the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Reconstructing 3D subsurface salt flow
- 1Geological Institute, EMR, RWTH Aachen University, Germany
- 2Institute of Geology and Geochemistry of Petroleum and Coal, EMR, RWTH Aachen University, Germany
- 3Forschungszentrum Jülich GmbH, Projektträger Jülich, Germany
- 1Geological Institute, EMR, RWTH Aachen University, Germany
- 2Institute of Geology and Geochemistry of Petroleum and Coal, EMR, RWTH Aachen University, Germany
- 3Forschungszentrum Jülich GmbH, Projektträger Jülich, Germany
Abstract. Archimedes' principle states that the upward buoyant force exerted on a solid immersed in a fluid is equal to the weight of the fluid that the solid displaces. In this 3D salt-reconstruction study we treat Zechstein evaporites in the subsurface of the Netherlands, Central Europe, as a pseudo-fluid with a density of 2.2 g/cm3, overlain by a lighter and solid overburden. 3D sequential removal (backstripping) of a differential sediment load above the Zechstein evaporites is used to incrementally restore the top Zechstein surface. Assumption of a constant subsurface evaporite volume enables the stepwise reconstruction of base Zechstein and the approximation of 3D salt-thickness change and lateral salt re-distribution over time.
The salt restoration presented is sensitive to any overburden thickness change irrespective if caused by tectonics, basin tilt or sedimentary process. Sequential analysis of lateral subsurface salt loss and gain through time based on Zechstein isopach difference maps provides new basin-scale insights into 3D subsurface salt flow and redistribution, supra-salt depocentre development, the rise and fall of salt structures, and external forces' impact on subsurface salt movement. The 3D reconstruction procedure described can serve as a template for analyzing other salt basins worldwide and provides a stepping stone to physically sound fluid-dynamic models of salt tectonic provinces.
Stefan Back et al.
Status: final response (author comments only)
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RC1: 'Comment on se-2021-153', Frank Peel, 01 Mar 2022
se-2021-153 Review 'Reconstructing 3D subsurface salt flow....' Stefan Back et al
To the editors: I have uploaded a pdf document with reviewer comments and sketch diagrams, which is addressed to the authors, and it should be shared with the authors. See "se-2021-153 Back et al reviewer comments v002.pdf"
This is a worthy manuscript containing good work. I recommend that it should be accepted subject to revisions that are described in detail in the attached document. However, there are problems with it as it stands , which need to be addressed.
The main weaknesses are
1. the restoration process used by the authors is not adequately described
2. the concepts that are described in words in the text are difficult to understand as written, but if simple explicative diagrams in the form of cross sections were added, this could be made very much clearer
3. the results of the analysis are shown ONLY in the form of maps of salt thickness changes. Again, if combined with geological cross sections extracted from the model, the results would be very much clearer and the impact would be easier to grasp.
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AC1: 'Reply on RC1', Stefan Back, 14 Apr 2022
Response to the comments made by Frank Peel (referee #1)
Dear Frank,
first of all thank you very much for all the work you put in reviewing “Reconstructing 3D subsurface salt flow” (se-2021-153)! We have rarely received such a through, informative and constructive review! Also many thanks for your sketches. These are super informative! We currently work on a revised document in which we will incorporate all suggestions from your side.
Here the response to your comments:
- “Addition of cross sections to the paper” (your page 1):
Please find in the attached pdf a revision of all figures – now including 4 cross-section figures directly based on your cartoons.
- Figure 1 now includes two cross sections (b and c) that illustrate the difference between a basin in Archimedean equilibrium and an actively withdrawing supra-salt basin. We have furthermore added some simple formulas for further understanding. We will use this figure in the revised document to better discuss why we use the static equilibrium model for our salt reconstruction (also see point 2).
- The cross sections of new figure 4 shows the restoration procedure of this paper, guided by sketch number 2 of your review. It is shown that the backstripping approach only includes unloading (and for scenarios 2 and 3 decompaction); this contrasts the classical vertical shear methods for palaeotopographic restoration.
- The cross sections of new figure 5 are 2D restorations extracted from the 3D restoration along seismic lines Norg XL 8000 and Twente IL 9000 (location on figures 2, 6, 7, 8, 9, 10).
- The cross section of new figure 9 shows the difference between the restoration approach of this paper and classic Airy-isostatic balancing based on a figure you sent informally by email.
- “Applicability of Archimedean equilibrium approach” (your pages 1 and 2):
We will adress this important point in the revised ms in the two last paragraphs of the introduction. Revised Figures 1b and 1c (see above and attachment) now allow to show and discuss the difference between a basin in static equilibrium and one which is not. You suggested that salt withdrawal and lateral salt flow in the Zechstein basin was rather small (in comparison e.g. to the GOM) and therefore likely allowed application of the equilibrium model; we will discuss this, and point out at the same time that the model procedure forwarded is also in the study area limited, e.g. when reaching piercement structures or in the very initial phase of post-salt sedimenttation (Early Triassic).
- “Better explanation of restoration procedure” (your page 4):
We will provide an improved description of the restoration procedure based on new figure 4. We will add all necessary detail to the originally too brief method section; e.g. your question about “the space above the top surface: air or seawater”. We originally restored everything subaerially. However, following our informal email exchange and looking at the restored top surfaces (residual topographies) of the model (-> please see new figure 10; might be shifted in place in the revised ms), we ran all restorations again using submarine conditions (seawater density above restored top surface). Remodelling was anyway necessary because of the use of wrong physical property values for the Chalk Group (see referee#2).
- “Restoration process independent of basement tectonics”:
We will clearify this in the revised paper. It is clear that any change in basement configuration will affect the current model; however, if known, where and what kind of change occurred, this could be introduced into the restoration procedure (lines 228 to 230 in the original ms).
- All comments on page 6:
We will go through these one by one and follow all; incase we disagree, we will state so explicitly in the cover letter accompanying the revised ms.
In summary, thank you very much for your very thorough and helpful review! Your review provided the base for 3 completely new figures, and one revised figure. The new residual topography figure 9 also results from a suggestion by you. The planned revision of the text will also benefit greatly from your comments.
Best wishes
Stefan et al.
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AC1: 'Reply on RC1', Stefan Back, 14 Apr 2022
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RC2: 'Comment on se-2021-153', Anonymous Referee #2, 10 Mar 2022
The paper presents an interesting novel methodology for understanding the general movement of salt during the evolution of a sedimentary basin. The manuscript is well written, and illustrated and acceptance is recommended following some minor corrections.
At the end of the introduction the motivation for conducting the study using the particular method of analysis could be addressed in a clearer way. It is more apparent in the abstract than the introduction.
Line 90 - In all cases the present-day cumulative average
density of the column of vertical overburden was lighter than the evaporate substratum (fluid 90 with ρ = 2.2 g/cm3)
So why does Table 1 show the overburden with higher densities except for the Chalk? I suspect it is due to porosity, but if this is the case it needs to be made clear in the text. I do wonder then about how the densities are represented – for example the density of the Upper North Sea Group is given as 2.65, which is the density of pure quartz…..so presumably porosity is not taken into account. But the 2.2 density for the Chalk is not close to the density of calcium carbonate and flint, so I presume microporosity is being taken into account for chalk. Hence the treatment of density seems to be inconsistent.
Line 129 minor changes in
144 FP comprise the main
145 200 m thickness of
146-147 – I am not sure that present tense is best here.
170-173 – change : and ; to full stops.
210 approximation of the top Zechstein as a………depth of the base
212 – the assumption that the top Zechstein formed at base level…………location of the base Zechstein determined from substruction of the restored………..from the present-day
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AC2: 'Reply on RC2', Stefan Back, 14 Apr 2022
Response to the comments made by referee #2
Dear referee #2,
thank you very much for your straightforward, constructive and positive review of the ms “Reconstructing 3D subsurface salt flow” (se-2021-153).
Here is our response to your comments:
- “Introduction, motivation for conducting the study can be addressed in a clearer way”:
We currently work on a revised document in which we will address the motivation for conducting the study more clearly. We will stress that the method proposed provides insights into 3D subsurface salt flow and redistribution on basin-scale, the rise and fall of salt structures and associated depocentre development, and external forces' impact on subsurface salt movement.
- “Table 1, density”, but also Young’s Modulus, Poisson Ratio of Chalk:
The physical properties for the Chalk were wrong. This error was not only in table 1, the same mistake was also in the lithological model used for backstripping and decompaction. We consequently re-ran all restorations of the study with revised chalk values (from onshore NL) provided by Hunfeld et al. (2021). Please find in the attached pdf the revised table and revised model results (e.g. in figures 5; 6; 7; 8; and 10). Please note that the revised models additionally contain a change suggested by reviewer Frank Peel: surfaces that were restored to a level below zero were treated as submarine, contrasting the original model (fully subaerial restoration). Yet, the new model results are quite similar to the original restoration results.
- “present-day cumulative average density”:
Yes, this is grain density + porosity; we will make clear in the upcoming revisions.
All other suggested changes (lines 129, 144, 145, 146-147, 170-173, 210, 212) will go into the revised manuscript.
Again, thank you very much for your helpful review, particularly for exposing the mistake in our lithological model!
Best wishes
Stefan et al.
Reference: Hunfeld, L.B., Foeken, J.P.T., and van Kempen, B.M.M.: Geomechanical parameters derived from compressional and shear sonic logs for main geothermal targets in The Netherlands. TNO: https://www.nlog.nl/sites/default/files/2021-12/data_selection_and_methods.pdf, last access: 11.04.2022.
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AC2: 'Reply on RC2', Stefan Back, 14 Apr 2022
Stefan Back et al.
Stefan Back et al.
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