This study was funded by the Interreg NWE Program through the Roll-out of Deep Geothermal Energy in North-West Europe (DGE-ROLLOUT) Project (www.nweurope.eu/DGE-Rollout) between 2018 and 2023, and lead by Geological Survey of North Rhine-Westphalia. The Interreg NWE Program is part of the European Cohesion Policy and is financed by the European Regional Development Fund (ERDF).

Introduction

Deep Geothermal Energy (DGE), which is considered a renewable energy source with ubiquitous availability, can play a significant role in the future energy mix. The Transnational EU Interreg North-West Europe (NWE) funded project DGE-ROLLOUT (“Roll-out of Deep Geothermal Energy in NWE), between 2018 and 2023, aimed to promote the DGE potential of Carboniferous carbonate rocks, which are expected to represent one of most favorable reservoirs for hydrothermal energy extraction. This reservoir is widespread in the NWE subsurface of the Rhenohercynian Basin, which was investigated following a multidisciplinary geoscientific approach. In the NWE, the carbonate rocks of the Lower Carboniferous are known by various names: Carboniferous Limestone Group, Zeeland Formation, 'Kohlenkalk', Dinantian or sometimes as 'Kulm'. For the project, we use the generic term ‘Dinantian’ when referring to this stratigraphic interval, equals to the lower part of the Mississippian series according to the international geological time scale of the International Commission of Stratigraphy (ICS). The project objectives were focused on three main axes: 1) to provide a reconciled and common knowledge baseline for DGE market development in the Dinantian reservoir rocks; 2) to fill existing information gaps by acquiring transnational 2D seismic surveys, conducting drilling operations, reprocessing vintage seismic data, and producing a variety of subsurface 2D and 3D models; and 3) to increase the efficiency of existing geothermal systems by implementing new or improved production techniques in the fields of reservoir behavior, cascading systems, and thermal energy storage. This paper presents an overview of the project and their results at the North-West Europe and the Hauts-de-France scales, without claiming to be a scientific article. More details are available on the project website and in the report Dezayes et al. (2024) for the Hauts-de-France part.

Geological context

Dinantian limestones are present in a large part of the Northwestern Europe (Figure 1) and in the British Isles. The sedimentary deposition and stratigraphy of the Dinantian were influenced by sea-level changes, local and global tectonics, and paleoclimatic conditions (Poty, 2016). This results in various lithologies from carbonates to dolomites and sandstones regarding sedimentation areas and facies (platform deposit, calciturbidites, siliciclastic flysch, …).

At the end of the Dinantian and during the Silesian times, the southern part of the study region has been constituted by lower Paleozoic and Devono-Carboniferous rocks, which were folded and thrusted to the North, formed the Variscan front.

Figure 1

In the Hauts-de-France area, a thin Meso-Cenozoic cover overlies the Paleozoic terrains because of the Midi thrust fault. Below this cover, Paleozoic formations are more than thousand meters thick of sediments and was strongly structured by the Variscan orogeny (Laurent et al., 2021). This northern front of the Variscan range is defined by large structural units (Meilliez & Mansy, 1990 ; Mansy et al., 1997 ; Lacquement et al., 1999 ; Mansy & Lacquement, 2006; Bélanger et al., 2012; Laurent et al., 2021) that are respectively from North to South (Figure 2):

• the Brabant Caledonian Basement or Brabant Massif.
• the Brabant Para-Autochthonous Unit, which consists of Carboniferous-age terrain affected by folds and north-verging thrusts overlying the undeformed terrains.
• the Overturned Thrust Sheet complex characterized by strongly deformed Silurian to Carboniferous series.
• the Ardenne Allochthonous Unit carried by a major crustal-scale thrust: the Midi Fault or Allochthon Main Basal Thrust (AMBT).

Figure 2

Methodology

Bulding of a shared lithostratigraphic chart

Although the lithology of DInantian is dominated by carbonate, local heterogeneities exist. To help to build the transborder maps, a lithostratigraphic chart was yielded to represent the variations in the lithological nature of the Dinantian along the variscan front in the Hauts-de- France region and Belgium.

The use of borehole data coupled with works on the lithostratigraphy field observations of the Carboniferous series makes it possible to propose synthetic litho-stratigraphic columns by sector (Paproth et al., 1983 and 1999). Whatever the area, the columns can be divided into 5 distinct stratigraphic units (Figure 3). Detailed methodology for building this chart is presented in the report project D1.1.5 (Veldkamp et al., 2023).

 

At the base, the Lower Tournaisian (Hastarian) is characterized by an alternance of limestones and sandstones. The proportions vary according to the sites, particularly on the edge of the Brabant massif. A shaly horizon (Pont d'Arcole Formation) is also present throughout the region. The upper part of the Tournaisian (Ivoirian) is characterised by presence of a carbonate platform and formation of biostromes. Only the northern part is more clayey with the presence of marly deposits. Dolomitic facies are abundant at the top of the Tournaisian. Thickness of the Tournaisian is globally constant, around 250 metres.

In the lower part of the Visean, dolomitic facies alternate with limestones. In the middle Visean (Livian), two levels of interstratified sedimentary breccias in the limestones are present and constitute the geothermal reservoir in the Mons productive wells (Licour, 2014) The thickness of the Visean is highly variable, from a few tens of metres to several hundred metres, depending on the areas. The top of the Visean is composed by shales indicating the transition to the detrital coal series of the Upper Carboniferous (Namurian).

Figure 3

Reprocessing vintage seismic profiles

Besides the lithostratigraphy definition, the subsurface structures were highlighted by the interpretation of seismic profiles. Vintage profiles, acquired in 1980’s, were reprocessed consisting of several steps to remove anthropic and acquisition noises, surface waves and other coherent events which do not correspond to primary energy. The aim of the seismic reprocessing is to obtain an image of the subsurface allowing a better definition of the geometry. In the Hauts-de-France, a depth migration and an elastic impedance inversion was performed. Depth migration allows to obtain the depth migrated images which serve for the interpretation and inversion of main structural elements at depth. These processed profiles were used to build a 3D geological regional model of the Variscan thrust front between Lens and Maubeuge (Laurent et al., 2021).

Moreover, an elastic impedance inversion allowing to derive information about the petrophysical characteristics of the reservoirs (Mougenot, 1999) has been done. For this quantitative interpretation, reprocessing of seismic profiles with amplitude preserving and well petrophysical data are needed to interpret impedances of P and S waves of each seismic lines. The detailed process and results are presented in the project report D2.1.15 (Capar et al., 2022).

Heat In Place calculation

In order to calculate the geothermal potential maps of the Carboniferous Dinantian in the wider Dutch, North-Rhine Westphalia, Belgium and French region, temperature, permeability and heat in place maps have been constructed and calculated. Maps are generated from information made available by the DGE Rollout consortium, and literature data. It builds on Dinantian depth and thickness maps that were constructed as parts of previous DGE Rollout deliverables. A temperature map for the Top Dinantian was constructed by integrating temperature data derived from regional 2D maps, 3D temperature models and well data. Because of the different data formats a workflow was adopted to (re)calculate available temperature data into (local) temperature gradients (°C/km), merge the gradients into a single map and then use the resulting cross-border temperature gradient map together with the Top Dinantian map to calculate the temperature at the Top Dinantian. In this way, the structural grain (presence and incorporation of (local) faults) is preserved, challenges related to sparse temperature data coverage are alleviated and effects resulting from differences in burial, compaction and lithology are negated. Regional permeability maps have been constructed for the Dinantian. Based on existing literature reviews, it is assumed that Dinantian permeability is largely governed by structural (fault permeability) and diagenetic (karstification) processes. Using literature data on fault damage zone width, fracture permeability, fault displacement and length, combined with carbonate rock properties (permeability measurements on Dinantian core plugs) and karst permeability assumptions), minimum, average and maximum permeability maps have been calculated. A Heat in Place (HIP) map was calculated for the Dinantian for areas where the Dinantian thickness is known. The HIP varies from 50–200 GJ/m² in the North-Rhine Westphalia and Wallonia regions to up to 400 GJ/m² in the Flanders region. For the most part, Dinantian thickness is unknown in the Netherlands, hence the HIP here is sparse

 

Using the ThermoGIS method with flow property (the permeability maps), maps of the geothermal power that can be extracted by a heat exchanger were calculated. Depending on the permeability scenarios, minimum, average and maximum potential power maps have been compiled. Limitations of the various compiled maps include:

• Uncertainty in the temperature map is largely caused by sparse temperature data (or models) in certain regions. With the addition of new temperature data, the map will improve in accuracy.
• Sparse permeability measurements on Dinantian rock plugs, as well as incomplete understanding of the spatial continuity of permeability in fault zones, requires that permeability concepts and/or assumptions need to be adopted in order to correlate permeability to structural concepts (fault displacement, fault damage zone) as well as sedimentological concepts (karstification). Calibrating the maps with new permeability measurements on rock plugs will improve the map.
• Only few of the faults used for the permeability assessment have been actually mapped in the Dinantian but rather extrapolated from the Base Permian/Top Carboniferous. Therefore additional uncertainty is related to the actual presence and precise location of these faults and associated assumed high permeability zones. 3
• The generated heat in place maps do not include flow property information. Therefore they do not provide information on the actual geothermal potential on a local scale. The HIP maps do provide regional information, and as such should only be used to for a first order, regional assessment of the heat potential

Uncertainties assessment

The diverse sources of input data influence the robustness and reliability of the interpreted top and base depth, thickness and temperature maps, the Heat In Place and ultimately the geothermal potential of Dinantian rocks at depth. Assessing the uncertainty of the resulting maps will provide the end-user with a first order assessment of the reliability of these maps. For this first order assessment, the focus was carried out on the top of Dinantian. The following major qualitative indicators for the uncertainty computation were identified that determine the uncertainty in depth for the top Dinantian:

1. Data quality and density
2. Total depth
3. Tectonic complexity

Various uncertainties result from the source of data that has been used to interpret the top Dinantian. Seismic data is often of different vintage with varying quality and is either 2D (digital or analogue lines) or 3D seismic. In addition, seismic data density varies within a country, but also between countries. Also, seismic data was interpreted by multiple individuals over longer periods of time, applying varying methods with different geological models/targets in mind.

Seismic data is acquired in time and converted to depth using velocity models of the for the overlying units. With cross-border interpretations, differences in the used velocity models will result in different time-depth conversions. This adds to the uncertainties.

In areas where seismic data is unavailable, the depth of the Top Dinantian may be derived from available well data of from geological concepts. Nearby wells, uncertainties in interpreting the top/base are less than away from the wells.

There are also areas where no seismic and no well data is available. Here, depth to the Top Dinantian may be inferred from regional geological knowledge and trends.

Structural complexity introduces another source of uncertainty. In The Netherlands and North Rhein-Westphalia, the structural setting of the Dinantian is relatively simple, with block faulting present. Further south in Northern France and Southern Belgium (Wallonia), Dinantian is more complex with thrust sheets and structural duplications present. Moreover, Southern and below Midi fault, Dinantian is deeply buried, and geometry is still poorly understood (Figure 2).

The complete methodology and maps developed by DGE-Rollout partners are available in the project report D.L.T.3.2: Reduce_uncertainties_of_geological_interpretation (Valdkamp & Foeken, 2023).

Socio-economic combined index

The importance of evaluating both topics, social and reservoir factors, is described in the technical approach pyramid by Moeck et al., 2020. It considers social and geological aspects in the exploration phase of geothermal projects and emphasizes the decisive surface and subsurface elements. The Combined Index of the socio-economic potential in DGE-ROLLOUT area was calculated to map areas where deep geothermal projects might have a larger chance of success from the surface view. The methodology of this project was mainly based on literature and adapted from previous surveys. The main categories Social, Economy and Environment are taken from the literature, (Moeck, et al., 2020; International Renewable Energy Agency, 2017; Chocobar, 2020). Those categories are the typical groups for the factors involved in socio economic studies.

The detailed methodology and calculation are available for the scale of North-West Europe in D1.2.3 NWE and for France in D.1.2.3 France.

Data

Date available for the NWE area

In the framework of the project, several seismic lines were acquired. Two lines were acquired by the Geological Survey of Belgium (Department of the Royal Belgian Institute for Natural Sciences) in Wallonia, the line L2 is located to the east of Charleroi and the line L1 is to the east of Namur.and the latter crossed the deepest borehole of Belgium (Havelange: 5648 m MD) (Figure 4). In Flanders three lines were acquired by VITO near and across the Dutch border, near Bree and Lommel. EBN acquired several lines in the Netherlands, which some of them reached into Germany (Figure 4).

Figure 4

Due to many new cross-border lines, the DGE-ROLLOUT campaign helped to gain some insights into geological ambiguities in particular in the border areas, allowing the linkage of different mapping areas, facilitating the exchange of knowledge on potential geothermal aquifers in the subsurface.

The borehole data of the studied area were gathered in a database in which the main information of Dinantian layers is highlighted (total depth, coordinates, lithological facies, Dinantian top and thickness, nature of the contact, etc.. The Dinantian boreholes database covers Northern France, Belgium, the Netherlands and North-Rhine Westphalia, and is downloadable from the EGDI/DGE-Rollout webGIS:

Data in Hauts-de-France

In the Hauts-de-France region, no seismic acquisition has been carried out, but six seismic lines, acquired in 1980 and 1981 between Lens and Maubeuge, were reprocessed for a combined length of around 172 km (Figure 5).

Figure 5

To perform the seismic quantitative interpretation, a petrophysical analysis from 2 wells Jeumont 1 and Epinoy 1, located on seismic lines (Figure 5) was carried out. Five main facies have been identified by the petrophysical analysis: limestone, dolomite, anhydrite, sandstone and clay. The Dinantian formations, in Epinoy 1 well, are located between 2100 meters depth and the bottom of the well (3952 meters), in a reverse series. The petrophysical analysis revealed the absence of porous levels. In the dolomite facies located around 2640 meters depth, the porosity is around 1 to 2 % (Figure 6).

Figure 6

In Jeumont 1 well, the Dinantien formations are located between 3650 meters depth and the bottom of the well (4315 meters) in a normal series (Figure 7). The petrophysical analysis shows permeable dolomitic facies between 4115 and 4230 meters depth with a porosity between 5 and 8% (Figure 7).

Figure 7

The elastic impedance inversion of the seismic lines provided the impedances P and S for each line (see more explanation in the project report D2.1.15 (Capar et al., 2022) and Dezayes et al. (2024). The quality of the data and the low porosity do not allow to establish a relationship between impedance and porosity. However, a further analysis of the data allowed to establish a qualitative differentiation of the facies from the Vp/Vs ratio. The Vp/Vs ratio for dolomites has empirical values between 1.78 and 1.84 (Domenico, 1984). Clay has Vp/Vs ratio below 1.78 and it can be lower than 1.7. For anhydrites the Vp/Vs ratio is 1.75, and for limestones the Vp/Vs ratio is rather between 1.84 and 1.99.

Temperature data

Temperature data are available in wells to complete the geothermal characteristics of the Dinantien, but, except for Mons area in Belgium, are scarce in the Hauts-de-France region (Dezayes et al., 2024). Observations in mine galleries sometimes indicate anomalous values, probably due to water rising along faults (Licour, 2014). Only the Epinoy 1 well has usable data indicating a normal geothermal gradient (30°/km).

Figure 8

Socio-economic data in Hauts-de-France

Data allowing the assessment of the socio-economic potential of DGE comes from national and European database (de los Angeles Gonzalez de Lucio et al., 2022). These databases are open access form various institutes and detailed in Table 1.

Table 1

Factors to be considered during the assessment process of a geothermal energy project
Socio	Political orientation	Political parties map / Election maps
	Country Information	Density of population
		Income
		Employment / Forecast
		Social level map
	Heat demand
Economic	Infrastructure	District heating
	Investment	Official national geothermal development plan
		Promotion by the ministry/interest of municipalities/major of the city/strategy plan
		Level of debt of municipalities
	Regulation	Legal framework
		Financial risk management (funding/investment)
		Recommendations on legal framework and risk management
Environmental	Land access	Land ownership
		Assigned land usage
		Environmentally sensitive areas
	Greenhouse gas emissions

Databases used for socio-economic mapping of France. Sources in appendix.
Bases de données utilisées pour la cartographie socio-économique en France. Lien sources en annexe.

Results

NWE Dinantian maps

Based on pre‐existing national grids, a first version of the NWE transnational maps of the top and thickness of Dinantian formations was prepared by stitching together regional maps. This approach revealed multiple trans‐border issues which were evaluated and finally solved thanks to insights gained from the interpretation of newly acquired and reprocessed seismic lines and well data made available during the project. The result is harmonized maps of the top and the thickness of the Dinantian formations (Figure 9), the description of the maps and methodology is provided on DGE-Rollout website (see report of D1.1.5).

Figure 9

Although maps show important improvements from previous ones, large uncertainties about the depth and especially the thickness of the reservoir still exist due to the limited availability and quality of the data in some areas of the study area. These uncertainties were assessed (Veldkamp et al., 2023) and are illustrated below (Figure 10).

Figure 10

Dinantian maps in Hauts-de-France

Dinantian is widely present in the eastern part of the Hauts-de-France region, whereas it is observed only to a band about 20 km wide in the western part from Lens (Figure 11).

Overall, the Dinantian slopes southward at a shallow angle (5°). The northern limit corresponds to an erosion zone where the Dinantian gradually thins. Its southern extension corresponds to a structural limit. It is truncated by south-sloping faults bearing the Ardenne Allochthon (AMBT).

 Figure 11

Important thickness variations affect the series, which can reach 2700 metres in thickness ( Figure 11 10). These variations are associated with two phenomena:

• Either linked to tectonic overthrusting with thicknesses of up to 1200 metres. Indeed, the presence of thrust faults such as those to the west of Lens and to the south of the thrust faults located in the zone between Douai, Valenciennes and Cambrai allows the series to be duplicated locally.
• Or associated with significant local subsidence, as in the area west of Mons, but also south of Jeumont and south of Cambrai.

Heat In Place map

The HIP map has been computed based on Dinantian thickness maps and temperature maps, derived from borehole temperature collected in the investigation area and interpolated in volumetric 3D (Nelskamp  et  al., 2022).

The HIP is calculated as energy per unit area (Figure 12).

Unlike over the London Brabant Massif in Belgium where the aquifer does not occur, the large grey areas in the Netherlands are not related to its absence. However, the data quality in central part of the Netherlands is such that it was not possible to determine the thickness of the aquifer (Figure 12).

Several areas attract attention as having HIP around ~500 GJ/m², the highest heat content. However, these are not necessarily the most attractive areas for geothermal production because heat content alone does not say anything about the possibility that the subsurface energy can in fact be produced using usual extraction technique (i.e. enough permeability) or having economic return (i.e. drilling cost). For example, at the highest heat content in the south part of the Hauts-de-France (Figure 12), the top of Dinantian reaches 6 km depth, which brings a high temperature but deep and expective boreholes for exploitation. On the contrary, western part of Hauts-de-France have very low HIP in the Dinantian (less than 25 GJ/m²) due the thinness and shallowness of the layer (Figure 11). Except these zones, the HIP of most parts of the Hauts-de-France is between 200 and 400 GJ/m², which makes it an attractive area for deep geothermal energy development.

Figure 12

Socio-economic maps

The different quantifiable factors of the socio-economic potential for deep geothermal energy were combined into a joint index (see detail in the project report DT1.2.3 de Los Angeles Gonzalez de Lucio & Theil (2022)). The factors population density, social progress index, acceptance of renewable energies, availability of district heating, gross domestic product, public debt, environmentally sensitive areas and greenhouse gas emissions were considered to produce a map (Figure 12).

In the Hauts-de-France, except Lille area, the socio-economic index is low (around 20 000) in regards of the rest of the NWE (Figure 12). Other urban areas have a socio-index around 60 000, mainly due to the high heat demand (including the high interest demonstrated by the private and public sectors the last past years) and the presence of district heating (Figure 12).

Figure 13

Discussions

The combination of the Heat In Place derived from the subsurface data and the socio-economic maps allows the definition of Hotspots, showing the best places for the development of deep geothermal energy in the North-West Europe from the Dinantian formations (Figure 13). This result shows the important potential of this reservoir in the studied area (Figure 13).

Figure 14

In the Hauts-de-France region, we identify a large Hotspot between Douai and Valenciennes (Figure 14). This zone is characterized by a high population density, a high heat consumption and the presence of a district heating network. Form a resource point of view, the top of Dinantian lies between 2000m and 4000m depth with a temperature between 75°C and 125°C (Bonté et al., 2010) and a dolomite facies. A second Hotspot is located around Maubeuge, with rather the same characteristics, but the top of Dinantian is deeper, around 3500m. The areas around Liévin and Lens, and Arras are also potential Hotspots but the Dinantian facies could be less permeable.

Figure 15

Conclusions

The DGE-Rollout project, funded by EU Interreg NEW between 2018 and 2023, explores the most promising geothermal reservoir int the North-West Europe and allows develop and de-risk geothermal energy projects from the North-West Europe scale to the Hauts-de-France scale by providing:

• Identification of facies variations of Dinantian carbonates
• Better knowledge of Dinantian carbonates structures in depth, with top and thickness maps
• Petrophysical characteristics of the Dinantian carbonates
• Computation of Heat in Place map for the Dinantian geothermal reservoir
• Socio-economic index for better targeting geothermal project development
• And, finally, favorability maps, so called Hotspots map, to help stakeholders & investors of geothermal energy in North-West Europe

In the Hauts-de-France, the most favorable places for developing deep geothermal projects are located between Douai and Valenciennes, and the Maubeuge area, with high demands and district heating network available.

This study brings new data and knowledges to contribute for future deep geothermal development to play a significant role int the energy mix in a dynamic transnational region.

Pour aller plus loin…

https://www.nweurope.eu/dge-rollout

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PAPROTH E., DUSAR M., BLESS M. J. M., BOUCKAERT J., DELMER A., FAIRON-DEMARET M., HOULLEBERGHS E., LALOUX M., PIERART P., SOMERS Y., STREEL M., THOREZ J. and TRICOT J. (1983b).- Bio and Lithostratigraphic subdivisions of the silesian in Belgium, a review, Annales de la Société Géologique de Belgique, 106, 185-239.

POTY E. (2016)-. The Dinantian (Mississippian) Succession of Southern Belgium and Surrounding Areas: Stratigraphy Improvement and Inferred Climate Reconstruction. Geologica Belgica 19, n^o 12 (2016): 177200. https://doi.org/10.20341/gb.2016.014.

VELDKAMP H., ARNDT M., BROOTHAERS M., DEZAYES C., LACQUEMENT F., LAURENT A., VANBRABANT Y. & TEN VEEN J. (2023).- Thickness map of the Dinantian in North-West Europe. DGE Rollout Report D1.1.5. https://vb.nweurope.eu/media/21105/2023_08_dge_rollout_dt115_updated_transnational_harmonized_depth_and_thickness_map_of_deep_geothermal_potential_in_project_area.pdf.

VELDKAMP H. & FOEKEN J. (2023).- Uncertainties related to the depth mapping of the Dinantian aquifer. DGE Rollout report DLT 3.2. https://vb.nweurope.eu/media/21135/2023_06_dge_rollout_dlt32_reduce_uncertainties_of_geological_interpretation.pdf

Appendix

Résumé long en français

Dans le Nord-Ouest de l’Europe, bien que cette région ne présente pas d'anomalie thermique positive connue, les calcaires du Dinantien, situés au-delà de 1 000 m de profondeur, pourraient constituer un réservoir géothermique intéressant pour produire de la chaleur dans une région densément peuplée.

Dans les années 1970 et 1980, cette formation a été traversée par des forages scientifiques en Belgique. Des venues d’eaux chaudes à débits élevés au niveau du Dinantien dans le Hainaut belge ont permis de transformer ces forages en puits géothermiques productifs pour le chauffage urbain (Saint-Ghislain, Douvrain et Ghlin). Côté français, le forage profond pétrolier de Jeumont et le forage prospectif de géothermie de Condé-sur-l’Escaut, qui ont également traversé cette formation, n'étaient pas assez productifs et donc non économiquement viables. La formation du Dinantien possède donc des caractéristiques de réservoir fortement hétérogènes liées à l’histoire géologique complexe de la région.

Afin d'accroître la connaissance du Dinantien et d'en évaluer le potentiel géothermique, une étude de sa structure et de ses propriétés réservoir a été menée dans le cadre du projet européen INTERREG NWE DGE-Rollout (Déploiement de la géothermie profonde dans l’Europe du Nord-Ouest) sur la partie nord-est de l’Europe. En étroite collaboration avec les équipes belges, une carte transfrontalière de la structure du Dinantien a pu être réalisée, bénéficiant des résultats de la thèse d’A. Laurent (collaboration BRGM-Université de Lille) sur la structuration du front nord-varisque.

Une charte lithologique a été construite pour représenter les variations de la nature lithologique du Dinantien dans la région des Hauts-de-France et la Belgique, ainsi que les autres pays impliqués (Allemagne et Pays-Bas). Bien que dominée par des roches carbonatées, dans le détail le Dinantien présente des hétérogénéités lithologiques. Ainsi, au niveau du Viséen moyen (Livien), qui constitue le réservoir géothermique identifié dans la région de Mons en Belgique, deux niveaux de brèches sédimentaires sont présents avec une épaisseur très variable (quelques dizaines de mètres à plusieurs centaines de mètres).

Afin d’imager la structure de ce réservoir en profondeur, plusieurs lignes sismiques ont été acquises en Belgique, Pays-Bas et allemagne. Dans les Hauts-de-France, six lignes sismiques acquises en 1980 et 1981 entre Lens et Maubeuge (longueur totale de 172 km) ont été retraitées en amplitude préservée. En association avec les données des puits de Jeumont 1 et Epinoy 1, une interprétation quantitative a permis d’obtenir des informations sur les caractéristiques pétrophysiques du Dinantien.

Cinq faciès principaux sont identifiés par l'analyse pétrophysique : calcaire, dolomie, anhydrite, grès et argile. Dans le puits Epinoy 1, les calcaires du Dinantien sont situés entre 2 100 et 3 952 m de profondeur (fond du puits), en série inversée et appartiennent à une écaille tectonique. Le faciès dolomitique situé vers 2 640 m de profondeur montre une porosité très faible, estimée de l'ordre de 1 à 2 %. Pour le puits Jeumont 1, les calcaires du Dinantien sont situés entre 3 650 et 4 315 m de profondeur en série normale. À l’intérieur de la formation, l'analyse pétrophysique montre un faciès dolomitique entre 4 115 et 4 230 m de profondeur avec une porosité comprise entre 5 et 8 %.

La qualité des données et la faible porosité de la formation ne permettent pas d'établir une relation entre l'impédance acoustique et la porosité. Toutefois, une analyse plus poussée des données a permis d'établir une différenciation qualitative des faciès à partir du rapport Vp/Vs. Ce rapport est interprété en termes de variations de faciès et de valeurs de porosité en cohérence avec les variations lithologiques observées à l’affleurement.

Une corrélation fine entre les séries décrites à l’affleurement et les profils sismiques les plus proches a permis de différencier quatre faciès sédimentaires à l'intérieur du Dinantien, dont la cible géothermique, caractérisée par des évaporites et des brèches carbonatées (Livien). L’analyse détaillée des résultats montre que les intervalles dolomitiques sont plus présents dans les parties nord et est des profils sismiques. Comme les dolomies présentent des valeurs de porosité plus importantes, cette zone pourrait avoir de meilleures propriétés de réservoir que la partie sud-ouest.

Pour compléter les caractéristiques géothermiques du Dinantien, des données de température en puits sont disponibles, mais peu nombreuses dans les Hauts-de-France. Des observations dans les galeries de mines indiquent parfois des valeurs anomales probablement en raison de remontées d’eau le long de failles. Seul le puits Epinoy 1 dispose d'informations exploitables indiquant un gradient géothermique normal (30°/km).

Afin de déterminer les zones les plus favorables pour le développement de l’exploitation géothermique, les besoins en énergie ont été identifiés pour les différents secteurs d’activité, ainsi que les réseaux de chaleur urbains existants. Les zones les plus favorables sont définies par la combinaison du besoin d’énergie et de la disponibilité de la ressource géothermique (profondeur, épaisseur, propriété du réservoir du Dinantien). Dans les Hauts-de-France, deux zones sont identifiées, la plus importante en termes de superficie se situant entre Douai et Valenciennes. Cette zone est caractérisée par une forte densité de population, une forte consommation de chaleur et la présence d'un réseau de chauffage urbain. Du point de vue des ressources, le sommet du Dinantien, présentant un faciès dolomitique potentiellement plus poreux, se situe entre 2 000 et 4 000 M. de profondeur avec une température estimée entre 75 °C et 125 °C. Une seconde zone favorable est située autour de Maubeuge, avec sensiblement les mêmes caractéristiques, mais le toit du Dinantien est plus profond, vers 3 500 m.