Originally published as: Fuchs, S., Balling, N., Mathiesen, A. (2020): Deep basin temperature and heat-flow field in Denmark – New insights from borehole analysis and 3D geothermal modelling. - Geothermics, 83. DOI: http://doi.org/10.1016/j.geothermics.2019.101722 1 Deep basin temperature and heat-flow field in Denmark – 2 new insights from borehole analysis and 3D geothermal modelling 3 Sven Fuchs, Niels Balling, Anders Mathiesen 4 5 6 7 8 9 10 11 12 13 Received: 28.06.2019 14 Accepted: 20.08.2019 15 Published online: 02.10.2019 16 Authors 1 17 Sven Fuchs, Niels Balling, Anders Mathiesen 18 19 Affiliations 20 Sven Fuchs (corresponding author) 21 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 4.8 22 Geoenergy, Telegrafenberg, 14473 Potsdam, Germany 23 24 Niels Balling, Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000 25 Aarhus C, Denmark 26 Anders Mathiesen, Geological Survey of Denmark and Greenland, Copenhagen, Denmark 27 28 Abstract 29 We present a 3D numerical crustal temperature model with inverse optimisation methodology 30 and analyse the present-day conductive thermal field of the Danish onshore subsurface. The 31 model is based on a comprehensive analysis and interpretation of borehole and well-log data 32 for thermal and petrophysical rock properties and their regional variability and spatial 33 distribution across the country. New values of terrestrial surface heat flow derived from 21 deep 34 well locations are 65–76 mW∙m-2 (mean: 72±3) for the Danish Basin, 77–86 (81±5) for the 35 Danish part of the North German Basin, and 61–63 (62±1) for the Sorgenfrei-Tornquist- 36 Zone/Skagerrak-Kattegat Platform, respectively. The observed heat flow variations are 37 consistent with the position of the Danish area in the transition zone between the old 38 Precambrian Baltic Shield (low heat flow) and central European accreted terrains and deep 39 basin systems (significantly higher heat flow). 40 For the temperature modelling, conductivities and heat flow are constrained and validated (rms: 41 1.2°C, ame: 0.7°C) by borehole temperature data covering a depth range of up to 5 km (137 42 values from 46 wells). Significant modelled temperature variations are caused by (i) complex 43 geological structures (thickness variations, salt structures) and (ii) the variation of rock thermal 44 conductivity between and within geological formations as well as lateral variation in 45 background heat flow. Modelled temperature for major geothermal reservoirs indicate 46 substantial potential for low enthalpy heating purposes. Reservoir temperatures above 130°C, 47 of interest for the production of electricity, are observed for some local areas, however, likely, 48 too deep for non-stimulated sufficient reservoir quality. 49 50 Keywords: Deep basin temperature field, heat-flow density, borehole analysis, 3D calibrated 51 thermal modelling, core-log integration, uncertainty analysis, geothermal energy 2 52 53 Highlights 54 55 This paper presents a 3D numerical temperature and heat-flow study of Danish 56 onshore areas including deep sedimentary basins. 57 We present 21 new heat-flow values with significant variation according to regional 58 structural and tectonic background. 59 Most of Denmark provides suitable conditions for low enthalpy geothermal utilization; 60 local potential for electricity production. 61 A spatial variable thermal parameterization excels the homogeneous approach in terms 62 of temperature prediction accuracy. 63 Acknowledgements 64 This study was performed within the framework of geothermal energy projects, funded by the 65 Danish Council for Strategic Research (geothermal energy, project # 2104-09-0082) and the 66 Innovation Fund Denmark (project GEOTHERM – “Geothermal energy from sedimentary 67 reservoirs – Removing obstacles for large scale utilization”, project # 6154-00011B). 68 Additional financial support from the University of Aarhus and the Geological Survey of 69 Denmark and Greenland (GEUS) is gratefully acknowledged. We are grateful to GEUS for 70 providing the basic structural data for the applied geological model as well as background data 71 from boreholes, logging data and core material. We thank Lars Ole Boldreel (University of 72 Copenhagen) and Morten Sparre Andersen (GEUS) for providing access to the 3D digital 73 structural seismic model and Rikke Weibel (GEUS) for providing mineralogical and 74 petrophysical data from the Gassum Formation. Project coordination by and discussions with 75 Lars Henrik Nielsen (GEUS) are gratefully acknowledged. This modelling study was based on 76 the utilization of the commercial FEFLOW® code, and we kindly thank the support of DHI 77 Wasy for continuous help, where help was needed. Thanks to Yuri Maystrenko (Geological 78 Survey of Norway) and Magdalena Scheck-Wenderoth (GFZ Potsdam) as well as to Irinia 79 Artemieva (University of Copenhagen) and Hans Thybo (Istanbul Technical University) who 80 kindly provided structural data for the Pre-Zechstein crustal units of the study area. 3 81 1.0 Introduction 82 General knowledge of heat flow and thermal structure of sedimentary basins is of critical 83 importance for the understanding of basin formation and their tectonic evolution. Most basin 84 generating mechanisms include important thermal components and may leave significantly 85 different thermal signatures to be interpreted together with crustal and lithospheric structure for 86 a proper understanding of basin formation and evolution (Allen and Allen, 2013). Furthermore, 87 information about the internal thermal structure of basins is essential for the treatment of many 88 practical and economic aspects. Examples include the assessment of geothermal resources, 89 hydrocarbon maturity, and energy storage potential. Particularly, the recent year’s awareness, 90 that waste amounts of geothermal energy resources are present in sedimentary basins and may 91 play an important role in future sustainable energy supply (e.g. Lund and Boyd 2015; Antics et 92 al. 2016), has resulted in the need for accurate thermal information and thermal models. 93 This study presents a 3D numerical crustal temperature and heat-flow model for onshore 94 Denmark. For the first time on a countrywide scale, a comprehensive analysis of well-log data 95 provides well-constrained input for a fully parameterized and calibrated numerical subsurface 96 temperature model. Early subsurface temperature models for the Danish area (e.g. Balling et al. 97 1981; 2002) were based on a dense grid of 1D analytical temperature-depth profiles. Now, 3D 98 numerical models have been developed (Balling et al. 2016; Fuchs and Balling, 2016b; Poulsen 99 et al. 2017), with emphasis on the parameter inverse calibration methodology and its 100 application. Inverse parameter calibration procedures are widely used in groundwater 101 modelling (e.g. Hill & Tiedeman 2007), but, so far, with little application for subsurface thermal 102 modelling. Valuable exceptions include Wang and Beck, (1986), Gemmer and Nielsen (2001) 103 and Vosteen et al. (2004). Such parameter estimation, or optimization procedures, were 104 demonstrated to be of great importance, in particular in applying borehole-temperature data for 105 constraining the thermal rock properties (Fuchs and Balling, 2016b; Poulsen et al., 2017) and 106 are applied in the present study. 107 Our model builds on lab-constrained well-log derived rock thermal parameters (thermal 108 conductivity [TC], specific heat capacity [SHC], and radiogenic heat production [RHP]), a 109 procedure by Fuchs et al. 2016), new heat-flow determinations for 21 deep-well sites as well as 110 on a new digital structural geological model. This fully updated structural model, based on a 111 reinterpretation of all available reflection seismic lines across the country (Vosgerau et al. 112 2016), provides information on depth levels and thicknesses of 15 sedimentary units used as 113 ‘model input layers’ for the sedimentary succession. With a model base at 15 km depth, also 114 the upper parts of the crystalline crust are included. 4 115 As outlined in the following section, the Danish subsurface is characterised by thick sequences 116 of sedimentary rocks forming deep basins separated by basement highs. So far, onshore, 117 hydrocarbons have not been found in commercial amounts; however, the basins contain 118 reservoir units with vast amounts of geothermal resources (e.g. Balling et al. 2002; Nielsen et 119 al. 2004; Mathiesen et al. 2009). At present, three geothermal district heating plants are in 120 operation in Denmark (positions in Fig. 1) and several more at planning stage. Current 121 production is from Lower Jurassic and Triassic sandstone reservoirs at depths of 1.2–2.6 km 122 and temperatures of 44–75°C and with plant thermal capacities between 7 and 14 MW (Røgen 123 et al. 2015). Resource assessments and modelling studies indicate that geothermal energy has 124 the potential for supplying the Danish district heating network with sustainable energy for 125 centuries (Mathiesen et al. 2009; Mahler and Magtengaard 2010; Poulsen et al. 2017). 126 127 128 Figure 1. Left: Map of study area with structural elements and deep wells (colour code – red dots: with accurate 129 equilibrium temperature logs; orange: with formation test temperatures; dark blue: with corrected bottom-hole 130 temperatures (BHT; equilibrium estimates); light blue: wells with uncorrected BHTs (only minimum 131 temperature estimates); small black