Gaseous Vs Aqueous Fluids: Travale (Italy) Case Study Using EM Geothermometry

PROCEEDINGS, Thirty-Ninth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 24-26, 2014 SGP-TR-200 Gaseous vs. Aqueous Fluids: Travale (Italy) Case Study using EM Geothermometry Viacheslav V. Spichak, Olga K. Zakharova GEMRC, POB 30, 142190, Moscow, Russia E-mail: [email protected] Keywords: electromagnetic geothermometer, fluids, resistivity, temperature, reservoir, Travale ABSTRACT It is often necessary to recognize the type of the heat carrier circulating in the geothermal system. In particular, it is difficult to distinguish between hot aqueous and gaseous fluids solely basing on the resistivity and/or seismic velocity cross-sections without prior information, which may come from geology, geochemistry, well logs, etc. Even joint analysis of the resistivity and seismic velocity data does not always provide enough information for drawing sound conclusions on the heat sources or type of geothermal fluids. In particular, electromagnetic sounding is mainly aimed at detecting low resistivity anomalies, which are interpreted as hydrothermal fluids circulating in the system. In doing so, we can miss the resistivity anomalies that could reflect the gaseous/steam/supercritical fluids. Meanwhile, joint analysis of the resistivity and temperature models preferably accompanied by the gravity data may not only provide the location but also identify the type of the fluids (aqueous / gaseous or liquid / steam). In this paper we study the feasibility of mapping the supercritical geothermal reservoir and discriminating the type of fluids circulating in the geothermal system by a joint analysis of the resistivity and temperature models of the Travale area up to the depth of 10km, the latter being revealed using the indirect electromagnetic geothermometer. The comparison of the resistivity and temperature cross-sections indicates that an extensive resistive area located in the depth range of 3 - 5.5km is characterized by temperatures higher than 500˚C. Considering the negative Bouguer anomaly over this area, it could be interpreted as a fractured granitic massif filled by supercritical mixture of gases and vapor, which is in agreement with geochemical data. Another remarkable feature of both structures is the shallow (0-2km) inclined slab detected both in the resistivity and temperature cross-sections. It is connected with a deep reservoir and could be considered as a channel for transporting the hot steam to the surface rather than as a separate near-surface reservoir predicted here by the interpretation of seismic data. 1. INTRODUCTION Geothermal deposits are perfect objects for their study by electromagnetic methods since they cause significant variations in electric resistivity of the geological medium. However, in contrast to the high temperatures, which are a necessary condition for searching for a geothermal reservoir, the low or high electric resistivity itself is neither indicative of the presence of the reservoir nor of the character of the fluid (liquid or gaseous) circulating in the hydrothermal system. Despite of this, in many studies devoted to identifying the geothermal reservoirs from the ground-based geophysical (including electromagnetic) data (e.g., see the review of Spichak and Manzella (2009) and references therein), interpretation of the deep resistivity sections is mainly focused on revealing the highly conductive areas, which are then considered as the geothermal reservoirs or the pathways of circulation of a liquid geothermal fluid. A characteristic example comes from the interpretation of the magnetotelluric data in the Travale geothermal area in Italy, which is presented in (Manzella et al., 2006). Travale is a part of the Larderello geothermal region in Italy. Its location is shown in Figure 1. Here, two geothermal basins characterized by very high temperatures and high vapor release are exploited. The shallow basin is located at a depth of the cataclastic horizons of carbonate evaporite deposits belonging to the Toskana complex. The deeper reservoir is larger; it is hosted by the fractured metamorphosed rocks at a depth below 2 km (Barelli et al., 2000). In order to restrict the range of uncertainty and to conduct the interpretation of the geophysical data in the terms of geothermal application, it would be desirable to analyze the geoelectric cross-sections together with the distributions of temperature (e.g., see (Spichak et al., 2013)). However, the difficulty in reconstructing the temperature sections is associated with the fact that temperature logs can only provide temperature distributions up to a depth of 1-2 km, while the heat flux modeling yields only the regional-scale reconstructions (e.g., (Khutorskoi et al., 2008; Ollinger et al., 2010; Limberger and Van Wees, 2013)), which are lacking the necessary accuracy. Meanwhile, Spichak et al. (2011) suggested the new method for indirect estimating the temperature in the Earth's interior. This method relies on the electromagnetic sounding data and is suitable for reconstructing the temperature distributions on the scale of the geothermal reservoirs. Using this method, the authors reconstructed the two-dimensional (2D) and three-dimensional (3D) temperature models for the well-known geothermal areas of Soultz-sous-Forêts in France (Spichak et al., 2010) and Hengill in Iceland (Spichak et al., 2013). In the present work, by the example of the Travale geothermal area, we explore the possibility of searching for geothermal reservoirs and estimating the type of the heat carrier (gaseous or aqueous fluid) by the joint analysis of resistivity cross section and temperature distribution yielded by this approach. 1 Spichak and Zakharova 2. GEOLOGICAL SETTING The following tectono-stratigraphic complexes (Figure 1a) are distinguished in the area of the study (Brogi et al., 2003; Bellani et al., 2004): (1) the Miocene-Pliocene and Quaternary sediments filling the vast tectonic depressions; (2) the Ligurian complex that includes Jurassic ophiolites, the overlying Jurassic-Cretaceous sediments, and Cretaceous-Oligocene flysch formations; (3) the Tuscany complex composed of the sedimentary rocks, Cretaceous and Oligocene evaporite and carbonate rocks up to the Late Oligocene-Early Miocene turbidites; (4) the underlying strata revealed by the geothermal drilling. These rocks include two units. The upper unit is related to the Monticiano-Roccastrada formation, which is mainly composed of the Triassic quartzites and phyllites (the Verrucano group), Paleozoic phyllites and mica schists. The lower unit corresponds to the gneiss complex. The rocks composing the upper unit bear the footprints of the Apennine orogeny, whereas the gneiss complex does not show any signs of such transformation. This probably indicates that these rocks were part of the foreland crustal area ahead of the Apennine orogen Figure 1. Simplified geological map, showing site location the study area (a) (Manzella, 2004); MT sites (triangles), boreholes N1 and N4, and the main profile A-A’ (b). After the stages of convergence and collision (in the Late Cretaceous-Early Miocene), which determined the further structural development of the northern Apennines, the Larderello region experienced three episodes of tectonic extension. The first and second events occurred in the Miocene and resulted in the overlapping of the Ligurian complex onto the Triassic evaporites, the rocks of the Verrucano group, and Paleozoic phyllites along the gently dipping normal faults. The third (latest) episode (Pliocene to the present) is marked by the development of the normal faults steeply dipping northeast. In this region, the surface heat flux reaches its maximal intensity (above 500 mW/m2) along several normal faults (Bellani et al., 2004). This fact was interpreted as the result of the fluid motion in the localized zones of deformation corresponding to the main shear zone. This process is continuing up to the present and can be associated with both the liquid and overheated vaporous geothermal fluids. The tectonic extensions in Tuscany were accompanied by acid magmatism. Drilling in the Larderello region exposed numerous acid dikes and granitoids with the age of cooling of 2.25-3.80Ma, which thread through the Paleozoic mica schists and the gneiss complex. Intrusion of these magmatic bodies at some places resulted in overlapping of the contact-metamorphic mineral associations onto the earlier ones. The deep structure of the Larderello region is traced by the seismic reflection data. A sharp contrast between the weakly reflecting upper crust and strongly reflecting middle and lower crust is observed along the seismic profiles. Based on the seismic data and the results of drilling, Brogi et al. (2003) supposed that the younger steeply dipping faults are the surface manifestations of the extensional shear deformations, which tend to flatten out with the increasing depth. The deep structures in the Travale geothermal field are determined by the behavior of the so called H- and K- horizons located at the depths of 1-2 and 5-6 km, respectively (see their locations in Figure 2). Both horizons are marked by the high amplitudes and intermittent lateral distribution of seismic reflections (Bertini et al., 2005). Here, the H-horizon is more discontinuous than the K- horizon, which is located at a larger depth. The boreholes penetrating this horizon show that it is a fractured zone filled with vapor and located near the top portions of the granite intrusion. 2 Spichak and Zakharova Figure

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