the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Mar 2025
The influence of burial history on physical properties of claystones – Overview of a systematic research program across scales
Abstract. The search for a suitable host rock for the deep geological disposal of high-level radioactive waste is one of the major geological challenges of our time. In Germany, alongside rock salt and crystalline rock, claystones are considered a promising geomaterial and the subject of intensive research within the scope of the site selection process. Particular focus is placed on those rock properties that are intended to prevent the migration of radionuclides into the environment effectively, referred to as barrier properties. These primarily include low permeability, self-sealing efficiency with respect to fractures, sorption capacity, and mechanical properties for long-term stability of the underground infrastructure.
However, these properties are dependent on numerous factors such as mineralogical composition, temperature and stress conditions and water content. Among these factors, the burial history and thus compaction affects porosity, permeability, and mechanical properties. Within the framework of the MATURITY project, the impact of burial history on barrier-relevant properties of claystones is investigated in a detailed multidisciplinary investigation approach across scales. For this purpose, a Lower Jurassic claystone formation (the Amaltheenton-Formation (Fm)) was investigated which was subjected to variable maximum depth and subsequent uplift during its burial history. Eight shallow boreholes at the margin area of the Lower Saxony Basin (Germany) were drilled through the formation with varying degrees of maturation. Comprehensive field and laboratory investigations are aimed to analyze burial-induced alterations of claystone barrier properties, and thereby advance the current understanding of these processes. With this contribution, we aim to establish a framework for a series of detailed parametric studies that will systematically approach the dependencies between burial history and petrophysical (e.g. density, permeability), geochemical (e.g. cation exchange capacity), hydrogeological (e.g. transitivity, storativity) and mechanical (e.g. rock strength, elasticity) properties.
We present the first results of different project steps that show (a) a relatively homogeneous clay dominated mineralogical composition of the Amaltheenton-Fm across the boreholes, (b) an increase of max. burial temperatures (83 °C–169 °C) over a lateral distance of ~50 km within the investigation area, (c) a gradual increase in bulk density accompanied by a reduction in porosity and permeability for normally compacted Amaltheenton-Fm sequences along increasing max. burial temperatures, (d) a reverse trend of those parameters for a potentially under-compacted Amaltheenton-Fm sequence, and (e) hydraulic conductivity determined from in-situ hydraulic tests that span two orders of magnitude (10-5 m/s to 10-7 m/s).
- Preprint
(3782 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 10 Oct 2025)
- RC1: 'Comment on egusphere-2025-579', Daniel Minisini, 04 May 2025 reply
-
CC1: 'Comment on egusphere-2025-579', Giacomo Medici, 28 Jul 2025
reply
General comments
Good hydro-geophysical research that needs additional detail before publication. Please, follow my specific comments to fix the minor issues.
Specific comments
Lines 39-41. “Their suitability to act as natural barriers is mainly due to their favorable physical matrix properties, such as very low permeability (down to 10-21 m2), preventing significant focused fluid flow and related advective mass transport of radionuclides in aqueous solutions”. Insert recent literature on the low permeability/hydraulic conductivity of shales/claystones that behave as aquitards and aquicludes:
- Medici, G., Munn, J.D., Parker, B.L. 2024. Delineating aquitard characteristics within a Silurian dolostone aquifer using high-density hydraulic head and fracture datasets. Hydrogeology Journal 32, 1663-1691.
- Liu, Z., Fan, Z., Zhang, Y. 2019. Fracture characteristics of overlying bedrock and clay aquiclude subjected to shallow coal seam mining. Mine Water and the Environment 38, 136-147.
Line 131. Clearly say the specific objectives of your research by using numbers (e.g., i, ii, and iii). Such objectives are not clear in the following sentences.
Line 200. Please, insert more detail on the geometries of the faults that characterize the study site.
Line 200. How many boreholes have been drilled?
Line 200-250. Depth of the water table?
Line 280. Wireline logs in the saturated portion of the aquifer?
Line 280. Overall how many meters have been logged?
Line 282. “temperature” of the fluid/groundwater? Please, specify.
Figures and tables
Figure 2. Make the figure larger.
Figure 5. I can see several extensional faults. Please, provide detail in the relevant sub-paragraph.
Figures 13 and 16. Make the numbers and lables larger.
Citation: https://doi.org/10.5194/egusphere-2025-579-CC1 -
RC2: 'Comment on egusphere-2025-579', Ben Laurich, 29 Sep 2025
reply
Short summary to reflect my understanding
The authors report on current efforts within the research project MATURITY. MATURITY examines the relation of the thermal maturity of a clay formation to its physical rock properties. The authors state relevance of their work in eventually finding and comparing potential repository sites for radioactive waste.
The manuscript refers to samples of the Amaltheenton-Fm., gained from eight relatively close‑by boreholes (~50 km) in the Lower Saxony Basin (Germany).
The authors state that the close-by sample selection ensures only marginal inter-sample divergence on mineral phases and consequently argue that the found variable physical rock properties must instead be caused by the variable maximum burial depth and uplift histories of the samples, with max. burial temperatures in the study region altering by up to ~80 °C. The authors claim a clear, continuous relation of VRr% to several physical rock properties. However, they also acknowledge a limitation of that claim for higher maturity cases (> 0.87 VRr%), where presumable location-specific hydrocarbon generation inversed the otherwise stated continuous porosity decrease with depth.
The burial history is referred to by current depth and by VRr% of the samples as well as by citing a regional 3D model of Castro-Vera et al. (2024), which seems the recent of several similar studies by the same Aachen-Group around Prof. Littke.
Properties that where related to depth derive either from borehole tests (among others: Th/K and Th/U ratios by gamma-ray spectr., vp by fullwave sonic, ρbulk by gamma-gamma density and K and T by hydraulic packer tests) or from laboratory tests (among others: Tmax by Rock Eval analysis, mineral phases by Rietveld-XRD, Φ by He-Pycn. and k by radial N2 uptake).
General appreciation
I suggest to accept this manuscript after a major revision.
The manuscript is generally well-written and the studied boreholes and samples make it a relevant and timely topic, surely interesting to many readers in the German site-selection procedure. It makes appetite for the next, currently gained results of MATURITY. Below, I outline several points that I would encourage the authors to revise.
Risk of Misinterpretation
The compelling question of a general reader could likely be: Which positive and negative effects come with increasing max. burial for the site-selection procedure? Can these help to discriminate one setting/region over another? I recognize that this manuscript, as correctly stated, focuses ‘only’ on the relation of max. burial to physical rock properties. Hence, to guide the general reader, I would hint that this relation is one of many essential criteria to consider in that compelling question.
Foremost, to me, that the effect of mineralogy and rock physical properties must necessarily be related to distinct boundary conditions. For instance, if a local shift to lower salinity pore water is allowed, that enhances the effect of swellable clay minerals, or, if uplift reduces the acting stress to be significantly lower than that of max. burial, it enhances brittleness (see studies on “over-consolidation ratio” (OCR) for this matter). Again, I acknowledge that this specification is beyond the scope of this study, I just fear that not stating its necessity can trick the audience into over-simplifications like: the deeper it has been (without early lithification or hydrocarbon generation), the less porous, the better for a repository site. In part, my fear is rooted by statements such as “[...] focus on […] properties […] referred to as barrier properties” (line 15ff, line 108ff and others, see line comments).
Improving Conciseness
- Broader categorizations of this kind (“barrier properties”) are frequently invoked but often lead to less precise formulations, without improving the clarity (see in-line comments).
- Similarly, the introduction gives broader, unspecific objectives, e.g. line 125 “These correlations will provide a particular emphasis for the applicability to potential site areas”, which are sometimes clearly biased, e.g. line 126 “The thermal maturity shall serve as key proxy for the burial history.” or line 671 “In the future, […] studies will reveal […]”.
A formulation similar as below would give the reader a more clear line of argumentation:
Our study addresses four aims:
- report on first measurement results of MATURITY and on how and where they were recorded,
- comparison of borehole and laboratory derived measurement results,
- comparison of max. burial values from VRr% and 3D modelling,
- discussion of the influence of max. burial on physical rock properties.
These aims could be followed by truly specifically describing their benefit to the site selection procedure for a radioactive waste repository. For instance: Does the max. burial trend allow to extrapolate from shallow boreholes to deeper regions of the same fm. if belonging to the same depositional centre? How is that helpful in the site-selection?
3. I encourage to rename and reorder some sections.
“Site Identification”
It would be advisable to delete this section and integrate the essential information into the Introduction, Previous Studies, or Geology sections (ensuring that it is not redundant with material already presented there).
In addition, the phrasing “was selected” (line 164 and similar arguments) leaves unclear from what pool of sites this particular one was chosen. My impression is that the site and its maximum burial difference as well as its mineralogy were already known before developing MATURITY, and that the aim was subsequently framed to match the site. To be clear: if true, this does not undermine the relevance of the subject—the topic remains important—but the wording should be revised to better reflect this relationship. Maybe just state in the introduction: “For these aims, the Amaltheenton Formation in the Hills and Sack Syncline area provides an excellent study opportunity. First, the nearby well sites (~ 50 km) exhibit largely similar mineralogy, while their Tmax values differ significantly by up to 80 °C. Second, the formation is of particular interest because it is also considered a potential host formation for nuclear waste disposal (among several other formations; see Fig. 3 for those that are clay-bearing).”
“Drillings, Sampling, Borehole Installations” & “Laboratory investigations”
Commonly, these are collectively given under “Methods”. If the authors fancy, they could consider an overview table that lists all the derived value types in the first column, a Boolean field for “Lab/Borehole” as well as, in a third column, the method(s) applied for that value type. Maybe wise to list properties first and then derived index parameters. The table can be followed by “Below we describe the drilling/sampling procedure first and subsequently explain each method of table X separately.” Apart from this extra table, the methods part can be shorted as it repeats reasoning that is given in the introduction already (see line comments).
“Site Characterization”
This is “Results”.
“Implications and Outlook”
This is the “Discussion” section, which would benefit from a clearer line of argumentation if it were subdivided according to aims 1–4 outlined above. At present, it is sometimes unclear what merely serves as a qualification of the measurements and what, from the large number of measured values, actually contributes to the maximum burial trend concept.
“Concluding Remarks”
This is “Conclusion”. Again, the line of argumentation and the clarity would win, if the conclusions regarding aims 1-4 were addressed sequentially.
Arguments concerning the content
- The introduction refers to a low-min. difference being an a-priori reason for site identification, while in the discussion section it partly reads like an additional aim not stated initially. XRD and Th/K, Th/U ratios are discussed as ‘minor variability’ (line 536) or ‘relative homogeneity’ (line 578). Yet, these phrases lack definition regarding dimension and value range. Without such clarification, the conclusions risk being vague. For instance, two samples with identical carbonate content may differ strongly in strength depending on whether carbonate occurs as finely distributed cement or as fossil clasts (cf. Klinkenberg et al., 2009). Is this now a “minor variability”? If the authors better clarify that they address explicitly the variability of mineral phases, then, in turn, they should also (1) explain why they consider a variability of “58-75 wt.%” in clay minerals minor (line 536) and (2) hint that a mineralogical uniformity does not necessarily grants physical rock properties to be equal. First, this holds true as it is microstructure that defines physical rock properties (including porosity, porosity distribution, cementation, etc.), not mineralogy. Second, it would be necessary to adept to environmental boundary conditions – a certain mineralogy can have different implications (based on distinct depth, fluid flux and fluid type, etc.). In other words: the relation of mineralogy to physical rock properties can be non-linear, with an interdependence on the state of other controls. The authors state a “complex interplay” themselves, yet they do not fully acknowledge this important circumstance.
- Sample alteration / desaturation: Have the fresh samples been directly weighted at site? What is their water loss when unpacked in the lab? Are there differences from storage time (older sample sets vs. recent sample sets)?
- Burial history: The burial trend is recognized for a distinct depth-window (1,400 m - 2,440 m), below which hydrocarbon generation invokes a slight trend inversion. The aims are claimed to help the site-selection procedure, yet such regions with hydrocarbon generation will explicitly be excluded in that endeavor. Or have I gotten this wrong? This hypothesis of hydrocarbon generation is also not stated in the conclusion. After reading, I feel unsure if uplift has happened or not. The section “geology” clearly says "yes" (line 190 and Figure 5), but in later manuscript parts, in particular in the discussion, the effect of uplift is not examined anymore, simply the max burial and the “expected” trend that goes “along” within the identified window (line 33). For uplift: Would a clay rock have a “memory” of its max. burial depth, keeping its properties until getting somehow overprinted? What could make that “memory”? Internal cohesion/cementation? What could cause that overprinting? Long-term unloading? (It cannot be short-term unloading as that has happened to all samples during retrieval). Does it remain enigmatic? In this regard: have the upper most samples really seen “under compaction” (line 34) or more unloading due to uplift than the other samples? I encourage the authors to examine the interplay of burial and uplift, not just the Tmax / max. depth values (see comment on OCR above). Or, a bit drastic, is the authors’ line of argumentation deliberately challenging the OCR theory? If so, that should be discussed and underpinned by arguments.
I am not an expert in the local geology nor in the borehole design and logging. Hence, I have made only a few comments regarding these chapters.
I would like to thank the authors—the effort required to compile this dataset must have been considerable. I am confident that the manuscript has the potential to develop into a strong publication and will be of great interest to many readers, particularly within the radioactive waste community. I do not wish to remain anonymous.
All the best,
Ben Laurich
In-line comments
Title:
The title “The influence of burial history on physical properties of claystones – Overview of a systematic research program across scales” illustrates the difficulty to address the multiple aims. Moreover, “systematic” in what sense? MATURITY seems a many-methods endeavour, apparently not all designed to examine the burial history. Plus “across scales” what scales are meant? Sample to regional? Is this wording actually meaning the third aim above (VRr% vs 3D model data)?
L6 & L7 “Chair” – I guess “Institute for / Department of” as the authors do not hold the chair positions themselves, correct? (in German, “Lehrstuhl für” would be correct).
Abstract:
L 13 “geological challenge” consider rewording “a major societal challenge.”.
L 14 “geomaterial” commonly refers to a synthetic material made from natural products, consider rewording or simply shorten to “In Germany, clay-bearing formations are under investigation to potentially host a repository for radioactive waste (alongside rock salt and crystalline rock).”
L 14ff “barrier properties” - What comes of defining that term? I’d suggest to avoid it and state specifically k and CEC. What is your argument for the other rock properties to fall in that category? Is swelling not also a bad thing if it behaves uncontrollably at stresses higher than Sig3? Is high mechanical strength not also favouring brittleness and fractures? Is lower porosity not also emphasising thermal conductivity? Otherwise: is not every single property relevant to the barrier functionality? I do not repeat this concern in following occurrences.
L 21 “multidisciplinary” would be more precise, if the authors name the aims 3 and 4 above. What is “across scales” referring to?
L 22 What thickness has the fm.?
L 23 “Eight boreholes” for the claim of comparing max burial, it seems more important that 5 locations were examined.
L 24 reword to “that show varying degrees of maturation”.
L 26 “With this contrib….” delete – redundant.
L 40 “physical matrix properties” – lack of definition, avoid term.
L 46 “Physical and ..” – delete; circular reasoning (and vague “barrier attributes”).
L 50 “The general impact…” – delete, vague and redundant.
L 54 “elastic wave velocity” replace with “increase in E-Modulus”. The velocity refers to a measurement, not a “fundamental change in claystone”.
L 66 “involves” – replace with “invokes”. The cause (which could also be meant by “involves”) for S->I is sometimes falsely attributed to compaction only. If it is “causes” what is meant to be conveyed here than also state, as with later parts of the manuscript, availability of K+, time, and T.
L 77 “control larger scale behaviour” – Counterargument: Fractures in clays have impermeable side walls. Calcite veins prove to have isotope signatures unrelated to close-by Ca-fossils that microstructurally often seem intact. Moreover, tracer profiles across faults (e.g. Main Fault in Mont Terri) show now deviation from an diffusion profile.
L 80 delete – too many too broad, less specific claims.
L 83 “self-sealing” this is an often missed opportunity to state what the term actually means. With ductility, as stated here, the fm. is “self-sealing” in the sense that it hinders larger brittle fractures to form by easily giving in to stress in a viscous manner. This is strictly different to “self-sealing” of fractures, where clay swelling CAN play a crucial role if the fm is not already saturated and/or if a pore fluid change to lower-ionic strength is at play.
L 89ff See Rutter et al. 2001 for a good differentiation on what controls the mechanical behaviour, to become more precise here. Applies to L 93ff, too. (Evirn. controls vs. material intrinsic ones)
Fig. 3 – inset refers to Fig 4, should be Fig 5
L 109ff “general changes” & “systematic and quantitative studies”, “set the outlines for a series of detailed parameter studies designed to improve our understanding of […] critical claystone properties” – all seem unnecessary broader categorizations, hindering to be specific and concise.
Fig. 5 b) can the well be indicated, so to help spot the target formation? Maybe also indicate Amaltheenton fm as an indented annotation to “Lower Jurassic” in the legend.
L 245 So Rybacki is contradicting the authors hypothesis? What might he have over looked?
L 248 So Mann & Müller are contradicting studies cited just above, where sec. porosity increases with maturity and hence must lower the density?
L 256ff There is a study by Eseme et al. 2007, that examined the mechanical properties of Oil Shales. Eseme, E., Kroos, B. M., Littke, R., & Urai, J. L. (2007). Review of Mechanical Properties of Oil Shales: Implications for Exploitation and Basin Modelling. Oil Shale, 24(2), 159. https://doi.org/10.3176/oil.2007.2.06
L 264 “Amalthea Clay Fm” English now?
L 308 What fluid at what P? N2 I guess?
L 347 “decompaction, volume expansion” same thing.
L 347 list also Sr reduction
L 357 “immediate Geochem. testing” – on site?
L 388 “he-pyc..” – capitalize “h”
L 400 However, the lithological division, as in Fig.7, was made by naked-eye, right?
Table2: Can Pyrite be listed, too? That is crucial in weathering
Fig. 13b: delete “gamma” from x-axis – correctly given in legend
L 593 food for the discussion: Is this revesible? Is porosity decreasing after a P-depletion?
L 601 state also N = 4 to illustrate relevance of R²
L 608 “900 m deeper burial depth during its burial history” – maybe to replaced by “900m larger max. burial depth, following…” meant?
L 610 “while porosity decreases” - does it? Fig. 14 shows otherwise
L 611 “A partly strong deviation” – reword.
L 611 what is expected normal trend? This formulation occurs at multiple occasions. Is it and , as mentioned in the introduction? For OPA at Mont Terri, for instance such a deviation from “expected normal” due to cementation has also been reported by Corkum et al. (2007).
Corkum, A.G., Martin, C.D., 2007. The mechanical behaviour of weak mudstone (Opalinus Clay) at low stresses. International Journal of Rock Mechanics and Mining Sciences 44, 196–209. https://doi.org/10.1016/j.ijrmms.2006.06.004L 613 “and 2,440 m to 3,300 m” – depth derived sense Castro-Vera et al. (2024)?
L 624 citations are repeated
L 626 Can this line be reformulated to match “From a depth of xx m on, K is usually lower than the measurement accuracy and the declining trend is thus not recognizable anymore (References as given).”?
L 633 Give a definition of “decompaction zone”. Is this what is meant elsewhere by “under-compacted”?. Again, decompaction / unloading alone can lead to fracturing if a certain brittleness is given.
L 635 see comment above on “memory”, as all samples are tested unconfined and reveal the same values. Or do the authors imply that K is low with all sorts of samples always and hence laboratory K-measurements can be neglected in the site-selection procedure? Maybe illustrate: If a flux of X Bq is allowed to leave, how large, given the K values would a corresponding area need to be? How would that change in size for the different K values?
L 640 Provide a reasoning for “within” and differentiate to or replace by “across”. Subtle difference is that radioactive waste within the fm. would be immobile even when fractures develop - as long as no fracture hits the waste itself.
L 643 “self-sealing processes” – (1) overburden pressure is a sealing process (sensu Dehandschutter et al. and Bock et al.), (2) if I got it right, then at this location fractures in deeper levels are not closed by overburden but rather have not developed in the first place, see “decompaction zone” above.
L 652 “The initial findings […] require validation […]”: For results, I reckon the authors should replace “initial” with “first” and “require” with “will be complemented by…” as they do not intend to change but to compliment them. For “findings” meant in terms of “deduced theories”, the authors should state explicitly which ones are more of preliminary hypotheses that require a validation. Holds true for L 670, L 681.
L 656: ‘properties particular for site selection’ – This phrasing suggests a hierarchy of importance. Which property is considered more important than the others, and by whom was this order established? Perhaps the authors mean to say that they ‘address those parameters which, in their view, would most severely compromise the repository’s suitability if they fall outside a certain range.’ If so, that would still require argumentation.
Conclusions:
This chapter can again be more specific, sticking with well-defined aims and avoiding boarder categorizations. For example “[…]physical rock properties such as[…]”, can be avoided by directly mention “the E-modulus, density, porosity and permeability values (or what others were correlated?) show a linear relationship to VRr% for the range of […]”.
L 668: “[…] eight boreholes” + “from five locations […]”
L 671 “parameter study”, to my understanding, is a study that alters a certain (set of) parameter(s) to study the effect. I guess here it is meant “empirical study”, which is based on observations and measurements of the real world. If so, it will not “reveal the complex dependencies and interactions of burial-induced changes” (for which a parameter study might indeed help) but it would simply – and of course also valuable – complement the here presented results for the given set of parameters at the study location.
L 689 All specimen have been tested for k unconfined. Discuss, if there will be a trend if subjected to depth-respective confining pressures.
L 689 and others: The authors seem a bit cautious in the formulation of their own results, pointing to the need for confirmation by ongoing MATURITY studies. However, they do not state what hypothesis or theory of this manuscript is going to be tested. If the upcoming works are rather (more detailed) repeated measurements, than there seems no need to be cautiously referring to them.
L 692 “valid” – replace by “recognized”
L 699 reformulate to be specific.
Figure 16: What results can be drawn from (b) that are not given from (a)? Delete (b)? (a): is “chemical compaction” a defined term? It does not “compact” in the strict sense, is “chemical diagenesis” better suited? What justifies the 70 °C line? Is that from a reference? I guess that the y-axis caption should be named “max. burial depth / temperature” with 163 °C in the figure and 169 °C in the caption being a mistake. Or do the authors mean current depth / temperature ? If so, then this figure would not provide a conclusion to the studied aim.
Citation: https://doi.org/10.5194/egusphere-2025-579-RC2
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 1,427 | 122 | 18 | 1,567 | 103 | 106 |
- HTML: 1,427
- PDF: 122
- XML: 18
- Total: 1,567
- BibTeX: 103
- EndNote: 106
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Thanks for allowing me to participate in the peer-review process of this work. Below my comments written in the spirit to enhance clarity and avoid contradictions in the content of the manuscript.
Sincerely, Daniel Minisini.
This paper focuses on rock units that could host disposal of radioactive waste. The paper prepares the ground for future contributions on the same topic and same area. Its main goal is to prove that the Amaltheenton Fm represents an optimal stratigraphic unit to study how a rock unit, originally having self-similar characteristics, is affected by different burial histories. In fact, the authors would like to investigate the impact of burial history on barrier-relevant properties of claystones (“low permeability, self-sealing efficiency with respect to fractures, sorption capacity, and mechanical properties for long-term stability of the underground infrastructure”).
This is emphasized in passages like the following:
Although the authors explain how they understand the different thermal maturity in the area of study, they do not explain well how they understand that the rocks investigated in 5 wellbores are/were homogenous. The text says that the Amaltheenton-Fm is “a regionally relatively homogeneous claystone formation within the study area”. Homogenous is a clear adjective that is not used with its significance in the paper. The quote itself says “relatively homogenous”, which is an oxymoron. Actually it includes the term “regionally”, as if zooming in, the formation would be heterogenous. In fact, the authors state that the Amaltheenton Fm “exhibits only slight variations between the individual borehole locations”. But “slight” is not described in the text and their nebulous concept of “homogeneity” (among the 5 sites and within each site) rises a flag, as we are talking about the foundational work for potential nuclear waste disposal sites.
The authors could take advantage of the deep dives into these fine-grained rocks made by the Energy companies working on Unconventional resources. They might expand their views on fractures (permeability) and early diagenetic processes (porosity) affecting these claystones/mudstones.
Since the authors themselves mention that they still need more ”analyses….to assess the elemental and mineralogical composition of the Amaltheenton Formation at each location. …and quantify spatial variability, which is crucial for understanding basin-wide depositional patterns and potential lateral facies changes”, it seems this contribution might benefit from another round of readings by the authors.
Also, there authors should explain better to the reader the reason why the chose the Amaltheenton Fm (i..e, prove that the Amaltheenton Fm represents an optimal stratigraphic unit to study how a rock unit, originally having self-similar characteristics, is affected by different burial histories).
Finally, although the authors focus on burial history, they mention other factors (“mineralogy, temperature, stress and water content”) that affect the barrier properties they want to study. They should better explain why, among the several factors, they focus on burial history (beyond writing that “burial history and thus compaction affects porosity, permeability, and mechanical properties”).
Introduce why you focus on “claystones”, in other words, explain why this is the most appropriate among the several different rock types (sandstones, limestones, evaporites, magmatic, metamorphic rocks).
52: why “up to 1 Ma”?
53 and 92-94: although most literature support the idea of “the deeper the less porous”, the last decade dedicated to study the claystones/mudstones for Unconventional Resources demonstrated that some of these rocks get lithified before compaction, this understanding has implications for the idea of “the deeper the less porous”. My colleague Macquaker and I wrote few weeks ago a piece about it in Marine and Petroleum geology and I hope this can help you with a different point of view on the same rocks we are analyzing in different industries. Maybe something to tie with your comments in lines 68-70. Also lines 75-79 may take in consideration the new literature derived from the Unconventional plays (it would be wonderful from their point of view if fractures would contribute to permeability, as they would produce more hydrocarbon resources; unfortunately the role of fractures does not go into that direction).
71 -537: examples where nomenclature may be improved/refined. E.g. use “as muds are compacted to lower porosity mudstones…” 537:“ as argillaceous mudstone”. In sum, the rocks you are describing are mudstones (grains up to 64 micrometers, including grain size of clay -up to 4 micrometers- and silt -between 4 and 64 micrometers). They are not claystones (whose components are up to 4 micrometers). As an example, if a rock is composed by large clasts made 100% by clay minerals is not considered a claystone, it is a conglomerate.
117: “mineralogically homogeneous claystone” explain the scale you are thinking about when using the term homogenous (e.g. bed scale, lamina scale, grain scale, cement scale, pore scale). Mention also the geological time encapsulated in the thickness of the formation taken in consideration (Amaltheenton Fm). This would make you wonder on the likelihood of “homogenous” environments of deposition (i.e. if the formation was deposited in 1 Million years, is it likely that the env of dep remained the same?)
118: “This formation should be i) accessible with shallow drillings (less than 100m)” explain why.
119: “and ii) covered by rock strata to minimize the influence of weathering processes “ Report the thickness of the “strata” covering the Amaltheenton in each borehole (use Table 1) and explain what is the assumed safety thickness to avoid weathering (if possible, reporting data supporting the thickness declared). Adjust with statement in line 165. Be ready to tackle the data from BO4.0 where the Amaltheenton is only covered by 6 m of Quaternary strata.
146 : report the duration of the “safe long-term conditions” in years
152: northern not Northern
Fig 3 and Fig 5: Specify what formation the vitrinite maps refers to. If vitrinite refers to the Amaltheenton Fm, then note figure 1c of this article https://link.springer.com/article/10.1007/s00531-024-02477-9 showing a data point with 0.8 vitrinite in the Amaltheenton formation that does not match with your maturity map in fig 3. I see there is no discussion around that specific value, but hope you may find the information useful.
232-248: clarify better that all the data are referred to another formation, not the object of study. Add a sentence highlithing the imporantce of the data from the Posidonia for the overlying Amaltheenton formation.
250 and 591: the measurements of porosity and permeability on rocks recovered more than 40 years ago (“core materials from the 1980´s drilling campaign”) rise a red flag. Rock was very likely dissecated and unsuitable for those measurements. Geologists and engineers working in Unconventional reservoirs have proven that those measurements are not reliable and so they spend millions of Euros/Dollars to collect fresh samples to have right values for their expectations on hydrocarbon production. Sealing radioactive waste might be considered more important than hydrocarbon production, hence this point should be clearly highlighted to the readers.
253: “Their results align with the previous studies on the organic-rich Posidonienschiefer-Fm” If you investigate the range of values both for the studies made by Gaus et al. (2022) on the Amaltheenton, and for the studies made by Mann (1987) on the Posidonia, you’ll discover that their different values represent different rock fabrics, in turn related to different environments of deposition. In this view, remember your previous statements on the “homogenous” environments of deposition (see comment on line 117).
260-264: are these statements conclusions from your work? Are they inferences? Or are they statements derived from previous studies?
552: are you sure that TOC “documents the increase in thermal maturity”?