# Porosity and pore and throat size distributions in carbonate-rich salt caprock of halite diapirs: effect of deformation and geochemical processes

P Závada, M Staněk, M Machek, S Adineh, Yves Géraud, J Bruthans, S Heuss-Aßbichler, M Zare

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## ► To cite this version:

P Závada, M Staněk, M Machek, S Adineh, Yves Géraud, et al., Porosity and pore and throat size distributions in carbonate-rich salt caprock of halite diapirs: effect of deformation and geochemical processes. Journal of Structural Geology, 2025, 199, pp.105483. 10.1016/j.jsg.2025.105483. hal-05309990

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# Porosity and pore and throat size distributions in carbonate-rich salt caprock of halite diapirs: effect of deformation and geochemical processes

P. Závada<sup>a,∗</sup>, M. Staněk<sup>a</sup>, M. Machek<sup>a,†</sup>, S. Adineh<sup>b</sup>, Y. Géraud<sup>c</sup>, J. Bruthans<sup>b</sup>, S. Heuss-Aßbichler<sup>d</sup>, M. Zare<sup>e</sup>

## ARTICLE INFO

### Keywords:

Salt caprock

Porosity

Permeability

Pore throat size distribution

Carbonate stringer

Salt glacier

## ABSTRACT

Microscopic porosity of salt caprock on two salt diapirs, Karmostaj and Siah Taq in southern Iran, was studied on a range of samples that represent the major lithological types of salt caprock in the area: the gypsum matrix-supported and clast-supported dissolution breccia, the dolomite and limestone stringers of the Hormuz Formation and gypsum mylonites. Mercury intrusion porosimetry alongside the microstructural analysis with digital image analysis of resin-saturated thin-sections was employed and revealed a large variation in porosity of 2–35 % and in the median throat size of 0.1–50 µm. The highest porosity values are associated with vuggy dolomites and limestones interspersed by a dense network of gypsum veins and gypsum matrix-supported breccia with low degree of deformation. In contrast, low porosity is linked with strongly deformed gypsum matrix-supported breccias and dark micritic carbonate stringers that were not interspersed with the gypsum. Large variation in porosity and pore throat size and pore shape is attributed to 1) metasomatization of carbonate to gypsum due to reaction of H₂SO₄ with carbonate after oxidation of H₂S or elemental sulfur in caprock and subsequent dissolution of gypsum in metasomatized carbonates creating the abundant vuggy porosity and 2) deformation of the caprock that was responsible for recrystallization and mechanical closure of pores in the frontal, downslope margins of both reactivated diapirs. While the deformed and gypsum-rich parts of the diapir caprocks (~30–60 % of caprock on the diapirs) are associated with estimated permeabilities of ~10⁻¹⁶ – 10⁻¹⁵ m², caprocks in central parts of reactivated diapirs with less deformed caprock (undeformed gypsum- and clast-supported breccia) display higher permeabilities ranging between ~10⁻¹⁵ m² and 10⁻¹³ m².

## 1. Introduction

Salt caprock represents a solid residue that forms by dissolution of rock salt in salt diapirs at contact with meteoric fluids or deeper connate fluids from the surrounding rock formations (Posey and Kyle, 1988; Warren, 2016). It usually envelops the roof and shallow lateral margins of salt diapirs (Fig. 1). The term ‘salt caprock’ is essentially different from ‘caprock’, which is in applied geosciences attributed to a layer of hard impermeable rock overlying and often sealing a deposit of oil, gas, coal or artificial repository of CO₂. Quite the opposite, the salt caprock can be locally highly porous and can itself become a reservoir of

hydrocarbons, or host the deposits of sulfur or metals (cits. Halbouty, 1979; Kyle and Posey, 1991; Warren, 2016). For simplicity, we will refer to the “salt caprock” as “caprock” in this study.

Salt caprocks were first documented in onshore salt diapirs of the Mexican Gulf salt province (Goldman, 1933; Posey and Kyle, 1988) and indicated a simple structure of laminated anhydrite overlain by a layer of gypsum and variegated calcite. The calcite forms by replacement of the anhydrite layers when the hydrocarbon-charged fluids enter the salt-caprock environment (Fig. 1)(Posey and Kyle, 1988). In contrast to the caprocks developed from the Louann salt at the Mexican Gulf or the Zechstein salt in Germany (Borneman et al., 2008), emergent salt diapirs

<footnote><sup>a</sup> Institute of Geophysics ASCR, Boční II 1a/1401, Prague, Czech Republic</footnote>

<footnote><sup>b</sup> Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Praha 2, Czech Republic</footnote>

<footnote><sup>c</sup> GeoResources Laboratory, University of Lorraine, 54500, Nancy, France</footnote>

<footnote><sup>d</sup> Department of Geo and Environmental Sciences, Ludwig-Maximilians-University of Munich (LMU), Munich, Germany</footnote>

<footnote><sup>e</sup> Department of Earth Sciences, Shiraz University, Shiraz, Iran</footnote>

<footnote>This article is part of a special issue entitled: Janos L. Urai published in Journal of Structural Geology.</footnote>

<footnote>* Corresponding author.
E-mail address: zavada@ig.cas.cz (P. Závada).</footnote>
in the Zagros Fold and Thrust Belt (ZFTB) in Iran reveal a complex mélange of sedimentary breccia, which consists of various fragments or stringers of intrasalt layers embedded in gypsum or anhydrite-rich matrix (Warren, 2015; Závada et al., 2021). Most abundant stringers in the Iranian salt diapirs consist of carbonates, primarily the dark dolomites or limestones, siliciclastic layers - purple colored or green marlstones, siltstones and sandstones. The sequence also contains various exotic blocks of igneous rocks (Fig. 1A)(Kent, 1979; Bosák et al., 1998; Bruthans et al., 2009; Závada et al., 2021). In general, two types of caprock were identified in the diapirs of southern Iran, which are investigated in this study (Warren, 2016; Závada et al., 2021): the sulphate matrix dominated breccia, called floatbreccia, and the clast supported dissolution breccia, called packbreccia (Fig. 1). While the floatbreccia is likely formed after the dissolution of rock salt with anhydrite, the packbreccia accumulates after dissolution of the salt with abundant clastic components and only minor sulphates (anhydrite).

In general, the structure, composition and physical properties of salt caprock are still poorly known and likely depend on the composition of the source salt layer, the degree of deformation and the alteration processes driven by external fluids that percolated through the caprock. Physical properties of the salt caprock, namely the permeability, are significant for safety assessment of various facilities constructed in salt diapirs for storage of liquid or gas hydrocarbons or waste materials worldwide (Karably et al., 1983; Seni et al., 1985; Hansen and Leigh, 2011; Borneman et al., 2008; Lord, 2013; Duffy et al., 2023). Earlier technical reports (Karably et al., 1983; Seni et al., 1985) identified large differences in the salt caprock permeability between the basal anhydrite layer section and the overlying upper calcite zone. Locally large porosity was reflected in the poor drillcore recovery in the calcite rich part above the anhydrite section. The drillcores manifested porosity of 1.3–13 % and strongly anisotropic permeability: up to $1.6 \times 10^{-17} \text{ m}^2$ in the vertical and $10^{-20} \text{ m}^2$ in the horizontal direction, respectively. In contrast, the porosity and permeability measured on the recovered drill cores in the anhydrite zone was very low, 0.8–3.3 % and $10^{-20} \text{ m}^2$, respectively (Karably et al., 1983). Karably et al. (1983) also suggested that both the brittle and ductile deformation may play a significant role in modifying the initially low porosity in the sulphate-rich zone. The combined brittle and ductile deformation of gypsum was identified in the salt caprocks of the subsurface Gorleben salt diapir in Germany (Borneman et al., 2008) and emergent diapirs in Iran (Závada et al., 2021). In addition, the fluids that percolate through the caprock can promote further porosity development during deformation by overpressure driven fracturing. This is manifested, for example, by the densely interspersed gypsum veinlets developed in the close surroundings of thrust zones formed by

mylonites in the frontal margins of the caprock bearing salt glaciers (Závada et al., 2021).

In this paper, we quantify the porosity from salt caprock on salt diapirs in Southern Iran, the Karmostaj and Siah Taq, respectively, described in detail by Závada et al. (2021) (Fig. 2), using microscopic methods and mercury intrusion porosimetry (MIP) on hand specimens (5 cm wide drill cores). We evaluate this porosity in terms of rock composition, relative degree of deformation and the role of permeating fluids. The study aims to present an overview of microporosity for the major rock types forming the salt caprocks in Southern Iran and the physical processes related to formation and modification of this microporosity. The study focuses on the two groups of sedimentary breccias which represent the most abundant rock types in the caprocks of these two studied diapirs - the gypsum-matrix supported floatbreccia and clast-supported "packbreccia". In addition, microporosity was also investigated in the occasional carbonate stringers embedded in the caprock and gypsum mylonites. Our analyses indicate that the original caprock porosity in gypsum-matrix dominated caprock (floatbreccia) decreases with increasing strain manifested by development of deformation layering. In case of the carbonate stringers formed by both dolomites and limestones, their permeability was likely promoted by $H_2S$ rich fluids derived from the sulphate reduction reactions. Replacement of carbonates by gypsum (sulphates) is a direct consequence of the intervening fluids in oxidizing conditions.

## 2. Geological setting

Zagros Fold and Thrust Belt in Southern Iran hosts about 130 emergent salt diapirs (Edgell, 1996; Bosák et al., 1998; Jahani et al., 2009). These structures stem from the originally up to 2500 m thick layered evaporite sequence of the Precambrian-Cambrian age at the base of the Phanerozoic sedimentary sequence of the Persian Gulf Basin - the evaporitic Hormuz Salt Series (HSS, Fig. 2). The HSS consists of two subunits, 1) the Lower Hormuz Salt containing thick beds of multicolored salt and anhydrite interbedded with dark banded dolomites and thin beds of purple or green colored sandstones, magenta colored siltstones and marlstones, and 2) the Upper Hormuz Salt consisting of thick beds of coarsely crystalline multi-colored pure rock salt without any carbonate or siliciclastic interlayers (Talbot and Alavi, 1996; Závada et al., 2015). Dismembered layers of bedded sediments, called stringers, represented by the dolomite, siliciclastics (sandstones and siltstones) or blocks of igneous rocks are frequently found in the caprock of the many salt diapirs presently exposed in the ZFTB (Talbot and Alavi, 1996; Bosák et al., 1998; Závada et al., 2015).

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Schematic diagram illustrating the structure A) and deformation style of the caprock on salt diapirs in Iran during their Cenozoic reactivation (B,C). The caprock consists of the dissolution breccia comprising large blocks of carbonates and siliciclastics and was derived from dissolution of the layered evaporite sequence of the Infracambrian Hormuz Complex.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Position of investigated salt diapirs, the Karmostaj and Siah Taq, in the Zagros Fold and Thrust Belt in Iran (A,B) and their simplified geological map (C). The map in (C) also includes the simplified structural cross-section through the Siah Taq diapir (X-X') and locations of the studied samples (C). Map in (A) modified after Talbot and Alavi (1996) and Tavakoli-Shirazi et al. (2013).</figcaption></figure>

The two diapirs investigated in this study, the Karmostaj and Siah Taq diapirs represent extrusive salt landforms (Fig. 2) that consist of rock salt around their summit portions and deformed caprock in their lower lying portions (Závada et al., 2021). The caprock is formed by the gypsum matrix supported dissolution breccia that surrounds the large blocks or various deformed stringers of the Hormuz Salt Series. Závada et al. (2021) also identified a strain gradient along a profile on the Siah Taq diapir with deformation of the caprock culminating in a zone of imbricated salt and caprock slivers, several meters thick gypsum mylonites and gypsum-coated thrusts, all compatible with radial spreading of the caprock over the synclinal deposits in front of the advancing extrusion. Deformation of the caprock is primarily manifested by zones of fluid-assisted ductile deformation of the fine-grained gypsum, the gypsum mylonites, alternating with layers of the gypsum matrix-supported dissolution breccia (floatbreccia).

# 3. Methodology

## 3.1. Sample collection

Total 14 samples were collected during structural mapping of both diapirs (Závada et al., 2021) (Fig. 2) using a portable accumulator powered drilling machine with a 5 cm wide diamond core drilling bit. Samples were excavated from 5 cm depth of the outcrop walls. They were cut with a water-cooled diamond blade and immediately dried with a paper towel and a hair dryer and then dry-polished. This procedure is similar to the processing of rock salt samples for thin-sections (e.g. Schléder and Urai, 2007). To remove the powder in cavities after dry-polishing, the samples were vacuum-cleaned.

A total of 30 specimens prepared from the samples were successfully analyzed by the mercury intrusion porosimetry. The preparation of the specimens consisted of cutting prisms of approximate dimensions of 1x1x2 cm out of the sample by a diamond blade saw. Where available,
several specimens per sample were prepared in order to capture any
possible variation of measured properties based on sample microstruc-
ture and composition in subdomains of the rock or to obtain backup
specimens in case of failed experiment. Except for the sample of VL
(Vuggy Limestone), the specimens from a given sample are considered
representative of the sample as a whole. From the sample VL, a specimen
from each of its two distinct parts - a gypsum rich and calcite rich,
respectively, was prepared.

Five samples of massive carbonate stringers were collected in the Siah Taq diapir. Among them, two represent the massive dark dolomite (D1, D2), two were collected in the southern part of the Siah Taq diapir from blocks of dark vuggy dolomite with microscopically observed porosity (VD1, VD2), one is a vuggy limestone (VL) (Fig. 3A and 4A,G). The other samples represent the matrix-supported ‘floatbreccia’ with abundant gypsum matrix (e.g. Fig. 4B and C; in total five samples), clast-supported ‘packbreccia’ (two samples, P1, P2 Figs. 3B and 4E) and laminated fine-grained and strongly deformed gypsum with abundant fragments of carbonates aligned in planes parallel to the deformation layering - ‘gypsum mylonite’ (two samples, GM1, GM2 Fig. 4D,F). The set of floatbreccia samples can be divided into two subgroups - A) relatively undeformed samples with homogeneous texture or with only locally distinct deformation layering (‘floatbreccia undeformed’ - FU1, FU2) and B) strongly deformed samples characterized by well-defined planar fabrics of gypsum layers anastomosing around fragments of carbonates or siltstones. The genetical terminology related to sample description is adopted from Warren (2016) and Závada et al. (2021). Location of the sampling sites is indicated in the geological map of Fig. 2.

3.2. *Thin-section preparation and image analysis*

Samples were first saturated with a fluorescent epoxy resin in a
vacuum chamber and standard polished thin-sections were prepared.
Micrographs mosaics of thin-sections were collected 1) under plane-

polarized light and UV light using a Nikon 80i polarizing microscope and 2) under a microscope equipped with cathodoluminescence CITL Mk5-2 CL microscope stage (http://www.cathodoluminescence.com/) working at 700 mA with a capture time of 1-1.5 s at the Czech Geological Survey in Prague. We adopted the terminology after Scholle and Ulmer-Scholle (2003) for the microstructural description of the caprock samples. Only the pores infilled with epoxy resin (yellow in transmitted light) are considered to be primary.

Image analysis using ImageJ Fiji image processing package distri-
bution was carried out on the transmitted light micrographs of various
microstructures of the vuggy limestone and dolomite, the packbreccia,
undeformed floatbreccia, deformed floatbreccia and gypsum mylonite
(Schindelin et al., 2012). HSB color thresholds were used to produce
black and white images of the epoxy resin saturated pores. The accuracy
of the filtering has been visually confirmed. The resulting images were
digitized using automated procedure in QGIS Desktop 3.40.4 software to
get polygon shapefiles representing the pores. The latter were further
analyzed using PolyLX, a Python package for visualization and analysis
of digitized 2D microstructures (Lexa and Landa, 2024). The pore size is
represented by the Equal Area Diameter (EAD) and the shape of the
pores is described using the Aspect Ratio (AR). The EAD is defined as the
diameter of a circle with equal area as the polygon. The AR is the ratio of
the long axis to the short axis of an ellipse fitted to the polygon outline.
The AR has been used to separate vuggy and fenestral types of pores.

3.3. *Scanning electron microscopy*

The thin-sections were also submitted to Back Scattered Electron
(BSE) imaging and phase identification using scanning electron micro-
scope (SEM Tescan VEGA) equipped with the energy dispersive spec-
troscope (EDS X-Max 50, Oxford Instruments) at the Institute of
Petrology and Structural Geology (Faculty of Science, Charles Univer-
sity, Prague) with 15 kV accelerating voltage and 1.5 nA beam current.

<figure><img src="image_5.png" /></figure>

**Fig. 3.** Outcrop in the southern part of the Siah Taq diapir (for VD1, VD2 sample locations see the map in Fig. 2) that exposes the interface between the rock salt containing numerous fragments of magenta colored arcosic sandstone and carbonates ('dirty salt', B) and the overlying caprock. The salt caprock consists of locally folded (B) layered anhydrite-rich caprock with block of dark vuggy dolomite blocks with faint layering (A) and macroscopic porosity of up to millimetre-wide pores (vugs) visible on freshly broken surfaces. Locations of corresponding samples of vuggy dolomite and anhydrite-rich packbreccia are indicated. Domains with visible layering in the vuggy carbonate block are indicated by white dashed line in (A). Layering in the anhydrite-rich caprock (B) is indicated by black dashed contours, blocks of siltstone in the dirty salt are marked by magenta colored contours and the interface between anhydrite caprock and dirty salt is indicated by a solid black line. Planar fabrics in the anhydrite-rich section are locally folded (B) and contain numerous dark carbonate fragments, partly replaced by sulphates. The overlying dark vuggy dolomite block is characterized by alternating dark gray layers of the carbonates and white/gray layers that are enriched in gypsum (A, inset).
<figure><img src="image_6.png" /><figcaption>Fig. 4. Outcrop photographs of the different rock types sampled for the current study. Fragmented dark dolomite stringers surrounded by gypsum with remnants of dolomite layers (A), outcrop of a floatbreccia with relatively low degree of deformation and surrounding clasts of carbonates with variable amount of gypsum (Floatbreccia Undeformed) (B), deformed floatbreccia with lense-shaped aggregates of gypsum surrounding abundant clasts of siltstone and dolomite (C), layers of gypsum mylonite alternating with packbreccia (D), homogeneous domain of packbreccia crosscut by gypsum veinlets in middle-frontal part of Siah Taq (E), alternating layers of gypsum mylonite and packbreccia in the frontal portion of Siah Taq (F), and vuggy limestone stringer interspersed with a dense network of gypsum (G). Corresponding sample numbers and their locations are indicated.</figcaption></figure>

## 3.4. X-ray powder diffraction (XRD)

Fragments of the samples were pulverized and homogenized and analyzed by XRD at the Geological Institute of the Czech Academy of Sciences. Powder X-ray diffraction data were collected with a Bruker D-8 DISCOVER θ-2θ diffractometer in reflecting Bragg-Brentano geometry. The diffractometer is equipped with a Johansson-type primary monochromator and a linear silicon strip detector LynxEye. Long-line fine-focus copper tube was operated at 40 kV and 40 mA. The powder specimens were deposited on low-background silicon sample holders and data acquired within the angular range from 5 to 70° 2q with a 0.02° 2q step and counting time of 1.2 s per a step. The detector opening was 2.93°. Mineral identification was carried out with DIFFRAC EVA v. 4.2 coupled with the PDF-2 database (version 2011). Semi-quantitative estimates of modal phase compositions were carried out by a Bruker proprietary procedure based on a reference-intensity-ratio approach implemented in the DIFFRAC EVA software.

## 3.5. Mercury intrusion porosimetry (MIP)

MIP was used to measure the connected total and free porosity, the pore throat size distribution and the bulk and skeletal density using the

Micromeritics Autopore IV porosimeter at the GeoRessources Laboratory (University of Lorraine, France). The principles of the method are summarized in Fig. 5. The MIP reflects the size of microscopic apertures connecting the pores in the rock - the pore throats - using the relation between the pore throat diameter *D* (m) and the pressure applied on mercury based on the Young–Laplace equation (Lenormand et al., 1983; Washburn, 1921):

$$D=(-4\gamma \cos\theta)/p \quad (1)$$

where $γ$ is the mercury surface tension (485 dyn cm⁻¹), $θ$ is the contact angle between mercury and solid (130°) and $p$ is the pressure (Pa). Taking into account the minimum and maximum pressure enabled by the porosimeter, the resulting volume vs. throat size curves cover throat-size range from 300 µm, which corresponds to the vacuum at the start of the experiment, down to 5 nm at the highest pressure (Fig. 5B). Any void space within the specimen accessible from its external surface by connections of higher or lower size than these measurement limits is therefore not measured by the method.

It has been demonstrated that if large pores are accessible by small throats, the mercury forced into the pore remains in it after the mercury pressure is decreased to atmospheric pressure (Wardlaw and McKellar,
<figure><img src="image_7.png" /><figcaption>Fig. 5. Mercury intrusion porosimetry (modified after Staněk and Géraud, 2019). The MIP takes advantage of the fact that mercury is electrically conductive non-wetting liquid. During the measurement, mercury is forced into the voids of an air-evacuated specimen by step-wise application of external pressure. At each pressure increment, the volume of mercury intruded into the specimen is calculated from measured changes in electrical capacitance of the metal-coated stem of a penetrometer (specimen holder) due to the replacement of mercury by electrically not conductive air or oil (A). Plot of an idealized cumulative porosimetric curve to explain the measurement of connected total, trapped and free porosity by mercury reintrusion (B); note the position of the volumetric median throat size (MTS) - half of the connected total porosity volume is accessible by larger throats than the MTS, half by smaller throats; a–d relate to (C). Schematic drawings of void spaces in a rock specimen to explain the phenomenon of mercury entrapment with respect to different arrangements of throats in relation to pores (C). DT: throat diameter, DP: pore diameter, arbitrary units.</figcaption></figure>

1981; Yu and Wardlaw, 1986a, 1986b; Staněk and Géraud, 2019). To take advantage of this phenomenon called trapping, the MIP was executed in three phases (Fig. 5B): (I) intrusion – the pressure increases from vacuum to the maximum, (II) extrusion – the pressure decreases down to atmospheric and (III) reintrusion – it increases again to the maximum value. The volume of intruded mercury after the intrusion corresponds to the total connected porosity (stage c in Fig. 5C). The volume that remains in the specimen at ambient pressure at the end of the extrusion corresponds to the trapped porosity represented by por-e-throat arrangements with low throat-to-pore size ratio. Analogously, voids with high throat-to-pore size ratio, including cracks, are free of mercury at this stage since they do not entrap mercury (stage d in Fig. 5C). Finally, when the pressure is increased again, the mercury can intrude only voids where trapping has not occurred and thus intrudes the free porosity only. The free porosity is calculated as the difference between the measured porosity at the end of the reintrusion and the measured porosity at the start of the reintrusion. However, since the mercury re-intrusion is technically available during the high pressure analysis only, the free porosity values represent only the microporosity of the specimens (i.e. accessible by throats smaller than 6 µm). The total porosity and its volumetric median throat size (MTS) value were used as input for the calculation of permeability using the model of Katz and Thompson (1986, 1987):

$$k=(0.61/178) \Phi D^2 \quad (2)$$

where k is the permeability (m²), Φ is the porosity (dimensionless) and D is the throat size (m). This approach including comparison of

permeability values calculated from MIP data with values measured directly was used for various rock types (e.g. Comisky et al., 2007; Gao and Hu, 2013; Okazaki et al., 2014; Staněk and Géraud, 2019). For a more detailed overview of various techniques for calculating the permeability from the MIP data see Costa (2006), Buiting and Clerke (2013), Nooruddin et al. (2014), Ghanizadeh et al. (2015) and Zhao et al. (2015).

## 4. Results

### 4.1. Sampling sites and rock description

The sampling strategy was to collect a range of samples from the major caprock types of the caprock and analyze the evolution of their physical properties related to the structural evolution recorded at Kar-mostaj and Siah Taq diapirs described in detail by Závada et al. (2021). The sampling comprised zones of relatively low and high deformation representing the different sections of the crestal central plateau area and the frontal portions of these flow structures, respectively (Závada et al., 2021). Although the bulk strain intensity increases to the frontal portions of these structures, the local deformation intensity is strongly variable. Lower deformation intensity can be found for example in strain shadows of more competent blocks (e.g. carbonate stringers). The deformation intensity on outcrop and hand-sample scale is estimated only approximately on the basis of the developed deformation layering, manifested by elongated gypsum aggregates surrounding the fractured clasts.
## 4.2. Rock composition and microstructure

The microstructure of salt caprock samples from Karmostaj and Siah Taq diapirs was first documented using optical microscopy, the cathodoluminescence and scanning electron microscopy. The microstructure is described according to subdivision based on composition and deformation degree to “Carbonate Caprock”, “Undeformed Caprock” and “Deformed Caprock”. The “Undeformed Caprock” represents gypsum-matrix or clast supported caprock breccia types with absent or slight evidence of deformation manifested by deformation banding and comprises both the floatbreccia and packbreccia samples. The “Deformed Caprock” comprises both the strongly deformed floatbreccia characterized by gypsum bands enclosing fractured clasts and the “Gypsum Mylonites”.

### 4.2.1. Carbonate stringers

The unaltered carbonate sample D1 is a micritic dolomite with grain size of 5–10 µm characterized by alternating bands rich in dolomite (53 vol%) and microcrystalline quartz (36 vol%) (Fig. 4A and 6). The dolomite aggregates contain minor interstitial grains of muscovite, apatite, pyrite and Fe- and Ti-oxide, fluorite and quartz. Intergranular pores, up to 15 µm in diameter, were identified using the SEM (SI-File, SI-Fig. 1A and B). The massive dark dolomite sample D2 consists of micritic dolomite (86 vol%) and microcrystalline quartz (12.5 vol%) intercalated with dolomite or forming lenses parallel to lamination traces and small amount of fine-grained white mica (1.5 vol%) (Fig. 6, Table 1).

Dark vuggy dolomite stringers are characterized by alternating dark gray layers of the carbonates and white/gray layers enriched in gypsum (Fig. 3A, inset). The vuggy dark dolomite samples VD1 and VD2 exhibit

complex replacement microstructures of dolomite by gypsum (Fig. 7A and B). The amount of replacement varies at the microscale from almost unaltered dolomite to zones formed solely by gypsum. The porosity of these samples is of vuggy or fenestral to channel-type with length up to 1 mm. In places dominated by gypsum, subangular dolomite relics are surrounded by medium-grained gypsum laths and elongated pores or elongated angular dolomite relicts separated by subparallel arrays of gypsum-filled fractures (sample VD1; Fig. 7A and B). In these fractures, the gypsum crystals are aligned with their axes perpendicular to the fracture walls. Porosity is predominantly present within the gypsum domains as intercrystal vugs or intercrystal channels oriented parallel to boundaries of gypsum grains and carbonate relics. The vuggy limestone sample VL also displays replacement microstructures of calcite by gypsum along fractures and of the vuggy and fenestral to channel-type porosity along the gypsum grains (Fig. 7C and D). The increasing degree of calcite replacement by gypsum is associated with increase in pore size and in proportion of porosity that becomes more interconnected. Quartz, chlorite, muscovite, anhydrite and iron oxide were observed as minor phases using the SEM in the vuggy calcite (SI-File, SI-Fig. 1C and D).

The size of pores in vuggy dolomite and limestone is characterized by size (EAD) distribution with median between 22 a 39 µm and mean between 30 and 54 µm as documented by image analysis of epoxy resin saturated thin sections (SI-File, SI-Fig. 2, SI-Tab. 1). The finer grained parts of microstructure exhibit smaller sizes of pores showing 14 µm as median and 22 µm as mean of the pore size distribution. Fenestral to channel-type of pores show systematically larger pore size, while vugs display smaller median and mean values than the other pore types. Networks of grain boundary and intergranular microfractures are

<figure><img src="image_8.png" /><figcaption>Fig. 6. Scanned images of the specimens selected for the microporosity study in the caprock of the salt diapirs.</figcaption></figure>
<caption>**Table 1**</caption>

Volumetric modal mineral composition estimates (vol%) of the analyzed samples acquired by X-ray powder diffraction (XRD) and semi-quantitative modal analysis.

<table><thead><tr><th rowspan="2">XRD data</th><th colspan="4">Carbonate stringers</th><th colspan="4">Undeformed caprock</th><th colspan="3">Deformed caprock</th></tr><tr><th colspan="2">Dolomite</th><th colspan="2">Vuggy Dolomite</th><th colspan="2">Vuggy limestone</th><th colspan="2">Packbreccia</th><th colspan="2">Floatbreccia</th><th colspan="2">Floatbreccia Deformed</th><th>Gypsum mylonite</th></tr><tr><th>Sample</th><th>D1</th><th>D2</th><th>VD1</th><th>VD2</th><th>VL</th><th>P1</th><th>P2</th><th>FU1</th><th>FU2</th><th>FD1</th><th>FD2</th><th>FD3</th><th>GM1</th></tr></thead><tbody><tr><td>Gypsum</td><td>1</td><td></td><td>48.0</td><td>67.9</td><td>55.3</td><td>24.3</td><td>8.5</td><td>88.5</td><td>33.7</td><td>48.1</td><td>30.3</td><td>57.5</td><td>96.8</td></tr><tr><td>Quartz</td><td>36</td><td>12.5</td><td>1.6</td><td>3.2</td><td>6</td><td>21.9</td><td>2.5</td><td>2.5</td><td>4.7</td><td>4.1</td><td>26.4</td><td>12</td><td>4.3</td></tr><tr><td>Dolomite</td><td>53</td><td>86</td><td>49.2</td><td>26.9</td><td></td><td>19.1</td><td>3.5</td><td>8.5</td><td>52.0</td><td>7.0</td><td>20.2</td><td>8</td><td></td></tr><tr><td>Calcite</td><td>2</td><td></td><td>1.2</td><td>1.9</td><td>38.7</td><td>1.2</td><td>13.5</td><td></td><td>4.6</td><td>1.4</td><td>7.8</td><td>5.5</td><td></td></tr><tr><td>Chlorite</td><td>3</td><td></td><td></td><td></td><td></td><td>4.0</td><td></td><td></td><td></td><td></td><td>6.2</td><td>8</td><td></td></tr><tr><td>Muscovite</td><td></td><td>1.5</td><td></td><td></td><td></td><td>5.6</td><td></td><td>70.2</td><td>3.2</td><td></td><td></td><td>3.5</td><td></td></tr><tr><td>Anhydrite</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>1.0</td><td></td><td></td><td>1.9</td></tr><tr><td>Celestine</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>1.8</td><td>0.7</td><td></td><td></td><td></td></tr><tr><td>Chabazite</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Orthoclase</td><td>5</td><td></td><td></td><td></td><td></td><td>7.8</td><td></td><td></td><td></td><td>6.1</td><td>3.2</td><td>4</td><td></td></tr><tr><td>Illite</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>1.6</td><td>3.8</td><td></td><td></td></tr><tr><td>Sylvite</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.4</td><td></td><td></td></tr><tr><td>Bassanite</td><td></td><td></td><td>1.7</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Albite</td><td></td><td></td><td>14.6</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Halite</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1.8</td><td></td><td></td><td>0.9</td><td></td><td></td><td></td></tr><tr><td>Clinochlore</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>3.7</td><td></td><td></td><td></td></tr><tr><td>Talc</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2.5</td><td></td><td></td><td></td></tr><tr><td>Hematite</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>1.5</td><td></td></tr></tbody></table>

connecting isolated large pores in the vuggy dolomite and the limestone (SI-File, SI-Fig. 3).

### 4.2.2. Undeformed caprock

The packbreccia sample P1 consists of subangular polymict lithoclasts of an arkosic sandstone (SI-File, SI-Fig. 1E) interspersed by gypsum veinlets. The fine-grained material between the lithoclasts is formed by smaller fragments of the same sandstone together with hematite (SI-File, SI-Fig. 1E and F) and porous aggregates of euhedral dolomite crystals with gypsum forming the cement of the lithoclasts' fractures (SI-File, SI-Fig. 1F). The lithoclasts contain small interclastic voids and intraparticle microfractures (Fig. 7E and F). Minor amount of small vuggy pores was also observed inside the gypsum-filled veinlets. The anhydrite-rich packbreccia P2 (Fig. 7G) is formed by large subangular to subrounded anhydrite lithoclasts consisting of a large subangular anhydrite and small rounded calcite grains, and subangular lithoclasts of arkosic sandstones with subrounded quartz grains and dolomite, K-feldspar and Fe-oxide grains intercalated with fine-grained anhydrite matrix (SI-File, SI-Fig. 1K and L). Bent carbonate lithic fragments consist of bands of fine-grained dolomite and coarse-grained calcite overgrown by anhydrite (SI-File, SI-Fig. 1J). The fine-grained matrix between the lithoclasts consists of gypsum, anhydrite, calcite, dolomite, K-feldspar, quartz, Fe-oxide and celestine (SI-File, SI-Fig. 1K). Up to 3 mm large zoned euhedral crystals of dolomite reveal bright fluorescent rims in the CL images (Fig. 7H). Large channel type pores line the lithoclast boundaries (Fig. 7G). Within the lithoclasts, significantly smaller intercrystal pores are present at mineral grain boundaries. The distribution of pore size (EAD) obtained for packbreccia is characterized by 10 µm median and 14 µm mean (SI-File, SI-Fig. 2, SI-Tab. 1). Larger size of pores is observed for more elongated fenestral to channel-type of pores. Thin epoxy resin infilled microfractures connecting large pore types are observed under the UV light (SI-File, SI-Fig. 3).

The undeformed floatbreccia samples FU1 and FU2 (Fig. 7I–L) are characterized by a coarse-grained gypsum matrix embedding large subangular lithoclasts of dolomite and micaceous siltstone, in addition to euhedral to subangular dolomite and calcite grains, subangular to subrounded quartz grains, magnetite, composite opaque grains of magnetite and pyrite together with celestine and baryte (Fig. 7I–L, SI-File; SI-Fig. 1M-R). The weak deformation of the gypsum matrix of sample FU1 is visible on the sample section (Fig. 6). In the thin-section, deformation is recognizable only by local alignment of lath shaped gypsum grains and aligned band-like aggregates of mixed fine-grained talc, K-feldspar, quartz, dolomite, chlorite, celestine and magnetite.

The intercrystal porosity is concentrated in the pressure shadows of lithic fragments and within the foliation parallel high porosity bands (Fig. 7I). Larger clasts or euhedral crystals of dolomite are lined by irregular channel porosity and some intercrystal porosity is present also in the gypsum matrix (sample FU2, Fig. 7K and L). The distribution of pore size (EAD) obtained for undeformed floatbreccia is characterized by median value between 35 and 51 µm and mean value between 45 and 63 µm (SI-File, SI-Fig. 2, SI-Tab. 1). Thin epoxy resin infilled grain boundary and intergranular microfractures connecting large pore types are observed under UV light (SI-File, SI-Fig. 3).

### 4.2.3. Deformed caprock

The deformation microstructure of the deformed floatbreccia (Samples FD1, FD2, FD3, and FD4) is characterized by foliation defined by large gypsum grains and foliation perpendicular tensional fractures filled by gypsum (Figs. 6 and 7M,N). The porphyroclasts of the floatbreccia are formed by large subangular lithoclasts of fine-grained dolomite and arkosic sandstone together with euhedral dolomite and baryte, subhedral calcite crystals and subrounded quartz grains (SI-File, SI-Fig. 1S and T). Fine-grained aggregates form bands parallel to the gypsum foliation. The mineral composition of these bands is similar to that of the dolomite and sandstone lithoclasts. Two types of porosity are recognized in the deformed floatbreccia. First, intercrystal vuggy or fenestral to channel-type porosity marks the pressure shadows of the porphyroclasts (Fig. 7M and N). These pores are aligned with the recrystallized gypsum grain boundaries and foliation parallel bands formed by aggregate of fine-grained dolomite, calcite, quartz, K-feldspar, muscovite, chlorite, tourmaline, magnetite and apatite (SI-File, SI-Fig. 1U–W). The other type of porosity is represented by pronounced and open foliation parallel fractures (Fig. 7M and N, SI-File, SI-Fig. 3) lining the band forming fine-grained aggregates. The pore size (EAD) distribution in deformed floatbreccia is characterized by lognormal distribution with median value between 35 and 37 µm and mean value between 40 and 48 µm (SI-File, SI-Fig. 2, SI-Tab. 1).

The microstructure of the gypsum mylonite samples GM1 and GM2 (Fig. 6) is characterized by elongated and up to 250 µm long gypsum laths forming the mineral foliation (Fig. 7O,P,Q). Relics of dolomite aggregates form either large porphyroclasts or fine-grained particles enclosed by the gypsum laths (Fig. 7Q). The microstructure of the mylonite GM1 reflects an intense non-coaxial deformation manifested by shear bands formed by highly elongated gypsum grains, asymmetric gypsum strain fringes in dolomite porphyroclasts pressure shadows and thin bands of fine-grained, silt-like material distributed along planes
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Micrographs of the selected specimens using plane-polarized light (PPL) and the cathodoluminescence (CL) of equivalent areas in the thin-sections. Note the decreasing abundance and size of the intercrystal and interparticle porosity from the vuggy carbonate (A,B) and vuggy calcite (C,D) to the anhydrite caprock (G,H), the undeformed (I–L) and deformed floatbreccia (M,N) and the gypsum mylonite (O,P). While the vuggy carbonates are characterised by the vuggy to fenestral</figcaption></figure>
porosity, interparticle cracks, channels and irregular voids are typical for the packbreccia and anhydrite-rich packbreccia (E-H) and the undeformed floatbreccia. Floatbreccia contains isolated intercrystal/particle vuggy porosity in pressure shadows of the clasts and clearly developed foliation parallel fractures (I-L). Mylonite contains isolated pores at triple point junctions of gypsum grain boundaries (O,P,Q). The black contour in C and D demarcates the boundaries of residual dolomite fragments surrounded by gypsum. Specimen location for the MIP in vuggy limestone sample VL (D) is indicated by dotted rectangles. PV-vuggy type pore, PF-fenestral type pore, PCH channel type pore, PVo-void type pores at grain boundaries and triple points, F-fracture, G-gypsum, Qtz-quartz, Dol-dolomite, Cc-calcite, LC-lithoclast, SC-siltstone clast.

parallel to the shear bands and foliation. Subrounded quartz grains and siltstone lithic clasts were also observed in the GM2 mylonite (SI-File, SI-Fig. 1X and Y). In both mylonite samples, the fenestral, vuggy and void types of porosity occur along gypsum grain boundaries while the fracture type porosity follows the deformation foliation. The distribution of pore size (EAD) obtained for gypsum mylonite is characterized by 7 µm median value and 9 µm mean value (SI-File, SI-Fig. 2, SI-Tab. 1). Network of epoxy resin infilled grain boundary and intergranular microfractures is connecting larger fenestral and vuggy pores (Fig. 7O and P)(SI-File, SI-Fig. 3).

### 4.3. Porosity and permeability properties

All acquired porosimetry data are summarized in plots presenting the 1) throat size distributions (TSD) (Fig. 8), 2) total porosity vs. median throat size (MTS) (Fig. 9A), 3) proportion of free porosity on the microporosity (throat size between 6 µm and 5 nm) versus free porosity (Figs. 9B) and 4) calculated permeability (Fig. 10). The measured bulk values and the calculated permeabilities for each specimen are indicated in the SI-File (SI-Tab. 1).

#### 4.3.1. Carbonate stringers

The **dolomite** (samples D1 and D2) features the lowest porosity within the sample set with an average value of 2.9 % and its average TSD is unimodal with a peak at 0.1 µm. In contrast, the **vuggy dolomite** (Samples VD1 and VD2) is characterized by very high porosity from 28 to 33 % connected by throats with a large span of sizes. The related TSDs dominate in the range from 6 to 60 µm and their average MTS (median throat size) is 23 µm. Very high porosity and MTS are associated also with the **vuggy limestone** (sample VL) as related to the whole sample set. The two MIP specimens of sample VL (Fig. 7D) show important mutual differences. While the porosity of the calcite-rich specimen VL_A is concentrated within two peaks of the TSD - one between 1 and 6 µm and the other between 20 and 200 µm - the gypsum-rich specimen VL_B features only the latter peak with slightly higher increments than for the specimen VL_A (Fig. 8A). In consequence, the specimen VL_A has higher porosity but lower MTS than the specimen VL_B. In addition, the free porosity of the specimen VL_A is higher by a factor of 6 than that of the specimen VL_B. The free porosity of the calcite-rich specimen constitutes 41 % of its microporosity as compared to 19 % for the gypsum-rich one. The dolomite is the least permeable rock group in the sample set with an average permeability of 6 x 10<sup>-19</sup> m<sup>2</sup>. In contrast, both vuggy carbonate rocks show the highest permeabilities in the sample set of 5 x 10<sup>-13</sup> m<sup>2</sup>, i.e. 6 orders of magnitude higher than the micritic dolomite.

#### 4.3.2. Undeformed caprock

The three rock types of undeformed caprock have intermediate porosities between 15 and 23 % (Fig. 9). The average porosities are 16 % for the **packbreccia** (sample P1), 18 % for the **undeformed floatbreccia** (samples FU1 and FU2) and 23 % for the **anhydrite-rich packbreccia** (sample P2), respectively (Fig. 8). The three rock types differ markedly in the position of the dominant peak of their TSDs and in consequence they have their MTDs of different orders of magnitude: 0.7 µm for the packbreccia P1, 2.8 µm for the anhydrite-rich packbreccia (P2) and 20 µm for the undeformed floatbreccia (FU1, FU2). The TSD of the undeformed floatbreccia is very similar to the one of the vuggy dolomite; the main difference are the higher increments for the vuggy dolomite within the dominant peak at 20–30 µm. The free porosity (i.e.

crack porosity, c.f. section methodology) is high (4–6 %) in both types of packbreccia and relatively low (1–2 %) in the undeformed floatbreccia. The undeformed caprock features intermediate to high permeability ranging from 3 x 10<sup>-16</sup> m<sup>2</sup> for the packbreccia to 2 x 10<sup>-13</sup> m<sup>2</sup> for the undeformed floatbreccia.

#### 4.3.3. Deformed caprock

The **deformed floatbreccia** (samples FD1, FD2 and FD3) contains between 15 and 20 % porosity and has a unimodal TSD with a peak at 0.1 µm (Fig. 9). This sample set is unique with intermediate to high porosity and relatively low MTD with respect to all other samples. It also features high free porosity, 5 % on average, and the highest proportion of free porosity on the microporosity of 44 % on average and more than 50 % for some of the specimens. The **gypsum mylonite** (samples GM1 and GM2) displays low porosity between 6 and 10 % and a bimodal TSD (Fig. 8). While both specimens of the sample GM2 have a common peak around 0.01 µm near the low-size end of the spectrum, one of them has another peak between 6 and 40 µm. In contrast, Sample GM1 shows no peak for small throats and its porosity is accessible by throats of sizes between 0.1 and 30 µm. The deformed floatbreccia is a rather low permeable rock with an average permeability value of 1 x 10<sup>-17</sup> m<sup>2</sup>. The gypsum mylonite has an average permeability of 3 x 10<sup>-16</sup> m<sup>2</sup> and features important differences between the specimens in the values of the TSDs. While the specimen GM2_C with unimodal TSD and with very small throats only has the lowest permeability within the sample set of 7 x 10<sup>-20</sup> m<sup>2</sup>, the other specimen from the sample GM2 shows permeability more than five orders of magnitude higher (Fig. 10).

### 5. Discussion

The large modal content and porosity shape variation of the measured sample set summarized in Fig. 11 reflects the variable composition, texture and also the different processes involved in the formation and closure of the interstitial pores in the salt caprock on Siah Taq and Karmostaj diapirs. The void space of the samples is characterized by sample averaged median throat size (MTS) from 0.1 to 50 µm (Fig. 9A). In terms of the total porosity, the samples can be divided into three groups. The carbonates and gypsum mylonites with total porosity below 10 %, the deformed and undeformed floatbreccias and packbreccias where porosities range between 10 and 25 % and the vuggy dolomite and limestone associated with porosities above 25 % (Fig. 9B). In the following discussion, we first interpret the limits of the methodological approach and the results of the MIP together with microscopic porosity analysis from the thin-sections (Fig. 11).

We argue that the alteration of carbonate-rich and sulphate rich layers of the carbonate stringers (formed both by dolomite or limestone), reflect the partial metasomatization of carbonate into gypsum. Subsequently, deformation in the caprock is responsible for deformation-induced closure of the interstitial pores, but also for the formation of new fluid-pressure induced fracture networks sealed by gypsum (Fig. 12).

#### 5.1. Methodological limits

The porosity of the studied rocks may have been affected by interaction with meteoric water as the caprock is presently exposed at the surface. However, given the low porosity and the low MTD of the strongly deformed rocks, it is unlikely that the latter were largely
<figure><img src="image_10.png" /><figcaption>Fig. 7. (continued).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Incremental porosimetric curves showing throat size distributions of the connected porosity of different caprock types. Thick lines in (A) are averages of the curves of the elementary specimens plotted in (B). The number of specimens analyzed by the MIP is indicated in the individual plots in (A) and (B) in brackets after the sample names. Ap - apatite, Cc - calcite, Chl - chlorite, Dol - dolomite, Gp - gypsum, Mag - magnetite, K-Fsp - K-feldspar, Tur - tourmaline, Qz - Quartz. Range between mean and median size of pores (EAD) obtained by image analysis marked by gray rectangle.</figcaption></figure>

influenced by meteoric water flowing over the surface of the outcrops as the erosion of caprock is also relatively fast (Bruthans et al., 2008). Although it can be expected that the porosities get slightly enlarged during sample preparation, the fact that gypsum mylonites, as the weakest and the most soluble examples of the studied rock types has one of the lowest porosity and smallest pores observed, indicates that the sample preparation influence is limited. Given the MIP has an upper throat-size detection limit of 300 µm while our microscopic observations show larger voids in the samples, the total porosity and the MTD

measured by the MIP as well as the permeability calculated based on these two MIP parameters should be understood as the minimum values. The sample preparation and epoxy resin vacuum impregnation followed the recommended procedures (Scholle and Ulmer-Scholle, 2003) to avoid development of any artificial porosity during sample collection and processing.

In all the caprock samples, except the dolomites (D1, D2), the size of micropores quantified using the digital image analysis (DIA) fall within the range of MIP incremental porosimetric curves of populations of large
<figure><img src="image_12.png" /><figcaption>Fig. 9. Plots of measured bulk porosimetric values of the individual specimens showing (A) volumetric median throat size versus total porosity and (B) the pro- portion of free porosity on the microporosity versus free porosity. Gp - gypsum, Cc - calcite.</figcaption></figure>

throat sizes. The peaks of the incremental porosimetric curves in the range of 6–80 µm can be related to the pores of void, vuggy and fenestral types of porosity in corresponding caprock types by their size deduced from the digital image analysis. The throat size corresponding to 0.1 µm of the incremental porosimetric curves (Fig. 8, Fig. 11) corresponds well with the typical width of the microfractures in other salt caprock types and rocks in general (LeRavalec et al., 1996; Reuschle et al., 2003). Those microfractures impregnated by epoxy resin are captured in both the micrographs and the MIPs for all the packbreccia, floatbreccia and gypsum mylonite samples. In contrast, the MIP curves of undeformed floatbreccia and vuggy dolomite samples (FU and VDs, Fig. 11) do not display pores that would correspond to microfractures observed under the microscope. These fluorescent epoxy resin impregnated micro- fractures clearly connect the larger vuggy and fenestral pore types (SI-File, SI-Fig. 3a,b,d,e).

## 5.2. Porosity distribution in the caprock

The MIP of the unaltered micritic dolomites (D1, D2) yielded an overall porosity of several percent and a unimodal throat size distribu- tion with a peak at 0.1 µm, which can be associated with the void space along the grain boundaries (Fig. 8)(SEM Images, SI-File, Fig. SI-Fig. 1A and B). In contrast, the **vuggy dolomite** and the **vuggy limestone** display abundant pores elongated parallel and perpendicular to the original layering of the carbonate stringers (Fig. 7A–D) associated with throat sizes of 10–100 µm as indicated by the MIP (Fig. 8). While the unaltered dolomite displays porosity of only <5 %, the porosity in the **vuggy dolomite** and **vuggy limestone** is ~30 %. The porosity also differs in the gypsum-rich and calcite-rich domains, respectively, of the **vuggy limestone** sample VL (Fig. 7C and D). The calcite-rich domain is associated with a bimodal TSD and a dominant peak at 20–200 µm (Fig. 8A), total porosity of 34 % and free porosity of 6 %. The gypsum- rich domain has a unimodal TSD with a peak at 20–200 µm (Fig. 8A) and is associated with a very low free porosity of ~1 % suggesting the absence of fractures.

The dominant peak of the TSD of the packbreccia P1 at 0.04 µm probably corresponds to the porosity of the fine-grained matrix (Fig. 7E and F and 10). The broad subdominant peak of the TSD (0.5–10 µm) is attributed to the interclastic voids, vugs and fractures with larger throats. The broad distribution of throat sizes of the anhydrite-rich packbreccia P2 corresponds to the large channel-type pores lining the

lithoclast boundaries (Fig. 7G) and significantly smaller intercrystal pores that are present at grain boundaries of polycrystals (Fig. 8A). The fact that the TSD displays the dominant peak in the micron scale, i.e. a significant fraction of the porosity is accessible through connections of such size, corresponds to our observation that even the large channels are connected to the void network via small throats.

Despite the difference in the chemistry and structure of the solid phase, the **undeformed floatbreccia** exhibits similar porosity charac- teristics as the **vuggy carbonates** (represented primarily by the dolo- mites) as shown by both microscopy and MIP. The dominant porosity structures are channels with throat sizes on the order of tens of microns and larger. A common feature of the vuggy dolomites and of the unde- formed floatbreccia is low free porosity, suggesting their porosity is made up of voids with a high ratio of pore-size to throat-size (ink-bottle pores) (Fig. 5C).

The TSD of the **deformed floatbreccia** (FD) features a dominant peak centred around 0.1 µm. In the microstructure, this is likely related to long and relatively open fractures present at the edges of the fine grained aggregate bands. In contrast, the vuggy and fenestral types, which were identified in pressure shadows of larger clasts are only locally interconnected by microfractures (SI-File, Fig. SI-Fig. 3). Because these types of pores do not show in the MIP curves (Fig. 8, Fig. 11), they are not connected with the larger-size fracture network. We can specu- late that the dynamically recrystallized gypsum bands, where no porosity is observed, serve as barriers to mercury injection and thus most of the porosity reached by MIP is represented by the long and open fractures.

The TSD of the **gypsum mylonite** (GM1, GM2) is characterized by two distinct peaks - one narrow at the low end of the measured spectrum (0.01 µm) and one broad spanning from the submicron range to several tens of micrometers (Fig. 8). The large throats are attributed to the visible vugs, voids and fenestral porosities both in the gypsum matrix and the dolomite clasts. Large pores are interconnected by micro- fractures, which is indicated by both the MIP and the thin microstruc- tures (SI-File, Fig. SI-3). The small throats may correspond to the grain boundaries and cleavage microfractures within the highly deformed gypsum matrix.

## 5.3. Origin of the vuggy carbonates

The development of abundant porosity in the vuggy carbonates
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Permeability of the specimens calculated using the model of Katz and Thompson (1986, 1987). The pale colored rectangles show the average value for each group.</figcaption></figure>

(dolomite and limestone stringers) is interpreted to reflect the consequence of two processes: 1) the replacement of carbonate by gypsum and the development of a dense network of gypsum veins and 2) the dissolution of the gypsum aggregates of the gypsum vein network by sulphate unsaturated fluids, leaving behind the abundant vuggy porosity (Fig. 12B). The replacement of carbonate by gypsum can be explained by interaction of the carbonates by sulphuric acid. This acid is formed by oxygenation of hydrogen sulfide percolating from depth of the diapiric system for example from sites of thermochemical sulphate reduction (TSR)(Worden and Smalley, 1996):

$$H_2S + O_2 \Rightarrow H_2SO_4, \qquad (3)$$

or by oxygenation of the native sulfur ($S_0$):

$$S + H_2O + 3O \Rightarrow H_2SO_4. \qquad (4)$$

The sulphuric acid then reacts with the carbonate (calcite, dolomite) to produce the gypsum (Fig. 12B):

$$H_2SO_4 + CaCO_3 \Rightarrow CaSO_4 + CO_2 + H_2O. \qquad (5)$$

The dense network of gypsum veins permeating the dolomite or limestone stringers may be the consequence of an increase in volume as densities of carbonate and gypsum are 2.7–2.84 g/cm³ and 2.3 g/cm³, respectively. The vuggy porosity development in the carbonates can be attributed to the dissolution of gypsum in porosity precursors by water, which is also the product of the above indicated reaction, and to the draining of the sulphate enriched aqueous fluids from the host carbonate-sulphate aggregates (Fig. 7A–D). The small throats of the bimodal TSD of the calcite-rich part of sample VL (Fig. 7C and D; Fig. 8) likely correspond to the microstructure of the residual calcite mass while its large throats correspond to the subdomains rich in gypsum.

The primary evidence of sulphate reduction by hydrocarbons is provided by findings of galena at the site of sampling at locality VD1 (SI-File, Fig. SI-4C) and the occurrence of zebra calcite textures and radial aggregates of calcite that grew at the expense of sulphates and which are also associated with macroscopic native sulfur crystals (SI-File, Fig. SI-4A,B,D,E)(Posey and Kyle, 1988; Machel, 1987, 2001). It is also possible that the metasomatization of the carbonate layers into gypsum occurred soon after their deposition in the Precambrian sea or during their diagenesis. This is because the zebra calcite textures (or the radial aggregates of calcite growing at the expense of sulphates; SI-File, Fig. SI-4A,B,E) may have formed as stratabound replacement reactions in the hydrothermal system of the original evaporitic basin along the sulphate rich layers (Adineh et al., 2023; Wallace and Hood, 2018). The sulphate reduction along these layers would be promoted by CO₂ or hydrocarbons from organic matter dispersed in the dark carbonate layers during their interaction with hydrothermal fluids (dolomite or limestone in the evaporitic succession) (Zhang et al., 2019). In this case, the sulphates in metasomatized carbonates may have transformed from anhydrite and gypsum during diagenesis and exhumation within the salt diapir. Related reaction volume changes may have further promoted fragmentation of these carbonate stringers. Since the dark carbonate stringers interspersed by gypsum veins and zones of fragmentation along gypsum-rich shear zones are common in the frontal deformed part of the caprock, we suggest that the replacement of carbonate by gypsum contributed to significant mechanical weakening of the caprock as the strength of gypsum is much lower than that of the carbonates (Fig. 12A; SI-File, Fig. SI-4E) and deforms by solution-precipitation creep (Závada et al., 2021).

### 5.4. Impact of deformation in the caprock on throat-size distribution and interstitial fluid redistribution

Comparison of the porosity properties in the relatively undeformed floatbreccia samples with their deformed equivalents clearly indicates decrease of total porosity from 18 % to 13 % and of the MTS from >10 µm to ~0.1 µm (Figs. 8 and 12B). Similarly, we suggest that gypsum-rich floatbreccia and gypsum mylonites with carbonate fragments developed by reaction weakening and deformation of carbonate stringers. This is supported by the bimodal distribution of the TSD of the mylonites, where the higher size peak corresponds to that of the vuggy dolomite (Figs. 8 and 12B). Given the presently high porosity of the vuggy carbonates, it is possible that they served as temporary storage sites for sulphate-rich aqueous fluids that were redistributed during caprock deformation along the stress-gradients in the deforming caprock (Závada et al., 2021). Such zones of deformation are manifested by gypsum-rich shear zones surrounded by vein-rich packbreccia domains (Fig. 12A) or large folded stringers of sandstones and carbonates interspersed by gypsum veins oriented at a high angle to the fold-axes of these folds (Závada et al., 2021).

### 5.5. Permeability calculation of the caprock from MIP

Measurements on core samples from the onshore salt diapirs in the Mexican Gulf province indicated permeabilities in the range of $10^{-20}$ m² and $10^{-17}$ m², with low values typical for the laminated anhydrite in the
<table><thead><tr><th></th><th>Type</th><th>Sample</th><th>Clasts</th><th></th><th>Gp# (%)</th><th>Cb (%)</th><th>Anh. (%)</th><th>Pore types</th><th>Poro.* (%)</th><th>P. thr.† (µm)</th></tr></thead><tbody><tr><td rowspan="4">Carbonate Stringers</td><td>Dolomite</td><td>D1<br>D2</td><td>—</td><td></td><td>n/a</td><td>53 dol<br>86 dol</td><td>—</td><td>microfracture</td><td>4.2<br>2.5°</td><td>0.035<br>0.10°</td></tr><tr><td>Vuggy Dolomite</td><td>VD1<br>VD2</td><td>dolomite</td><td></td><td>48-80</td><td>1.5-3 cal<br>15-49 dol</td><td>—</td><td>vuggy<br>fenestral<br>channel<br>microfracture</td><td>29°</td><td>23°</td></tr><tr><td>Vuggy Limestone</td><td>VL</td><td>calcite</td><td></td><td>55-68</td><td>24-39 cal</td><td>—</td><td>vuggy<br>fenestral<br>channel<br>microfracture</td><td>26-34</td><td>11-48</td></tr><tr><td>Packbreccia</td><td>P1</td><td>arkosic sandstone;<br>carbonates</td><td></td><td>24</td><td>1 cal<br>19 dol</td><td>—</td><td>microfracture<br>vuggy<br>void</td><td>16°</td><td>0.69°</td></tr><tr><td rowspan="3">Undeformed caprock</td><td>Packbreccia</td><td>P2</td><td>anhydrite;<br>arkosic sandstone;<br>calcite; dolomite;<br>euhedral dolomite</td><td></td><td>8.5</td><td>13 cal<br>3.5 dol</td><td>70</td><td>vuggy<br>fenestral<br>microfracture</td><td>23</td><td>2.8</td></tr><tr><td>Floatbreccia<br>Undeformed</td><td>FU1<br>FU2</td><td>dolomite;<br>euhedral dolomite;<br>calcite; quartz;<br>mica. sandstone</td><td></td><td>34-89</td><td>5 cal<br>9-52 dol</td><td>—</td><td>vuggy<br>fenestral<br>microfracture</td><td>15-19</td><td>16-23</td></tr><tr><td>Floatbreccia<br>Deformed</td><td>FD1<br>FD2<br>FD3<br>FD4</td><td>dolomite;<br>arkosic sandstone;<br>baryte; calcite;<br>quartz</td><td></td><td>30-60</td><td>1.5-8 cal<br>8-20 dol</td><td>&lt;0.5</td><td>fracture<br>vuggy<br>fenestral</td><td>11-14</td><td>0.10-0.41</td></tr><tr><td>Deformed caprock</td><td>Gypsum mylonite</td><td>GM1<br>GM2</td><td>dolomite;<br>quartz;<br>sandstone</td><td></td><td>80</td><td>15 dol</td><td>—</td><td>microfracture<br>vuggy<br>void</td><td>6.6-9.5</td><td>0.018-7.8</td></tr></tbody></table>

<figure><img src="image_14.png" /></figure>

* Mineral abbreviations according to Whitney (2010) * Poro. - porosity † P. thr. - pore throat diameter Ø Average

<caption>**Fig. 11.** Overview chart characterizing the microstructural and quantitative features of the porosity in investigated samples and content of the major minerals. Gp - gypsum, Cb - carbonate, Anh - anhydrite.</caption>
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Structural profile of the Siah Taq diapir indicating the increasing intensity of caprock deformation from the apical part of the structure to the frontal, northern part (A). Deformation in the frontal part of the extrusion is strongly partitioned into gypsum rich layers that deform by solution-precipitation creep assisted by sulphate-rich aqueous fluids. These fluids also promote local overpressure-driven fracturing in the packbreccia domains (PB-F) in the surroundings of the shear zones. Deformation in the caprock is responsible for closure of the porosity in the floatbreccia (B) and local transformation of the vuggy carbonates to mylonites and pore destruction by recrystallization of gypsum that encloses ghosts (fragments with faint layering) of carbonate clasts interspersed by gypsum veins (CFG - carbonate fragment ghosts) embedded in gypsum rich matrix (A). GM – gypsum mylonites, PB-F – fractured packbreccia, G - gypsum.</figcaption></figure>

lower part of the caprock (Karably et al., 1983) and higher permeabil-
ities in the variegated calcite zone in the upper part of the caprock. In
our study, higher permeabilities are attributed primarily to the disso-
lution of gypsum in carbonate, where the gypsum formed by oxidation
of H₂S or the elemental sulfur in the vuggy carbonates (Figs. 11 and 12).

Based on the evaluation of the permeability pattern throughout the caprock of the diapirs, we can suggest that it may be strongly zonal, with low permeability at the marginal, deformed and gypsum-rich parts (~10⁻¹⁶ - 10⁻¹⁵ m²) of the diapir caprocks (~30 % of caprock volume at Siah Taq, ~60–70 % of caprock on Karmostaj diapir). Permeability of the caprock in the central part of Siah Taq, which is dominated by packbreccia (Fig. 2), is ~10⁻¹⁵ m². The permeability should be higher in the central and less deformed part of Siah Taq in outcrops of gypsum dominated caprock (~10⁻¹³ m²). However, in general, proportions of the different rock types in the caprock are difficult to estimate for the quantification of the total permeability of the caprock on the investigated salt diapirs. For example, the low deformation floatbreccia was not identified on the Karmostaj diapir. We are aware of the limitations of our study, as it cannot capture the mesoscale and map-scale porosity pattern, which would require detailed mapping of fracture width, orientation and density.

**6. Conclusions**

Mercury intrusion porosimetry alongside the quantitative micro-
structural analysis of the major salt caprock types was carried out to
decipher the processes related to development of microscopic porosity at
two salt diapirs in Southern Iran. This is a first study that addresses the
microporosity of salt caprock in such detail and in an area where the
deformation evolution of the salt caprock was already established
(Závada et al., 2021). The caprock analysis revealed a wide range of
total porosities and of the sample averaged median throat size values
ranging from 2 to 35 % and from 0.1 to 50 µm, respectively. The lowest
observed porosity below 5 % associated with very narrow throat size
distribution in micritic dolomite carbonates is attributed to the void
space along grain boundaries. In contrast, the highest recorded porosity
around 30 % of the vuggy dolomite and limestone is related to the large
vuggy, fenestral, or channel-type porosity. The undeformed caprock
packbreccias and floatbreccias have similar intermediate porosities be-
tween 15 and 23 % but differ markedly in their throat size distributions.
The undeformed floatbreccia exhibits similar porosity characteristics
and microstructural relationships as observed for the vuggy carbonates.
The packbreccia, on the other hand, contains several types of micro-
porosity including tiny pores in the fine-grained matrix, interclastic
voids, vugs as well as fractures interconnected by pores with rather small
throats. The porosity observed in the deformed floatbreccia and gypsum
mylonite is made up of intercrystal vugs and channels and foliation parallel fractures. However, their throat size distribution suggests that the interconnectivity of the porosity network is provided by very narrow throats - likely grain boundaries or microfractures.

Two main processes have been recognized to modify the porosity properties of the studied caprock types. The first is the development of the abundant vuggy porosity in the carbonates attributed to the replacement of the carbonate minerals by a dense network of gypsum veins and further dissolution of gypsum by sulfide unsaturated fluids during diagenesis and exhumation within the salt diapir. The second process changing significantly the porosity properties of the caprock is the deformation during diapir extrusion and radial spreading of the caprock leading to the decrease in the total porosity and interconnectivity and thus also permeability due to significant decrease in the median throat size.

### CRediT authorship contribution statement

P. Závada: Writing – original draft, Visualization, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. M. Staněk: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Data curation. M. Machek: Writing – original draft, Visualization, Investigation. S. Adineh: Investigation. Y. Géraud: Supervision, Methodology, Investigation. J. Bruthans: Writing – review & editing, Supervision, Investigation. S. Heuss-Aßbichler: Writing – review & editing, Supervision, Investigation. M. Zare: Investigation.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work was supported by a Czech Science Foundation grant No. GA24-11150S. P.Z., M.S., M.M. and S.A. were also supported by the Czech Academy of Sciences institutional support to the Institute of Geophysics of the Czech Academy of Sciences (RVO 67985530). The MIP analyses were executed in the frame of the mobility project of M.S. supported by the Czech Ministry of Education, Youth and Sports programme CZ.02.2.69/0.0/0.0/18_053/0016986.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsg.2025.105483.

### Data availability

Data will be made available on request.

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