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The effects of rock heterogeneity on compaction localization in porous
carbonates
Antonino Cilona a , *, Daniel Roy Faulkner b , Emanuele Tondi c, Fabrizio Agosta d,Lucia Mancini e, Andrea Rustichelli c, Patrick Baud f, Sergio Vinciguerra g
a Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, United Statesb University of Liverpool, Liverpool, United Kingdomc School of Environmental Sciences, University of Camerino, Camerino, Italyd Department of Science, University of Basilicata, Potenza, Italye Elettrae Sincrotrone Trieste SCpA, SS 14, Km 163,5 in Area Science Park, 34012 Basovizza (Trieste), Italy
f EOST Strasbourg, Strasbourg, Franceg Department of Geology, University of Leicester, Leicester, United Kingdom
a r t i c l e i n f o
Article history:
Received 18 December 2013
Received in revised form
8 July 2014
Accepted 16 July 2014
Available online 25 July 2014
Keywords:
Discrete compaction bands
Grain sorting
Triaxial compaction experimentsMechanical twinning
Porosity reduction
a b s t r a c t
Recent eld-based studies document the presence of bed-parallel compaction bands within the
Oligocene-Miocene carbonates of Bolognano Formation exposed at the Majella Mountain of central Italy.
These compaction bands are interpreted as burial-related structures, which accommodate volumetric
strain by means of grain rotation/sliding, grain crushing, intergranular pressure solution and pore
collapse.
In order to constrain the pressure conditions at which these compaction bands formed, and investigate
the role exerted by rock heterogeneity (grain and pore size and cement amount) on compaction local-
ization, we carried out a suite of triaxial compression experiments, under dry conditions and room
temperature on representative host rock samples of the Bolognano Formation. The experiments were
performed at conning pressures that are proxy of those experienced by the rock during burial (5e35 MPa). Cylinders were cored out from a sample of the carbonate lithofacies most commonly affected
by natural compaction bands. Natural structures were sampled and compared to the laboratory ones.
During the experiments, the samples displayed shear-enhanced compaction and strain hardening
associated with various patterns of strain localization. The brittleeductile transition occurred at 12.5 MPa
whereas compaction bands nucleated at 25 MPa conning pressure. A positive correlation between
conning pressure and the angle formed by the deformation bands and the major principal stress axis
was documented. Additional experiments were performed at 25 MPa on specimens cored oblique
(parallel and at 45) to the bedding. Detailed microstructural analyses, performed on pristine and
deformed rocks by using optical microscopy, scanning electron microscopy and X-ray computed
microtomography techniques, showed that grain crushing and mechanical twinning are the dominant
deformation processes in the laboratory structures. Conversely, pressure solution appears to be dominant
in the natural compaction bands. Experimental results highlight the strong inuence exerted by
bedding-parallel rock heterogeneity on both orientation and kinematics of deformation bands in the
studied carbonates.
2014 Elsevier Ltd. All rights reserved.
1. Introduction
Porous rocks form important reservoirs for water, hydrocarbons
and, potentially, the storage of greenhouse gases. Post-depositional
processes (i.e., mechanical, chemical, physical and biological) may
strongly affect their uid ow properties and, hence, are important
to determine. For this reason, the analyses of both deformation
mechanisms and ow properties of siliciclastic porous rocks have
received a good deal of attention, both in the eld (e.g.,Aydin et al.,
2006; Fossen et al., 2007) and in the laboratory (e.g., Wong et al.,
1997; Wong and Baud, 2012). Less attention has been paid to
porous carbonate rocks, which still constitute a large proportion of
oil reservoirs.* Corresponding author. Tel.: 1 6507259355, 1 6502007418 (mobile).
E-mail address:[email protected](A. Cilona).
Contents lists available atScienceDirect
Journal of Structural Geology
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c om / l o c a t e / j s g
http://dx.doi.org/10.1016/j.jsg.2014.07.008
0191-8141/
2014 Elsevier Ltd. All rights reserved.
Journal of Structural Geology 67 (2014) 75e93
mailto:[email protected]://www.sciencedirect.com/science/journal/01918141http://www.elsevier.com/locate/jsghttp://dx.doi.org/10.1016/j.jsg.2014.07.008http://dx.doi.org/10.1016/j.jsg.2014.07.008http://dx.doi.org/10.1016/j.jsg.2014.07.008http://dx.doi.org/10.1016/j.jsg.2014.07.008http://dx.doi.org/10.1016/j.jsg.2014.07.008http://dx.doi.org/10.1016/j.jsg.2014.07.008http://www.elsevier.com/locate/jsghttp://www.sciencedirect.com/science/journal/01918141http://crossmark.crossref.org/dialog/?doi=10.1016/j.jsg.2014.07.008&domain=pdfmailto:[email protected]8/10/2019 Cilona Et Al JSG 2014
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In carbonates, as well as any other rock, strain localization on
both small-scale laboratory samples and large-scale crustal fault
zones, signicantly inuences the stress eld (Paterson and Wong,
2005), strain partitioning (Olsson, 1999; Issen and Rudnicki, 2000)
and uid-transport properties of the deformed rocks (see Aydin,
2000; Faulkner et al., 2010a for full reviews). Previous studies
accurately investigated the deformation mechanisms associated to
the formation faults and fractures in carbonates by means ofeld
and/or laboratory analyses (e.g., Baud et al., 2000; Agosta et al.,
2007; Antonellini et al., 2008). Since these strain localizations are
mainly associated with dilatancy, scientic attention has focused
on the study of this phenomenon in tight carbonates. In contrast,
systematic eld and laboratory investigations of compaction-
assisted strain localization have been carried out only in recent
times.
Field-based studies (Tondi et al., 2006; Agosta et al., 2009;
Cilona et al., 2012; Rustichelli et al., 2012; Tondi et al., 2012;
Antonellini et al., 2014) described compaction localization within
limestones characterized by a wide range of porosities
(15 < f < 45%). These authors documented strain localization
occurring along narrow tabular bands oriented oblique and parallel
to bedding (i.e. compactive shear bands and compaction bands,
respectively; sensu Aydin et al., 2006). Bed-parallel compactionbands are characterized by a local porosity reduction and do not
show any macroscopic shear offset (Aydin et al., 2006). Based upon
the results ofeld and microstructural analyses, these compaction
bands have been interpreted as burial-related structures which
accommodate volumetric strain by the complex interplay of grain
rotation/sliding, grain crushing, intergranular pressure solution
and pore collapse. With the exception of intergranular pressure
solution, these micromechanisms are analogous to those previously
described in sandstones (e.g., Mollema and Antonellini, 1996; Aydin
et al., 2006; Fossen et al., 2007; Aydin and Ahmadov, 2009;
Eichhubl et al., 2010; Shultz et al., 2010; Deng and Aydin, 2012;
Alikarami et al., 2013).
Laboratory-based studies have investigated the parameters
controlling the mechanics of compaction of porous carbonates(10< f < 46%). Among these parameters, attention has been drawn
on uids type and/or chemistry (e.g., Homand and Shao, 2000;
Risnes et al., 2005; Zhang and Spiers, 2005 Croizeet al., 2010b),
temperature (e.g.,Croizeet al., 2010a) and pore size/type (e.g.,Zhu
et al., 2010). Other studies described the evolution of failure modes,
microstructures and micromechanism at different conning pres-
sures (Vajdova et al., 2004; Baxevanis et al., 2006; Baud et al., 2009;
Dautriat et al., 2011b; Cilona et al., 2012; Vajdova et al., 2012). In
contrast to sandstones for which compaction localization has been
reproduced in laboratory (e.g., Besuelle, 2001; Cuss et al., 2003;
Vajdova and Wong, 2003; Baud et al., 2004; Fortin et al., 2005;
Louis et al., 2006, 2009), porous carbonates mainly showed ho-
mogeneous deformation associated with the ductile regime (e.g.,
Vajdova et al., 2004, 2010; Baud et al., 2009; Zhu et al., 2010;Dautriat et al., 2011b; Vajdova et al., 2012). A recent pilot study
described compaction bands formed during laboratory experi-
ments performed on a carbonate grainstone under wet conditions
(Cilona et al., 2012). These authors suggested that a more system-
atic work was needed to understand fully the parameters control-
ling compaction localization in these rocks.
In this paper we perform a systematic set of triaxial compaction
experiments on specimens cored from the Bolognano Fm.
(Crescenti et al. 1969) to constrain the conditions at which natural
compaction bands nucleate and to investigate the role of rock
heterogeneity on compaction localization in porous carbonates.
The study rocks crop out at the Majella Mountain (central Italy) and
have been lately investigated by Rustichelli et al. (2012)and(2013).
The authors correlated compositional, sedimentological and pore
network characteristics to development and distribution of bed-
parallel compaction bands. In this work, an attempt to replicate
in the laboratory these natural features is performed. Dry experi-
ments, conducted under a range of conning pressures (5e35 MPa)
on samples cored in different orientations with respect to the
bedding (i.e., perpendicular, parallel and at 45 degree), are aimed
at evaluating the role exerted by primary rock constituents (e.g.,
grain and pore size/shape and cement amount/type) on any
compaction localization. The effect of rock heterogeneity and
anisotropy on the strength and failure modes of sandstones has
been previously studied by Louis et al.(2009) and Baud et al. (2012).
Both intact and deformed samples are characterized by means of
detailed microstructural analyses, performed by integrating optical
microscopy, Scanning Electron Microscopy (SEM) and X-ray
computed microtomography (mCT) techniques (Baker et al., 2012).
The results of microstructural analysis of experimentally-deformed
rocks arethen compared withthoseobtained from naturalstructures.
2. Methodology
2.1. Laboratory experiments
The experiments were carried out at the Rock DeformationLaboratory of Liverpool University. Nine cylindrical specimens
(20 mm diameter and 50 mm length) were cored, perpendicular to
bedding, from a boulder-sized hand sample. Moreover, two addi-
tional specimens (same shape) were cored parallel and oblique
(45) with respect to bedding. Each specimen was ground, to make
its bases parallel to each other, and then oven-dried for 48 h at
80 C to eliminate any residual humidity. The starting porosity
values were determined by means of helium multipycnometer
measurements using Quantachrome Instruments (MVP D150E).
The pore-throats distribution of one intact sample was computed
by mercury injection. The specimens were then placed in a 3.4 mm-
thick PVC jacket, in order to isolate them from the conning me-
dium (silicon oil). Eleven specimens were deformed in a conven-
tional triaxial conuration, under dry conditions and at roomtemperature, at conning pressures ranging from 5 to 35 MPa (cf.
Faulkner et al., 2010bfor details on the experimental apparatus).
The axial force applied on the sample was measured by an in-
ternal force gauge with a 0.02 kN resolution. The axial displace-
ment was measured using a linear variable differential transformer
(LVDT) attached, outside of the pressure vessel, to the electro-
mechanical servo-controlled ram for axial loading. The axial load
was applied at a xed rate of 0.5 mm s1 that corresponds to a
nominal strain rate of 105 s1. Since the studied rock was too
porous to enable the use of strain gauges, the volumetric strain of
the samples was recorded with a conning pressure volumometer
with a 0.1 mm3 resolution. The volumometer was calibrated by
loading a steel blank specimen up-to 20 kN while keeping the
conning pressure constant.
2.2. X-ray microtomography (mCT)
A selection of samples (i.e. deformed and pristine) was vacuum
impregnated with epoxy resin and imaged at the Elettra synchro-
tron light laboratory in Basovizza (Trieste, Italy) by two different
instruments. Each cylindrical sample (diameter of 20 mm) was cut
in two parts: one half-cylinder was imaged by conventional
microfocus X-raymCT at the TomoLab station (Zandomeneghi et al.,
2010). A smaller parallelepiped-shaped sample was cut from the
remaining half cylinder to be investigated by using phase-contrast
synchrotron radiation (SR) mCT at the SYRMEP beamline (Tromba
et al., 2010). Details about samples investigated by X-ray mCT are
reported inTable 1.
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At SYRMEP, the SR is provided by a bending magnet source; the
sample is placed at a distance of about 23 m from the source and is
illuminated by a monochromatic, nearly parallel X-ray beam with a
maximum area of about 160 6 mm. The monochromator is based
on a double Si(111) crystal system making it possible to tune the X-
ray energy in the range 8.3e38 keV. The high spatial coherence of
the X-ray beam permits to benet of phase-contrast effects
employing a free-space propagation based technique (Cloetens
et al.., 1996). The sample projections were recorded with a 12 bit,
water-cooled CCD camera (4008 x 2672 pixels, 4.5 4.5 mm
effective pixel size). The SR mCT scans were acquired in the
following conditions: X-ray energy 34 keV, sample-to-detector
distance 180 mm, exposure time/projection 4.8 s, pixel
size9.0 mm. For each scan 1440 projections were recorded over a
180 rotation. The reconstruction of the slices was performed using
the SYRMEP_Tomo_Project 4.0 software and the GRIDREC algo-
rithm (Dowd et al., 1999) with an isotropic voxel size of 9.0 mm.
The TomoLab instrument is equipped with a sealed microfocus
source (Voltage range: 40e130 kV, maximum current 300 mA,
minimum focal spot 5 mm) and it is based on a cone-beam ge-
ometry. A water-cooled, 12 bit CCD camera was employed as de-
tector (4008 2672 pixels, effective pixel size 12.5 12.5 mm2). The
sample was imaged in the following conditions: Voltage 130 kV,current 61 mA, 1.5 mm thick Al lter, exposure time/
projection 5.8 s, pixel size 17.2 mm. For each scan 2400 pro-
jections were recorded over a 360 angular rotation. The com-
mercial software COBRA 7.4 (Exxim) was used for slice
reconstruction with an isotropic voxel size of 17.2 mm. By using this
software, a beam-hardening correction based on a polynomial
tting of the measured nonlinear relationship between object
thickness and the log ratio of the intensities was used for slice
reconstruction. In this study, the use of two different mCT in-
struments allows a multi-scale and complementary analysis of the
imaged samples. Phase-contrast SR mCT technique outperforms
conventional mCT in pore space determination. This is due to the
higher spatial and contrast resolution related to the use of a nearly-
parallel, monochromatic high-intensity X-ray beam leading toreconstructed slices free of beam hardening and magnication ef-
fects (Kak and Slaney, 1988) and with a high signal-to-noise ratio.
Moreover, by working in phase-contrast mode (edge-detection
regime), all the interfaces corresponding to phase changes in the
sample show an enhanced visibility (Cloetens et al., 1996; Mancini,
1998). Compared with absorption images, the objects appear
sharper and very small details can be detected (also smaller than
the pixel size of the detector) Cloetens et al. (1997). The epoxy,
partially invading the pores of the sample, could be visualized and
easily distinguished from the empty space (within the limit of
detail detectability of the employed technique) giving a higher
accuracy in the image analysis procedures used to separate the
solid phase from the porous space. However, conventional mCT
imaging allowed us to obtain a quantitative characterization ofmacroporosity (pores with sizes of the order or larger than 30 mm)
on a larger volume than SRmCT (Table 1). The quantitative analysis
of the volumes was carried out by means of the Pore3D software
library developed at Elettra (Brun et al., 2010; Zandomeneghi et al.,
2010). The same software was used to lter the slices reconstructed
by conventionalmCT in order to reduce the ring artefacts present in
the images.
The reconstructed slices were visualized by using the ImageJ
software (Abramoff et al., 2004) while the volume renderings of
raw and processed images were obtained by the commercial soft-
ware VGStudio MAX 2.0.
In order to separate the different phases present in the sample, a
segmentation process has to be applied to the reconstructed vol-
umes. In the literature, several authors treated the problem to es-
timate macro- and micro-porosity from X-ray mCT images in
carbonates spanning across several tens of length scales (Bauer
et al., 2011;Blunt et al., 2013; Wildenschild et al., 2013). Different
approaches have been proposed based on multi-scale analysis
combining mCT with SEM imaging and/or on ltering and seg-
mentation of mCT images into three phases. The three resulting
phases represent the resolvable porous space (macro-porosity), the
micro-porosity (below the resolution of the mCT image), and the
solid region, respectively (Sok et al., 2009; Ji et al., 2012).
We distributed the voxels in two phases, i.e. solid and void, by
manually selecting a threshold value in 3D from the histogram of
the intensities (Zandomeneghi et al., 2010) in the analysed Volume
Of Interest (VOI). In this way, gray scale images were converted into
a binary images from which it was possible to estimate the 3Dporosity of the samples. Because this approach allows to quanti-
tative characterize only the pores with sizes compatible with the
spatial resolution of the applied mCT instrument, we integrated this
technique with image analysis of SEM images.
2.3. Microstructural analysis
The epoxy-impregnated samples were cut along a plane parallel
to the axial direction to prepare petrographic thin sections. The thin
sections were polished to be analysed using an optical polarizing
microscope (Nikon Eclipse E600 Pol microscope) and eventually
carbon coated to be observed under Scanning Electron Microscope
(JEOL JXA-8230). 2D porosity values were obtained on micropho-
tographs (6 mm pixel size) by means of the open source softwareImageJ 1.32, (see Rustichelli et al., 2012 for details on the meth-
odology). Quantitative microstructural analysis was carried out on
representative samples to determine micro-cracks density. After
identifying the portions of the samples where localization
occurred, we selected 88 microphotographs (3 mm of pixel size)
representing different degree of deformation. A rectangular grid of
9 mm2 (3.25 2.75) of area was superimposed on each analyzed
microphotograph, using stereological techniques, the number of
micro-crack intersections with a test array of 15 parallel lines
spaced at 0.2 mm was manually counted. Measurements were
made in two orthogonal directions parallel and perpendicular to
the s1 axis, respectively. We denoted the linear intercept density
(number of micro-crack intersections per unit length) for the array
oriented parallel to s1 by PkL , and that for the perpendiculararray by
PL . Previous studies (Underwood, 1970) have demonstrated that
since the spatial distribution of damage is approximately axisym-
metric in a triaxially deformed sample, the crack surface area per
unit volume (SV) can be inferred from linear intercept measure-
ments along two orthogonal directions, see Equation(1):
Table 1
List of the analyzed samples and the acquisition parameters.
Sample code Description Facility Voxel size [mm] Imaged volume: geometry Imaged volume [mm] VOI [mm3]
BoloH Host rock TomoLab 17.2 Half cylinder Diameter 20; Height 21 6.7 5.1 3.0
Natural CB Compaction Band SYRMEP 9 Parallelepiped 4 4 8 3.5 3.8 3.7
Bolo5 Deformed at 25 MPa Pc TomoLab 17.2 Half cylinder Diameter 20; Height 21 4.4 10.1 4.3
Bolo5 Deformed at 25 MPa Pc SYRMEP 9 Parallelepiped 4.5 4.5 8.0 3.8 3.8 2.3
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Fig.1. Geological setting of Majella Mountain. a) Schematic geological map (modied afterGhisetti and Vezzani, 1998). b) Stratigraphic scheme of the carbonate succession (modie
(modied afterVezzani and Ghisetti, 1998).
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Sv P
2PL
2
P
2
P
jL (1)
In addition, we used the Equation(2) to dene the value of the
micro-crack anisotropy factor (U23,Underwood, 1970):
U23 PL P
jL
PL 4=P 1PjL
(2)
The parameterU23 represents the ratio between the surface area
of micro-cracks parallel to s1and the total micro-crack area.
The density of twinned calcite grains was obtained using the
methodology described byVajdova et al. (2010). Microphotographs
of intact and deformed samples (i.e., localization zones and sur-
rounding parts) were selected, an array of squares (#35) was then
superimposed on each picture (#39 in total, 6 mm pixel size). The
number of twinned calcite crystal was counted within each square
(area of 1 mm2) and mean values were obtained for the localization
zones and the surrounding parts of the specimens.
3. Geological framework
The Majella Mountain is an east-verging, thrust-related anti-
cline located in the external zone of the central Apennines, central
Italy (Fig. 1;Ghisetti and Vezzani, 2002). The stratigraphic succes-
sion includes a 2 km-thick sequence of Cretaceous to Miocene
Fig. 2. a) Backscattered Electron image of the skeletal grainstones of the facies B, legend: Bryozoan fragment (B), Echinoid plate (E), Syntaxial overgrowth cement (S), intergranular
(Pi) and intragranular (Pii) porosities. Laminae dominated by intergranular or intragranular pores are highlighted by red and blue box, respectively. b) Pore-throat statistics from
mercury injection on a host rock sample; Microfocus X-raymCT images (voxel size 17.2mm) of intact Bolognano grainstones. c) an example of a 2D axial slice; d) volume rendering
of a parallelepiped-shaped volume (height in the zdirection 21 mm) showing both pores (black) and grains plus cement (grey); e) volume rendering of the same sample region
illustrated in d) after segmentation: only the pores are shown. The porosity values computed within the blue and red polygons are also reported. (For interpretation of the ref-
erences to colour in this gure legend, the reader is referred to the web version of this article.)
A. Cilona et al. / Journal of Structural Geology 67 (2014) 75e93 79
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carbonate formations related to various depositional settings
(platform and slope/ramp) originally pertaining to the northern-
most sector of the Apulian Platform realm (Vecsei et al., 1998). The
Apulian carbonates are overlain by marine siliciclastic and evapo-
ritic deposits of Messinian-to-Pleistocene age, the majority of
which originally deposited within the Peri-Adriatic foredeep basin
of the central Apennines fold-and-thrust belt (Vezzani and Ghisetti,
1998). According toGhisetti and Vezzani (2002)and Agosta et al.
(2009), the development of the Majella thrust-related anticline
occurred during the Middle-to-Late Pliocene. The internal defor-
mation of this box-shaped fold, characterized by two steeply-
dipping to overturned limbs, mainly consists of high-angle
normal, strike-slip and oblique-slip faults, small folds and
different sets of mode I fractures (e.g., Marchegiani et al., 2006;
Tondi et al., 2006; Antonellini et al., 2008; Agosta et al., 2009,
2010; Aydin et al., 2010). Conversely, based upon crosscutting re-
lationships with Messinian-to-Pleistocene sediments topping the
carbonates, Scisciani et al. (2002) interpreted faulting activity as
Messinian.
We analyzed structures pertaining to the Oligocene-Miocene
ramp carbonates of the Bolognano Formation cropping out in the
Roman Valley Quarry area of the MajellaMt. (Fig.1). The carbonates
of the Bolognano Formation consist of i) relatively shallow-waterskeletal grainstones and packstones, which are made up of frag-
ments of Larger Benthic Foraminifera (LBF), bryozoans, red algae
and lamellibranches, echinoid plates and spines, and ii) deeper
water marly wackestones and mudstones with planktonic forami-
nifera (Fig. 2a; Vecsei and Sanders, 1999; Pomar et al., 2004;
Rustichelli et al., 2013). These marine carbonates accumulated on
an isolated, gently dipping, carbonate ramp under sub-tropical
conditions and nutrient-rich sea water (Westphal et al., 2010;
Rustichelli et al., 2013).
For the laboratory experiments, we selected the lithofacies B in
Rustichelli et al.'s (2013)stratigraphic section because it is the one
that is most densely crosscut by natural bed-parallel compaction
bands (Rustichelli et al., 2012). This lithofacies consists of medium-
grained (mean grain size0.3 mm), moderatee
(Fig. 2a blue box)to well-sorted (Fig. 2a red box) skeletal grainstones composed by
more than 99.5% of calcite (Rustichelli et al., 2012). Some of these
grainstones contain hydrocarbon residues in the form of bitumen
(Agosta et al., 2009). This lithofacies is dominated by bryozoan
fragments and minor amounts of LBF (mainly Amphistegina and
Operculina, rareMyogipsina), echinoid plates and spines, red algae
and lamellibranch fragments (Fig. 2a;Rustichelli et al., 2012; 2013).
The lithofacies is upto 15 m-thick, and it is madeup of stacks of 10 s
of cm- to 4 m-thick crossbed packages bounded by sub-horizontal
truncation surfaces (Rustichelli et al., 2012; 2013).
The pore-throats distribution, obtained from mercury injection
performed on one intact sample (26 mm in diameter and 29 mm in
height) and interpreted based on thin section analyses, revealed a
bi-modal porosity: larger (0.1e
0.01 mm) intergranular pores andsmaller (0.01e0.001 mm) intragranular pores (Fig. 2b).Rustichelli
et al. (2012) showed that most of the intragranular micro-pores
(10 s-to-1 mm) are localized within echinoid and bryozoan frag-
ments, additional intergranular micro-pores are encompassed be-
tween microscparry cement particles. To quantify the amount of
intragranular and intergranular micro-pores, we applied image
analysis techniques on high-magnication (150) SEM images
(0.5 mm pixel size). Within the echinoid and bryozoans fragments
we measured an average porosity of 5.5% 0.95 (standard devia-
tion), whereas we measured an average porosity value of
51.5% 5.4 (standard deviation) between the microsparry cement
particles. Because echinoid and bryozoans fragments represent on
average the 60% of the grains in the studied lithofacies and the
microsparry cement isz
1.2% of the of the rock volume (Rustichelli
et al., 2012), we estimated that z4% of the total rock volume is
constituted by micro-pores not detectable by the SR mCT facility
(Table 1).
A host rock sample was imaged by microfocus X-ray mCT and
porosity values were obtained from selected VOIs (seeTable 1). At a
sample scale the host rock shows rhythmic alternation of laminae
(0.2e2 cm thick) where larger pores are dominant (red polygon,
Fig. 2e) and layers where smaller pores are more abundant (blue
polygon Fig. 2e). The presence of different pore-size distributions is
responsible of the different porosity values (up to 4% of difference)
measured within different laminae sub-parallel to bedding
(Fig. 2dee).
4. Natural bed-parallel compaction bands
At the Roman Valley Quarry the bed-parallel compaction bands
(CBs) dip 10-15 towards north and represent the rst structures
that developed within the studied carbonates (Agosta et al., 2009;
Rustichelli et al., 2012). At outcrop scale, the bed-parallel
compaction bands appear lighter-coloured with respect to the
surrounding host rock. In thin section, the inner texture of the CBs
consists of tight fossil fragments and a very little amountof residual
porosity mainly localized within the single grains (Fig. 3f). Acompaction band was investigated by phase-contrast SR mCT
(Table 1), an axial slice and the volume rendering of a sub-volume
are shown inFig. 3cee. A porosity ofca. 12% was calculated within
the CB from a segmented VOI, whereas adjacent to the CB the
measured porosity was ca. 16%. Across the thickest compaction
bands, a progressive grain-size reduction was observed from the
external to the more internal portion of the structure, in agreement
withCilona et al. (2012). The grain-size reduction was determined
by the interplay of intergranular pressure solution and Hertzian
cracking (Zhang et al., 1990) that occurred at the grain-to-grain
contacts (Fig. 3f). Adjacent and within individual compaction
bands, calcite twinning was observed on overgrowth syntaxial
cement, which behave like a single crystal.
5. Laboratory deformation experiments
Experiments were conducted on samples cored perpendicular
to bedding at various conning pressures (5e35 MPa) with the aim
of determining the yield points for the Bolognano Formation.
Microstructural investigations were performed to identify if any
compaction localization had occurred, or whether the deformation
was distributed. As will be shown later, compaction localization did
occur within certain pressure conditions. Then, based on the rst
results, we performed further experiments at the pressure corre-
sponding to compaction localization, using samples cored at
different orientation with respect to the bedding. These latter ex-
periments were useful to discuss possible the effect of rock het-
erogeneity on the compaction localization.
5.1. Bedding-perpendicular samples
5.1.1. Mechanical data
In the following section, we consider the compressive stresses
and compactive strains (i.e., shortening and porosity decrease) as
positive. The maximum and minimum (compressive) principal
stress axes are denoted by s1and s3, respectively. The mean stress
(s1 2s3)/3 and the differential stresss1 e s3 are denoted by Pand
Q, respectively. We dene as brittle a failure mode in which sig-
nicant strain softening is recorded immediately following the
peak stress. We classify as ductile a failure mode in which a
stressestrain curve displays strain hardening immediately
following yield (cf.Jaeger et al. 2006).
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Mechanical data pertaining to the experiments (Table 2), are
shown inFig. 4. The differential stress as a function of axial strain is
shown in two different graphs: in Fig. 4(a) are the results of ex-
periments carried out at a conning pressure (Pc) comprised be-
tween 5 and 12.5 MPa; in Fig.4(c) the results of experiments
conducted at Pc ranging between 15 and 35 MPa. Volumetric strain
as function of the mean stress is also displayed in two other graphs:
inFig. 4(b) are shown the results of experiments with Pc ranging
between 5 and 12.5 MPa; inFig. 4(d) those from experiments with
Pc comprised between 15 and 35 MPa.
Because it was not possible to measure volumetric strain with
strain gauges, the onset of pore collapse under hydrostatic
compression P* (see Wong et al., 1997) could not be directly
measured on dry Bolognano grainstone. To circumvent this prob-
lem, we followed the procedure applied byBaud et al. (2009) and
performed another test at a constant differential stress of 3 MPa.
The conning pressure was slowly increased and the onset of pore
collapse (55 MPa) could be discerned on the axial strain measured
by a DCDT (Direct Current Differential Transformer; Fig 4e). This
test allowed us to infer P* to be 58 MPa (Fig. 4f).
The experiments performed at 5e12.5 MPa Pc, showed typical
brittle behaviour (Fig. 4a); the stressestrain curves reached a peak
beyond which strain softening followed. Two experiments were
performed at 5 MPa Pc and these were in excellent agreement
Fig. 3. Compaction bands (CB) parallel to bedding (B) analyzed along the walls of the Roman Valley Quarry. aeb) CB affecting the grainstones of the lithofaciesB. Pen is for scale. The
grainstones are strongly invaded by hydrocarbons which highlights the whitish CB (afterRustichelli et al., 2012). Phase-contrast sy nchrotron X-raymCT images (voxel size 9.0mm):
c) 2D axial slice showing grains, cement, pores and the resin inside the pores; d) volume rendering of a sub-volume (size 2.7 3.6 4.3) mm3 showing both pores (black) and grains
plus cement (grey); e) volume rendering of the corresponding segmented volume where the pores are illustrated, the dashed contour highlights the CB. f) Backscattered Electron
images collage of a CB (red dashed), pressure solution and grain crushing are responsible of the local grain-size reduction. (For interpretation of the references to colour in this gurelegend, the reader is referred to the web version of this article.)
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(Fig. 4a). While the peak stresses were quasi-constant (53 MPa) for
both experiments, some variability in the post-peak stress drops
was observed (Fig. 4a).
The experiment performed at 12.5 MPa Pc did not show a sig-
nicant stress drop after the peak stress, just a gentle strain soft-
ening (Fig. 4a). This behaviour suggests that around this Pc the
brittle/ductile transition occurs (Paterson and Wong, 2005). Beyond
15 MPa conning pressure, the specimens experienced ductile
failure (Fig. 4c). Although the results of the latter experiments
documented various stress vs. strain curves shapes, in all cases
stress drops of a few MPa were recorded in the post-peak phase,
which is generally consistent with the occurrence of strain locali-
zation (Vajdova and Wong, 2003; Baud et al., 2004).
The volumetric strain data indicate that the mechanical
behaviour of the deformed carbonate specimens was always
compactant (Fig. 4b and d). This result is in agreement with those
previously obtained after dry experiments on carbonates with
comparable porosity values (Baud et al., 2009; Zhu et al., 2010;
Dautriat et al., 2011b).
Wong et al. (1997) showed that the hydrostatic and non-
hydrostatic loadings are coupled in a triaxial compression
experiment. In the compactive regime, these authors identiedthe yield point as the critical pressure C*, at which the deviatoric
part of the loading results in an acceleration of the compaction
with respect to the hydrostatic behaviour. Since we could not
directly compare the tests with the hydrostatic one, we deter-
mined theC* based on the change in slope of the stress vs. strain
curves (see Fig 4b as example). A shear-enhanced compaction
(sensu Curran and Carroll, 1979) behaviour was observed during
all of the performed experiments (Fig. 4bed). The computed
yield point values are reported in Table 2. InFig. 4(f) all experi-
mental data are shown in the stress space (mean stress vs. dif-
ferential stress), in order to document the yield points computed
for this carbonate lithofacies.
5.1.2. Structural analysis of experimentally-deformed specimens
5.1.2.1. Macroscopic observations. After being deformed, the sam-
ples were unloaded and then retrieved from the pressure vessel to
identify if strain localization occurred along deformation bands
(DBs). Deformation Bands appeared lighter-coloured with respect
to other portions of the specimens. The angles formed by the DBs
ands1varied according to the magnitude of the conning pressure
(Fig. 5). At 5 MPa Pc DBs formed angles of 40 and 30, respectively
(Fig. 5, Bolo1 and Bolo7). At 12.5 MPa of Pc, strain localization
occurred along a conjugate pair of DBs oriented at 53 and 77 with
respect to s1, respectively. At higher conning pressures (15 and
20 MPa), we documented the formation of clusters of sub-parallel
DBs, in agreement with Mair et al. (2000). At 15 MPa Pc, these
features were oriented at 55
with respect to s1, whereas they
formed an angle of 72 at 20 MPa Pc Sub-perpendicular (87) to s1DBs nucleated at 25 MPa Pc At the highest Pc used (35 MPa) strain
localization did not occur and we only documented homogeneous
deformation (Fig. 5). These results are in agreement with those
from previous studies conducted on carbonates (Baud et al. 2009;
Cilona et al., 2012) and sandstones (Baud et al., 2004) for which a
general positive correlation was found between the conning
pressure and the angle formed by DBs and the maximum
compression axis.
5.1.2.2. Microstructural analysis. During deformation, shear-
enhanced compaction behaviour was followed by the three main
failure modes: shear failure, compaction localization and homo-
geneous deformation. We present the results of ve selected
samples (i.e. Bolo H; Bolo7; Bolo11; Bolo5; Bolo9) that represent
different stages of deformation, from 0 to 3% of axial strain
(Table 2). With the exception of Bolo H (host rock), each analyzed
sample was subjected to a different failure mode: low-angle to s1compactive shear bands (Bolo7 and Bolo11); compaction bands
(Bolo5) and homogeneous deformation (Bolo9).
The analyzed DBs were less than 3 mm-thick, their internalstructure was characterized by heterogeneously cracked grains,
twinned calcite crystals and collapsed pores (Figs. 6 and 7).
Deformation bands presented a local porosity reduction (Fig. 8c) in
their internal portions associated with pore and grain size reduc-
tion (Figs. 6 and 7). Despite deformation bands generally appearing
planar and continuous features, up to 30% of thickness variation
was documented along the same DB (Fig. 6i). During the experi-
ments conducted in ductile regime the DBs generally tended to
cluster within laminae characterized by a better grain sorting,
larger pores and relatively richer in bryozoan fossils ( Figs. 6 and 7).
This result is in agreement with previous eld (Rustichelli et al.,
2012) and laboratory observations (Cheung et al., 2012).
Both distribution and density of cracks were calculated in
different portions of the samples (see Section2.3). The values ofdamage obtained for all the analyzed samples are relatively high
(e.g., Wu et al., 2000). Indeed the host rock, which we used as a
baseline value of the pre-experimental deformation damage,
showed a crack density of 6.8 and 8.3 parallel and perpendicular to
bedding, respectively. The anisotropy factor U23 in this sample is
0.14 meaning that a higher number of cracks is oriented perpen-
dicular to bedding.
All the samples, except for Bolo9, showed crack density values
two-to-three times higher than the values of the host rock ( Fig. 8).
Moreover, the crack density was higher within the regions where
strain localization occurred with respect to the surrounding parts of
the samples.
Bolo7 and Bolo5 have a more anisotropic crack distribution
within the strain localization areas than the surrounding parts.
Table 2
Summary of the triaxial experiments performed of the Bolognano grainstones.
Sample Angle to
bedding []
Porosity [%] Conning
pressure [MPa]
Axial strain [%] C* Microstructure
Diff. Stress [MPa] Mean stress [MPa] Respect tos1
Bolo1 90 27 5 0.8 44.7 19.9 Low angle
Bolo7 90 27 5 1.8 44.42 19.8 Low angle
Bolo11 90 26.3 12.5 3 49.150 28.88 Low angle high angle
Bolo3 90 28.3 15 3 48.175 31.06 High angle
Bolo4 90 28 20 2.85 42.24 34.08 Very high angle
Bolo5 90 27.3 25 2.85 40 38.3 Perpendicular
Bolo 45 to bed 45 26.7 25 2.85 49.07 41.35 45
Bolo 0 to bed 0 26.6 25 2.6 46.17 40.35 Low angle high angle
Bolo9 90 26.8 35 2 28.1 44.37 Homogeneous deformation
Bolo12 90 32 8 3 55 Homogeneous deformation
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Conversely, Bolo11 displays a more isotropic crack distribution in
the localization area respect to the surrounding parts. Bolo9 shows
a marked anisotropy of the crack distribution, but the standard
deviation of these values is high.
The internal structure of sample Bolo5 was investigated by
means of X-raymCT both using the conventional and the SR sources
(seeTable 1for details). X-ray mCT data were used to calculate the
porosity within and outside the compaction band. The porosity
Fig. 4. Mechanical data for triaxial compression experiments on Bolognano grainstones. a) Differential stress versus axial strain for experiments at conning pressures up to
12.5 MPa. b) Mean stress versus volumetric strain for experiments at conning pressures up to 12.5 MPa. c) Differential stress versus axial strain for experiments at conning
pressures up to 35 MPa. d) Mean stress versus volumetric strain for experiments at conning pressures up to 35 MPa. e) Mean stress versus axial strain for a constant differential
stress experiment. f) Yield points of the experiments represented in the stress space, different symbols for different orientation respect to bedding.
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values calculated within the compaction band are 16% with the
conventional mCT data (voxel size 17.2 mm) and 20% with SR mCT
data (voxel size 9 mm) (Fig. 9).
The porosity calculated outside the compaction band are 23%
and 25% with conventional and SRmCT, respectively. Relevant pore-
size reduction is documented within the laboratory compaction
band (Fig. 9bee). Within the localization zone, where small pores
are more abundant, the high accuracy of the phase-contrast SRmCT
technique (see Section2.2) gives a more precise approximation of
the porosity with respect to the conventional mCT technique (Fig. 9).
5.2. Bedding-oblique samples
With the aim of investigating possible effects of rock hetero-
geneity on strain localization, we cored two specimens at different
orientations with respect to sedimentary bedding (parallel and at
Fig. 5. Pictures of the deformed samples, in white the superimposed interpretation of the formed deformation bands.
Fig. 6. Compilation of photomicrographs (cross-polarized nicols) and SEM images from the samples Bolo H (a, b) Bolo7 (c, d, e, f) and Bolo11 (g, h, i). Red arrows indicate some of the
micro-cracks, T represents twinned calcite crystals, pores are black, DBs are evidenced with red dashed lines. In all the images of deformed samples the direction of s1 is
horizontal. a) micro-cracks; b) pore-emanated cracks. The images represent different portions of the samples: c and f are relatively-undeformed areas; d and h are areas adjacent to
deformation band; e and i show the deformation band. (For interpretation of the references to colour in this gure legend, the reader is referred to the web version of this article.)
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Fig. 7. Compilation of photomicrographs (cross-polarized nicols) and SEM images from the samples Bolo5 (a, b, c) and Bolo9 (d, e, f). Red arrows indicate some of the micro-cracks,
T represents twinned calcite crystals, pores are black. In the images the direction of s1 is horizontal. The images represent different portions of the samples: a relatively-
undeformed area; b area adjacent to compaction band with twinned calcite; c) within the compaction band. d) twinning-rich zone, e) bryozoans-rich and twinning-free
portion, f) strength difference between echinoid (E) and bryozoans fragments. (For interpretation of the references to colour in this gure legend, the reader is referred to the
web version of this article.)
Fig. 8. Quantitative data ofSv(a); U23(b) and 2D porosity (c). Each color represents a different sample, the error bars represent the standard deviation. The data are divided in two
groups in order to differentiate the measures within the strain localization area from those in the rest of the sample. (For interpretation of the references to colour in this gure
legend, the reader is referred to the web version of this article.)
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45). We run these two additional tests at 25 MPa Pc because it
corresponds to the pressure at whichs1-perpendicular compaction
bands formed (Fig. 5).
5.2.1. Mechanical data
The mechanical data are shown inFig. 10. The differential stress
as a function of axial strain is shown inFig.10(a), the mean stress as
the function of the volumetric strain inFig. 10(b). Those data arecompared to that obtained, at the same Pc, for the specimen cored
perpendicular to bedding.
For the bed-parallel specimen we documented a peak stress of
59 MPa followed by a 2 MPa-wide stress drop. On the specimen
cored at 45 to bedding, we recorded a peak stress of 61.5 MPa
before a stress drop of 8 MPa occurred. In this latter experiment
strain-hardening was more pronounced and further stress drops
were recorded in the post-peak part of the curve.
5.2.2. Structural analysis of bedding-oblique specimens
5.2.2.1. Macroscopic observations. As shown inFig. 11macroscopic
deformation was documented in both samples. On the specimen
cored at 45 to bedding, strain localization occurred and 45 to54-
to s1 DBs formed. Macroscopic shear offset (z0.3 mm) wasaccommodated along DBs parallel to the bedding (Fig. 11).
Discontinuous areas of deformation were documented on the bed-
parallel specimen as well. The general angle formed by these
patches of deformation and the s1 direction ranged between 65
and 85.
5.2.2.2. Microstructural analysis. During both experiments shear-
enhanced compaction was associated with two different failure
modes: strain localization and homogeneous damage. Both sam-
ples (i.e., Bolo 45 to bedding and Bolo 0 to bedding) were deformed
up to 3% axial strain (Table 2).
The thicknesses of the DBs documented within sample Bolo45
to bedding ranged from 1.7 to 3.3 mm (Fig. 11), their internal
structure showed highly-cracked grains, twinned calcite crystals
and collapsed pores (Fig. 12bec). Local porosity reduction, associ-
ated to pore and grain sizes reduction, was documented within the
DBs formed in Bolo45 to bedding (Fig. 8c). In this specimen the DBs
localized within the layers characterized by higher presence of
bryozoans fossils (Fig. 11). Based on the lower porosity of these DBs
with respect to the host rock (Fig. 11) and the macroscopic shear
offset they resolved, we classied them as compactive shear bands
(CSB;sensu Aydin et al., 2006).
The texture of sample Bolo0 to bedding was caused by diffusedeformation, strain localization did not occur in this sample but the
Fig. 9. aeb) Volume rendering of a parallelepiped-shaped volume (height of the volume in the zdirection 210 mm) of sample Bolo5 obtained by microfocus X-ray mCT (voxel
size 17.2mm): in a) both pores (black) and grains plus cement (grey) are visible. The red dashed polygon highlights the CB area and measured 3D porosity values are also shown. In
b) the corresponding segmented volume is visible with the pore network within and outside the compaction band. c) Volume rendering of a parallelepiped-shaped volume of
sample Bolo5 obtained by phase-contrast synchrotron X-raymCT (voxel size 9.0mm): the red dashed polygon highlights the CB area. Measured 3D porosity values are also shown.
e) Backscattered Electron images mosaic, red dashed polygons highlight discrete CBs. (For interpretation of the references to colour in this gure legend, the reader is referred to the
web version of this article.)
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central part of the sample was highly deformed (Fig. 11). The
texture of this specimen was tighter than the other two samples
deformed at the same Pc condition (Fig. 11). Pore collapse and
mechanical twinning occurred within this sample. The mechanical
twinning of calcite crystals was enhanced where grains pile-up
occurred (Fig. 12d). For sample Bolo45 to bedding crack density
was up-to-two times higher than the host rock (Fig. 8). Highercrack
density was documented in the area of localization with respect to
the surrounding parts of the sample. Within the strain localization
area, the crack distribution was less isotropic than the surrounding
parts. The crack density values for sample Bolo0 to bedding were
slightly higher than the host rock (Fig. 8). Although the damagewas
homogeneously distributed and the crack distribution was aniso-
tropic, high standard deviation values of the crack distributionsuggest isotropy in some analyzed pictures and anisotropy in others
(Fig. 8).
Fig. 12(a) shows the results of twinning density by means of
column diagrams. Both samples, where strain localization took
place, presented higher twinning densities values within the
deformation bands with respect to surrounding portions of the
samples. Among the three samples the maximum twinning density
was record in the sample 45 to bedding. The minimum density of
twinning was found to correspond to the sample perpendicular to
bedding.
6. Discussion
6.1. Mechanical behaviour
The carbonate grainstones discussed in prior sections presented
post-peak stress drops in both brittle and ductile regimes (Fig. 4).
We documented a brittle behaviour up to 10 MPa Pc whereas the
brittle/ductile transition was observed at 12.5 MPa Baud et al.
(2009), in their experiments on carbonates with comparable
porosity, documented this transition at similar Pc conditions.
Ductile failure occurred beyond 12.5 MPa Pc associated to
strain-hardening. Beyond the peak stress, and during the strain-
hardening part of the curves, stress drops events were recorded:
the stress drops are generally associated to the occurrence of strain
localization (Baud et al., 2004). These observations are consistent
with the stressestrain curves of laboratory-deformed carbonates
described byCilona et al. (2012)and previous work on sandstones
(e.g., Vajdova and Wong, 2003; Baud et al., 2004; Fortin et al.,
2006).
Fig. 10.Mechanical data for triaxial compression experiments (performed at 25 MPa) on samples of Bolognano grainstones cored at different orientations with respect to bedding.
a) Differential stress versus axial strain. b) Mean stress versus volumetric strain.
Fig. 11. Pictures of the sample deformed at 25 MPa, in white the superimposed interpretation of the formed deformation bands. For the two oblique-to-bedding samples (Bolo45 to
bed and Bolo0 to bed) a mosaic of SEM images shows their internal texture. The red dashed lines highlight the CSB. (For interpretation of the references to colour in this gure
legend, the reader is referred to the web version of this article.)
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Temperature may condition the mechanics of carbonates
(Paterson and Wong, 2005): higher temperatures generally pro-
mote ductility and increase the sensitivity of carbonates to strain
rate variations. Croize et al. (2010a) performed experiments on
bioclastic carbonate sand at temperatures up to 70 C (corre-
sponding to a depth >2 km, under a normal geothermal gradient:
25 C/km). Their results showed that within that range the tem-
perature does not affect the mechanical response of carbonate
sands under strain rates comparable to those of our experiments.
Based upon the latter evidence, we suggest that the mechanical
strength and failure behaviour of our samples, acquired at a room
temperature, were not affected by thermally activated deformation
mechanisms, in agreement withLiteanu et al. (2013).
Our data are coherent with a purely compactant mechanical
behaviour of Bolognano grainstones. Shear-enhanced compaction
behaviour was documented, in agreement with previous data ob-
tained under dry conditions on carbonates with a porosity of about
30% (e.g.,Baxevanis et al., 2006; Baud et al., 2009; Zhu et al., 2010;
Fig.12. a) Column diagram showing the density of twinned calcite crystals in the different samples. The data are grouped in two clusters to show the difference between the strain
localization areas and the rest of the sample; Microphotographs (cross-polarized nicols) and SEM images from sample 45 to bedding (b ec) and 0 to bedding (dee), the arrows point
out to some micro-cracks and the Tindicates the twinned crystals. For all the images the direction ofs1is vertical. (b) and (c) are taken within the CSB, (d) shows collapsed pores
and diffuse deformation, high density of twinned calcite is documented in picture (e).
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Vajdova et al., 2012). Conversely,Cilona et al., (2012) documented
shear-enhanced dilation at 5 MPa ofPeff.under wet conditions and
strain rates of 107 s1, suggesting the occurrence of chemical
processes such as subcritical crack growth (seeBrantut et al., 2013
for a recent review).
Fig. 13compiles the yield points data in the stress space (PeQ)
and compares them with dry date fromVajdova et al.'s (2004)on
Indiana limestone (f 16%) and with Baud et al.'s (2009) on
Orfento and S. Maximin grainstones (f30 and 37%, respectively).
While the yielding strength of the carbonates from literature is
inversely proportional to porosity and grain size, Bolognano
grainstone turn out to be much stronger than one would predict by
considering only its grain size and porosity values. Since more
variability in porosity and grain size is present in these carbonates,
predicting the plastic yield envelops and the conditions for strain
localization, is not as reliable as it is in sandstones (Shultz et al.,
2010and references therein). For example, the strength of Bolog-
nano grainstones is higher than that of Orfento limestone (Baud
et al., 2009) and may be a consequence of a more abundant pres-
ence of echinoid fragments (Rustichelli et al., 2013). Indeed echi-
noid fragments promote the growth of syntaxial cement, which is
responsible of a strengthening of the grain contacts.
During the deformation experiments, both low- and high-angleto s1 deformation bands developed. Moreover, in agreement with
carbonates (Baud et al. 2009; Cilona et al., 2012) and sandstones
(e.g., Baud et al., 2004; Fortin et al., 2006; Wong and Baud, 2012 )
studies, a general positive correlation was found between the
conning pressure and the angle formed by DBs and the maximum
compression axis (Fig. 5). These latter results are consistent with
the statement that to form high-angle to s1 deformation bands
higher conning pressures and lower differential stresses are
needed with respect to low angle ones (i.e., Baud et al., 2004;
Fossen et al., 2011; Cilona et al., 2012).
6.2. Microstructural characterization of deformation bands
A systematic microstructural analysis was carried out on sam-ples deformed at different stress and strain conditions. For the
analyses, thin section observations and X-ray mCT data were
integrated. For comparison, the analyses were performed on host
rock, experimentally, and naturally deformed samples.
InFig. 2(a) we showed that the host rock contains alternating
laminae characterized by different values of sorting (Fig. 2a) with
porosity contrasts ofz4%. These contrasts are likely determined by
the variable concentration of bryozoans fossils, which are rich in
intragranular macro-pores (Rustichelli et al., 2013). A few percen-
tiles difference in porosity may strongly affect the mechanical
behaviour of a layer with respect to another in carbonates ( Cilona
et al., 2012) as well as in other rocks (e.g., Baud et al., 2012). Also
Louis et al. (2009) documented up to z8% of porosity difference
between low- and high-porosity layers in the Rothbach sandstones.
The crack surface area values we calculated for the undeformed
sample are one order of magnitude higher than those documented
by Wu et al. (2000) in their unstressed sample of Darley Dale
sandstone. The values measured on Bolognano are so high because
of the contribution of 10 s ofmm-long cracks emanating from pores
(Figs. 6 and 7). Indeed, in addition to the lower strength of calcite
with respect to quartz, the abundant intragranular pores in
Bolognano might have promoted the formation of pore-emanating
cracks during the natural deformation phases experienced by the
rock (Fig. 6aeb; Vajdova et al., 2004; 2010). Despite we docu-
mented high values of crack surface area also for the naturallydeformed sample, we highlight that the density within the
compaction band was similar to that in the surrounding host rock
(Fig. 8). This result suggests that within natural compaction bands
pressure solution prevails respect to grain crushing, in agreement
withTondi et al. (2006)and Cilona et al. (2012).
We compared the crack-density area values measured in this
study with those obtained by previous author in samples deformed
at similar levels of plastic strain. Our results were two-to-three
times lower than those published for porous carbonates (Vajdova
et al., 2010; Cilona et al., 2012), and up to a factor six higher than
those measured in sandstones (Wu et al., 2000).
Unlike other studies (Wu et al., 2000;Vajdova et al., 2010), we
did not aim to describe how the damage increases with the axial
strain; thus we did not calculate the Sv at different stages ofdeformation and constant Pc. Despite the different levels of plastic
strain experienced by the samples, a negative trend between the
conning pressure and Sv can be observed in Fig. 8(a). We docu-
mentedthe highest valuesofSv in the sample deformedat 5 MPa Pc
and the lowest (factor three less) in the sample deformed at 35 MPa
Pc. This difference might be causedby an interplayof grain crushing
and pore collapse: in the ductile regime the latter process is
dominant. After comparing the samples deformed at 25 MPa Pc
which experienced the same amount of plastic strain, we docu-
mented higher Sv values in the two samples where strain locali-
zation occurred. In the host rock, the 2D porosity values
underestimated the 3D ones by a factor of z1.5; Rustichelli et al.
(2012)documented similar ratios. The 2D porosity within the lab-
oratory compaction band underestimated the 3D one by a factorthree-to-four. The underestimation is higher within the DBs
because of the smaller pore size: image analysis accuracy is
strongly related to the spatial and contrast resolution of the
analyzed pictures (Fig. 9). Respect to the host rock, the porosity
decreased up-to-one fourth within the laboratory deformation
bands and up-to-one fth in the natural CB. Similar porosity re-
ductions are documented byCilona et al. (2012).
6.3. Rock heterogeneity and compaction localization
The specimens deformed at 25 MPa Pc showed ductile behav-
iour. Although in these experiments the recorded peaks were
almost equal, the bed-perpendicular specimen had the lowest
strength whereas, the bed-oblique specimen showed the highest
Fig. 13. Compilation of yield points of different porous carbonates: Bolognano (this
study), Indiana (Vajdova et al., 2004); Orfento Majella Limestone and St. Maximin
(Baud et al., 2009).
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strength. Lower strength of the bed-perpendicular samples with
respect to the bed-parallel ones was also measured byBaud et al.
(2012) for the Diemelstadt sandstone. Conversely, Louis et al.
(2009) documented the highest strength in the bed-
perpendicular specimens and the lowest in the bed-parallel ones.
These latter authors claim that the distribution of grain contacts
controls the strength of the Rothbach sandstone. Our experimental
results are consistent with Bolognano grainstones promoting strain
localization within laminae sub-parallel to bedding characterized
by higher porosity and better sorting (Fig. 2). Additionally to
porosity, grain sorting plays a fundamental role on strain localiza-
tion: better-sorted layers are more keen to develop compaction
bands (Fossen et al., 2011; Cheung et al., 2012; Rustichelli et al.,
2012; Skurtveit et al., 2013).
The microstructures we documented varied with the orienta-
tions of sample with respect to bedding (Fig.11). These results are
similar to those presented by Louis et al. (2007) for Rothbach
sandstone, however the same authors demonstrated that one of the
45-to bedding samples, deformed at the conning pressure at
which CBs were predicted, did not develop compactive shear bands
but short and discontinuous compaction bands conned within
porous layers (Louis et al., 2009). This latter event likely did not
happen for the 45-to bedding sample of Bolognano because amacroscopic shear offset was detected on that specimen.
Despite few examples of bed-parallel compactive shear bands
are documented in some of the cross-beds of Bolognano Fm.
(Fig. 13a; Agosta et al., 2009), we suggest that during its burial
history the studied rocks unlikely experienced a differential stress
of magnitude equal to that one shown in Fig. 10(with vertical s1).
Conversely we cannot exclude that similar stress conditions were
reached during Middle-to-Late Pliocene when Majella Anticline
formed (Ghisetti and Vezzani, 2002). This scenario would imply
diffuse damage caused by the horizontal s1 and would justify the
high crack density measured in the host rock sample (Fig. 6aeb and
8).
We documented the highest twinning density in the 45-to
bedding sample, this value is justied by the occurrence of shearfailure; indeed calcite requires low shear stresses to initiate me-
chanical twinning and dislocation (Vajdova et al., 2010, 2012;
Cilona et al., 2012). In this study, mechanical twinning was
mainly documented within the overgrowth syntaxial cement
(Rustichelli et al., 2013 for detailed information). We suggest that
sample-scale variations of echinoids fragments amounts affect the
density of twinning (Fig. 12) and can inuence the interplay be-
tween grains crushing and mechanical twinning from one sample
to another. Although Vajdova et al. (2012)describes the mechanical
effects of the predominance of one or the other process, further
systematic analyses should be performed to address this issue.
In this paper we propose a scenario for compaction localization
in carbonates that is very different from what previously presented
for sandstones, for which rock homogeneity appears to promotethe development of compaction bands (Wang et al., 2008; Cheung
et al., 2012). Laboratory experiments on sandstones documented
CBs oriented perpendicular to s1, independently from the angle
formed by the specimens and the sedimentary bedding (e.g., Baud
et al., 2006; Fortin et al., 2006), the main effect of bedding in-
terfaces was to inhibit the propagation of compaction bands
through the sample (Louis et al., 2009) or increase their tortuosity
(Baud et al., 2012).
In limestones, to our knowledge, natural CBs have been mainly
documented parallel to bedding, onlyTondi et al. (2012)describes
bed-oblique CBs at the tips of well-developed faults. Bed-parallel
compaction bands tend to localize within the most porous and/or
coarser layers (Tondi et al., 2006; Agosta et al., 2009; Cilona et al.,
2012; Rustichelli et al., 2012; Tondi et al., 2012), and pre-existing
mechanical interfaces (e.g., carbonate hardground or bedding)
may also promote compaction localization (Cilona et al., 2011;
Rustichelli et al., 2012). These eld observations provide a
possible clue to explain why in previous laboratory studies, per-
formed on homogeneous porous carbonates, pure compaction
bands did not nucleate (seeWong and Baud, 2012for a full review).
Many systematic experimental studies documented either diffuse
deformation (Vajdova et al., 2012) or high-angle to s1 compactive
shear bands/shear-enhanced compaction bands (Baud et al., 2009;
Vajdova et al., 2010) associated to ductile failure. We suggest that
the absence of strong bedding-parallel heterogeneity inhibited the
systematic development of compaction bands in most of the
laboratory-deformed carbonates. Cilona et al. (2012) deformed
different strata of the Orfento Fm. from Madonna della Mazza
Quarry of the Majella Mountain, Italy (Tondi et al., 2006) and were
able to produce experimentally compaction bands in some of the
deformed strata. Our results are consistent with the observations of
Dautriat et al. (2011a)and conrmed the hypothesis ofCilona et al.
(2012): rock heterogeneity may enhance compaction localization.
A stress-independent kinematics of natural deformation bands
in the studied carbonates should be considered. Bolognano grain-
stones are characterized by alternations of horizontal and crossed
beds (Fig. 13;Rustichelli et al., 2013).Agosta et al. (2009)describedcompactive shear bands parallel to cross beds adjacent to bed-
parallel compaction bands localized within horizontal beds
(Rustichelli et al., 2012). Based upon our experimental data, and
comparing the orientations and kinematics of the deformation
bands developed in bed-oblique and bed-perpendicular samples,
we propose that under the same stress conditions (i.e. mean and
differential stress;Fig. 10) both compaction bands and bed-parallel
compactive shear bands can nucleate at the same time in strata
oriented oblique or perpendicular s1. It actually means that rock
heterogeneity can be responsible of the switching from a purely
volumetric deformation to a shear/volumetric one (Fig. 14b).
The strain localization into compaction bands or compactive
shear bands would then cause different types of stress perturbation
at the tips of these structures with implications on orientation anddistribution of secondary dilatant tail structures (e.g., dilation
bands, joints). From the prospective of reservoir exploration and
production it can be postulated that, due to the sedimentary ar-
chitecture, some portions of a reservoir could increase their con-
nectivity if a higher effective stress would cause the formation of
compactive shear band.
7. Conclusions
The presented study integrated eld and laboratory approaches
to investigate the effects of rock heterogeneity on compaction
localization in porous carbonates. A systematic set of triaxial
compaction experiments was performed on the Oligocene-
Miocene skeletal grainstones of Bolognano Fm., central Italy. Thecarbonates displayed shear-enhanced compaction and strain
hardening associated with various patterns of strain localization.
The brittle/ductile transition occurred at 12.5 MPa conning pres-
sure, and discrete compaction bands nucleated at 25 MPa. A posi-
tive correlation between conning pressure magnitude and the
angular value formed by individual deformation band and the
major principal stress axis was observed.
As natural compaction bands also the laboratory ones localized
within bryozoan-rich laminae, because of the z4% higher porosity
and better sorting.
We compared internal structure as well as the micromechanism
of laboratory compaction bands to those of natural one. Despite the
internal texture appeared to be very similar for both structures, in
natural compaction bands pressure solution dominates with
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