Integrated assessment and prospectivity mapping of geothermal resources for EGS in Brazil

Abstract

Studies in Brazil related to geothermal resources have been carried out mainly by the National Observatory. Based on the results vast areas of Brazil have been delimited with a surface heat flow of over 60 mW/m2 and an excess temperature above 100 °C at depths between 3 and 6 km. The nature of the heat source and the mode of heat dissipation in these areas are largely unknown. Geothermal resources that might be associated with these areas can occur in hot sedimentary aquifers (HSA) and units of hot dry rocks (HDR) that are hosted, respectively, in intracratonic sedimentary basins and high heat producing granites (HHPG). The focus of our study is on finding resources of high enthalpy (above 150 °C) in HHPG, which regardless of their hydrological properties (porosity, permeability) could be considered as potential targets for enhanced geothermal systems (EGS). The structural provinces of Tocantins and Borborema were chosen as pilot regions based on both the indication of HDR and the wealth of open access georeferenced information. A comprehensive database was compiled and processed under GIS that includes geology, faults, soil typology, radiogenic heat, heat flow and geothermal gradient from wells, thermal spring locations, airborne gamma-ray spectrometry and magnetic surveys, terrestrial gravimetric surveys, and earthquake locations. In parallel, various critical factors were defined primarily to identify and characterize the most promising HHPG under insulating sedimentary rocks for EGS. These critical factors were grouped into three main categories labelled as: ‘thermal source’, ‘thermal insulation’ and ‘active pathways’. In the absence of exploratory data on existing EGS in Brazil these critical factors were used in this study along three separate work streams to develop a knowledge-driven model based on fuzzy logic for improving the knowledge on the prospectivity of geothermal resources. The developed model was further tested for its validation. The selected approach is expected to be applicable to mapping areas with EGS potential in other regions with similar geology and open access georeferenced databases.