Quartz, albite, calcite, hematite, wairakite, chlorite (mostly clinochlore) and illite appear consistently in fine-grained andesitic ignimbritic tuff (Unit III) considered as the caprock.

Although epidote is the main mineralogic indicator of reservoir temperature, its occurrence is only minimal and observed only in three wells: AH-16A, AH-33B and AH-34. Its presence indicates reservoir temperatures of 250?C to 260?C.

Wairakite has been found associated with high permeability and its first appearance indicates the top of the permeable reservoir.