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.