Geïntegreerde analyse van geologische factoren die de reservoirkwaliteit van Middle Buntsandstein-zandstenen beïnvloeden: inzichten uit ondergrondgegevens (Zuid-Nederland) en outcrop-analogieën in Elzas (Frankrijk) en Trier (Duitsland)
Year: 2025
Type: dissertation
Abstract: The Lower Triassic Middle Buntsandstein Formation, widespread across Northwest European Basins, offers significant reservoir potential for groundwater, hydrocarbons and increasingly for geothermal energy as well as CO2 and hydrogen (H2) storage. Despite decades of subsurface exploration, understanding and predicting reservoir quality in the Middle Buntsandstein sandstones remains a major challenge. These fine- to coarse-grained sandstones are lithologically heterogeneous because of complex fluvial-aeolian depositional processes and multi-phase diagenetic overprinting. In particular, porosity and permeability vary significantly within the same stratigraphic units due to grain-scale variability, facies distributions and diagenetic cementation. Furthermore, the extent to which well-exposed outcrops from the Alsace (France) and Trier (Germany) regions are representative of depositional, diagenetic and petrophysical conditions for subsurface equivalents is still poorly constrained. This study investigates the geological controls on reservoir quality of the Middle Buntsandstein by integrating sedimentological, petrographical, geochemical and petrophysical data of subsurface cored intervals from the Broad Fourteens and West Netherlands Basins (southern Netherlands) and nearby outcrop sections from the Alsace and Trier areas of the Germanic Basin. The research goal is to evaluate how depositional facies, diagenetic transformations, and fluid-rock interactions influence porosity and permeability at scales ranging from the microscopic pore network to the centimeter-scale core plug (~2.5 cm), and how outcrop analogues compare with buried reservoirs in predicting subsurface quality. A total of 107 core plugs and 80 thin sections were examined from subsurface and outcrop sections, with supplementary data from 122 additional samples obtained from the publicly accessible well database (https://www.nlog.nl/). A multi-method approach was employed by combining core description, thin section petrography, scanning electron microscopy (SEM), cathodoluminescence (CL), X-ray diffraction (XRD), stable isotope analyses (δ13C and δ18O), with quantitative porosity and permeability measurements including helium porosity, air and gas permeability, nuclear magnetic resonance (NMR), and mercury intrusion capillary pressure (MICP). Well-to-well stratigraphic correlation was performed across 116 wells using Petrel software, focusing on the West Netherlands Basin. The studied Middle Buntsandstein sandstone comprises complex lithofacies associations, including braided to shallow ephemeral fluvial channels, sheetflood sands, distal floodplains, sandy desert lake margins, playa lakes, damp aeolian sandflats, dry aeolian sandflats and aeolian dunes. Lithostratigraphic correlation and mapping of geobodies reveal that their spatial distribution is influenced by local topography and the distance from source areas. Porosity and permeability range widely, from < 5% and < 0.01mD in fine-grained compacted sandstones to > 22% and > 1000 mD in coarse-grained clean sandstones. Subsurface plug-scale results from Well-X show porosity from 6.7% to 13.0%, with corresponding permeability varying between 0.02 and 14.4 mD, reflecting the strong influence of grain size, dolomite cements and burial compaction. Stable isotope analysis of dolomite cements from Well-X yielded δ18O values between -4.54 to -2.97 ‰ VPDB and δ13C values between +1.95 to +0.59 ‰ VPDB, which are consistent with early reflux dolomitization from evaporated marine or sabkha-type brines. In contrast, the isotopic signatures of dolomite cements from the surrounding wells are more depleted both in δ18O and δ13C values and suggest a source from meteoric waters containing atmospheric or soil-derived CO2. One of the key diagenetic phenomena observed in Middle Buntsandstein sandstones is bleaching. Bleaching is characterized by the loss of the inherent reddish coloration in these sandstones. Bleaching features were systematically categorized into three genetic types based on field relationships and petrographic context, i.e. (i) Laminae-bound bleached sandstones (LBBS), which follows the sedimentary bedding and reflects meteoric fluid infiltration during shallow burial, (ii) Patchy bleached sandstones (PBS), interpreted as localized alteration of clay clasts, and (iii) Stratiform bleached sandstones (SBS), likely associated with fault-controlled CO2-rich fluid migration. Furthermore, discontinuous bleached sandstone (DBS) is present along fractures and fault zones. The latter, however, was not incorporated in this study. The majority of subsurface cored sections also reveal widespread bleaching phenomena in the southern Netherlands. Petrophysical analysis of bleached and unbleached (red) sandstones reveals a systematic difference in pore structure. Bleached sandstones, particularly those of the stratiform (SBS) type with larger grain size (average grain size >150 µm), show higher porosity (up to 22%) and good pore connectivity. In contrast, the red sandstones, where Fe-oxide/hydroxide rims are preserved, exhibit poor pore connectivity despite similar total porosity values (up to 24%). The LBBS and SBS samples, which display a fine-grained composition (average grain size <150 µm) exhibit low porosity and permeability. Comparison of outcrop and subsurface data shows that sedimentary architecture and detrital compositions are broadly comparable. However, the diagenetic histories diverge significantly because of a difference in burial depth, interaction with basinal fluids and tectonic overprinting. The early diagenetic processes, predominantly influenced by depositional fabrics and surface climate conditions, exhibit analogous features in both outcrop samples and their subsurface equivalents. The presence of mesodiagenetic cements and overgrowths in subsurface strata suggests a higher thermal exposure than that observed in outcrop samples. In addition, telodiagenetic modifications, including dissolution of carbonates and the precipitation of Fe-oxides/hydroxides and Mn-oxides, caused a significant distinction between outcrop and subsurface samples. Secondary porosity development in outcrop sections related to telodiagenetic dissolution processes would lead to overestimated reservoir properties if these would be used as direct analogues without correction. Overall, this study shows that reservoir quality in the Middle Buntsandstein is governed by the interplay between depositional facies, early diagenetic cementation, and post-depositional fluid-rock interactions, with significant implications for pore connectivity and storage potential. By combining petrography, geochemistry, and advanced pore network characterization (e.g., MICP, NMR, µCT), this research offers a quantitative framework for evaluating heterogeneity in these fluvial-aeolian Triassic red-bed sandstones. These insights can be applied directly in the exploration and screening of red-bed sandstone reservoirs for hydrocarbon exploration, geothermal energy, CO₂ sequestration, and hydrogen storage.
Source: Lirias (KU Leuven)
Author: Yousaf, Husnain
Language: English
Subfield: Mechanics of Materials
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7112870744
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