Evaluating the geothermal reservoir potential often requires fracture analysis, as fractures serve as key pathways for fluid flow in subsurface formations. Borehole images (BHIs) are essential for this analysis, providing 2D representations of boreholes with millimetre-scale resolution. However, their interpretation is highly subjective, leading to uncertainties in the results and the subsequent quantitative assessment of the fracture networks. In the West Netherlands Basin (WNB), accurate fracture characterization is critical for assessing the geothermal viability. However, the traditional manual interpretation of BHIs has shown inconsistencies. This study introduces a supervised deep learning (DL) approach to support fracture analysis using high-resolution formation micro-imager (FMI) data from the Naaldwijk well (NLW-GT-01). The proposed DL-based system integrates a U-Net model (PickNet) for segmentation and a fully connected convolutional network (FitNet) for automated feature extraction. Initially trained on synthetic low-resolution BHIs, the model has been adapted for FMI data using two approaches: (1) transfer learning and (2) a simplified adaptation method that involves resizing the FMI input, leading to some resolution loss. A comparison of these approaches has revealed that the simplified adaptation produces better results, closely aligning with conservative manual interpretations calibrated with core samples while enabling more detailed fracture detection. To enhance reliability, we propose a semi-automated human–machine collaboration framework, where experts validate or refine the automatically detected features. This approach leverages human expertise to improve interpretation accuracy while addressing challenges related to robustness and redundancy in the supervised learning model.

The Main Buntsandstein Subgroup in the Roer Valley Graben in the southern Dutch subsurface is a sand-prone sedimentary interval deposited in a fluvial-aeolian environment, and is currently investigated for its suitability as target for low-entalpy geothermal exploration. The current depositional models in the Roer Valley Graben do not fully address the facies heterogeneities within and between Buntsandstein sedimentary units and their impact on the prediction of reservoir architecture. A detailed analysis of the Main Buntsandstein sedimentary facies heterogeneities to de-risk future sustainable energy operations is therefore crucial. In the present study, the sedimentology and lithostratigraphy of the Buntsandstein are assessed in a multidisciplinary analysis by use of a subsurface dataset composed of well cores, gamma-ray logs, and thin section data. The deposition of the Main Buntsandstein sediments in the Roer Valley Graben is dominated by different fluvial processes, with minor aeolian reworking. River planform style evolved through geological time from highly mobile and ephemeral to more perennial in nature. These changes in river style seem to be dictated by a decrease in climatic aridity along with a decrease in tectonic activity. The depositional processes resulted in the development of six lithofacies associations, developing three different types of reservoir architectures with their own set of heterogeneities at different spatial scales. Amalgamated, stacked sandstones have the highest net-to-gross (N/G) with a high degree of lateral and vertical connectivity, and the highest average porosity and permeability. Compensational-stacked sandstone reservoirs have a lower N/G and are the most heterogeneous due to the frequent occurrence of cemented intervals as well as mud drapes in the sandstone bodies. Marginal isolated sandstones show a well preserved relationship between reservoir properties and depositional facies, while more data are needed to resolve the spatial connectivity and lateral continuity of these sandstone bodies. The results of this study enhance the understanding of Lower Triassic reservoir architecture and sedimentary heterogeneities in the Roer Valley Graben that can be applied well beyond the area and provide a solid basis for future investigation of the relationship between sedimentary facies, diagenesis, and reservoir quality.

Sandstones from the Triassic Main Buntsandstein Subgroup represent a promising deep geothermal target in the subsurface of the Netherlands. These sands have a widespread distribution and temperatures that locally reach up to 140-150°C at depths of ~ 4 to 5 km. The Main Buntsandstein Subgroup is a sand-prone interval, but the reservoir quality of these sandstones is known to be heterogeneous as a result of the interplay between depositional and diagenetic processes. Recent drilling campaigns at depths greater than 4 km have confirmed the poor reservoir quality in certain areas. Nonetheless, successful geothermal projects have been realized in the southern Netherlands at shallower depths of 2 to 3 km, where temperatures of 70 to 90°C can meet local heating demands for greenhouses and district heating. To facilitate further geothermal exploration, it is important to understand the geological conditions that control reservoir quality and identify areas with favourable subsurface conditions. The Main Buntsandstein sediments have been extensively studied at a regional scale in the Netherlands. The factors controlling reservoir quality remain however poorly understood in the southern Netherlands, particularly in the southeastern region where data are scarce.

This thesis aims to improve the geological understanding of the Main Buntsandstein sediments by conducting an integrated geological study leveraging an extensive hydrocarbon dataset and newly acquired data from geothermal exploration in the southeastern part of the Netherlands. The study characterizes the structural, sedimentological, and diagenetic heterogeneities of the Main Buntsandstein Subgroup and evaluates their control on porosity, permeability, and, ultimately, geothermal potential. The first part of the thesis (Chapter 2) assesses the structural evolution of the Roer Valley Graben and the distribution of the Main Buntsandstein sediments through detailed seismic and well-data interpretation. A 2D palinspastic restoration was performed to evaluate the burial history and basin geometry during the Early to Middle Triassic. The analysis reveals that the central and southern parts of the Roer Valley Graben were active depocenters during the Early to Middle Triassic, while the northern part was a more marginal area where predominantly fine-grained sediments were deposited. After deposition, the sediments were significantly impacted by faulting, with burial depths reaching 4-5 km in the central graben, while the flanks experienced shallower burial of 2-3 km, making them promising targets for future geothermal investigations given the higher likelihood of preserved primary reservoir properties.

Next, in Chapter 3, the sedimentology and lithostratigraphy of the Main Buntsandstein are examined using a subsurface dataset of well cores, gamma-ray logs, and thin sections. The study identifies six lithofacies associations deposited through different fluvial processes with minor aeolian reworking. Overall, the different depositional processes are linked to tectonic and climate changes and led to the development of three distinct types of reservoir architectures, each with its own set of heterogeneities at different spatial scales. At the scale of the study area the heterolithic sediments deposited as result of playa-lake expansions can hamper the vertical connectivity of sandstone units given their confluency at km scale. Within the sandstone units, cemented zones or mud drapes are the most common fluid baffles.

Furthermore, most of the sandstone types do not preserve a primary relationship with reservoir properties because of post-depositional diagenetic processes. The diagenetic processes that control porosity and permeability in the Main Buntsandstein Subgroup were analyzed through detailed petrographic studies of available and a series of new thin sections (Chapter 4). The results show that illite, quartz, and dolomite are the dominant cements in these sandstones, with their influence on reservoir quality varying according to the sedimentary facies. In areas such as the Roer Valley Graben flanks, where the maximum burial history during Triassic and Early to Mid-Jurassic was largely shallower than 2 km, lower compaction and cementation rates favour the preservation of primary reservoir properties.

A structural study was conducted to analyse the distribution and characteristics of natural fractures and to investigate the mechanical stratigraphy of the Main Buntsandstein Subgroup (Chapter 5). This study used a dataset from the West Netherlands Basin as the Roer Valley Graben lacks the needed datasets. It revealed that natural fractures are favourably oriented with respect to the present-day in-situ stress, increasing the likelihood that these fractures are open. Fracture density was found to be higher in the heterolithic sedimentary successions, suggesting a link to the depositional environment and Main Buntsandstein Subgroup stratigraphy.

The geological insights gained from these studies were then employed to assess the geothermal potential of the Main Buntsandstein Subgroup in the Roer Valley Graben (Chapter 6). Porosity and permeability were evaluated, and calculations for Heat Initially in Place (HIIP) and Geothermal Power (GP) were made at well scale. A sensitivity analysis identified reservoir thickness and permeability as the parameters that most influence these calculations. The results are contextualized within the broader geological knowledge developed throughout the thesis, and three case studies corresponding to three types of potential geothermal plays are presented and discussed.

A conclusive synthesis is presented in Chapter 7, aimed at summarizing the main findings of this thesis and discussing how these results should be used in the future to reduce uncertainty and mitigate risks in geothermal exploration within the Main Buntsandstein of the southern Netherlands. To keep pace with the growing heat demand and the transition away from hydrocarbons as a primary energy source, the geothermal industry must make fast progresses. The Main Buntsandstein Subgroup has the potential to serve as a promising reservoir, particularly in regions like the northwestern Roer Valley Graben, where geological conditions suggest more favorable reservoir properties. Future exploration and production in these areas could play a crucial role in meeting the Netherlands sustainable energy targets.

Sandstones from the Main Buntsandstein Subgroup represent a promising deep geothermal target in the subsurface of the Netherlands considering their widespread distribution and temperatures locally reaching 140-150 °C at depths of ~ 3 to 5 km. The Main Buntsandstein Subgroup is a sand-prone interval, but the reservoir quality of these sandstones is known to be heterogeneous as result of an interplay between depositional and diagenetic processes. This makes the Buntsandstein sediments an uncertain and risky geothermal play.

In this project, we assess the syn- and post-depositional history of these sediments. The aim is to define structural, sedimentary, and diagenetic heterogeneities within the Main Buntsandstein sediments and assess their impact on reservoir quality. This will help reduce uncertainties for geothermal operations in the Triassic in the southern Netherlands and beyond.

The structural analysis of the study area using seismic and well data reveals that the Main Buntsandstein sediments represent an early syn-rift sequence and that their present-day distribution is strongly controlled by faulting. In parallel, the study of the sedimentology and stratigraphy conducted on core and wireline data indicates that the depositional environment evolves through the Buntsandstein stratigraphy, resulting in the development of different reservoir architectures. Diagenesis has largely altered the primary relationship between sedimentary facies and porosity and permeability. Overall cementation seems to have a larger impact on reducing reservoir quality than compaction, with quartz, dolomite, and illite representing the most abundant types of cement. The analysis of fractures using core and image logs suggests that the fracture density is driven by the lithological variability within the Main Buntsandstein and that fracture joints and stylolites locally may contribute to enhancing the system permeability.

The integrated assessment of the results allows the development of prospect play maps for the Buntsandstein in the southern Netherlands, addressing uncertainties and providing future recommendations for further exploration and optimizing geothermal operations in the Triassic.

The lower Triassic Main Buntsandstein Subgroup represents a promising, but high-risk geothermal play in the Netherlands. Although the gross thickness in boreholes locally exceeds 200 m, the spatial distribution, geometries and preservation of these sedimentary units remained uncertain due to the lack of seismic data with sufficient resolution and the sparse borehole network. This creates uncertainty in the quantification of the aquifer dimensions that is essential for the planning of geothermal operations.

In this study, seismic interpretation and 2D palinspastic restoration of new and reprocessed seismic data were conducted and combined with borehole data to assess the tectonic evolution of the Roer Valley Graben in the southeastern Netherlands and its control on the spatial distribution of the Main Buntsandstein Subgroup sediments. Our results show that the central and southern parts of the Roer Valley Graben were active depocenters in the Early to Middle Triassic times dominated by fluvial sandstone deposition, providing important play elements for prospective leads on geothermal exploration. The northern part of the basin was a more marginal area where mostly fine-grained sediments were deposited. To the northwest, differential subsidence resulted in the development of areas where the Buntsandstein thickness is reduced to ∼150 m.

After deposition, the Main Buntsandstein sediments were compartmentalised by faulting related to post-depositional tectonic activity, locally reducing the lateral extent of the geothermal target areas down to 1–2 km in a ∼NE–SW direction. On the platform areas adjacent to the Roer Valley Graben and to the southeast, Jurassic sediments are largely absent and the Main Buntsandstein sediments are present at depths shallower than 2 km. These platforms are promising targets for further investigation, as the relatively shallow burial depths, compared to the central part of the Graben, may have contributed to the preservation of more favourable reservoir properties.

The Lower Triassic Main Buntsandstein Subgroup represents one of the most promising deep geothermal plays in the Netherlands. Reservoir zones consist of sandstone units deposited in an arid to semi-arid alluvial plain reaching total thicknesses of over 200 m in the Roer Valley Graben. However, the lack of a comprehensive reservoir model integrating stratigraphy, sedimentology, and petrography makes the Buntsandstein a high-risk target. Here, we present a reservoir architecture model of the spatial and temporal variation of sedimentary facies and inherent permeability barriers and baffles, based on the integration of data from 33 wells from different stratigraphic levels and different areas of the Roer Valley Graben.

Core samples and thin sections analysis revealed two major reservoir facies that were interpreted as the products of transport and depositional processes in braided and sinuous river settings. The identified facies were then coupled to wireline logs to assess the spatial and temporal variation in sedimentary architecture. The lower part of the stratigraphy is dominated by braided river reservoir facies with a high degree of connectivity, where regional lacustrine-playa lake sediments represent the main potential permeability barriers. By contrast, the upper part of the stratigraphy is characterized by an increase in the proportion of sinuous river complexes. These latter yield a lower degree of connectivity with different types of baffles such as intercalated fine-grained overbank sediments, abandonment plugs, bar-draping fines, and cemented dolocrete scour fills. These are much more localized compared to the braided complexes-related barriers, making the prediction of the upper stratigraphy architecture uncertain.

In this study, a re-evaluation is performed of the well data of the NLW-GT-01 and VAL-01 wells in the Lower Triassic sandstones in the West Netherlands Basin. Core, geophysical and image logs, are compared to document the characteristics of natural fractures distribution, and investigate the geological parameters influencing their development. The main control on the distribution is identified. Natural fractures are concentrated in heterolithic lithological intervals of the VAL-01 and NLW-GT-01 wells. The identification of more fractures in VAL-01 compared to NLW-GT-01 can be explained by the difference in basin location. VAL-01 was located in the centre of the basin where distal playa environments produced fine-grained material alternating with coarser sands. The more proximal NLW-GT-01 was dominated by fluvial sands. The lithological variability produces Young’s modulus variability that seems to be driving increased fracture density rather than that the absolute value of the Young’s modulus. These fluctuations could be the result of concentrated compressional stress, which is supported by the existence of stylolites, indicative of high compressional stresses in the same intervals. The identification of controls on fracture distribution in the targeted stratigraphic interval can be used to optimize the planning of future geothermal doublets and to de-risk upcoming operations.

The Buntsandstein subgroup in the southeastern part of the Netherlands represents one of the most promising, but risky, geothermal plays. To understand the main controls on Buntsandstein reservoir quality, we combine petrophysical (porosity and permeability) and petrographic (point counting) data derived from different wells and different depth levels. Results show that porosity ranges from 2 to 18.5 and permeability from 0.001 to 285 mD. Dolomite represents the most abundant cement and show an inverse correlation with porosity. Illite occurs in higher concentrations in samples with values of permeability below 20 mD, while kaolinite becomes the most dominant phyllosilicate cement in samples with higher permeability. By looking at the main cement distribution over the sedimentary facies, it appears that dolomite is strongly related to depositional facies and has a positive correlation with grain size, while illite and kaolinite yield a negative correlation with grain size. Pedogenic dolomite nodules are often reworked as detrital grain into the channel scour deposits and are the main source for dolomite cementation. The current study has shown how diagenesis makes Buntsandstein reservoir complex and heterogeneous, and how reservoir quality is strongly related to the depositional environment.

A combined study of depositional facies and diagenesis variation was carried out to understand the main controls on aquifer quality of the Middle Buntsandstein in the southeastern part of the Netherlands. Heterogeneities in continental sandstone bodies occur at different spatial scales, ranging from micrometers to hundreds of meters. Commonly, such heterogeneities result from the interaction of depositional processes at various spatial and time scales. These processes partially also influence subsequent diagenetic evolution, hence present-day aquifer properties. Understanding the role of the resulting architectural heterogeneities in controlling the dynamic reservoir behavior is key in determining aquifer properties and improving pre-drilling prediction. The sandstones of the Main Buntsandstein subgroup in the southeastern part of the Netherlands provide an excellent example where different detrital compositions, internal sedimentary architectures, and diverse burial histories have resulted in a wide range of present-day aquifer properties. In the study area, the aquifers are composed of stacked heterogenous alluvial sandstones bodies intercalated with mud-prone intervals deposited in arid to semi-arid conditions. Differences in sediment sources, transport mechanisms, and intrabasinal conditions resulted in a wide distribution of composition and texture. Additionally, the effect of post-depositional burial diagenesis in a basin with complex tectonic history created diverse burial histories across the basin. The study aims to investigate the variation of present-day aquifer hydraulic parameters about changes in aquifer facies and architecture, detrital composition, as well as compaction and cementation during burial. Core sample analysis unfolded a diverse spectrum of sedimentary facies and lithic fragments, which differ between formations. Thin section analysis provides insights about mechanical compaction, cementations, and authigenic phases. By combining these results with petrophysical data on permeability and porosity of core samples, the major controls on present-day aquifer quality can be assessed.