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
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:
- “Collectively, the initial findings … characterize the Amaltheenton Fm as a … homogeneous claystone formation... Thermal maturity quantifications…corroborate variable … burial depths, following a SE-NW trend. Both findings are vital for the MATURITY project and highlight the suitability of the Amaltheenton-Fm for the stated objectives.”
- “The homogenous mineralogical composition and the gradually increasing trend of maximum burial are the two key prerequisites to conduct detailed parameter studies on the relationship between burial history and changes in the physical rock properties.”
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”?
Citation: https://doi.org/10.5194/egusphere-2025-579-RC1 -
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
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 577 | 113 | 16 | 706 | 63 | 72 |
- HTML: 577
- PDF: 113
- XML: 16
- Total: 706
- BibTeX: 63
- EndNote: 72
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1