European Geologist Journal 59

Bleaching Mechanisms and Reservoir Quality in Buntsandstein Sandstones: Limitations of Outcrop Analogue Use

 

by Husnain Yousaf 1, Hannes Claes 1, Gert Jan Weltje 1, Jean-Marie Mengus 2, Remy Deschamps 2, Fadi Henri Nader 2,3, Rudy Swennen 1

1 Department of Earth and Environmental Science, KU Leuven, Belgium.

2 Division of Earth Sciences and Environmental Technologies, IFP Energies Nouvelles, France.

3 Department of Earth Sciences, University of Utrecht, the Netherlands.

Contact: Husnain.yousaf@kuleuven.be

Abstract

Understanding and predicting heterogeneous reservoir properties of Lower Triassic sandstones is crucial for sustainable resource management. This study investigates the petrographical and petrophysical characteristics of red and bleached sandstones to address bleaching mechanisms and their implications on reservoir quality in the context of gas storage and geothermal exploration. Fieldwork and laboratory observations reveal three distinct bleaching patterns, i.e., stratiform bleached sandstones (SBS), lamina-bound bleached sandstones (LBBS), and patchy bleached sandstones (PBS). Each represents very different diagenetic processes across geological timescales. Diagenetic cement dissolution is much more pronounced in outcrop samples than in their subsurface equivalents. Multiple analytical techniques provide a comprehensive understanding of rock and pore types, and overall pore network architecture, enabling reservoir characterisation from the microscale to the macroscale. While bleaching processes intrinsically improve porosity, the associated (silica) overgrowths negatively impact pore interconnectivity. Telodiagenetic dissolution significantly enhances the petrophysical properties in outcrop samples. Therefore, it is essential to correct these modifications for the accurate prediction of subsurface reservoir properties. By improving the reliability of subsurface reservoir assessment, these findings can support geothermal energy development and underground gas storage in Buntsandstein reservoirs and contribute to the advancement of the UN Sustainable Development Goals.

Cite as: Yousaf, H., Claes, H., Weltje, G. J., Mengus, J.-M., Deschamps, R., Nader, F. H., & Swennen, R. (2025). Bleaching Mechanisms and Reservoir Quality in Buntsandstein Sandstones: Limitations of Outcrop Analogue Use. European Geologist, 59. https://doi.org/10.5281/zenodo.16443062

Creative Commons License This work is licensed under a Creative Commons Attribution 4.0 International License.

1. Introduction

The term Buntsandstein (translated from German as colourful sandstone) was introduced by German geologist Abraham Gottlob Werner in the late 18th century [1]. The red colours are because of the precipitation of iron-oxides/hydroxides into detrital clay rims during a shallow burial [2-12]. These colourful sandstone facies have been valuable sources for building stones since (pre-) Roman times [13,14]. Porous reservoir lithologies in the Buntsandstein serve as groundwater aquifers [15-18] and hydrocarbon reservoirs [19,20]. In recent decades, interest in the Buntsandstein lithologies has been increasing given their potential as (shallow-deep) geothermal reservoirs [7,8,12,21-26] and potential for underground CO2/H2 storage [27-29]. Over time, reducing fluids (such as hydrocarbon, methane, organic acids, hydrogen sulphide, acidic meteoric water, etc.) at low pH and elevated salinities, may lead to changes in colour and affect reservoir properties [2,4,6,7-11]. As a result, the removal of the iron oxides/hydroxides coating process gives rise to more pale colours and is often referred to as “bleaching” [2,4,6,7,9-11,30-33].

In order to de-risk underground applications, reservoir quality assessment of these continental fluvial-aeolian deposits is essential but remains quite challenging because of their vertical and horizontal heterogeneous nature. These heterogeneous properties are often attributed to variations in depositional settings [34-36], and diagenesis [2-4,6-12,37-39], as well as structural features, such as presence/absence of fractures and faults [3,11,40,41]. The growing emphasis on sustainable energy requires a more thorough qualitative and quantitative characterisation of the Lower Triassic sandstones from a sedimentological, diagenetic, and structural point of view [2,5-9,11,12,22,25]. Subsurface reservoir characterisation commonly utilises data from cores, sidewall cores, boreholes, wells, and seismic surveys. Understanding lateral and vertical variations in reservoir architecture remains challenging due to the limited availability of data at the intermediate scale. Analogue outcrop studies can effectively augment limited subsurface data by providing measurements of structural dimensions, shapes, orientations, and overall geo-body architecture, all essential for reservoir evaluation [11,42–46].

An appropriate analogue should have a similar tectonic setting, geological age, subsidence rates and sedimentary environments [47,48]. The Buntsandstein outcrop sections in the border area between France and Germany, apart from their uplift modifications, seem to fulfil these criteria for the Lower Triassic sandstones of the Germanic Basin, and hence are the focus of this study. Based on a multi-methodological petrographical and petrophysical approach, seven Buntsandstein outcrops (e.g., Haut-Barr, Kronthal, Soultzerkopf, Loegel, Adamswiller, Petersbach, and Barrois) along the western graben shoulder of the Upper Rhine Graben (URG), Vosges area in NE France and two (e.g., Kordel and Butzweiler) in the Trier area in SW Germany are not only critically assessed for reservoir analogue suitability, but, in addition, help to delineate depositional and diagenetic reservoir controlling parameters. Particular focus is given to one of the most discussed reservoir enhancing processes in the literature, i.e., bleaching. We present a process-based understanding of the depositional and diagenetic heterogeneities on newly acquired petrographic and petrophysical data, with new insights for future exploration of reservoirs, to support the sustainable energy development goals in the region.

2. Geological setting

The Vosges and Trier areas offer 200-600 m thick outcrop sections of the Lower Triassic Buntsandstein deposits (Fig. 1A) [35,50]. They make an ideal accessible setting to investigate the geological controls on lithofacies, bleaching and porosity evolution inside Buntsandstein lithologies. During Permian-Triassic times, the study area was surrounded by the Variscan-Appalachian Mountain Belt in the south, the Fennoscandia Mountains in the north, and the Gallic-Armorican and London Brabant Massifs in the west [35,51-55]. During the early Triassic, the evolution of these intracratonic basins was dominated by thermal subsidence and/or uplift, resulting in variations in lithofacies and the thickness of lithostratigraphic units [55–61].

Buntsandstein outcrop sections near URG are composed of very fine- to large-grained clastic fluvial successions with intercalations of lacustrine and aeolian sediments (Fig. 1B) [35,58-63]. Early Triassic Induan is missing from the study area and Middle Buntsandstein (Olenekian) deposits unconformably overlie Permian sediments [54]. Middle Buntsandstein is predominantly composed of braided fluvial deposits from the Conglomerate basal, Grès Vosgien, and Conglomerate principal Formations, inter-fingering with aeolian and reworked deposits. Top of the Middle Buntsandstein is marked by the Zone Limite Violette (ZLV), where it is present. The Conglomerate Principal Formation is truncated northward by a major tectonic (Hardegsen) unconformity, which locally cuts through the Grès Vosgien Formation. The Anisian Couches intermediaries Formation unconformably overly the Hardegsen Unconformity. Finally, the upper part of Lower Triassic succession, the Grès à Voltzia, influenced by the marine transgression in some parts of the Germanic Basin [34,54,60,64].

Figure 1: (A) Geological map of the study area, in NE France (Vosges) and SW Germany (Trier) (source: infoterre). White circles showing the location of studied outcrop sections. 1 = Kronthal, 2 = Haut-Barr, 3 = Rauscher quarry Adamswiller, 4 = Rauscher quarry Petersbach, 5 = Loegel quarry in Rothbach, 6 = Soultzerkopf quarry, 7 = Barrois quarry, 8 = Kordel and 9 = Butzweiler. Red stars showing the location of pervasive bleached outcrop sections studied by Soyk (2015) and Busch et al. (2022). Dashed red box area of the studied wells by [11] and Soultz‑sous‑Forêts, a geothermal site studied by [25]. (B) The chronostratigraphic and lithostratigraphic column of the study area, modified from [54].

3. Material and methods

Samples from outcrops at Kronthal (48°37’48.92″ N, 7°28’15.78″ E), the Loegel quarry in Rothbach (48°54’51.17″N, 7°30’53.51″E), the Soultzerkopf quarry (48°58’39.3″ N, 7°49’35.9″ E), the Barrois quarry (49° 9’54.06″N, 6°46’29.91″E) and the Kordel (49°50’30.90″N, 6°37’51.34″E) are classified as Middle Buntsandstein Grès Vosgien [34,54,64]. The outcrop sections vary in colour from red to reddish-white to greyish-white sandstones. Samples collected from the Haut-Barr (48°43’27.06″N, 7°20’16.71″E) are part of Conglomerate Principal Formation. It comprises red to greyish-white sandstones and conglomerates [e.g., 34]. The Rauscher quarry in Adamswiller (48°54’37.04″N, 7°12’26.63″E), the Rauscher quarry in Petersbach (48°53’11.54″N, 7°16’3.75″E) and the Butzweiler outcrops (49°49’31.31″N, 6°26’7.53″E) are attributed to the Upper Buntsandstein Grès à Voltzia. They comprise red, brown, and local grey-whitish sandstones. Sixty samples were carefully selected to cover all sedimentary textures and bleaching patterns observed in the nine natural outcrop exposures.

These samples are classified as red sandstones (RS, n=22), stratiform bleached sandstones (SBS, n=16), lamina-bound bleached sandstones (LBBS, n=11), and patchy bleached sandstones (PBS, n=11). Red sandstone (RS) samples serve as a reference for comparison in petrographic and petrophysical changes compared with their bleached equivalents. Cylindrical plugs (2.54 cm diameter) were drilled (35 vertical and 25 parallel to the bedding plane) for textural observation and various analytical measurements (e.g., water saturation (WS), Helium porosity (HP), nitrogen gas permeability (GP), air permeability (AP), nuclear magnetic resonance (NMR). To compare results across complementary techniques, the same samples were used for the different analyses. Sub-samples (1.8 cm in diameter), selected to represent each red and bleached lithofacies, were used for 3D X-ray micro-computer tomography (X-µCT) imaging and Mercury Injection Capillary Pressure (MICP) analysis.

The trimmed ends of prepared cylindrical plug samples for petrophysical analyses were impregnated with blue dyed epoxy resin to aid identification of porosity and prepared as 30 μm thin sections for petrographic analyses. Thin sections were stained with Alizarin red-S and potassium ferricyanide to distinguish the carbonates [65]. Samples were analysed with a conventional optical microscope (Olympus BX60 equipped with a Zeiss Axiocam 305 colour digital microscopic camera) and scanning electron microscope (SEM-BSE, EDS; TESCAN MIRA Field Emission Microscope) to assess the modal composition, texture, grain size, sorting and optical porosity. To conduct the SEM analysis, polished thin sections were coated with carbon using a Quorum Q150T ES Plus. A Nikon Optiphot with a modified Technosyn Model 8200 MkII stage microscope Cold Cathodoluminescence (CCL) was used to identify carbonate cement phases and to assess detrital quartz and feldspar [66].

Mineral constituents and optical porosity were quantified based on 1000 points counted per thin section [67] by the use of JMicroVision software [68]. Folk’s QFR diagram [69] was used to classify the analysed samples. Average grain sizes (AGS) were determined from scanned thin sections using a grid adjusted to the maximum observed grain size. Measurements were conducted on a minimum of 100 grains per sample to ensure statistical reliability [70]. Intergranular volume (IGV) was calculated based on point counting results, according to Paxton et al. [71]. Compaction porosity loss (COPL) and cementation porosity loss (CEPL) were derived based on the IGV, according to Lundegard [72]. Initial porosity of the initially unconsolidated sand was assumed to be 45% according to Wilson & McBride [73].

Field-based AP measurements were performed on hand specimens and outcrop walls using a Tinyperm 3 device [74]. The GP and HP were measured by conventional core analysis at Panterra Geoconsultants B.V., Leiderdorp, the Netherlands. Water-saturation porosity experiments were conducted on all plugs at KU Leuven. Samples were dried at 60°C until changes in weight were less than 0.1%. Samples were water-saturated under vacuum conditions.

The NMR transverse relaxation time of the studied sandstone samples, each ~2.54 cm in diameter and ~3.5 cm in length, was measured using an in-house constructed fixed-field (0.196 T; 8.33 MHz) Halbach-based rock core analyser at the Sakellariou NMR lab of the KU Leuven. The device was calibrated with known quantities of water and brine (CuSO4) solutions, as well as various plugs. The NMR T2 distributions were retrieved through the Carr-Purcell-Meiboom-Gill (CPMG) measurement method. The basics of NMR and pore size characterisation are given in [75-79].

X-µCT was employed to characterise the pore network of selected mini plugs using a microfocus TESCAN UniTOM XL at the KU Leuven X-CT core facility [79,80]. Scans were conducted at 140 kV X-ray source voltage, 280 µA current, and an isotropic voxel resolution of ~4 µm. After smoothing, filtering, and segmentation of the X-µCT images, the pore network was analysed by using Avizo 2019.1 (Thermo Fisher Scientific, Hillsboro, Oregon, USA). It is important to highlight the implications of the 4 µm X-µCT resolution. When comparing to NMR and MICP data, pores and connectivity below the resolution are not resolved, resulting in underestimated reservoir quality. MICP was used to determine pore throat radii distributions on mini plugs using a Micromeritics Autopore IV instrument. Pore throat radii were calculated from mercury capillary pressure curves using the Young-Laplace equation [81]. The MICP pore throat size distributions are complementary data to the pore body size distributions from NMR [79].

4. Results

Macroscopic observations

Bleaching patterns were initially not always clear on weathered outcrop sections and required closer examination of the outcrop walls and hand samples. These bleaching patterns dominantly occur in the sandy reservoir rocks, hence adjacent silt- and claystones were not further examined as part of this study. Sandstones are categorised by bleaching type in the following paragraphs to better understand bleaching processes and their relation to petrophysical properties.

In the studied Buntsandstein outcrop successions, four distinct types of bleached lithofacies were identified (Fig. 2A-F), namely:

  • Lamina-bound bleached sandstones (LBBS) exhibit two patterns, which are mainly observed in the Kronthal and Loegel quarry. These bleached laminae predominantly comprise top-set of fluvial deposit (Fig. 2E).
  • Patchy bleached sandstones (PBS) are characterised by mm-cm sized isolated white spots within red bed sandstones (Fig. 2F). This type of bleaching is relatively uncommon and has only been documented in a few outcrop sections (e.g., Loegel quarry).
  • Stratiform bleached sandstones (SBS) typically extending along bedding planes and sometimes affecting the entire formation (Figs. 2B, C, D). SBS can be observed in cm to m thick beds (e.g., Haut-Barr, Loegel quarry and Butzweiler) to complete outcrop sections (e.g., Soultzerkopf and Barrois).
  • Discontinuous bleached sandstone (DBS) is present along with fractures and fault zones. The latter, however, were not studied.

 

Figure 2: Macroscopic appearance of unbleached red sandstone (A), stratiform bleached sandstones (B and C), lamina-bound bleached sandstones: bleached lamina along the bedding planes (D), bleached lamina parallel to sedimentary structures such as cross-bedding and erosive boundaries (E), patchy bleached sandstones (F) and locally observed red sandstone with concretions (Wadflecken) (G).

Petrography

The average grain size of the RS (120 to 283 μm) is not significantly different from the SBS (93 to 282 μm) and PBS (180 to 225 μm), while they are significantly larger than the LBBS (from 80 to 139 μm) and exhibit a Kruskal-Wallis p-value of 5.9e-05 (Fig. 3A; Supplementary Data). All examined samples range from moderately well to well sorted with subrounded to subangular grains.

The modal mineralogical composition and microscopic image of each type of sandstone are given in Supplementary Data, figs. 3 and 4, respectively. In all examined samples, the most prominent detrital components are quartz grains (including mono- and polycrystalline quartz) with a significantly different content (p = 0.0046) particularly when comparing red and bleached samples (Figs. 3B and 4). Total quartz content in the RS samples (ranges from 41 to 69%) is significantly lower than the PBS (59 to 69%), LBBS (54 to 67%) and SBS (53 to 66%). No significant difference is observed between the SBS and LBBS, nor between the LBBS and PBS. The quartz content in SBS is significantly less than that observed in PBS (Fig. 3B). Quartz grains exhibit light blue or dark violet luminescence (Fig. 5A). K-feldspars are prominent in both red and bleached samples, and do not exhibit significant variation (p = 0.46; Fig. 3C). K-feldspar dissolution is common (Figs. 4A, D, G, J) and partially dissolved K-feldspar exhibits blotchy blue luminescence under CL (Fig. 5A). Rock Fragments comprise sedimentary, plutonic and/or metamorphic rocks (Figs. 4A, G), and display a statistically significant difference (p = 0.0057) between red and bleached samples, while bleached sample series exhibit an insignificant difference between each other (Fig. 3D). Mica is rare (<1%) in all sample series.

The RS samples are plotted as sub-arkoses to arkoses; bleached lithofacies, however, are classified as sub-arkoses to lithic arkoses (Fig. 6A). Most of the analysed sandstones form a closely grouped cluster, indicating a strong correlation between outcrop sections and across stratigraphic units, as summarised in Supplementary Data.

Figure 3: Violin plots comparing different lithofacies based on the Average Grain Sizes (A) and major compositional parameters: Quartz (B), Feldspar (C), Rock Fragments (D), Fe-Oxide (E) and Mn-Oxide (F). Kruskal-Wallis testing was used to determine statistical significance (p ≤ 0.05 for significance, p > 0.05 for non-significance), denoted by ‘s’ and ‘ns’, respectively. RS = Red Sandstones, SBS = Stratiform Bleached Sandstones, LBBS = Lamina-Bound Bleached Sandstones, and PBS = Patchy Bleached Sandstones samples.

 

Figure 4: Overview of mineral assemblage in red and bleached sandstone samples. Thin section microphotographs under plane polarised light (PPL) and cross polarised light (XPL). Blue resin showing the pore space (SP). (A, B and C) RS samples predominantly composed of detrital quartz (Qz) with typical Fe-oxide/hydroxide coatings around the grains. Quartz overgrowth (black arrows) is identifiable by Fe-oxide/hydroxide rims around the detrital quartz. K-feldspar (Fsp) is the secondary dominant mineral. Secondary porosity (SP) developed because of the dissolution of cements and grains. Dark red Fe-oxide/hydroxide (FeOx) (green arrows) precipitated in the pore spaces. Dark brownish Mn-oxide (MnOx) (orange arrows) are also present in the pores. The RS samples comprise fine-grained quartz (Qz) and K-feldspar (Fsp) with dark brownish Fe-oxide/hydroxide rims (FeOx) around the grains and in the pores. Medium to large-grained sandstone exhibit floating to point grain-to-grain contact in contrast to fine-grained sandstones.

(D, E and F) SBS samples showing lack of Fe-oxide/hydroxide rims around the detrital quartz (Qz) and K-feldspar (Fsp) grains. Partial dissolution of K-feldspar (Fsp) is clear, with oversized pores corresponding to secondary porosity (SP). SBS with fine-grained display no dark or brownish Fe-oxide/hydroxide coating around the detrital grains. Medium to large-grained sandstone exhibit floating-to-point grain contact in contrast to fine-grained sandstones, which predominantly exhibit long to concave-convex grain contacts.

(G, H, I) LBBS predominantly consists of (very) fine-grained sandstones with tight packing and low porosity (SP). The sandstone predominantly comprises quartz (Qz), K-Feldspar (Fsp) and deformed rock fragments (RF). Remnants of Fe-oxide/hydroxide coatings around the grains can be observed. However, Fe-oxide/hydroxide (FeOx) is also occluding secondary pores (SP).

(J, K and L) PBS samples are composed of detrital quartz (Qz) and K-feldspar (Fsp). Quartz overgrowths (black arrows) can be differentiated through clay remnants between the detrital and authigenic quartz overgrowths. Kaolinite (K), replacing K-feldspar grains can be observed. Dark red Fe-oxide/hydroxide (FeOx) (green arrows) precipitated in the pore spaces. Mica (M) is also present.

Figure 5: A) Cathodoluminescence image (colour corrected to enhance the image quality) showing the light-dark violet luminescence of quartz (Qz) grains and K-feldspar (Fsp) showing blue shades that have been slightly etched/altered. (B and C) Reflected light microscopic image showing a relict of rhombohedral dolomite or siderite cements, which are presently replaced or overprinted by Fe-oxides/hydroxides (SP). Mn-oxide (MnOx) overprint or replace the Fe-oxide/hydroxide (FeOx). Oversized secondary porosity (SP) developed because of the dissolution of grains or cements. (D) Fe-oxide (FeOx) occluding the pore space, decreasing the porosity in some red sandstone horizons. (E) SEM image showing pore-occluding Fe-oxide/hydroxide (FeOx-black arrows) and Mn-oxide (MnOx-white arrows) cements. Sharp contacts with secondary dissolution pore (SP) show that these phases formed before secondary porosity development. (F) Tangential illite (black arrow) is present around the detrital quartz grains blocking the pore throats and meshwork or fibrous illite (white arrow). Kaolinite booklets (K) can also be observed in the pore space.

The primary authigenic constituents within the RS samples comprise iron oxide/hydroxide, manganese oxide, syntaxial quartz and feldspar overgrowths, illite, kaolinite, and residuals of dolomite/siderite cements. Fe-oxide/hydroxide and Mn-oxide content exhibit the most pronounced statistical variations (p = 0.00091 and p = 0.00017, respectively) in the examined sample series. RS samples (4 to 26%) have significantly higher Fe-oxide/hydroxide contents than SBS (0 to 8%), with the exception of three outliers and PBS (3 to 12%). In contrast, LBBS shows relatively elevated contents of 1 to 17%, displaying no statistically significant difference from the RS (Fig. 3E). A comparison of Mn-oxide content (Fig. 3F) reveals significantly higher levels in RS (0 to 12%) compared to those of SBS (0 to 6%) and PBS (0 to 7%), with no significant difference observed in comparison to LBBS (0 to 3%).

Quartz overgrowths are the second most common cement type. There is a notable increase in quartz overgrowths within SBS (max. 5%) compared with RS samples (max. 3%). In LBBS and PBS samples, its concentration is below 2% (Supplementary Data). K-feldspar overgrowths are observed with a low frequency (0 to 2%) across all samples analysed. Rhombohedrally shaped Mn- and Fe-oxide structures suggest the former presence of dolomite or siderite cements (Figs. 5B, C). However, no direct evidence of carbonate cementation was identified within the analysed sandstone series.

Illite is a major authigenic clay mineral and is present in two main forms, i.e., as pore lining, parallel to detrital grain surfaces, and secondly as platy to fibrous pore filling illite (Fig. 5F). Illite ranges from 0% to 3% with an average of 1% in all sample types. Kaolinite booklets are observed within the pore space, accounting for less than 1% in the analysed samples (Figs. 4J and 5F).

Figure 6: (A) Ternary plot of compositional parameters according to Folk (1980). QFR: quartz, feldspar, and rock fragments. The Buntsandstein sandstones are predominantly (sub) arkoses to lithic arkoses in composition. (B) Violin plot comparing the intergranular volume (IGV) and showing a pronounced difference (p = 8.5e-06) between red and bleached sample series.

Compaction

The RS IGV values are significantly higher (21–43%) than those observed in the PBS (13–25%), LBBS (8–31%), and SBS (21–32%) sample sets. Bleached sandstone samples exhibit no significant variations, except for those between SBS and PBS samples (Fig. 6B). Statistically significant differences (p=3.5e-05 for CEPL and p=8.5e-06 for COPL) are observed within the examined sample series. RS samples display higher CEPL values relative to SBS, PBS, and LBBS samples, whereas bleached sandstones display homogenous characteristics (Fig. 7A).

COPL values in RS are significantly lower than in PBS, SBS, and LBBS samples. PBS samples show significantly higher COPL values than those in SBS samples (Fig. 7B). Analysis of the sample series reveals a cluster primarily in the upper section of fig. 7C; however, some RS samples are present in the lower section.

Figure 7: (A and B) Violin plots comparing the calculated CEPL and COPL values in the red and bleached lithofacies. The cement porosity loss (CEPL) and compaction porosity loss (COPL) exhibit significant differences (p = 3.5e-05 and p = 8.5e-06, respectively) between the examined samples. (C) Compactional porosity loss (COPL) is plotted against cementation porosity loss (CEPL) for all examined lithofacies.

Petrophysical analysis

Multiple analytical techniques were used for microscale to macroscale reservoir characterisation of red and bleached sandstones. Petrographic and scanning electron microscopy (SEM) analyses were conducted for rock and pore characterisation. Total porosity was determined via point counting (PC), Helium porosity (HP), and water saturation (WS) measurements. Pore size distribution was analysed using nuclear magnetic resonance (NMR), pore throat sizes via mercury injection capillary pressure (MICP), and pore network architecture via X-ray micro-computed tomography (X-µCT). Nitrogen gas permeability (GP) and air permeability (AP) methods were employed to determine permeability.

Porosity in the red and bleached lithofacies varies considerably, spanning from 10% to 26%. Analysis of RS samples (15–26%) showed no statistically significant difference from SBS samples (10–22%); however, RS samples exhibited significantly greater porosity than LBBS (10–15%) and PBS (11–16%) samples (Fig. 8A). A comparative analysis of HP values against other applied techniques reveals a significant correlation with WS porosity, demonstrating a strong positive correlation (R² = 0.86, Fig. 8B). NMR porosity mirrors this pattern, yielding an R2 value of 0.69 (Fig. 8C). Conversely, the results of PC analysis indicate lower porosity values than other methods, with a coefficient of determination (R²) of 0.44 (Fig. 8D). This disparity is likely attributable to the inherent limitations of optical microscopy in resolving microporosity. Given these results and for comparison with literature, we used Helium porosities and nitrogen gas permeabilities to compare results between the lithofacies.

Permeability values show substantial variability throughout the sample series (Supplementary Data), with a statistically significant difference observed (p = 5.4e-08; Fig. 9A). A determination coefficient (R²) of 0.75 is obtained by comparing the measured AP and GP values, as shown in equation (9B).

The porosity and permeability data indicate RS samples possess higher porosity and permeability than bleached lithofacies, although six SBS samples (9C) deviate from this trend. The permeability-average grain size (AGS) correlation shows that grain size significantly influences reservoir quality (Fig. 9D). Samples containing larger grains (AGS >150 µm) and low diagenetic cement content show high permeability, as shown in fig. 9D. The LBBS samples display a fine grain size (AGS < 150 µm) composition and exhibit minimal permeability. In contrast, the PBS sample is characterised by a substantially larger average grain size (AGS > 150 µm) and reduced permeability resulting from diagenetic pore-filling cements (Fig. 9D).

Figure 8: (A) Violin plot comparing the porosity values in the red and bleached lithofacies. A comparative analysis of various porosity analyses is presented. Helium porosity, used as a standard technique, is plotted against porosity results from water saturation (WS) porosity (B), NMR porosity (C) and point counting (PC) porosity (D), and yield a determination coefficient (R2) of 0.86, 0.69 and 0.44, respectively.

Figure 9: (A) Violin plot comparing different lithofacies based on the permeability data. (B) Air permeability values are compared with gas permeability values, yielding a determination coefficient (R2) of 0.75. The overall trend is the same, but the air permeameter measurements show a wider range of values. Cross plots of He porosity (C) and average grain size (D) against permeability of red and bleached outcrop samples.

Pore types and size distribution

NMR relaxation time T2 measurements were performed on fully water saturated sandstone samples. T2 relaxation time exhibits a direct proportionality with pore radius; larger pores correlate with higher T2. Analysis of the samples revealed diverse pore size distributions, characterised as uni-, bi-, or trimodal, with peak occurrences approximated at 0.01, 0.04, and 0.7 ms for RS (Fig. 10A). SBS samples showed consistent values (0.01, 0.05, and 0.6 ms), with three exceptions exhibiting peak values near 1 ms. The LBBS samples display uni- to bimodal distribution with modes around 0.007 and 0.2 ms, including a weak tail due to the presence of a few larger pores. While PBS samples show peaks at around 0.004, 0.02 and 0.2 ms with uni-, bi- and trimodal pore distribution. The measured surface relaxivity coefficients for the examined sandstone samples fall within the previously reported range (1.89 to 160 μm/s) observed in sandstones [82-85]. Analysis of MICP data indicates that the pore throat size distribution is unimodal for both red and bleached samples, characterised by two separate size ranges. Sandstones with AGS > 150 µm exhibit pore throat sizes approximating 10 µm, while those with AGS < 150 µm display pore throat sizes near 0.2 µm. In contrast, the pore throat size distribution of the PBS sample exhibits bimodality, with two peaks at approximately 0.1 µm and 0.7 µm (Fig. 10B).

To establish a correlation between T2 NMR relaxation time distributions and optical pore characteristics, high-resolution images were acquired. Assuming a spherical pore geometry, pore sizes were determined and classified as micro-pores (< 1 µm), meso-pores (1-15 µm), and macro-pores (15-300 µm) in agreement with [86]. Fine-grained sandstones (AGS < 150 µm), characterised by a predominance of micropores, exhibit peaks on the left side in NMR plots. NMR plots reveal that large-grained sandstones (AGS > 150 µm), characterised by their meso- to macro-porosity, exhibit higher T2 values and peaks shifted toward the right side in NMR plots (Fig. 11).

X-µCT models show the differences in pore connectivity between red and bleached sandstones (Fig. 12). Three-dimensional visualisations of the examined samples reveal the presence of different types of pores. For the 4 µm resolution of the reconstructions, unconnected pores are represented by multiple colours, while the connected pore network is depicted as one colour, in this case blue (Fig. 12). All studied samples display significantly reduced pore connectivity above 4µm except a few SBS samples. The pore connectivity is relatively higher in coarse-grained sandstones as compared to fine-grained sandstones. The SBS samples, which consist of fine-grained sediments (AGS < 150 µm) display a pore network along layers. In comparison, the LBBS sample shows an isolated pore network, and similar behaviour is observed in PBS samples (Fig. 12).

Figure 10: A) NMR T2 displays unimodal, bimodal, and trimodal pore size distributions. The main peak of the pore size distribution is found between 0.1 and 1 µm, bimodal samples have a secondary peak between 0.02 and 0.1 µm, and trimodal samples have a third peak, which is between 0.001 and 0.02 µm. B) MICP pore throat data showing the unimodal and bimodal distribution in the examined samples.

 

 

Figure 11: (A) NMR T2 pore size distribution associated with large-grained sandstones (average grain size > 150 µm) and (B) fine-grained sandstones (average grain size < 150 µm) in all examined sample series.

 

 

Figure 12: Microscopic images (left) display segmented two-dimensional pore structures, while three-dimensional X-ray micro-computed tomography (µCT) models (right) represent the pore structures of all examined samples. Φ = Helium porosity and K = gas permeability.

 

5. Discussion

Bleaching patterns

Middle Buntsandstein samples present in this study reveal a significant predominance of sandstone, with only a minor presence of silty and clayey facies. The sedimentological features such as planar to trough cross bedding, horizontal to massive laminations, lenticular to wavy bedding, strongly amalgamated sand bodies, current ripples, abundance of pebbles and mud clasts, erosive basal boundaries and fining upward sequences show a predominantly fluvial depositional environment with intercalations of lacustrine and aeolian sediments. These observations are in line with [6-8,11,22,34,58,59,62,64,87,88].

The studied outcrop sections exhibit diverse bleaching patterns which suggest the involvement of both local and regional-scale diagenetic processes across geological times, discussed below:

Lamina-Bound Bleached Sandstones (LBBS) are characterised by two main patterns:

  1. Bleached lamina along the bedding planes is observed in the Kronthal and the Loegel quarry. These laminae are primarily composed of very fine- to fine-grained layers ranging from millimetres to sub-millimetres in thickness, which likely correspond to suspension settling laminae (Figs. 2E, 4H, I).
  2. Bleached lamina along the sedimentary structures such as cross-bedding and erosive boundaries are only observed in Loegel quarry and typically correspond to top sets (Figs. 2E, 4H, I).

The high concentration of LBBS in the top sets of the fluvial deposits (Fig. 2E) indicates a significant influence of near-surface conditions on the bleaching process. Solubility and subsequent removal of iron were likely facilitated by the infiltration of acidic meteoric fluids (potentially enriched with dissolved CO₂), as shown in fig. 13. Evidence of cyclical fluid migration within sedimentary structures (including cross-beds and erosional contacts) suggests that reducing conditions were preferentially formed within less permeable laminae. The restricted fluid flow allowed prolonged reaction periods, resulting in the removal of Fe-oxide/hydroxide coatings in these laminae. This novel bleaching pattern represents an unprecedented finding, as it has not been previously reported or described in any published scientific work.

Patchy bleached sandstones (PBS) are distinguished by millimetre-to-centimetre-scale isolated white patches (spots) within red-bed sandstones. This particular bleaching type is quite rare, having only been observed and documented in a few outcrop and subsurface sections. Previous studies [90,91] have attributed PBS to the occurrence of heavy minerals in these horizons. However, the absence of these minerals within the geographical study area directly contradicts the proposed hypothesis, thus suggesting the involvement of different diagenetic processes.

We propose that the transformation of (organic-rich) sedimentary clay-clasts likely generated localised reducing conditions, leading to the removal of Fe-oxides/hydroxides in and around these spots, giving rise to white patches or spots (Fig. 14). These observations align with previous findings in the literature [6,9,10], further supports the role of localised diagenetic alteration in PBS formation. The significantly higher concentration of sedimentary rock fragments observed in these samples provides further support for this interpretation (Fig. 3D; Supplementary Data). Alternatively, the observed bleaching may be correlated with, or a consequence of, alterations in the clay mineral content and/or the conversion of K-feldspar to kaolinite. However, the rarity of well-formed kaolinite booklets in the examined samples challenges this hypothesis.

Stratiform bleached sandstones (SBS) are characterised by a widespread occurrence of bleaching. Complete bleaching of the outcrop is apparent in Soultzerkopf and Barrois (Fig. 2B). In contrast, at Haut-Barr, Loegel quarry, and Butzweiler only individual beds are bleached (Fig. 2C), potentially related to various diagenetic mechanisms.

The lack of hydrocarbon residue found within the outcrop sections indicates hydrocarbons were probably not involved in the bleaching process in these sections. In addition, the absence of underlying Permian Zechstein sulphates in the study area, as noted by [92], also rules out their possible involvement in the bleaching process, as proposed by [4,93] from north Germanic Basins. Wendler et al. [93] proposed that volcanic and acidic (meteoric) fluids played a significant role in the bleaching processes in this region. Reported pervasive bleaching in Cleebourg and Neustadt an der Weinstraße [6] is closely associated with fault-related acidic fluid migration into these formations [11,94]. Our study area, situated roughly seven kilometers from the Cleebourg, lies along the URG faults, a significant geological feature of the region. Thus, it is proposed that the SBS bleaching in the studied outcrop section may result from the flow of CO2-rich fluids through faults (e.g., post-Jurassic lift or URG reactivation, spanning from the Eocene to Pliocene [50]), as illustrated in fig. 15. The remnants of Fe-oxide/hydroxide coatings between the detrital quartz and overgrowths in the bleached samples indicate that bleaching phenomena likely occurred during burial diagenesis. These findings are in line with observations of previous researchers [6,7,9,11].

Paragenesis

Inherited clays are mainly present in the form of smectite coatings around the detrital grains that originate prior to their arrival at the site of deposition [95]. Early diagenetic reddening of the studied sandstones is caused by the precipitation of Fe-oxides/hydroxides on the latter clay-rims [2,4,9,31-34]. According to Cornell and Schwertmann [96], Fe-oxides/hydroxides in recent soils precipitate initially as ferrihydrite, which alters to goethite (Fe-hydroxide) and/or hematite (Fe-oxide) under warm and wet conditions. These dark brown coatings are prominently encased in syntaxial overgrowth cements and at grain contacts (Figs. 4A, B, C, D, E, J). Walker et al. [89] reported similar early diagenetic processes in present-day sediments in hot deserts or semi-arid environments.

The presence of uncompacted fabrics (Figs. 4A, D, E), along with irregular outlines and embayments surrounding detrital quartz grains, indicates the prior existence of carbonate nodules and cements during the early diagenetic stage in the analysed outcrop samples. The carbonate-saturated fluids commonly acted as a corrosive agent, which led to the dissolution or etching of detrital minerals [97,98]. Early diagenetic carbonate nodules and cements inhibited mechanical compaction, which would have otherwise significantly reduced the volume of framework grains. The present findings are consistent with previous reports [11, 29].

The lack of K-feldspar syntaxial overgrowths indicates it likely occurred after the carbonate cementation during early diagenesis. These overgrowths probably originated from the early diagenetic dissolution of detrital K-feldspar [100]. Moreover, Bossennec et al. [99] reported 40Ar/39Ar dating results from feldspar overgrowths in the Buntsandstein, yielding a youngest age of 140 Ma, consistent with the Late Jurassic.

Subsurface burial induces alterations in temperature and pressure, resulting in a transformation of clay minerals (e.g., smectite to illite and kaolinite/illite), mineral dissolutions, and/or overgrowths; Tangential illite coatings are typically formed by the transformation of smectite coatings [101,102]. The presence of illite coating textures within syntaxial quartz overgrowths suggests their formation predates quartz overgrowth development. Continuous illite coatings on detrital quartz grains frequently prevent the formation of quartz overgrowths. The formation of fibrous and meshwork illite in the pores and illitisation of detrital K-feldspar occurred probably during the mesogenesis (Figs. 4J, 5F). Based on K-Ar isotopic ages, various authors [6,11,103] reported the different phases (210-185 Ma, 175-155 Ma, and 110-95 Ma) of illite formation in the Buntsandstein in SW Germany. The presence of kaolinite (Fig. 5F) implies its formation either through the alteration and/or dissolution of K-feldspar during early diagenesis [104], burial diagenesis [3], or uplift diagenesis [105].

Syntaxial quartz overgrowths are differentiated by preserved iron oxide/hydroxide clay coatings between detrital grains and the overgrowths (Figs. 4A, B, D, E, G, J, L). Quartz overgrowth formation in outcrop samples is inferred to have postdated carbonate cementation, as observed by Yousaf et al. (under review) from Southern Netherlands. These overgrowths may result from the dissolution of detrital K-feldspar or the recrystallisation of clay minerals, or both [106]. Chemical compaction at grain contacts covered by illite and involvement of mica can contribute to the formation of quartz overgrowths [107,108]. The continuous Fe-oxide/hydroxide clay coatings around the detrital quartz inhibited their overgrowths in the RS samples. However, the more significant presence of the aforementioned overgrowths in the SBS samples indicates that the bleaching processes removed the Fe-oxide/hydroxide clay coatings from the grains. The partial or complete dissolution of illite coatings further supports the hypothesis that the observed bleaching is due to acidic fluid migration within these sandstones, as previously noted.

Following the phase of burial diagenesis, the Buntsandstein successions were uplifted and exposed to meteoric waters. As a result, all carbonate minerals (e.g., dolomite/siderite), as well as the nodular cements (if any) underwent dissolution [108]. The presence of relict rhombohedral dolomite or siderite cements (Figs. 5B, C) suggests their prior existence in the analysed samples. Carbonate cement dissolution resulted in the release of Fe and Mn. These elements then precipitated as oxide phases and often formed concretions (e.g., Wadflecken) (Figs. 2G, 4B, 5D, E). Bauer [94] also reported the rhombohedral to nodular Fe- and Mn-oxide replacements after siderite or dolomite in the outcrop samples from the region. A summary of the main diagenetic processes identified in the outcrop sections is presented in fig. 16.

Most of the early diagenetic processes interpreted in outcrop samples show strong correspondence to those previously reported in the Buntsandstein subsurface samples in the region [2,4,10,11,25,26,29]. A key paragenetic difference lies in the timing and extent of cementation and subsequent dissolution. Subsurface samples generally exhibit a well-preserved record of early-late burial diagenetic processes compared to outcrop samples. These are significantly altered by subsequent telodiagenetic events most likely related to post-Jurassic uplift and tectonic reactivation in the URG. The secondary porosity resulting from carbonate dissolution is more developed in outcrops because of their extended exposure to meteoric fluids. Moreover, the presence of Fe-oxides/hydroxides as well as Mn-oxides, observed mainly in outcrop samples, suggests meteoric weathering under humid, oxidising conditions [11,30].

Reservoir Potential

The Buntsandstein sandstones exhibit a broad spectrum of petrophysical properties, shaped by depositional textures (e.g., fining-upward sequences) and subsequent diagenetic overprinting, including cementation and dissolution. The Buntsandstein successions exhibit a total porosity range from 10 to 26%, which is mainly controlled by grain size (Fig. 8D) and the extent of diagenetic modification (e.g., cementation and dissolution).

Bleached samples showed low reservoir properties, except some SBS samples, which possess large grain size (AGS > 150 µm). The remaining bleached sample series, which corresponds to either SBS or LBBS, is fine-grained (AGS < 150 µm). The PBS samples, characterised as larger-grained sandstones with an AGS exceeding 150 µm, exhibit lower reservoir quality (Fig. 9). Although the grains are larger, the lower reservoir quality is likely due to the precipitation of iron and manganese oxides in the pores.

Our analysis indicates that observed bleaching patterns in the examined outcrop sections predominantly correspond to top sets of the fluvial deposits (e.g., Kronthal, Loegel quarry). Therefore, higher content of fine-grained sediments in these lamina decreases their reservoir quality. In contrast, the bleached samples that represent main stratigraphic beds (e.g., middle part of a sequence), exhibit relatively larger grain sizes, show higher petrophysical properties and are comparable with RS samples. The present findings are consistent with those previously documented in [2,6,7,9-12,29-33]. Surprisingly, the RS samples present in this study comprise large-grained sediments (AGS > 150 µm) except one sample (AGS = 120 µm) (Fig. 9D). These RS samples exhibit higher porosity and permeability values except those which have higher content of pore-occluding cements (e.g., Fe-oxides/hydroxides and/or Mn-oxides) (Figs. 8 and 9).

Pore Size Distribution

Analysis of petrographic images reveals both intergranular pores (between grains) and intra-granular pores (formed by the partial or complete dissolution of grains), which vary in shapes and sizes. As previously noted, nuclear magnetic resonance (NMR) relaxation time (T2) is sensitive to variations in pore size, shape, and relative abundance. The distinct NMR peaks suggest the presence of unimodal, bimodal, or trimodal pore size distributions in outcrop samples (Fig. 10A). RS and SBS samples with AGS greater than 150 µm predominantly show bi- to trimodal distributions (Figs. 10A, 11A). The larger amplitude and relatively higher NMR T2 in some SBS samples (Fig. 11A), suggest a higher abundance of (meso-macro) pores compared to RS. This phenomenon is associated with the bleaching processes, leading to the removal of Fe-oxide/hydroxide coatings and partial or complete dissolution of feldspar grains. These observations support the previous studies, which indicate that bleaching processes improve reservoir quality, e.g., [2,6,9,10]. Interestingly, while the SBS samples showed a larger pore volume, their permeability was still lower than that of the RS samples, a contrast highlighted in fig. 9A. Point-count analysis of SBS samples reveals a significantly greater concentration of quartz overgrowths compared with the RS samples. It is likely that the relatively high quartz overgrowth content blocks the pore throats in these samples, thus resulting in a decrease in permeability values. This highlights the intricate correlation between bleached sandstones and reservoir quality. While the dissolution of iron oxides and hydroxides improves reservoir quality by creating larger pore spaces, the subsequent precipitation of silica, as quartz overgrowths, leads to a significant reduction in permeability, thus presenting a complex interplay of positive and negative effects on reservoir properties.

Analysis of MICP data from larger-grained RS and SBS samples reveals no significant variation in pore throat size, with both exhibiting a similar peak at approximately 10 µm (Fig. 10B). The pore throat size distributions of PBS samples are uniquely bimodal, displaying peaks centred on 0.1 µm and 0.7 µm (Fig. 10B). This is likely due to the presence of pore-occluding diagenetic cements in PBS samples, as mentioned earlier.

X-µCT 3D reconstructed pore network models at 4 µm resolution for both red and bleached sample series are illustrated in fig. 12. Insufficient resolution in X-µCT imaging often precludes accurate quantitative measurements of micropores and complex structures, particularly those characterised by low tomodensity [79]. Due to the significant presence of macro-pores within some SBS samples (Fig. 11A), a more extensive and interconnected pore network is observed in comparison to the RS samples (Fig. 12). In contrast, the SBS samples with an AGS <150 µm exhibit laminar distribution of pore networks (Figs. 4F and 12). The LBBS with their fine-grained composition, display tight grain packing (Fig. 4H), low porosity (Fig. 8B), and poor pore connectivity (Fig. 12). PBS samples show moderate porosity and permeability values (Figs. 8 and 9), but their isolated pore network (Fig. 12) designates these sandstones as poor reservoir rocks. While these conclusions on the limited reservoir potential of the LBBS and PBS are supported by petrographic observations and results from other techniques, they might locally be connected at a scale below the resolution of the X-µCT.

Reservoir Implications

The overall detrital composition, grain size, sorting, and sedimentary structures observed in the outcrop sections are comparable to previously described subsurface samples in the URG [11,25] and adjacent areas [26]. Early diagenetic features, such as carbonate nodules and grain-coating clays, which helped preserve the primary rock framework during mechanical compaction, are common in both outcrop and subsurface samples. Similarly, the distribution and influence of clay rims on inhibiting syntaxial quartz overgrowths in outcrop samples align with those observed in corresponding subsurface samples within the URG [11,25]. These features, however, display significant differences when compared to subsurface samples obtained from the West Netherlands Basin [26]. The absence of significant overgrowths in outcrop samples is consistent with extensive early pore-filling cementation (Yousaf et al., under review), substantial grain coating [11], and limited thermal exposure resulting from uplift.

The notable differences are largely attributed to telodiagenetic overprints, which mainly include the extensive dissolution of early- to late-diagenetic products, such as carbonates and probably K-feldspar. These telodiagenetic processes are likely driven by the influx of meteoric fluids during uplift, as well as by fault-controlled migration of later fluids and are absent in deeply buried subsurface samples [11,25,26].

Because of telodiagenetic modifications, outcrop samples may misrepresent the petrophysical characteristics of their subsurface counterparts, particularly in terms of porosity distribution, cement paragenesis and IGV. Due to their significant differences, absolute reservoir properties cannot be delineated accurately, only based on outcrop samples. Some authors (e.g., Busch et al. [11]) suggest that detected carbonate nodules in subsurface samples and their chemical properties could serve as a baseline for correcting IGV estimation in outcrop samples, however, such reconstructions require careful interpretation. Features like secondary intragranular dissolution pores may enable the precise reconstruction of carbonate nodules and cements, where present. In addition, the area of the dissolved carbonate nodules can be marked by a lack of syntaxial quartz overgrowths, as well as the areas with mostly floating to point grain-grain contacts. In this way, point count data may be corrected to calculate accurate IGV, as it is one of the key controlling factors of subsurface reservoir quality [7,11,12,25,26]. However, precise characterisation of the relative timing of formation and dissolution of early and burial diagenetic carbonate cements is impossible using outcrop samples only. Another major difference relates to burial depths. In view of the above observations, it is concluded that these surface outcrops are not reliable subsurface analogues for accurate estimations of reservoir quality.

Figure 13: A schematic diagram of Lamina-Bound Bleached Sandstones (LBBS) formation. (A) Continental red bed sandstones were deposited under semi-arid climatic conditions. (B) The solubility of iron was enhanced by acidic fluids, possibly containing dissolved CO₂, facilitating its removal from sedimentary structures (e.g., erosional surfaces). (C and D) Bleaching of laminae within the sedimentary structures is attributed to periodic acidic fluid infiltration. The cyclical patterns of bleaching are elucidated by observations of the Loegel quarry walls (Fig. 2E).

 

 

Figure 14: A schematic sketch of petrographically identified features associated with patchy bleached sandstones (PBS). (A) Red bed sandstone. (B) Isolated white spots resulting from localised reducing conditions, leading to removal or reduction of Fe-oxide/hydroxide coatings. The dissolved Fe-oxides/hydroxides subsequently precipitated in pore spaces.

 

Figure 15: Illustrates a conceptual sketch of the bleaching mechanism in stratified bleached sandstones (SBS). (A) Represents the red bed sandstones. (B) Complete outcrops with pervasive bleaching and (C) Bleaching of individual beds likely occurred because of fault-related fluid migration into the formations or along beds, respectively.

 

Figure 16: Paragenesis of major diagenetic processes observed in the studied red and bleached sandstones. The grey colour bars represent the inferred diagenetic phases.

 

 

6. Conclusion

Diverse bleaching patterns in the studied Buntsandstein outcrop sections in Vosges and Trier areas suggest the involvement of both localised and regional-scale diagenetic processes over geological times.

Lamina-Bound Bleached Sandstones (LBBS) typically correspond to top sets. Their local abundance indicates that the bleaching process was likely driven by the infiltration of acidic (meteoric) fluids, potentially enriched with dissolved CO₂, during shallow burial.

Patchy Bleached Sandstones (PBS) formation is linked to localised reducing conditions generated by the decomposition of organic matter in sedimentary clasts, resulting in the distinctive white, patchy morphology.

Widespread Stratiform Bleached Sandstones (SBS) are associated with fault-controlled migration of CO2-rich fluids, both across outcrop sections (e.g., Soultzerkopf and Barrois) and along individual beds (e.g., Haut-Barr, Loegel quarry, and Butzweiler).

Grain size plays a crucial role in determining reservoir potential. In the studied outcrops, bleached samples that comprise very fine to fine-grained layers (average grain size < 150 µm), exhibit tight packing and poor reservoir quality. In comparison, bleached sandstones of superior reservoir quality are characterised by larger-grained sediments (average grain size > 150 µm).

Bleaching processes enhance porosity through dissolution, but subsequent silica precipitation can reduce the permeability. Bleaching is a complex process with variable effects on reservoir quality, making it necessary to evaluate its influence on a case-by-case basis.

A significant correspondence exists between sedimentary features and early diagenetic processes observed in outcrop sections and their subsurface analogues within the region. Grain coating coverage and its influence on syntaxial quartz overgrowths in outcrop samples exhibit similarities to nearby subsurface samples yet differ significantly from subsurface data from the southern Netherlands. In addition, diagenetic alterations, including carbonate and likely K-feldspar dissolution and localised precipitation of iron and manganese oxides, differ from their subsurface counterparts. A comparative analysis reveals that outcrop samples are overprinted by telodiagenetic processes and thus may misrepresent the petrophysical characteristics of subsurface equivalents.

Supplementary Data: Supplementary data for this study can be found at https://doi.org/10.5281/zenodo.16443062.

Author Contributions: “Conceptualisation, Husnain Yousaf; method, Husnain Yousaf; software, Husnain Yousaf; validation, Husnain Yousaf, Dr. Hannes Claes, and Dr. Rudy Swennen; formal analysis, Husnain Yousaf, Dr. Hannes Claes; investigation, Husnain Yousaf; resources, Dr. Fadi Henri Nader, Dr. Rudy Swennen; data curation, Husnain Yousaf, Dr. Hannes Claes, Dr. Fadi Henri Nader, Dr. Rudy Swennen, Dr. Jean-Marie Mengus, Dr. Remy Deschamps; writing, Husnain Yousaf; writing-review and editing, Dr. Hannes Claes and Dr. Rudy Swennen, Dr. Gert Jan Weltje; visualisation, Husnain Yousaf; supervision, Dr. Rudy Swennen and Dr. Gert Jan Weltje; project administration, Husnain Yousaf, Dr. Rudy Swennen; funding acquisition, Dr. Rudy Swennen. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Acknowledgments: The authors extend their sincere gratitude to the IFP team for their invaluable support in geological fieldwork and sample collection, as well as for granting permission to publish this study. Special thanks go to reviewers for their expert guidance, constructive discussions, and valuable insights, which have contributed to the quality of this research.

Conflicts of Interest: The authors declare no conflict of interest.

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