Aiming to contribute to the energy transition, this study provides an integrated picture of the geothermal system hosted in the West Netherlands Basin and shows how the reconstruction of the basin's geological history can contribute to the correct exploration and exploitation of its geothermal resources. In the West Netherlands Basin, the main geothermal targets are found in the Cretaceous and Jurassic strata that were deposited during the rifting and post-rifting stages and were deformed during the subsequent basin inversion. Despite multiple studies on the tectonic setting, the timing and tectono-stratigraphic architecture of the rift system and its overall control on the development and evolution of geothermal systems are still to be fully deciphered. In this study, a detailed seismo-stratigraphic interpretation of the syn- and post-rift intervals in the West Netherlands Basin will be given within the framework of geothermal exploration. A recently released and reprocessed 3D seismic cube is used, covering a large portion of the onshore section of the basin. We identified two major Jurassic rifting episodes and a Late Cretaceous inversion event. During the Jurassic rifting phases, the compartmentalization of the basin and the creation of accommodation space led to the deposition of the Late Jurassic Nieuwerkerk Formation, which is the main regional geothermal producing target. Within this formation, we individuate growth synclines located in the central portions of the Jurassic half-grabens as sites that show good potential for geothermal exploration.@en

Aiming to contribute to the energy transition, this study provides an integrated picture of the geothermal system hosted in the West Netherlands Basin and shows how the reconstruction of the basin's geological history can contribute to the correct exploration and exploitation of its geothermal resources. In the West Netherlands Basin, the main geothermal targets are found in the Cretaceous and Jurassic strata that were deposited during the rifting and post-rifting stages and were deformed during the subsequent basin inversion. Despite multiple studies on the tectonic setting, the timing and tectono-stratigraphic architecture of the rift system and its overall control on the development and evolution of geothermal systems are still to be fully deciphered. In this study, a detailed seismo-stratigraphic interpretation of the syn- and post-rift intervals in the West Netherlands Basin will be given within the framework of geothermal exploration. A recently released and reprocessed 3D seismic cube is used, covering a large portion of the onshore section of the basin. We identified two major Jurassic rifting episodes and a Late Cretaceous inversion event. During the Jurassic rifting phases, the compartmentalization of the basin and the creation of accommodation space led to the deposition of the Late Jurassic Nieuwerkerk Formation, which is the main regional geothermal producing target. Within this formation, we individuate growth synclines located in the central portions of the Jurassic half-grabens as sites that show good potential for geothermal exploration.

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The West Netherlands Basin, which has a long history in exploration as a former prosperous hydrocarbon province, is currently a geothermal hotspot. Being exploited since the 1950’s, most of its oil and gas fields are now in their final phase of production. In the past decade, interest shifted to sustainable energy sources. The geothermal industry in the area is developing quickly, helped by the legacy of the hydrocarbon industry: a wealth of publicly available seismic and well data. Currently, the area has 14 realized, and at least 3 projects in the development phase, with the Late Jurassic Nieuwerkerk Formation being the main target.

Conversely to petroleum systems, in which anticlines are the preferential target for hydrocarbon exploration, synclines are the most suitable sites for geothermal exploration. They offer higher temperatures with respect to the limbs and anticlines, and possible remaining hydrocarbons are not expected to be located inside the central portions of the synclines.

The West Netherlands Basin is a former rift basin that developed during the Mesozoic in the framework of the North Sea rift, and subsequently inverted during the Late Cretaceous. The Nieuwerkerk Formation was deposited during the last major rifting phase. Thus, the thickest packages of its fluvial-deltaic deposits are fault-controlled and commonly located in the synclines. The heterogeneity of fluvial reservoirs causes lateral and vertical quality variations in porosity, permeability and net-to-gross ratios. With the hydrocarbon industry focussing on the stratigraphic highs, there is only limited well data available for the central portions of the synclines.

With reprocessed 3D seismic data, our study uses an image processing approach, coupling traditional amplitude mapping with seismic attributes. This will help to reconstruct the evolution of the fluvial architecture of the Nieuwerkerk Formation over time. By tying the seismic with well data, a better prediction of the quality of the sandy bodies per location can be made. These results can be implemented in de-risking geothermal well planning across fluvial reservoirs in inverted rift basins.@en

In rift basins, the spatial arrangement of extensional faults can influence the facies and the thickness distribution of the syn- and post-sedimentary infill, which can harbour good potential for geothermal systems. In this framework, unravelling the tectono-stratigraphic evolution of a rift basin is decisive, as it can influence one of the key parameters for planning geothermal doublets: aquifer thickness.

In our study, the West Netherlands Basin, located in one of the Netherlands most densely populated areas, is used as a case study. Up to 2022, 14 geothermal doublets were realized in the area, with the main target being the syn-rift deposits of the Late Jurassic Nieuwerkerk Formation. As a NW-SE oriented transtensional basin, the West Netherlands Basin developed as consequence of Mesozoic extensional tectonics, after which it became inverted during the Late Cretaceous and Cenozoic. Using publicly available seismic 3D and well data, our renewed interpretation of the study area shows two important rift events. The first one during the Early-Mid Jurassic and the second one, partly controlled by structures of the former, during the Late Jurassic, coinciding with the deposition of the Nieuwerkerk Formation.

Our study adds to the understanding of a multiple stage rifting history in the West Netherlands Basin. This is important, as the process influences reservoir thicknesses and with that, the amount of MW that can be extracted from geothermal aquifers. Therefore, this study forms a bridge between providing an integrated picture of the West Netherlands Basin and how the basins geological history affects its geothermal resources.@en

The positive impact that natural fractures can have on geothermal heat production from low-permeability reservoirs has become increasingly recognised and proven by subsurface case studies. In this study, we assess the potential impact of natural fractures on heat extraction from the tight Lower Buntsandstein Subgroup targeted by the recently drilled NLW-GT-01 well (West Netherlands Basin (WNB)). We integrate: (1) reservoir property characterisation using petrophysical analysis and geostatistical inversion, (2) image-log and core interpretation, (3) large-scale seismic fault extraction and characterisation, (4) Discrete Fracture Network (DFN) modelling and permeability upscaling, and (5) fluid-flow and temperature modelling. First, the results of the petrophysical analysis and geostatistical inversion indicate that the Volpriehausen has almost no intrinsic porosity or permeability in the rock volume surrounding the NLW-GT-01 well. The Detfurth and Hardegsen sandstones show better reservoir properties. Second, the image-log interpretation shows predominately NW-SE-orientated fractures, which are hydraulically conductive and show log-normal and negative-power-law behaviour for their length and aperture, respectively. Third, the faults extracted from the seismic data have four different orientations: NW-SE, N-S, NE-SW and E-W, with faults in proximity to the NLW-GT-01 having a similar strike to the observed fractures. Fourth, inspection of the reservoir-scale 2D DFNs, upscaled permeability models and fluid-flow/temperature simulations indicates that these potentially open natural fractures significantly enhance the effective permeability and heat production of the normally tight reservoir volume. However, our modelling results also show that when the natural fractures are closed, production values are negligible. Furthermore, because active well tests were not performed prior to the abandonment of the Triassic formations targeted by the NLW-GT-01, no conclusive data exist on whether the observed natural fractures are connected and hydraulically conductive under subsurface conditions. Therefore, based on the presented findings and remaining uncertainties, we propose that measures which can test the potential of fracture-enhanced permeability under subsurface conditions should become standard procedure in projects targeting deep and potentially fractured geothermal reservoirs.

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