Gravity Versus Thermal Gradient: Can We Use Gravity to Discriminate Potential Hydrogeothermal Areas ?

European Geothermal Congress 2019 Den Haag, The Netherlands, 11-14 June 2019 GRAVITY VERSUS THERMAL GRADIENT: CAN WE USE GRAVITY TO DISCRIMINATE POTENTIAL HYDROGEOTHERMAL AREAS ? Yassine Abdelfettah1,2, Jacques Hinderer1, Marta Calvo3, Eléonore Dalmais4, Vincent Maurer4, Albert Genter4. 1 Institut de Physique du Globe de Strasbourg (IPGS), CNRS UMR7516, University of Strasbourg, Strasbourg, France 2 Geophimage, Geophysical Consulting Company, 113 Rte de la Wantzenau, F-67000 Strasbourg, France 3 Observatorio Geofísico Central (IGN). C/Alfonso XII, 3, 28014 Madrid, Spain 4 ES-Géothermie, 5 rue de Lisbonne 67300 Schiltigheim, France [email protected] Keywords: Exploration, gravity, temperature, Rhine program to increase the likelihood for project success Graben, thermal gradient. by delineating more accurately promising geothermal areas. This could be considered as one derisking step. ABSTRACT Classically, the geophysical methods used in deep We present results obtained from new accurate gravity geothermal exploration are active-seismic and non- data showing average data uncertainties around 0.02 seismic methods, for instance electromagnetics, mGal. The data were acquired in 2013 and 2016 in the gravity and magnetics. Each of them present Upper Rhine Graben within the “Electricité de advantages and disadvantages. The choice of the used Strasbourg” geothermal exploration permits located in geophysical exploration methods depends on the northern Alsace (France). We achieved a qualitative scientific and technical challenges related to the study as well as a quantitative interpretation based on targeted geothermal reservoirs but also usually on the existing 3D geological models built mainly from 2D allowed financial support. seismic and borehole data. The interpretation of the Recent geothermal projects developed in the Upper mismatch between the observed and computed Rhine Graben (URG) tend to target fault zones at the Bouguer anomalies reveals high correlation between sediment – basement interface. Whereas seismic the density of the known fractures/faults and the high methods provide a useful and suitable underground and negative mismatch values. This discrepancy is image in the sediments, non-seismic methods give also interpreted in terms of negative and positive insights on nature and structure of basement rocks. As anomalies that express lack and excess of density permeable fractured zones in basement – targeted by values. The stripping approach conducted in the whole geothermal projects – present significant petrophysical area but more specifically applied onto seven thermal contrasts due to intense hydrothermal alteration, high gradient boreholes reveals that the density contrasts fracture density and natural permeability with their are often overestimated from the Jurassic but more background (Genter et al., 2000), their geophysical specifically for the Buntsandstein. This reveals high characterization should be refined by non-seismic fracture related porosity effect on the densities. The methods especially using gravity. stripping technique applied on the same location as those of the thermal gradient boreholes reveals that the In this paper, we present recent results obtained from boreholes F3 and F5 could present higher geothermal new gravity data analysis combined with 3D potential compared to the other boreholes. This geological modelling. These results are the continuity innovative approach using high accurate gravity data, of those discussed in Abdelfettah et al., (2016). The post-processing and 3D finite element gravity results obtained from gravity analysis will be modelling as well as 3D geological modelling leads to compared to the results obtained from borehole data in the same conclusion as the one obtained from thermal seven subareas located in the northern part. gradient borehole data analysis. The studied area is located in the northern of Alsace, 1. INTRODUCTION from Haguenau to the French-German boundary in the For a given geothermal project, the choice of the north and east including Soultz-sous-Forêts and drilling area is very important for project success. This Rittershoffen geothermal plants. To the west, it is step is very crucial for a project especially with limited by the main border fault of the Rhine Graben restricted financial support like in geothermal energy basin (Fig. 1). In total, 1033 gravity measurements (compared to oil/gas industry). Consequently, it is were used; among them 800 new measurements and important to undertake an adequate exploration 233 old ones, located mainly in the southern part of the study area (Fig. 1). 1 Abdelfettah et al. We show in (c) and the MP cross-section the used meshing. 3. OBSERVED AND COMPUTED BOUGUER ANOMALIES A preprocessing step was performed on the observed data removing the bad measured data. It is the quality control step. Then, the data were processed and we get accurate Bouguer anomaly with average uncertainties of 0.02 mGal, which is an acceptable value for discussing about an “accurate” Bouguer anomaly (Fig. 3a). Figure 1: Study area showing gravity measurements and the lateral footprint of the geological model. 2. GEOLOGIAL MODELLING During “EGS Alsace” and “ANR Cantare” French R&D programs, a 3D geological model covering approximately the same area than the gravity-studied area has been built based on reprocessed seismic data dating from the 80’s (Maurer et al., 2016). Due to the parameters used at this time, with the objective highlighting tertiary layers potentially bearing oil, and scarce information of seismic velocities, this model had an average depth uncertainty of 30 m (reaching a maximum of 103 m) at the Jurassic - Trias interface, which was assessed by comparing the obtained depth after time-depth seismic conversion with the depth measured at boreholes. The first order faults as well as some major geological layers (“Schistes à poisson”, Tertiary unconformity, top Trias, top Muschelkalk, and top Buntsandstein) have been mapped in 3D. The footprint of the geological model is shown in Figure 1 and its 3D views are shown in Figure 2. The 3D geological model was built by interpolating existing 2D seismic sections, constrained by borehole data located inside the study area. Figure 3: a) Observed Bouguer anomaly, b) computed Bouguer anomaly and c) discrepancy between observed and computed Bouguer anomalies. Note that the F3, F4.1, F4.2, F5, F6.1, F6.2 and F6.3 are thermal boreholes done by ESG (Maurer et al., 2018). In order to compute the theoretical gravity effect of the modeled geological model, we took it and meshed it using finite element approach. We affected homogeneous density value for each geological unit and compute its gravity effect. The gravity measurements were located on the real topography Figure 2: Geological model used in the quantitative and real coordinates (latitude, longitude) as observed and stripping studies. a) 3D view from NE, points. The used density values are shown in Table 1, b) sketch of cross-sections shown in c and d. which represent the average values obtained from 2 Abdelfettah et al. several sources. The computed Bouguer anomaly is Additionally, sequential and cumulative stripping shown in Fig. 3b. according to depth is very important to understand quantitatively the evolution of the anomalies In Figure 3c, we present the misfit or the discrepancy according to depth. The principle of the sequential between the observed Bouguer anomaly and the stripping is to strip geological units sequentially computed one. It is computed by subtracting the starting from the first layer, and then cumulate with theoretical value to the observed value for each the second one, and so on; for instance strip the Upper measured point. It means that this misfit could be also Rupelian firstly, then the Upper and the Lower interpreted in terms of negative and positive anomaly. Rupelian together until stripping the whole units of the The negative anomaly means that gravity attraction model. The results obtained from the sequential effect is overestimated and, on the contrary, for a stripping are shown in Figure 4. positive anomaly this theoretical gravity effect should be increased. In this misfit (Fig. 3c), we can observe mainly that its dynamical range is greater than the dynamical range of both observed and computed Bouguer anomalies. Moreover, the entire area is not explained by the computation, where a maximum of misfit reaches -30 mGal in the western part of the area, which is a huge value. In the central part of the model, we reach also a huge misfit value between -10 and -20 mGal. We can understand that the density values used in the forward modeling are very far from the real densities. Table 1. Density values assigned to the geological model used in the forward modeling. The names in brackets indicate the simplified name related to local geology used in the body of the paper. Geological unit Density -3 (kg.m ) Figure 4: Cumulative stripping results. a1) shows Plio-Quaternary and Serie-Grise (Upper 2100 the Upper Rupelian gravity effect and a2) Rupelian) shows the stripped Bouguer anomaly. b1) Pechelbronn formation and Dolomitic 2250 showed the cumulative gravity effect of the Complex comprises Lower Rupelien, Upper and Lower Rupelian units and b2) Priabonien and Bartonien (Lower shows the corresponding stripped Bouguer Rupelian) anomaly. c1) shows the cumulative gravity Jurassic mainly Lias (Jurassic) 2470 effect of the Upper and Lower Rupelian as well as the Jurassic units, and c2) shows the Keuper

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