Geothermics 57 (2015) 26–38 Contents lists available at ScienceDirect Geothermics jo ur nal homep age: www.elsevier.com/locate/geothermics Toward fracture porosity assessment by gravity forward modeling for geothermal exploration (Sankt Gallen, Switzerland) Pierrick Altwegg a,∗ , Eva Schill b , Yassine Abdelfettah b , Pier-Vittorio Radogna c , Guillaume Mauri a a Centre for Hydrogeology and Geothermics (CHYN), University of Neuchâtel, Emile-Argand 11, CH-2000 Neuchâtel, Switzerland b Institute of Nuclear Waste Disposal INE, Karlsruhe Institute of Technology KIT, Hermann-von-Helmholtz-Platz 1, Germany c RBR Geophysics GmbH, CH-6340 Baar, Switzerland a r t i c l e i n f o Article history: Received 19 December 2013 Accepted 25 May 2015 Keywords: Fracture porosity Gravity Stripping Reservoir characterization Fractured reservoir a b s t r a c t Fracture porosity is a crucial parameter in hydrocarbon and geothermal reservoir exploration and a major challenge in the absence of nearby exploration wells. The Sankt Gallen geothermal project targets a fault zone that affects Mesozoic sediments at a depth of about 4500 m. Spatial extension of these sediments, a major fault zone and indication for graben structures in the crystalline basement are observed in 3D seismic. Both the graben and the fault zone coincide with negative gravity anomalies acquired and ana- lyzed during this study. Forward modeling of gravity anomalies based on a 3D seismic survey is used to estimate possible fracture porosity. After stripping gravity effects of geothermally irrelevant geological units from the residual anomaly, most likely only local structures related to the fault zone account for remaining anomalies. Synthetic case study on the effect of density variation and considerable gas content in the well support possible fracture porosity between about 4% and 8%. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction In addition to the increasing exploration of fractured reservoirs in hydrocarbon and geothermal industry, fracture porosity turns out to be also a controlling factor in low-enthalpy hydrothermal regions such as the Alpine Molasse basin. The interplay between hydrothermal aquifer condition and fault zones is well-known in the Upper Malm (e.g. Signorelli and Kohl, 2006). This formation reveals aquifer properties in Germany and Austria to a consid- erable depth of >3300 m allowing for electricity production from thermal water at temperatures as low as 130 ◦ C and flow rates up to 150 l s -1 (e.g. at the Unterhaching power plant; Schellschmidt et al., 2010). In contrast, such aquifer conditions are not observed in wells in Northern Switzerland located about >50 km to the West of Sankt Gallen (e.g. Signorelli and Kohl, 2006). Following the con- cept of hydrothermal aquifer utilization as practiced in the Munich Molasse basin, fault zones are targeted in order to connect geother- mal wells to the reservoir and the regional flow field, which is possibly contained in the reef-facies (Böhm et al., 2013). Recent failures of geothermal projects which targeted the reef-facies, only, ∗ Corresponding author. E-mail address: pierrick.altwegg@unine.ch (P. Altwegg). confirm the increasing importance of fracture porosity. Other con- cepts in low enthalpy reservoir utilization are based on regional circulation along fault zones allowing for free convection such as the projects in Soultz-sous-Forêts and Landau in the Upper Rhine Graben (Bächler, 2003; Kohl et al., 2000). In both concepts, frac- ture porosity linked to fault zones is of major importance and may thus also be crucial for the Sankt Gallen geothermal well GT- 1. Recently, gravity has been mainly employed to carry out mass- balancing in this type of reservoirs (e.g. Oka et al., 2012). The determination of porosity in geothermal systems by means of grav- ity measurements has been applied, for example at the Geysers field, where according the phase of the fluid, porosity changes in the order of 0.5–1.6% have been attributed to density changes of 40–60 kg m -3 for reservoir zones of an extension of several kilo- meters (Denlinger et al., 1981). In this case, density changes were related to porosity through measurements of the interconnected porosity on samples. In Alpine low-temperature geothermal sys- tems, negative gravity anomalies have been linked to faults along which geothermal fluid rises naturally (Guglielmetti et al., 2013). First estimates on porosity by gravity in the European EGS reser- voir area in Soultz-sous-Forêts reproduce the order of magnitude obtained from logging data (Schill et al., 2010). In recent studies, gravity in combination with 3D geological modeling has proven to http://dx.doi.org/10.1016/j.geothermics.2015.05.006 0375-6505/© 2015 Elsevier Ltd. All rights reserved.