Borehole geology of well SJ9-2, San Jacinto - Tizate geothermal field, NW-Nicaragua

Abstract

Well SJ9-2 is located in the San Jacinto - Tizate high-temperature field in NWNicaragua. It is a directional well reaching a total depth of 1388 m with a sidetrack to 1725 m. The uppermost 1000 m were analysed through cuttings and thin sections; below 1000 m the analyses were based on thin sections. The lithology of well SJ9-2 comprises a volcanoclastic sequence which consists of tuff with intercalated andesite and basaltic andesite; three different types of tuff formations have been distinguished based on texture, crystallinity and compositional variation. Hydrothermal alteration is controlled by temperature, rock type and permeability. The mineral assemblage showed that the alteration temperature increased with depth, first with the appearance of minerals such as low-temperature zeolites, quartz, and smectite. The alteration reflects higher temperatures below 519 m depth with the appearance of epidote (sparse) and wairakite (≥230°C). The alteration increased below 900 m depth (epidote abundant, ≥250°C) and localized at 1250-1350 m depth (actinolite, ≥280°C). Four alteration zones were identified in the well: an argillic zone at 12-303 m, phyllic alteration at 303-519 m, a propylitic zone at 519-1281 m and a high-temperature propylitic zone at 1281-1329 m. The propylitic zone reappears at 1329-1725 m depth. The permeability in the well is negligible. According to the circulation losses and temperature profile, six small aquifers were identified in the production zone. Comparison of alteration mineral temperature and measured temperature showed that the geothermal system is overall in equilibrium with a reservoir temperature of 240-255°C.