Berlín geothermal field, Berlín, Usulután Department, El Salvadori

Regional Level Types
Berlín geothermal field	Geothermal Field (Active)
Berlín	Municipality
Usulután Department	Department
El Salvador	Country

The Berlín geothermal field is located 100 km east of San Salvador, on the flanks of the Berlín–Tecapa Quaternary basaltic–andesite stratovolcano, along the southern fault system of the Central American graben.

The Berlín–Tecapa volcanic complex is characterized by basaltic to andesitic lava flows and scoria, and andesitic to dacitic ignimbrites, which were produced during two major eruptions that accompanied the development of the Berlín and Blanca Rosa calderas. The Berlín caldera forms a keyhole-shaped crater that opens toward the north–northwest and encloses the geothermal field. Three distinct aquifers have been identified in the field: a shallow, low-salinity aquifer (1600 ppm NaCl equiv., 200 to 300 m a.s.l.); a moderately saline intermediate aquifer (6600 ppm NaCl equiv., near sea level) with temperatures of 150 to 200°C and a thickness of 500 to 700 m; and a deep (8000 to 12000 ppm NaCl equiv.,800 to 1200 m b.s.l.), saline aquifer with a pH of ~5 and a temperature of ~290°C, that is exploited for geothermal energy and is responsible for scaling in wells and pipelines. The reservoir is hosted by basaltic–andesite and andesite lavas interbedded with minor tuff layers; these rocks have undergone propylitic alteration, characterized by abundant quartz and epidote, moderate chlorite and sericite, and minor calcite and wairakite.

Composite samples, consisting of mixed scale, rock, and corroded pipe fragments, were obtained from solid collectors and drains attached to liquid-vapor and re-injection lines of wells, the water tank, and vapor lines during a maintenance operation in February 2000. The fragments constitute mixtures of material that originated in surface pipelines, at depth in the wells, and/or in the reservoir. The fragments were detached and transported by geothermal fluids to the solid collectors and drains, where they were cemented by amorphous silica.

Composite fragments consist mainly of sulphide- and electrum-bearing aluminium-rich amorphous silica scale, sulphide- and electrum-bearing saponitic/vermiculitic clay from the reservoir, and altered metallic pipe linings containing As–S-bearing iron oxide–oxyhydroxide grains. Siliceous and clay-rich precipitates contain concentrations of gold, silver, copper, lead, zinc, and antimony. Copper, lead, and zinc occur mainly as chalcopyrite, galena, and sphalerite, respectively, in amorphous silica and clay; near the surface, chalcopyrite transported from depth alters to bornite. Gold and silver occur mainly as electrum, which deposited with base metal sulphides in the clay precipitates, and amorphous silica at higher levels in the well. Electrum precipitates in the wells due to the rapid drop in temperature and loss of H2S associated with boiling (Raymond et al., 2005).

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