| Abstract |
Direct use of low enthalpy geothermal energy can face financial feasibility challenges, especially so in areas where volcanic activity is absent. In geothermal projects in sedimentary basins drilling costs have been identified as the main cost contributors [1-3]. In addition, reservoir permeability and local temperature gradients have the largest influence on power output [4].The increased heat conductivity of salt bodies [5] and their presence at shallower depths could help overcome the above mentioned challenges. In the southern Permian Basin in the Netherlands salt bodies have been found to influence the temperature gradient of existing nearby gas production wells [6]. Modelling of salt intrusions, within the same basin in Northern Germany, relates them to increased heat flow [7]. In the low enthalpy context of the Netherlands (average geothermal gradient 31.3°C/km [6]), higher heat flow at shallower depths can contribute to an economically more viable geothermal heat utilization. The Southern Permian Basin has been extensively studied for hydrocarbon exploration [8]. Permian (Zechstein) evaporite sequences in the North of the Netherlands have been subjected to halokinesis [9,10] and several salt intrusions and domes are present. Formation of the domes has caused fracturing of the anhydrite and or carbonate caprocks present on top of the (Zechstein) salt layers [5]. Brittle deformation resulting in boudinage and accompanying generation of neck fracture in anhydrite layers in a halite matrix has also been documented in lab conditions under deformation stress [11-13]. A fractured formation can act as an equivalent of porous media [14]. Several methods have been suggested to compute effective permeability of fractured media [15-17]. In the Dutch offshore, a caprock interval in this setting contains a proven and producing gas field (G16-FA). In our research project, 3D Pre-Stack Depth Migrated (PSDM) seismic data have been used to delineate salt bodies in the North of the Netherlands, above the currently producing Groningen gas field (expected to remain in production until 2080). We investigate the proof of concept of utilizing a fractured anhydrite layer at the top of a salt dome as a geothermal reservoir. Structural interpretation is carried out for a salt ridge covering 5.2km2 in a depth range between 1.7 and 2.0 km. At this depth, temperatures of ca. 65 °C are predicted. Lithostratigraphy of nearby borehole drilling logs suggests anhydrite thicknesses of 50m. Seismic attributes and structural analysis have identified the presence of directional discontinuities in the anhydrite layer. A workflow is suggested for a comprehensive analysis of fractured anhydrite as geothermal energy source. Efforts towards this are ongoing at the time of writing. References [1] Barbier E. 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Subsurface temperature of the onshore Netherlands: new temperature dataset and modelling. Netherlands Journal of Geosciences-Geologie en Mijnbouw 2012;91:491-515. [7] Agemar T, Schellschmidt R, Schulz R. Subsurface temperature distribution in Germany. Geothermics 2012;44:65-77. [8] Doornenbal JC, Abbink OA, Duin EJT, Dusar M, Hoth P, Jasionowski M et al. Introduction, stratigraphic framework and mapping. In: Doornenbal JC, Stevenson A, editors. Petroleum Geological Atlas of the Southern Permian Basin Area, Houten, The Netherlands: EAGE Publications b.v.; 2010, p. 1-9. [9] De Jager J, Geluk MC. Petroleum Geology. In: Wong TE, Batjes DAJ, de Jager J, editors. Geology of the Netherlands, Amsterdam, Netherlands: Royal Netherlands Academy of Arts and Sciences; 2007, p. 241-264. [10] van Gent HW, Back S, Urai JL, Kukla PA, Reicherter K. Paleostresses of the Groningen area, the Netherlands—Results of a seismic based structural reconstruction. Tectonophysics 2009;470:147-61. 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