Title: Borehole geology and alteration mineralogy of well PGM-5 Miravalles, Guanacaste, Costa Rica

Abstract

A description is given of the borehole geology and alteration
mineralogy of well PGM-5 in the Miravalles Geothermal Field
(Costa Rica). The drillhole intersected six stratigraphic units
consisting of lahars, andesitic lava flows, pyroclastic rocks,
acid- andesite, ignimbrite and volcano-sedimentary units. The
lithology of PGM-5 differs somewhat from other deep wells in
the area, in the presence of an acid-andesite and/or ignimbrite
units and a question is raised on the total subsidence of the
caldera floor, which may be about 1 km less than the 1.4 km
assumed by the presently accepted model. Further comparison
is recommended. By combining measured circulation losses and a sequence of
temperature profiles from the well, the position of 9 aquifers
was determined. On basis of the clay minerals, three alteration zones were
distinguished: 1) Smectite zone, 2) Mixed-layer clay mineral
zone and 3) Chlorite-illite-epidote zone, believed to be formed
at temperatures up to 200· C, 200-240· C, and from 240-300· C
respectively . The maximum reservoir temperature measured is 238 "
C. Comparison of the two, the mineral temperatures and the
measured, indicates that some cooling of the reservoir has taken
place in geological time. Further study and comparison with the
other wells is recommended.