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the Creative Commons Attribution 4.0 License.
| 23 Sep 2024
Poro-perm relations of Mesozoic carbonates and fault breccia, Araxos Promontory, NW Greece
Abstract. Aiming at assessing the porosity and permeability properties, we present the results of microstructural and laboratory measurements, including density, porosity, VP, VS, and electrical resistivity. These measurements were performed in dry and in saturated conditions on 54 blocks of Mesozoic carbonate host rocks and fault breccias collected in Greece. The host rocks consist of carbonate mudstones, wackestones, packstones, and sedimentary breccias from the Senonian and Vigla formations. These rocks exhibits average density values, low porosity values, and medium-to-high P- and S-wave velocities. Fault breccias originate from high-angle extensional and strike-slip fault zones, displaying a wider range of density, porosity values up to 5–10 times higher than host rock, along with ultrasonic velocities. Regardless of lithology, the carbonate host rocks might include vugs due to selective dissolution. Conversely, the fault breccia samples feature microfractures. Slight textural anisotropy is documented in the carbonate host rocks, while a higher degree of anisotropy characterizes the fault breccias. Selected samples were also tested in pressure vessels with confining pressure up to 80 MPa, revealing that transport properties along microcracks in fault breccias can significantly increase with increasing depth. To assess rock permeability and porosity-permeability relations, three different protocols were employed. Two of them were based on the Effective Medium Theory, where permeability was computed by inverting ultrasonic measurements, assuming an array of penny-shaped cracks embedded in an impermeable host matrix. The aspect ratio and crack width were obtained by the seismic measurements, modeling either by assuming all cracks as isolated or unconnected or all cracks connected into the network. The application of these two protocols showed a systematic variation of permeability with porosity. In contrast, the results of the third protocol, based on the digital image analysis outcomes only, did not exhibit systematic variation. This behavior was interpreted as a result of the not-selective dissolution of the outcropping carbonates causing a wide range of measured fracture aperture values. This study found that carbonate host rocks lacked a clear poro-perm trend due to the presence of stiff, sub-rounded pores and small vugs. On the contrary, fault breccia exhibited a linear increase in permeability with porosity due to a connected pore network including microfractures.
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RC1: 'Comment on egusphere-2024-2823', Anonymous Referee #1, 06 Oct 2024
This manuscript investigates the petrophysical characteristics of rock samples collected in NW Greece, with special attention to their role in geological formations Combining few structural analyses with laboratory measurements, the results of two-dimensional images and petrophysical analyses are synthesized to investigate various carbonate lithologies, calculate permeability values for different types of rocks, and compare them with experimental measurements. The results are discussed on the basis of direct observation of the connected pore space, and a scheme for the petrophysical analysis of carbonate rocks is proposed. The experimental part of the manuscript is well designed and supports the conclusions better, however, there are still some issues.
The first is the abstract section, which should begin with an introduction of the field to which this research belongs and highlight its importance in order to draw out the focus of this research. The current version is more oriented to what one did, which got some results, which will shed the interest of the general public and greatly reduce the impact of the manuscript. It is recommended that the authors that section be reorganized.
Sample collection and testing is an important part. The authors should consider to further describe the sampling strategy, I didn't see enough justification for the sampling choices in the manuscript, whether the samples are representative or not is an important thing.
Also, the discussion section should be emphasized. The discussion section of the current manuscript has four short summaries, which seem to be very detailed, but may have some problems. First, some contents are said and left out, and the discussion is not deep enough, such as 6.1 Deformation mechanisms is a good direction, but the content is too simple. Secondly, the discussion is limited to the direct analysis or simple extension of the experimental results, and the overall discussion lacks the part of the actual impact, which mainly stays at the level of the results of this experiment, which is a little bit unfortunate. Consideration should be given to exploring the impact of the results on carbonate reservoirs, such as potential applications for geothermal energy development.
Finally, there are some minor problems in the manuscript, such as problems with the citation format of the references. For example, line 100, MODIFIED FROM (Bourli et al., 2019). Please standardize the citation of references throughout the manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-2823-RC1 - AC1: 'Reply on RC1', Sergio Vinciguerra, 16 Dec 2024
- AC2: 'Reply on RC2', Sergio Vinciguerra, 16 Dec 2024
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RC2: 'Comment on egusphere-2024-2823', Anonymous Referee #2, 23 Nov 2024
The manuscript by Vinciguerra et al. comprises an experimental characterization of petrophysical properties, including density, porosity, Vp, Vs, electrical resistivity and microstructural analysis of carbonates and fault breccia in western Greece, with the aim to assess the porosity and permeability properties. The manuscript’s combined approach is of potential interest to EGUsphere readers but several issues must be addressed before it can be accepted for publication. Below I list some general comments and suggestions that might strengthen the authors’ interpretations of their results and several specific comments to be addressed when the authors revise the manuscript.
The main problem of the article is that the results and interpretations are presented quite segmented or isolated in a style of a working package results in a project and not in a synthesized overview, trying to link all the results of different analyses (analytical-experimental-modeling) and the uncertainties raised, in a complete interpretive model that would provide further recommendations for a proposed protocol that can be applied in similar case studies. In such an analysis, data don’t always need to be fitted perfectly in a graph, and even when they don't, most important is to decipher and discuss the underlying major factors that cause weak or non-systematic relationships. Furthermore, they don’t show the broader implication of their interesting approach with a comprehensive protocol for petrophysical carbonate analyses to adopt on similar case studies that can relate to potential applications on CO2 storage or geothermal energy as they state in their introduction.
It is well documented that carbonate rocks are highly heterogeneous and have undergone complex sedimentation under various depositional environments, affected by post-depositional diagenetic processes and tectonism and for all these reasons they develop a variable and complex pore type network with complicated combination patterns (vugs, fractures etc.) which create intricate pore-throat structures. The complex pore-throat structure leads to the complexity of poro-perm relationship. Commonly, for such analysis as presented in this study, except for thin section DIA and UPV measurements, there should be additional analysis of SEM images, mercury intrusion or μ-CT scan images, because it would give a much better insight on the 3D pore structure than the 2D-image analysis they performed.
Even reaching the end of the manuscript I am not quite convinced which are the main controlling factors of the weak poro-perm relationships. Are the different pore-throat structure types and their size that were not classified in detail? In this paper there is a lack of classification of the different pore-throat types. In order to have more reliable results for the poro-perm relationships there is a need to classify the different types of pore structure, their frequency distribution of pore- throat radius, (which can be multimodal, bimodal, centralized unimodal or asymmetric bimodal) and their connectivity. So effective porosity determination is crucial and is related to the determination of interparticle versus vuggy pores. Sometimes vuggy pores are evident, while other times are subjective and contain much uncertainty. In this study classification of vuggy pore space is not applied or described in more detail. Are there separate vugs interconnected only through the interparticle porosity or are touching vugs that form an interconnected pore system independent of the interparticle porosity? (see Lucia 1983). Thus pore segmentation is the first and also the key step for permeability estimation based on thin sections.
Another source of error may be the effective porosity determination which is related to pore size, since in images of thin sections obtained with optical microscopy, pores smaller than ∼10 μm are difficult to resolve. This is relative of course to the size of the dominant pore system in the samples. For a sample with dominant pore system of 50–100 μm or larger, smaller than 10 μm pores are either intraparticle pores or small interparticle pores, both of which do not significantly contribute to permeability but for a sample with smaller dominating pores (<50 μm), the inability to resolve pores smaller than 10 μm that can be part of the dominant pore network will lead to permeability underestimation. Thus, the effective porosity is more relevant to permeability than the total porosity. In contrast, poorly-connected pores, mostly vuggy pores, may account for substantial porosity, but do not affect permeability significantly thus should not be counted in effective porosity and need be excluded for permeability calculation. Consequently, only effective pores, which are interpreted to be interparticle pores in carbonates, should be included in both 2D and 3D permeability simulation, as several studies suggest. Those issues and uncertainties should be discussed at least in their analysis.
So, more information is needed about the pore-size distribution (PSD) and pore-throat types for each domain (Host rock-fractured host rock-breccia zone of fine breccia FZ) and which can be classified in different classes, as either cavity-fracture type, cavity type, pore-fracture type or simple pore type, and examined if each one type can be related to different poro-permeability relationship, instead of trying to fit all that in one best-fit line or curve.
Additionally, there is not much information about the various types of the diagenetic processes that these two main units (Senonian Limestones and Vigla limestones) have experienced, (i.e. early dolomitization or not, dissolution, cementation, etc.) which can have great influence in the pore-throat structure and consequently to the poro-perm relationship. Therefore, the authors should further examine and discuss the control of the various pore-throat types on the porosity- permeability relationship in order to establish a more reliable permeability calculation model.
Sampling uncertainties- Since those cubic samples only cover a small proportion of the reservoir study interval, this should be defined in which interval these measurements refer to. There is no field image showing how the samples were taken, in which Senonian or Vigla unit interval, how the samples are distributed along the brecciated fault zones and the sample locations on map or table with coordinates. Field images are not included to provide the reader with an objective overview of where the samples were collected. In such kinds of studies, it is important to provide some images of the outcrops, or at least representative ones in figs and a complete photo list of samples in a supplementary material file.
The interpretation of data in parts is not well presented or explained, and there is no correlation or at least discussion of their results with prior studies on poro-perm relationships in carbonates (e.g from Middle East or China etc), which could enhance the broader interest of the scientific community. As it stands now, it seems more like a site specific project outcome, which unlikely can give an insight or workflow protocol for a future study in other carbonate units.
Some parts of the analyses are not quite clear for what purpose they were conducted and what was the benefit for their analysis: for example the fractal dimension measurement and how significantly impacts the goals of this study should be mentioned. Except the quantitative aspect of it, what information can further give? In my point of view, fractal dimension can provide an indication of pore-surface complexity and scaling behavior of the object but this could be more reliable if it involves image analysis of SEM-BSE or results from m-CT 3D reconstruction of dense slices of images. In the same sense, the resistivity results were very surficially integrated to the rest of the results.
Further minor comments on text are listed below:
Line 21- textural anisotropy? What do authors mean by that? They assume that in different orientations the texture is different or they mean heterogeneity?
Line 22- imply that these selected samples are from different structural depths?
Often there is a wide variety of aperture width along the microfractures. From the studied thin sections is there any estimation of the % of the completely healed, partially filled or open microfractures?
Line 54- non-cohesive
Line 82- more relevant references should be referenced for the thrusting tectonics in FTB of Hellenides than the introductory article of Robertson and Dixon 1984, which deals mostly with ophiolite displacement and microplate tectonics in the eastern Mediterranean. In general, the Geological setting section needs an update and repolishing in references. There is nothing for the subsurface evaporitic diapirism that seems to play a crucial role for the deformation in western Hellenides FTB. Also only short reference is given for the units of Senonian and Vigla formations and their internal deformation, which are the main sampling units in this study.
Line 111- samples in proximity refer to fault damage zone (internal or external) or not?
Line 115- a cubic block taken normal to bedding comprises sections that are parallel to bedding strike-perpendicular to strike and parallel to bedding (x,y,z reference axes). Which sections were used for the 2D image analysis isn’t very clear for the reader. So, more details should be given on how the samples were chosen and which orientations were studied.
Additionally, at least a stereonet should be added in the main map, showing the regional bedding orientations, the fault zones that are discussed in the text and the main fracture set orientations for the Senonian and Vigla units.
Comments in sampling method.
Why were macropores avoided? And from what size and above? They don't contribute to the total porosity? (see also previous comments on porosity above). Since there are no graphs for pore size distribution for the samples, which is the dominant pore-size range for each sample? That is an important issue when you exclude specific pore size ranges.
Saturation of samples were not performed with vacuum? (less than 800 Pa for an hour). For grain mass Ms samples shouldn’t be dried? That I think refers to methodology of ISRM 1977, as far as I am aware.
Might be some issues with image analysis. See Fig. 3 bottom image pair where blue-epoxy pore space of two areas in bottom right are disconnected but in the bitmap image shown as well connected pores. Porosity measurements from images have a threshold limit, represented by the minimum detectable pore dimension. What is that limit in image analysis? What is the pixel size of thin-section images?
In 3.3.2. UPV section, what are the relative errors of the velocity measurements? Commonly a PCA analysis is done of density data and UPV values reordered along x,y,z direction to see if there are representative results for each block.
Line 268- something is missing from the sentence, please check.
Figure 4- samples should be rearranged in the figure either from FZ to the host rock or opposite and not mixed up (figs 4a-f). A scale or distance should be shown in the block model of the sampling area. Field image views of the outcrops that samples were collected from the fault zone and crush breccia zone would be valuable to add (either together with Fig.4 or in a Supplementary material section (together with sample coordinates)
Table 1 needs a legend explaining what is bis, pento a, pento b etc. AR 43 having identical % clast and % of matrix in both orientations seems a bit peculiar.
Line 307- is 2D optical porosity
Lines 308-329- all these data should be easier visualized if added in table 1 , showing which samples are from Senonian or Vigla, if they include stylolites, veins or fractures, porosity values and Do(pore) values.
Line 319- which sample is the fractured packstone?
Matrix and cement are considered the same here but are different in terms of the diagenetic history. What part (%) consists of the fine grained material and what part (%) is the binding cement material?
Authors mention pores aligned to veins? Or do they mean microfractures connecting moldic pores?
Line 354- normalization was performed with the formula xnormalized= (x-xmin)/range of x and then get the average?
Fig.6 I cannot see so clearly that trend which authors describe here. Most of the samples in FHR-FZ show higher density than HR-PFZ or HR-AFZ. This should be discussed further. Only CFB-FZ show significant decrease in density
Figs 6-7. A relative or average distance between main fault slip zone and rest of the HR domains should be shown.It comprises 10’s of meters or less? Are there any important shear zones in the FHR , CFB domains?
What are the differences of UPV measurements along the 3-orthogonal directions and how we can detect anisotropy along a specific orientation if all measurements are averaged for each sample? You discuss that in lines 394-395 but since you averaged all 3 directions this can't be validated. There is no information how the microfracture orientations are aligned with any of the 3-axes. Later in text, using equation 16 to calculate crack density requires cracks randomly oriented and distributed. I am not quite sure if your samples meet the conditions for perfect isotropic configuration.
Line 435- Cementation factor plays a more important role in the UPV and permeability relationship?
Line 491- ζ can't be aspect ratio and crack density also, something is labeled wrongly here. (or one is in italics and causes confusion?)
In Discussion, authors mention ‘’ the main mechanisms of samples experienced during deformation phases’’. Which are these deformation phases they refer to?
Citation: https://doi.org/10.5194/egusphere-2024-2823-RC2 - AC2: 'Reply on RC2', Sergio Vinciguerra, 16 Dec 2024
- AC1: 'Reply on RC1', Sergio Vinciguerra, 16 Dec 2024
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