Updated mapping of terrestrial heat flow in Brazil

摘要

An updated geothermal database is presented by providing a new mapping of terrestrial heat flow over the entire Brazilian territory. New estimates of this geothermal parameter were obtained using an indirect method based on groundwater silica content (silica geothermometry) measured in 4949 wells. The Brazilian Geothermal Database of the Geothermal Laboratory from the National Observatory currently comprises geothermal data obtained directly from temperature-depth profiles mostly in hydrocarbon wells and from several other records in groundwater wells using indirect temperature estimates. The procedure of indirect temperature estimates used here assumes that the amount of silica dissolved in groundwaters depends on the in-situ temperatures of the host rocks or geothermal reservoirs. Following this initial calculation procedure, the heat flow, the depth of the aquifer and the temperature at the reservoir bottom were calculated using a numerical relationship that was specifically modified and adapted to the Brazilian geological context. Measurements of silica content in groundwater wells were obtained from the Groundwater Information System of the Geological Survey of Brazil. When available in the database, the lithological profiles of these wells were additionally used to calculate weighted average thermal conductivity by considering the documented lithotypes and their respective thicknesses. For wells without lithological information, their thermal conductivity was reassessed by including information from the nearest documented well lithotypes and that belong to the same geological context. The results indicate that regions encompassing a system of fractured aquifers, with a mean silica content of 40 ppm, show the highest heat flow values of the order of 70mW/m(2) on average. Regions with karstic aquifers, characterized by a mean silica content of 29 ppm, display most of the lowest heat flow values = 40mW/m2. Regions with porous aquifers of mean silica content between 30 and 33 ppm present a range of intermediate heat flow values between 50 and 60 mW/m(2). Taking into account the different associated lithological types, the Brazilian structural provinces display values of thermal conductivity ranging between 2.1 and 2.7 mW/m(2). The refinement in mapping terrestrial heat flow in Brazil indicates various thermal anomalies at regional scale that can be further studied for prospecting geothermal resources of low-to-high temperature. These regional thermal anomalies are essentially found in the NW region of the Parana Basin, N of the Tocantins Province (western portion), south-central Tocantins Province (eastern portion), north-central part of the Sao Francisco Craton and the NE region of the Borborema Province.