Electrical Resistivity Applied to Understand the Fluid Flow Pathway and Define Borehole Targets in a Geothermal Area, Liquiñe, Chile

Abstract

For low-enthalpy geothermal projects spatially close to hot-springs, fluid flow in the shallowest portions (first 30 meters) is extremely important to make cheap boreholes, which can increase the temperature and/or the volumetric flow rate. Therefore, to assess a low-enthalpy system, it is critical to define the shallow geometry of geothermal system (e.g. geometry of conduits/barriers for fluid flow). The Liquiñe area in southern Chile presents diverse geothermal manifestations and thus, it represents an interesting study-case for understand the hot-fluids circulation. Hot springs are spatially associated with two different faults systems; 1) the Liquiñe-Ofqui fault system (LOFS) that is composed by NS and NE strike-slip trending faults; and 2) the Andean Transverse Faults (ATF) that represent NW strike-slip trending faults. In the selected site, we found 7 hot springs upwelling from fractured granodiorite underlying to fluvial deposits. Apparently, these hot springs are related with a recognized N71W-trending lineament, attributable to the ATF. We identified 3 representative geological units constituting the geothermal system: crystalline granitic rocks, fractured granitic rocks, and fluvial deposits. With the objective of determining the shallow geometrical structure (~30 meters) of this active geothermal system, we compiled available structural geological data and we conducted several Electrical Resistivity Tomography profiles (ERT). To estimate the electrical conductivity range of the hot-aquifer in a porous media, we measure the electrical conductivity of thermal water and apply the Archie’s Law (1942). Archie’s Law application constraint the electrical resistivity of a porous media fill with hot-water between 30-100 Ohm-m. ERT results shown that the electrical resistivity close to hot-springs manifestation is ranging between 10 and 180 Ohm-m. Outside of the hot-spring manifestation there are two main relative-conductivity units inside a high-resistivity media (~1000-5000 Ohm-m). The first one, is a horizontal and thin unit (~ 2-5 meter of thickness) at less than 10 m depth with resistivity ranging between 2-180 Ohm-m. Geological observations indicate that this unit may represent the soil-rock contact saturated with hot-water, which is consistent with reported electrical resistivity ranges. The second unit is composed by several vertical conductivity bodies ranging between 30-180 Ohm-m with depth between 10 – 40 m, and no more than 15 meter wide. This unit may represent the hot fluid circulation through fractured rock. These vertical conductivity bodies suggest that the area studied is composed by several hot-fluids conducts (spread in 450 x 50 m2 area), that is support by the ERT-profiles and the widespread hot spring surface manifestations. Archie’s Law allows to constraint realistic electrical-resistivity values for hot-fluid circulation but cannot explain the precise electrical resistivity of the hot-fluid circulation by itself, mainly due to hot-spring phenomena also include clay deposition, phase-changes process and not only porous medias. The ERT results indicate that there are two geothermal targets which could improve the geothermal capacity with a borehole: the horizontal and the vertical low-conductivity bodies that represent different geological units, and therefore different geothermal capacities.