Geologic Model of the Miravalles Geothermal Field

Abstract

The Miravalles geothermal field is located on the southwestern flank of the Miravalles volcano, within a 14 km diameter semi-circular structure known as the Guayabo Caldera.
Based on the information obtained from drilling fifty-two deep wells, a geologic model, which encompasses lithostratigraphic, structural and hydrothermal mineral alteration data has been generated.
Ten stratigraphic units related to the formation and filling of the caldera, including pyroclastic materials, lavas, debris avalanches and lacustrine deposits, have been defined.
The permeability of the field is mainly secondary in nature, controlled by structural systems striking NW-SE, N-S, NE-SW and E-W, due to a distensive tectonic setting that has formed a series of horsts and grabens in the deep units.
The use of illite and epidote as high temperature indicating minerals, along with the 220?C isotherm, has contributed to the understanding of the thermal conditions of the field, observing a good correlation in the central sector and varying towards the field boundaries, where the data indicate a marked cooling.