The Borinquen geothermal system (Ca˜nas Dulces caldera, Costa Rica)

Abstract

The geotectonic location of Costa Rica has allowed the development of an active volcanic arc, geological context associated with hydrothermal systems with geothermal potential. In the northwest of the country, on the Pacific slope of the Rincon de la Vieja volcano, inside the Cañas Dulces caldera, the presence of thermal manifestations motivated the development of geoscientific studies that culminated with the drilling of 4 geothermal exploration wells to depths of 2078–2594 m, as part of the research that seeks to corroborate the existence of a commercially geothermal reservoir. A date has explored an area of 31 km2 known as Borinquen where information on lithostratigraphy, mineralogy, electric resistivity and thermal gradients, as well as on the geochemical and isotopic composition of the fluids derived from the meteoric water, surface hydrothermal manifestation and exploration wells, was obtained. The distribution of the secondary mineral assemblages observed in the exploration wells suggests an increase in temperature with depth, which is confirmed by the distribution of the thermal gradient. The geoelectric structure hints at the existence of a cap rock and a geothermal reservoir, and the geochemistry and isotopic composition of the samples obtained from the exploration wells reveal the existence of neutral sodium chloride fluids with high enthalpy. This suggests exploitation could be economically viable, particularly in light of comparisons with similar reservoirs already being exploited in the neighbouring calderas of San Vicente and Guayabo. Taking into account all the available information, we provide a conceptual model of the Borinquen geothermal area that explains the main elements of its high enthalpy geothermal system – heat source, distribution of the thermal anomaly, recharge zone, reservoir, cap rock, main flow paths and flow directions. This will be useful as a guide for the exploration and future development and exploitation of this geothermal resource.