Along-strike fault core thickness variations of a fault in poorly lithified sediments, Miri (Malaysia)

Abstract

The degree to which a fault will impede fluid flow is only as great as its most permeable point. Processes that determine areas of the fault surface containing transmissible fault rocks must be utilized to produce reliable predictions of cross-fault fluid flow. We use a study site in Miri, Malaysia, to investigate in detail the fault-core thickness variations along-strike and down dip, and to quantify the risk of discontinuities in the clay-rich fault core. Four fault-core types have been identified: foliated clay-rich fault core, chaotic clay-rich fault core, anastomosing sandy shear zones and sandy breccia. We performed a geostatistical analysis, showing a correlation over 3 m scale, suggesting the presence of ‘patches’ of thin and thick fault core generally less than 3 m in length in profile. We interpret this geometry as superimposition of two or more different deformation processes at a smaller and a larger scale. We speculate about the processes that could produce the observed distribution of thickness and composition, and in particular, processes that could have disrupted the through-going clay-rich core.

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Journal of Structural Geology
journal homepage: www.elsevier.com/locate/jsg
Along-strike fault core thickness variations of a fault in poorly lithified
sediments, Miri (Malaysia)
Silvia Sosio De Rosa
a,∗
, Zoe K. Shipton
a
, Rebecca J. Lunn
a
, Yannick Kremer
a
, Titus Murray
b
a
Department of Civil and Environmental Engineering, University of Strathclyde, G1 1XJ, 75 Montrose St., Glasgow, UK
b
FaultSeal Pty Ltd, Sydney, Australia
ARTICLE INFO
Keywords:
Fault core
Along-strike thickness
Variogram
Clay smear
ABSTRACT
The degree to which a fault will impede fluid flow is only as great as its most permeable point. Processes that
determine areas of the fault surface containing transmissible fault rocks must be utilized to produce reliable
predictions of cross-fault fluid flow. We use a study site in Miri, Malaysia, to investigate in detail the fault-core
thickness variations along-strike and down dip, and to quantify the risk of discontinuities in the clay-rich fault
core.
Four fault-core types have been identified: foliated clay-rich fault core, chaotic clay-rich fault core, anasto-
mosing sandy shear zones and sandy breccia. We performed a geostatistical analysis, showing a correlation over
3 m scale, suggesting the presence of ‘patches’of thin and thick fault core generally less than 3 m in length in
profile.
We interpret this geometry as superimposition of two or more different deformation processes at a smaller
and a larger scale. We speculate about the processes that could produce the observed distribution of thickness
and composition, and in particular, processes that could have disrupted the through-going clay-rich core.
1. Introduction
Faults play a key role in controlling fluid flow in the shallow crust
(Caine and Minor, 2009;Faulkner et al., 2010). Predicting bulk fault-
zone petrophysical properties at depth is a key part of de-risking geo-
logical applications such as extraction of hydrocarbons, geothermal
heat, storage of CO
2
, or radioactive waste (Aydin, 2000;Douglas et al.,
2000;Shipton et al., 2004). To predict whether faults act as barriers,
baffles or conduits, structural geologists have attempted to develop
relationships between key parameters of fault architecture, for ex-
ample, using throw to predict mean fault thickness (Childs et al., 2007;
Shipton et al., 2006), using host rock type and throw to predict fault-
rock type (Faerseth, 2006), and then using these relationships to predict
petrophysical properties. However, data from oil and gas production
show that these estimates are not always reliable (Bretan et al., 2003).
Field data show that this lack of reliability could be a consequence of
significant along-strike variability within the fault plane due to fault-
scale processes that are not only related to throw (Caine and Minor,
2009;Kremer et al., 2018).
It is the size and location of relatively high-permeability fault rocks,
more than the mean low-permeability fault core thickness, that has the
strongest influence on the hydraulic behaviour of a fault (Heynekamp
et al., 1999;Lunn et al., 2008;Caine and Minor, 2009). Field studies of
fault architecture generally focus on down-dip sections to determine the
relationship between fault properties and stratigraphy. However, the
along-strike dimension has to be investigated to constrain the prob-
ability of high-permeability areas in a low-permeability fault core.
Significant lengths of along-strike sections are less commonly exposed,
and so a dearth of published observations exists about along-strike
variability.
Few studies of along-strike fault core composition have been pub-
lished in siliciclastic, poorly consolidated sediments. Along-strike clay
smear continuity was investigated by Lehner and Pilaar (1997) in a
fault of 70 m throw cutting a sand-shale deltaic sequence in the Frechen
mines (Germany). Doughty (2003) studied a 40–60 m throw growth
fault in conglomerate and sand, silt and mud fluvial and lacustrine
sediments. Van der Zee and Urai (2005) studies faults of up to a few m
slip in the deltaic sediments of the Miri Formation. Kettermann et al.
(2016) studied faults of 50–120 cm throw in deltaic sand-clay se-
quences. Lehner and Pilaar (1997) observe along-strike clay continuity
over considerable distances, up to 400 m. In contrast, Doughty (2003),
Van der Zee and Urai (2005) and Kettermann et al. (2016) found that
https://doi.org/10.1016/j.jsg.2018.08.012
Received 27 March 2018; Received in revised form 23 August 2018; Accepted 23 August 2018
∗
Corresponding author.
E-mail addresses: silvia.sosio-de-rosa@strath.ac.uk (S. Sosio De Rosa), zoe.shipton@strath.ac.uk (Z.K. Shipton), rebecca.lunn@strath.ac.uk (R.J. Lunn),
yannick.kremer@strath.ac.uk (Y. Kremer), titus@faultseal.com (T. Murray).
Journal of Structural Geology 116 (2018) 189–206
Available online 25 August 2018
0191-8141/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/BY-NC-ND/4.0/).
T

clay smears were not continuous along-strike at the 100-m, m and dm
scale respectively. The factors governing these differing observations
are not well understood.
We present data from a site in soft sediments that offers extensive
along-strike fault exposures and three sub-vertical cuts, giving an ex-
cellent 3D view of within-plane fault variability. We use spatial statis-
tics (e.g. variogram analysis, Matheron, 1971) to characterize de-
formation processes, and provide key information for robust
representation of these phenomena in numerical simulations. In
agreement with Caine and Minor (2009) and Kremer et al. (2018), the
variations that we observe cannot be related in a simple way to fault
throw. Instead we demonstrate that a number of processes for in-
corporating the host rock materials into the fault zone are responsible
for generating highly heterogeneous fault rocks along-strike.
2. Geological setting
The study area is located in Northwest Borneo, just outside the town
of Miri (Fig. 1a). The stratigraphic sequence of the Jalan Mukah outcrop
is part of the 456 Sand of the Upper Miri Formation, Middle Miocene.
The West Baram Delta is composed of up to 10 km of Middle Miocene to
recent deltaic sequences (Tan et al., 1999). Marine shale intervals are
separated by thicker coastal-fluviomarine sands, which constitute the
producing reservoirs of the oil Miri field (Fig. 1b; Tan et al., 1999).
Northwest Borneo is characterized by young gravity-related deforma-
tion: seaward (NW) and landward (SE) dipping growth faults, mobile
shales, and toe thrusts (Morley et al., 2003). The Late Miocene gravity-
driven extension is expressed in the Miri Hill structure as a set of listric
faults, the most important of which is the Shell Hill fault (offset 750 m;
Hutchinson, 2005), and its associated antithetic normal faults. The
extension was followed by NW-SE Pliocene compression, which re-
sulted in the reactivation of the Canada Hill Thrust and development of
the Miri anticline (Fig. 1c and d), causing uplift of the landward side of
the delta (Tan et al., 1999;Morley et al., 2003;Hutchinson, 2005;
Wannier et al., 2011). A strike-slip component of deformation asso-
ciated with compressional structures has been described by Sandal
(1996) for high-angle faults in the Northwest Borneo margin.
The Jalan Mukah outcrop is located on the southern apex of the Miri
anticline structure (Fig. 1c). Two sections of the outcrop were pre-
viously studied by Noorsalehi-Garakani (2015) for lateral clay injection
mechanism, and by Kessler and Jong (2017) for clay smearing. This
study is the first to describe all of the exposed sections and to analyse
the along-strike fault-core thickness data with variograms. Other parts
of this fault system have been previously studied (Burhannudinnur and
Morley, 1997;Tan et al., 1999;Van der Zee et al., 2003;Van der Zee
and Urai, 2005;Wannier et al., 2011). Van der Zee and Urai (2005)
describe the initial evolution of normal faults in layered sand-clay se-
quences in the same formation, at the Airport road outcrop, about
300 m from the Jalan Mukah outcrop. The Jalan Mukah outcrop offers
unprecedented along-strike exposure due to the clearing of an area of
land of 1 km
2
for the installation of a large water tank, and three sub-
vertical faces cut by bulldozer showing the faults in dip section
Fig. 1. a) Location of the study area in NW Borneo. b) Upper Miri Formation stratigraphy (modified after Wannier et al., 2011). c) Geological map and location of the
Jalan Mukah outcrop (after Wannier et al., 2011). d) SW-NE cross section of the Miri anticline (Van der Zee and Urai, 2005).
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
190

(Fig. 2a).
At the Jalan Mukah outcrop, the 456 Sand is dominated by grey fine
to medium sand beds, with minor orange sands and interbedded clay-
rich beds 0.2–2 m thick (Fig. 2b). The thickness and lateral distribution
of sand and clay beds varies. Fossil Ophiomorpha burrows 1–10 cm long
are common both in sand and clay beds. No calcite was observed either
as sand grains, as cement, or as fracture fill. The maximum burial depth
at the time of faulting was less than 1000 m, as indicated by clay por-
osity and vitrinite reflectance (Schmitz et al., 2003).
3. Methodology
A structural map of the whole outcrop (scale 1:500, resolution
50 cm), was compiled using high resolution photographs taken with a
drone (Fig. 2a). A map of the fault with the largest displacement, re-
ferred to as 'main fault', was made at the 1:50 scale in the field. This
map includes all structures with a length over 10 cm. The main fault
was logged with a measuring tape along its entire length at every 0.5 m.
The fault outcrop is exposed in both the planar and vertical dimensions
along strike, and therefore the outcrops are divided into seven sections,
which either expose the fault along strike or in a sub-vertical cliff. The
logging of the main fault is complimented by measurements of the fault
core (sensu Caine et al., 1996), and thickness and photographs of the
fault zone, taken every 0.5 m. Vertical and selected parts of the along-
strike exposures were mapped with a regular square grid of
0.5 × 0.5 m, placed with string and nails (Figs. 3–8). Photographs of
grid squares were used as a base for tracing structural elements and
data on site. Deformation band (DB), deformation-band cluster and
fracture intensity within the fault damage zone were measured every
0.5 m along seven transects perpendicular to the main fault. Oriented
samples were collected for microstructural analysis from the host rocks
and the fault zone with hammer and knife (locations on Fig. 2). Extreme
care was used to avoid the fragile rock crumbling into pieces. Footwall
and hangingwall stratigraphy were logged.
4. Jalan Mukah outcrop
4.1. Main fault
The main fault of the Jalan Mukah outcrop trends ENE-WSW and
dips SE between 40 and 62° (Fig. 2a and c). The main fault is traced for
a length of about 100 m of which 55 m are exposed, and is associated
with a conjugate set of normal faults with the same trend that dip
45–70° to the NNW and SSE (Fig. 2a and d). The main fault displaces
the entire exposed stratigraphy, therefore it is not possible to correlate
between the footwall and the hangingwall sequences which both belong
to the 456 Sand unit. The main fault offset is constrained to a minimum
offset by the thickness of the hangingwall stratigraphy (20 m), and the
thickness of the 456 Sand (122 m). Kessler and Jong (2017) suggest a
fault throw of 40–50 m based on litho-stratigraphic correlation, while
Noorsalehi-Garakani (2015) suggests a throw of 20 m. The bedding
consistently dips 10-15° west (Fig. 2d). Two synthetic faults in the
Fig. 2. Structural map of the Jalan Mukah outcrop showing the main fault (thick fault trace) and the secondary faults FI and FII (a). b) Footwall and hangingwall
stratigraphy. c) Aerial picture of outcrop (45° view from SW). d) Stereographic projections of fault core slip surfaces, bedding and shear zones, deformation bands,
fractures (lower hemisphere, equal area projections).
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
191

hangingwall have throws of 5.7–8 m each (faults FI and FII, Fig. 2a),
while all other conjugate faults have offsets smaller than 1 m. The lack
of slickenlines on the slip surfaces prevents the determination of the
sense of shear, however the inferred conjugate fault geometry indicates
extensional normal dip-slip faults. A dextral component of shear is
suggested by σ–shaped sand lenses in the clay-rich matrix of the fault
core within sections 1 and 5.
The damage zone of the main fault at Jalan Mukah contains de-
formation bands, shear zones and gentle folds of clay and sand beds
towards the fault core. Shear zones and deformation bands are defined
as tabular structural discontinuities having a continuous change in
strength or stiffness across a relatively narrow zone (Schultz and
Fossen, 2008). In this field area, we distinguish shear zones from de-
formation bands because shear zones contain clay as well as sand, and
generally have a larger offset for their width than deformation bands.
Deformation bands are dominant in clean sand, while shear zones
dominate where the host rock sands are more clay-rich. Deformation
bands occur in conjugate sets trending NE-SW, dip towards SE and NW
(34°–88°), matching the conjugate fault orientations, and have offsets of
a few mm up to 5 cm. Shear zones have a similar orientation to the main
fault, trending NE-SW and dipping 36 to 80° SE (Fig. 2d). Shear zones
have offsets greater than few cm, with a maximum of 54 cm in the
footwall and more than 1 m in the hangingwall.
The total damage-zone thickness varies from 1.5 to 7 m (core is
bounded by black dashed lines on Fig. 2a). The damage zone limit is
defined as the area where the deformation band density drops to a
background level (4–6 DBs per 0.5 m). This width for a damage zone for
the likely throw on the main fault is small, but is within the data po-
pulation presented by Shipton et al. (2006) and Childs et al. (2007). The
hangingwall damage zone is generally wider (3.5–7 m) than the foot-
wall damage zone (1.5–2.5 m). Post-faulting fractures overprint the
structures of interest with a variety of orientations (Fig. 2d) and vein-fill
is absent. Sections 1–6were used to characterize the fault core of the
main fault in detail.
4.1.1. Section 1–Along strike
Starting from the NE end of the main fault, the first exposure is the
along-strike, 25 m-long section 1(Figs. 2 and 3a). The host rock on both
Fig. 3. Along-strike exposure of the main fault at section 1(first half of the 25 m-long exposure). a) Location of section 1along the main fault trace. b) Main fault with
sand and silt-dominated fault core: picture (top) and map (bottom). Location of Fig. 9e is indicated and legend on the right. c) Continuation of the same outcrop
where fault core is clay-rich and location of Fig. 9c is indicated.
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footwall and hangingwall is grey sand with interlayered clay, silt and
organic matter. For metres 0 to 1.5, the fault core composition is sand
and silt (Fig. 3b), with slip surfaces bounding a cluster of anastomosing
shear zones, stained by dark red iron oxides. The rest of the section has
a foliated clay-rich fault core (Fig. 3c). Unfortunately a tree inside the
fault core between metres 1.5 and 3 obscures the transition from sandy
core to clay-rich core.
4.1.2. Section 2–Sub-vertical
Section 2has a hangingwall composed of grey silty sand. The
footwall is grey and orange sand separated by a hard iron-cemented
horizon that does not follow the stratigraphy. These footwall beds are
attenuated and gently curved towards the fault core through a set of
normal shear zones separating rock volumes where the bedding is
preserved, and gradually tilted in a stair-stepping geometry. The fault
core is composed of foliated dark clay matrix and elongated white sand
lenses (Figs. 2 and 4).
4.1.3. Section 3–along strike
For section 3between 29 and 32 m (Figs. 2 and 5), the clay-rich
fault-core thickness increases, and the layered structure becomes less
organised. The clay foliation and shear zones are wavy, and sand lenses
are elongate parallel to the fault walls or to the internal shear zones.
The foliation is absent from the fault core between 33 and 35 m where a
chaotic assemblage of irregular sand lenses is present (Fig. 5b).
4.1.4. Section 4–sub-vertical
Section 4is partially oblique (metres 57–59) and partially sub-
vertical (metre 59–60; Figs. 2 and 6). On the footwall side, grey and
orange sands occur with the hard iron-cemented horizon in between,
which can be traced to section 2, whereas hangingwall side comprises
grey sand overlain by a clay bed. The clay-rich fault core thins and
disappears between 59 and 60 m, being replaced by a breccia (fault
breccia, as defined by Woodcock and Mort, 2008) composed of sand,
silt, clay and organic matter rounded clasts in a sandy matrix.
4.1.5. Section 5–Along strike
Section 5has a footwall composed of 20–30 cm clay beds inter-
layered with grey sands, while the hangingwall is grey sand. Layered
clay smears are present on the footwall side (Figs. 2 and 7). Foliated
clay and attenuated sand lenses form the fault core.
4.1.6. Section 6–Sub-vertical
Section 6has a footwall with a 2 m-thick clay bed including
Fig. 4. Sub-vertical outcrop of the main fault, section 2. a) Picture showing footwall beds folded and attenuated towards the fault core through a set of shear zones
and location of Fig. 9a is indicated. Inset shows location of section 2on aerial photo. b) Map of the outcrop, see legend in Fig. 3 for reference. c) Plane-polarised-light
photomicrograph of a deformation band cutting sand, impregnated with blue epoxy. Note the lack of cataclasis and the presence of Fe-oxide within the band (sample
G6 on Fig. 2). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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(caption on next page)
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
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interlaminated sand and silt, overlain by an orange sand bed (Figs. 2
and 8). These beds form clay and sand smear, respectively (Fig. 8). Grey
sands form the hangingwall stratigraphy. The fault core is clay-rich and
foliated.
4.2. Secondary faults FI and FII
The secondary faults FI and FII are being characterized because they
provide information about core development in the host rock for small
displacement faults. Two synthetic faults, FI and FII, offset the hang-
ingwall of the main fault (Fig. 2a). FI outcrops are mostly along-strike,
Fig. 5. Along-strike outcrop of main fault at section 3. a) Metres 29–32: the fault core becomes thicker and the foliation becomes wavy in comparison with sections 1
and 2. b) Excavated ditch, metres 32.5 to 35.5. The fault core becomes chaotic, and the wavy foliation is present only in small areas and location of Fig. 9bis
indicated. See legend in Fig. 3 for reference. c) Plane-polarised-light photomicrograph of a shear zone cutting sand interlayered with clay, impregnated with blue
epoxy (sample G5 on Fig. 2). d) Location of section 3on aerial photo. (For interpretation of the references to colour in this figure legend, the reader is referred to the
Web version of this article.)
Fig. 6. Section 4: Sub-vertical outcrop in a small gully. Picture (a) and map (b) of the outcrop and location of Fig. 9d is indicated. Inset shows section 4location on
aerial picture. See legend in Fig. 3 for reference.
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
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and the offset increases from the SW towards the NE to a maximum of
about 7.6 m, inferred from the offset of clay marker beds. FI dies out in
a covered section just before the big cliffoutcrop (section 6of the main
fault). FII throw increases in the opposite direction to FI. The maximum
FII offset of 5.7 m is along the big cliffto the SW decreasing to 0.3 m in
the NE. The FII fault plane dip varies between 60° and a maximum of
85° on the big cliffto the SW of the study area. Both faults have several
minor strands.
The fault core of the secondary faults FI and FII have sand-rich core
mainly toward their NE ends, while clay-rich core is prevalent to the
SW. The sandy or sand and silt core consists of a cluster of anasto-
mosing DBs if the offset of few decimetres, whereas the core is weakly
foliated for offset greater than a metre. Foliation definition increases
with clay content in these two subsidiary fault cores.
4.3. Microstructural analysis
From microstructural analysis, the dominant deformation me-
chanism is particulate flow with minor cataclasis. Particulate flow is
defined as sliding and rolling of grains past each other (Borradaile,
1981). Deformation bands show very weak rearrangement of sand
grains, and deposition of Fe-oxide along them in places (Fig. 4c).
Conversely, the shear zones are characterized by sand and clay grain
rotation and sliding (Fig. 5c). The clay-rich fault core is characterized
by alignment of clay particles, which show maximum interference
colours at the same angle (Fig. 8c).
Fig. 7. Along-strike exposure of section 5: the footwall damage-zone topography is irregular, so only the area close to the fault core is mapped. a) Metres 86–90. b)
Metres 90–93 with clay smears present on the footwall. See legend in Fig. 3 for reference. c) Location of section 3on aerial photo.
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
196

Fig. 8. Sub-vertical cut at the SW end of the main fault (big cliff, section 6). Picture (a) and map (b). Sand and clay smear originate from footwall beds. Inset shows
location of section 6on aerial photo. See legend in Fig. 3 for reference. c) Plane-polarised and cross-polarised light photomicrograph of foliated clay-rich fault core,
impregnated with blue epoxy (sample G8 on Fig. 2). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of
this article.)
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
197

5. Main fault core architecture
At all sites, the main fault core is bounded by slip surfaces on both
sides (Figs. 3–8). The hangingwall slip surface is generally straight and
regular, while the footwall slip surface is curvy and in general less
regular. Sand lenses within the fault core can be either white sand, or
blue-grey silt and sand. The white sand is lighter in colour than the host
rock, likely indicating bleaching. Five distinct types of fault core have
been identified at the site:
a) Foliated, clay-rich fault core (2–35 cm thick) is the dominant fault
core type found in all sections. The foliation is marked by compo-
sitional banding, alignment of clay minerals in a dark clay-rich
matrix (pattern becomes visible upon drying) and by elongated sand
lenses, and does not involve any macroscopic mineralogical change.
Some sand lenses embedded in the clay matrix are boudinaged.
Layers of the fault core can sometimes be identified by a slight
difference in colour of the clay matrix, or by differences in the
number, size and orientation of sand lenses (e.g. Fig. 9a).
b) Thin, foliated clay-rich fault core (1 mm-2 cm thick) occurs
mostly at section 1, and at one location in section 5. The foliated
clay-rich fault core thins out to 1–2 cm or two branches of a few mm
Fig. 9. Fault core types identified along the main fault, picture on the left and sketch on the right. a) Foliated clay-rich fault core and elongate sand lenses (section 2).
b) Thin, foliated clay-rich fault core with sand lens in the middle (section 1). c) Chaotic clay-rich fault core with irregular sand lenses (section 3). d) Sandy breccia
fault core with clasts of different composition and orientation (section 4). e) Anastomosing sandy and silty shear zone fault core, with weakly deformed sand lenses
(section 1). Outcrop orientation: □indicates plan view; ﬩indicates sub-vertical view.
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
198

that enclose a sand lens (Fig. 9b). This type was identified because of
its relevance to discussions about cross-fault fluid flow.
c) Chaotic, clay-rich fault core (30–60 cm thick) is found only at
section 3. The chaotic core presents a less organised texture in
comparison to the foliated core. The foliation is partly present on the
hangingwall edge, while the rest of the fault core is dominated by
chaotic pods of sand with wavy or irregular geometry floating in a
clay matrix (Fig. 9c).
d) Sandy breccia (26–36 cm thick) is found only at section 4.Itis
composed of sand and clay clasts supported by a sandy matrix. The
clasts are sub-angular to sub-rounded. Sand clasts are mainly grey
sand and silt, but in places the sand is white. Sand clasts can be up to
10–20 cm in size. Clay clasts are generally smaller (few cm) and
mostly sub-rounded. Some clay clasts are very rich in organic matter
(Fig. 9d).
e) Anastomosing sandy shear zones (2–4 cm) occurs only at the top
end of section 1. Here, the fault core is formed by a set of anasto-
mosing shear zones. The shear zones are composed of sand and silt
from the host rock, and dark red iron oxides. Between the shear
zones there are elongated lenses of less deformed, and less eroded
sand, generally a few cm long (Fig. 9e). It was not possible to collect
an intact sample of this material to determine by microscopic ex-
amination the deformation mechanisms. Cataclastic deformation
bands often have relatively high relief on outcrop surfaces, and re-
duced porosity in comparison with the host rock (Antonellini et al.,
1994). In contrast, the sandy shear zones that we observe in the fault
core are eroded more than the surrounding rocks. We interpret the
relatively higher erosion and the presence of dark red iron oxides
inside the sandy shear zones, but not in the surrounding sands or in
the sand lenses, as an indication of enhanced shear zone perme-
ability during or after fault displacement.
5.1. The geometry of internal shear zones
Shear zones inside the fault core develop on the edge of sand lenses
and in the clay-rich matrix. The clay foliation and sand boudins define a
Riedel-shear geometry, both in plan view and in vertical view. Y-shears
are defined by the fault-core slip surfaces and parallel foliation. P-
shears are evidenced by offset of the preserved bedding inside of the
sand boudins, which were rotated, or by the elongation of the sand
lenses themselves. R-shears displace the sand boudins and quickly die
out in the clay matrix (Fig. 10a). In vertical cuts, the Riedel shears show
a normal sense of displacement for the main faults, but along-strike
they display a dextral component of displacement (Fig. 10b).
Minor, straight shear zones sometimes separate clays in the middle
of the fault core from clays filling an irregularity in the fault plane. Clay
filling the concavity usually shows a curved foliation, parallel to the
concavity edges, while the rest of the clay-rich core shows planar fo-
liation. This mechanism is observed both on the strike exposures of
section 1 and 7 of the main fault, and along the synthetic faults FI and
FII (see section 10). The length of the cut-offarea is between 30 and
50 cm, and the thickness is generally a few cm (mean of 3.5 cm).
6. Fault core thickness along-strike
The clay-rich fault-core thickness of the main fault varies from 0 to
60 cm (Fig. 11a and d). The mean clay-rich fault core thickness is
16 cm. The foliated clay fault core occurs along 80.5% of the total
mapped length of the main fault of which 2.7% (about 1.5 m) is the
length of very thin (≤2 cm) clay-rich fault core (Fig. 11a, blue dia-
monds). The clay-rich, chaotic core is 16% of the total length of the
main fault, while the total length of sand-on-sand juxtaposition is 3.5%
of the main fault exposure (2 m).
FI fault core thickness varies between 1 mm and 25 cm, whereas FII
core thickness is between 0.1 and 40 cm (Figs. 10b, 11c and 11e). These
thickness measurements include sand lenses. Clay-rich fault core
thickness histogram statistics for main fault and faults FI and FII are
presented in Appendix 1.
Fig. 12a shows the main fault-core thickness variations along strike
(as in Fig. 11a) and the relative fault-core type at each location where a
thickness measurement was recorded. Anastomosing sandy shear zones
(Fig. 12b) and attenuated, foliated clay core (Fig. 12c) constitute the
thinner core areas. Thicker fault cores are composed of a chaotic clay-
rich fault core (Fig. 12d), while the sandy breccia (Fig. 12e) has a
comparable chaotic texture to the chaotic clay-rich core but is thinner.
The rest of the outcrop for the main fault is dominated by the foliated
clay-rich fault core (Fig. 12f).
The main fault-core thickness variations along-strike and down dip
are not, at the scale of observation, related to the fault offset. In fact, the
offset increase of about 5 m from the NE to the SW (from section 1to
section 7) is not coincident with any notable core thickness increase
(Fig. 12a). Shipton et al. (2006) and Caine and Minor (2009) also failed
to find a consistent correlation between the thickness of the fault core
(or clay smear) and the displacement along-strike after a fault core
becomes well established during deformation.
The fault core thickness seems to be only partly related to the
nearby stratigraphy, in particular the footwall stratigraphy. At the lo-
cations where clay beds (20–200 cm thick) are incorporated from the
footwall into the fault, core thickness generally increases. This increase
is greater for a set of clay beds as opposed to a single bed. The core
thickness then diminishes down dip as the distance from these source
beds increases (section 5 and 6). However, the chaotic fault-core tex-
ture, which is present in the thickest fault core section, does not cor-
relate with any marked change in the footwall stratigraphy.
Fig. 10. Riedel shears evidenced by white sand lenses in the dark clay-rich matrix of the fault core. a) Fault core bounded by slip surfaces representing Y-shears,
partly preserved bedding inside of the sand lenses along P-shears and synthetic R-shears displacing them (section 2, sub-vertical exposure). b) 3D diagram of the
Riedel shears and related normal sense of movement along the main fault with minor dextral component.
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
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(caption on next page)
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
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Thickness variations in the clay-rich fault core cannot simply be
ascribed to stratigraphic variations. At this site, the clay and sand bed
thicknesses are small in comparison to the likely offset on the main
fault. Individual clay beds show some thickness variability, but they are
continuous at the outcrop scale. Hence, the footwall and hangingwall
stratigraphy that moved past each point on the fault, along the strike
section, must be very similar and could not have been the cause of the
large variability in fault core thickness.
Pore throats in clays are generally much smaller than in sand,
therefore a much higher across-fault fluid pressure is needed to reach
Fig. 11. Fault core thickness and composition variation along-strike. a) Main fault, with sections 1–7indicated both in the graph and in the topographic profile on
top. b) Secondary fault FI thickness variation along length and histogram of the clay-rich fault core thickness (all yellow diamonds correspond to zero clay-rich fault
core thickness). c) Secondary fault FII thickness variation along length; d) and e) show main fault and FII clay-rich fault-core thickness histograms. See supplementary
information for the histograms statistics in Appendix 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of
this article.)
Fig. 12. Fault core types related to the fault core thickness along-strike. a) Main fault core thickness variation along strike, as in Fig. 10; a cartoon of each fault core
type is related to the section where it is outcropping and to the fault core thickness at the specific location. b) Anastomosing sandy shear zones. c) Thin, foliated clay-
rich fault core. d) Chaotic clay-rich. e) Sandy breccia. f) Foliated clay-rich. g) Experimental and theoretical variogram derived from the along-strike fault core
thickness data in (a). The fitted variogram is for a spherical covariance distribution with a range of 2.9 and a variance of 75. h) and i) show the along-strike fault core
thickness variograms for secondary faults FI and FII respectively. Both variograms are fitted with a Gaussian covariance distribution with a range of 1.3 and a
variance of 2.6 (h), and a range of 4.5 and a variance of 275 (i); L is the distance at which 95% of the sill is reached; x- and y-axis are the same as in (g). Equations
from: Kitanidis, 1997.
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
201

the capillary entry pressure (Watts, 1987). Consequently, for predicting
capillary barriers, it is important to understand the distribution of high
permeability materials in the clay fault core. At this site, two sand-rich
fault core types, anastomosing sandy shear zones and sandy breccia,
occur and both are likely to have considerably lower pore entry pres-
sures than the clay-rich fault cores, because of their bigger pore throat
size. This behaviour means that such areas would be preferential
pathways for across-fault fluid leakage, with the leakage rate de-
termined both by the lateral and vertical extent of the patches of sandy
fault core and by the frequency with which they occur within the fault
plane. These observations underline the need for a mechanistic un-
derstanding of fault-core evolution so that the formation, frequency and
size of such high-permeability areas in the fault core can be predicted.
When simulating reservoir flow in production scenarios, complex
fault architectures must be up-scaled to provide a small number of
simplified parameters for application at the scale of a reservoir simu-
lator grid-cell. Using mean clay-rich fault-core thickness (16 cm) would
poorly represent the hydraulic properties of this fault. This outcome is
because geological material characteristics, such as fault-core thickness,
are not truly random, they are spatially correlated, and it is predicting
the patches of thin fault core that matters the most for hydraulic per-
formance. In a spatially correlated field, a known value of fault-core
thickness at one location on the fault makes it more likely for a
neighbouring second location to be similar. This spatial correlation
occurs up to a specific distance apart, called the correlation length,
beyond which the value at the second location is entirely independent
of the known value at the first. Visually, the correlation length is related
to the size of ‘thick’and ‘thin’patches on the fault. The method for
describing spatially correlated data is called a geostatistical variogram
analysis (Matheron, 1971) and it has been used extensively to describe
spatially varying geological properties such ore body percentage, por-
osity and permeability.
We performed a variogram analysis of fault-core thickness on the
along-strike sections of the outcrop, using 0.5 m spaced thickness
measurements. The variogram describes the square of the differences in
fault thickness between pairs of data points (plotted on the y-axis) at a
given distance apart (plotted on the x-axis). So, for locations on the
fault that are very close together (small values on the x-axis) fault
thickness is very similar (i.e. the difference in thickness between pairs
of adjacent points is close to zero). Then, as the distance between points
increases, the variogram value increases until it reaches a steady value,
at which the pair of thickness values are unrelated, and hence equal to
the background variance.
On the main fault, the experimental variogram is best fitted by a
spherical covariance distribution which reaches a steady background
variance at a distance of approximately 3 m (Fig. 12g.). This implies
that ‘patches’of thin and thick fault core are generally less than 3 m in
length, termed the correlation length. Similar results as in Fig. 12g are
obtained for the secondary faults FI and FII (Fig. 12h and i), however
for them, a gaussian covariance distribution is a better fit to the ex-
perimental data. The spacing between measured data points (0.5 m)
may be too large to capture the fault-core thickness variations of fault FI
(Fig. 12h), which would explain the lack of data between 0 and 1 m on
the experimental variogram. The occurrence of along-strike spatially
correlated variations in clay-rich fault-core horizontal thickness at
the < 3 m scale in the secondary faults FI and FII as well as in the main
fault means that the processes responsible for creating a system where
the significant changes in core thickness occur about every 3 m must
already be active at an early stage of fault evolution (offset 1–10 m).
Consequently, these fitted variogram functions can be used to generate
faults with realistic fault-core thickness variations along strike.
The observed spatial correlation (Fig. 12g) in along-strike core
thickness at the 0.5–3 m scale, alongside seemingly uncorrelated, but
highly variable fault thickness and composition at a > 3 m scale, im-
plies the superposition of at least two very different processes that
contribute to fault zone variability that will be discussed in sections 7.2
and 7.3.
A correlation length of 3 m may be related to fault geometry and
layer thickness, and other sites may contain thickness variations at a
different scale. Lunn et al. (2008) calculated a correlation length for
fault core thickness of 1.45 m for the Big Hole fault in aeolian sand-
stones (Utah). Responsible factors could also include the degree of host
rock consolidation, host rock composition, stratigraphic variations,
stress state and deformation history.
7. Discussion
7.1. Fault growth and clay smearing
The studied outcrops offer a rare opportunity for fault evolution
analysis to be investigated in soft sediments in 3D. Fault offsets at the
Jalan Mukah outcrop range from centimetre, to metre scale (secondary
faults FI and FII) to the tens of metre scale (main fault). Using greater
offset as a proxy for longer duration of fault evolution, we propose this
structural evolution:
•Incipient deformation occurred through the formation of deforma-
tion bands and shear zones. Shear zones formed in mixed sand-clay
sediments at offsets of a few cm, up to 1 m. Often, a shear zone was
composed of several strands of minor shears or deformation bands,
but no clear fault core-damage zone distinction was present. The
host rock maintained cohesion across the deformed volume of rock,
and no slip surfaces or fractures formed. The deformation bands
display a large range of orientations with variations in strike and dip
of up to 50° and shear zones vary in orientation for strike by up to
15° and dip by up to 45°, as the deformation is not well localised.
•At offsets between 1 and 10 m, the fault core-damage zone started to
form, however it was not yet well established. The fault became
simply composed of anastomosing shear zones. Fault core thickness
of a single strand was generally between 1 and 10 cm with a max-
imum observed thickness of 40 cm. Faults were composed of one or
more strands, and several segments linked together. At this stage of
evolution, one or two slip surfaces localised at the edge of the fault
core. A single slip surface can be located on either side of the clay or
sand-rich fault core or cut across it. As the displacement increased
from less than 1 m to few m, the slip surface orientations become
more similar, indicating a greater degree of deformation localisa-
tion. The fault-core internal structure is generally weakly foliated to
foliated, commonly with fault-bounded sand lenses. Clay smears are
observed deflecting into the fault both from hangingwall and foot-
wall stratigraphy. These observations are consistent with Van der
Zee and Urai (2005), who studied dip-slip exposures of normal faults
with throws up to 10 m at the Airport Rd outcrop in the same se-
dimentary sequence of the Jalan Mukah site. They reported a fo-
liated structure of sheared clay with sand in the fault core, locally
originating from thinly layered sand-clay beds. They also observed
that most faults formed by several non-co-planar segments linking
together so that increasing offset caused a strongly variable fault
zone thickness.
•For offset between 10 and 50 m, the fault core-damage zone archi-
tecture became well established, but the thickness of both fault core
and damage zone was quite variable. The fault core is bounded by
slip surfaces on both sides. The hangingwall slip surface is straighter
and more regular than the footwall slip surface, which is generally
corrugated at the decimetre-to metre-scale. As the deformation lo-
calised, the orientation of the main fault became less variable
(variation in strike of up to 19° and dip of up to 17°, hangingwall slip
surface) than the smaller-offset faults. A foliated-clay structure de-
veloped in most of the clay-rich fault core.
Several authors (Heynekamp et al., 1999;Sigda et al., 1999;
Rawling et al., 2001,2003;2006;Caine and Minor, 2009;Balsamo
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202

et al., 2010;Loveless et al., 2011) have noted the presence of mixed
zones along faults in poorly consolidated sediments. These studies are
in poorly consolidated siliciclastic units with grain size ranging from
gravel to clay that underwent deformation through particulate flow at
less than 1 km depth. These authors define mixed zones as being
composed of tectonically mixed material from either side of the fault,
and having a contrasting permeability distribution that varies widely
between the host rock sand and the fault core values (Sigda et al.,
1999). The sandy breccia observed at the study site could represent a
mixed zone from the textural point of view. However, it is located be-
tween the hangingwall and footwall damage zones, and the fault zone
lacks other structural features. Therefore, it has to represent the part of
the fault zone where the maximum displacement is accommodated.
Heynekamp et al. (1999);Sigda et al. (1999);Rawling et al. (2001) and
Rawling and Goodwin (2003) defined the mixed zone as being located
between the fault core and the damage zone rather than being the fault
core. Loveless et al. (2011) considers the mixed zone as representing the
fault core based on hydrological behaviour. Due to this ambiguity in the
meaning of the term mixed zone, we prefer to use the term breccia as a
geometrical description of the sandy pods texture.
7.2. Variations in fault wall geometry
Smooth variation in fault thickness could be explained by fault-wall
topography. It is well-known that fault-wall topography affects the
thickness of the fault core (Lindsay et al., 1993). A fault-wall asperity is
likely to be abraded away as displacement increases and the fault core
develops. The abrasion of a sand body from one of the fault walls may
also result in the increase of sand in the fault core system in the form of
sand lenses. However, if a relatively hard volume of sand extrudes from
either the footwall, the hangingwall or both, towards the fault core, it
could cause an interruption of the clay smear instead of being abraded
away. In this case, the clay smear would be squeezed outwards from the
extruding fault walls, and the clay-rich fault core would be replaced by
anastomosing sandy shear zones.
By contrast, if a fault wall contains a concavity, it can be filled by
clay from the fault core. At several locations, we observe planar shear
zones that separate clay within an irregularity in the fault plane, where
the clay was derived from the fault core. Clay filling a concavity usually
displays a curved foliation parallel to the surface of the concavity wall
(Fig. 13a). This geometry is observed both at the study site for dis-
placements at the metre- (FI and FII) to tens of metre-scale faults (main
fault), and for other clay-rich fault cores (Cowan et al., 2003;Kremer
et al., 2018). The curved slip surface of this structure has a similar
geometry to the sidewall ripout mechanism of Swanson (1989).He
observed asymmetric sidewall ripouts in pseudotachylite-bearing
strike-slip faults in metamorphic rocks, and attributed them to adhesion
wear between fault walls. The main differences are the host rock types
and the fact that we do not observe a systematic asymmetry in the
geometry of the curved slip surface.
Considering a schematic model for developing sidewall rip-outs, the
clay-filled concavity is likely to develop where the host rock is com-
posed of softer material, as compared to the rest of the fault wall
(Fig. 13b). We suggest that during fault movement, clay in the fault core
began to abrade and incorporate this softer material. It is likely that the
clay from the fault core was pushed and smeared laterally in the con-
cavity and downward by the fault movement. Clay platelets and sand
lenses deforming through particulate flow probably adhered to the
concavity walls forming a curved compositional banding and clay fo-
liation within the concavity (Fig. 13c). Increasing deformation possibly
determined the formation of a straight shear zone or slip surface cutting
offthe lobe-shaped concavity from the fault core, maybe favoured by a
relatively faster movement along the fault plane (Fig. 13d). This
abandonment of the clay inside the concavity reduces the overall vo-
lume of clay in the active part of the fault core system.
7.3. Disruptive fault evolution processes
The thickest fault core is associated with the chaotic clay-rich core.
Therefore, three key observations must be explained: 1) the transition
from well organised, foliated fault core to chaotic fault core happens in
a relatively short distance (2–3 metres) along strike; 2) this transition is
not matched by any observable lithological changes in the wall rock or
degree of lithification; and 3) the transition is not associated with a
geometric variation in the fault plane. It is possible that the chaotic
breccia formed by the same processes that formed reported examples of
“mixed zones”. Authors that have reported mixed zones describe that
these textures are usually formed by disaggregation and penetrative
particulate flow under low confining stresses. Rawling and Goodwin
(2006) speculated that faults form with a stair-stepping geometry in
lithologically layered sequences and that smoothing of these surfaces
results in the generation of the mixed zone. They argued that the fault
core subsequently localised within the centre of this zone because it is
weaker than the surrounding rocks. However, this explanation and
geometry do not match our observations in Miri. If this mechanism was
responsible for the formation of a mixed zone, we would expect it to
occur at all positions along fault strike, because the controlling me-
chanical variations in the stratigraphy are uniform along the fault trace.
However, we have a discrete occurrence of the chaotic core, and so
must invoke mechanisms that are localised. Three possible scenarios are
proposed:
Fig. 13. Field example and cartoon of the sidewall ripout mechanism. a) Picture (top) and map (bottom) of an along-strike section of section 1; Cartoon: a weaker
area on the hanging wall (b) is eroded away and incorporated as a sand lens (c) as the space is filled by foliated clay. d) The infill-area is subsequently cut-offfrom the
main fault core by a straighter slip surface or shear zone.
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
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7.3.1. Syn-faulting upward migration of buoyant methane from deep
sediments towards the seabed
In this first scenario, the pressure drop due to the decrease in depth
as the fluid rises would result in a phase change from liquid to gas, with
a related increase in volume (Fig. 14b). The gas expansion within the
fault walls could induce erosion of fault wall material and incorporation
into the core. The eroded material could be either incorporated into the
fault as randomly oriented lumps of sand, accounting for the chaotic
structure of the clay-rich fault core, or as an increased amount of dis-
aggregated sand filling the core from footwall to hangingwall. In the
Fig. 14. Schematic fault evolution at Jalan Mukah outcrop. a) At 1-10 m offset the fault core begins to form, there is clay smearing from both hanging wall and
footwall, and the principal slip surface can be replaced by a secondary slip (inset); b, c and d are three possible evolution of stage a) to an offset between 20 and 50 m,
that can explain the different fault core (FC) types observed in the main fault (1- Foliated clay-rich FC; 2- Chaotic clay-rich FC; 3- Sandy breccia FC). b) Upward flow
of natural gas at the liquid state during faulting, and passage from liquid to gas phase. c) A coalescence of gas bubbles at depth is attracted into an almost
instantaneous dilational jog during faulting. d) Overpressured clays from deeper beds are injected along the fault plane during deformation.
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
204

sandy breccia fault-core type, sub-rounded clasts of host rock (sand,
sand and silt, clay and organic matter-rich beds) with partly preserved
bedding are enclosed in the disaggregated sand material. Vertical mi-
gration of oil and gas along seismically resolvable faults has been de-
scribed by Haney et al. (2005) and Cartwright et al. (2007). In the West
Baram Delta system, hydrocarbon flow along faults has been reported
by Sandal (1996),Laird and Morley (2011) and Algar (2012). The ex-
solution process of gas bubbles from saturated CH
4
brines is illustrated
by Busch and Kampman (in press). They describe the formation of a
free-gas phase in brines, which migrates up fractures when the dis-
solved gas saturation exceeds the saturation limits. This process is
driven by a reduction in CH
4
solubility with decreasing hydrostatic
pressure. Further, the velocity of the gas bubbles increases considerably
with decreasing depth (Busch and Kampman, in press). This process is
driven by a phase change occurring local to the fault rocks.
Vitrinite reflectance data from Schmitz et al. (2003) show a max-
imum burial depth of less than 1000 m for the Airport Rd sediments
(the same unit of the study site). The temperature gradient reported for
the Baram Delta is between 25 and 36 °C (Sandal, 1996;Tan et al.,
1999;Madon, 1999). Morley et al. (2008) report a vertical stress gra-
dient of 19–22 MPa/km at 1500 m below sea level for offshore Baram
Delta Province. According to Ahmed (1989), pure methane at 25–30 °C
intersects the vapor-pressure curve (bubble point) at pressures of
29–32 MPa. These pressures would be too high for the phase change to
happen at 1000 m of depth. However, natural gas is a mixture of hy-
drocarbon components, and the overall physical and chemical proper-
ties depend on the individual components of the mixture and vary
widely with the composition of the gas (Ahmed, 1989). Therefore, if the
faults we studied were under the same temperature and stress gradient
described by Sandal (1996) and Morley et al. (2008) they could have
been at or close to the bubble point at the time of faulting.
7.3.2. Rapid opening of a dilational jog
In this second scenario, a dilational jog forms during a rapid slip
event, and the opening collapses almost instantaneously due to the
poorly consolidated sediments, creating a chaotic fault core fabric
(Fig. 14c). A similar process was invoked by Woodcock et al. (2006,
2014) for the slow collapse of dilational cavities in hard rocks. In poorly
consolidated rocks, such collapse would be immediate as they are so
much weaker than the rocks examined by Woodcock. The transient
pressure drop at such dilational cavities could further enhance disrup-
tion, sucking in local fluids (Sibson, 2000), perhaps accompanied by a
phase change to gas. As these fluids transit through the core, they
would disrupt the foliated clay-rich fault core structure, which is the
dominant texture along the rest of the fault. What is left behind would
be a more randomly-oriented clay matrix and irregular sand lenses. In a
normal fault, a dilational jog would ideally form a sub-horizontal
elongated opening. However, the dextral component of shear, demon-
strated by the internal structure of Riedel shears in the along-strike
exposures, could imply a slightly oblique conduit, able to channel the
gas upwards.
7.3.3. The presence of a locally overpressured clay bed deeper in the faulted
sequence
The final scenario is that a locally overpressured clay bed becomes
mobilised as it is entrained into the fault (Fig. 14d). The clays would
probably undergo rapid chaotic rearrangement during pressure release
as they are entrained into the fault core, possibly eroding sand from
fault walls (stoping) in the process. Such clay injection could explain
both the structure of the chaotic fault core and the along-strike increase
in fault core thickness. Parts of the foliated clay-rich core texture are
preserved towards the edges of the fault core as wavy foliated-areas.
This interpretation is supported by the fact that shale diapirs and pipes
are common features in the West Baram Delta province (Sandal, 1996;
and Morley et al., 2003), and Sandal (1996) reports injection of clay
along faults as well. The overpressures responsible for such features are
ascribed to undercompaction of clay units in the Baram Delta region,
possibly combined with folding and inversion during the generation of
hydrocarbons, particularly the cracking of oil to gas (Morley et al.,
2008).
A degree of interplay exists between these three proposed scenarios.
In fact, a dilational jog could attract not only a coalesced gas accu-
mulation, but also gas in liquid phase or mobilised plastic clays. All
three mechanisms could result in the erosion of fault walls. Bleaching of
sand lenses inside the clay-rich fault core, as observed at the field site, is
more likely in the first two scenarios, because hydrocarbons are a re-
cognised agent for iron bleaching (Chan et al., 2000;Beitler et al., 2003;
Parry et al., 2009).
All three of the scenarios discussed above invoke isolated sporadic
occurrences of an abrupt disruptive process. As observed within the
chaotic breccias, such processes are not likely to be spatially correlated.
Hence, the fault core thickness is well-modelled by a continuous spa-
tially correlated random field that has been overprinted and disrupted
in short sections at sporadic locations. Future research will be required
to estimate the spatial frequency and distribution of these sporadic
events.
8. Conclusions
The Jalan Mukah outcrop offers excellent 3D exposures of a normal
fault with 20–50 m offset. The fault core is exposed along-strike for
55 m. Five fault core types were identified. The fault core is generally
comprised of continuous, low-permeability foliated clay which, where
present, varies from 60 cm thick to a veneer of 1–2 cm. At some loca-
tions, the clay-rich fault core loses its foliated structure and becomes
chaotic. Two sections of the fault core are sand-dominated: composed
entirely of anastomosing sandy shear zones or of a sandy breccia. These
sections represent high-permeability zones in the otherwise low-per-
meability clay-rich fault core.
Two (or more) processes appear to govern the along-strike fault core
variability: one that controls the thickness in clay-rich fault core; and
one (or more) that results in abrupt changes in both fault core thickness
and composition. Controls on the thickness in clay-rich fault core could
include the geometry, layer thickness, degree of host rock consolida-
tion, host rock composition, stratigraphic variations, stress state and
deformation history. To characterize these variations we used spatial
statistics. The variogram of along-strike clay-rich fault core thickness
has a spatial covariance with a correlation length of 3 m, that implies
‘patches’of thin and thick fault core are generally less than 3 m in
length.
Abrupt variations in fault-core composition and geometry could be
explained by the occurrence of disruptive processes. We speculate that
such processes could include rapid degassing, slip events opening voids,
or sudden release of overpressure. Further investigation of faults in si-
milar geological settings is needed to obtain a better understanding of
the deformation processes acting at different scales that determine fault
core thickness and composition variations, and therefore the relative
changes in capillary pressure and sealing ability of the fault rocks.
Acknowledgements
This paper contains work conducted during a PhD study undertaken
as part of the Natural Environment Research Council (NERC) Centre for
Doctoral Training (CDT) in Oil & Gas [grant number NE/M00578X/1]
and the University of Strathclyde Faculty of Engineering PhD
Scholarships (SSDR), and NERC grant NE/N015908/1 (YK), whose
support is gratefully acknowledged. We thank Fabrizio Balsamo,
William Dunne and an anonymous reviewer for their positive and
constructive comments.
S. Sosio De Rosa et al. Journal of Structural Geology 116 (2018) 189–206
205

Appendix A. Supplementary data
Supplementary data related to this article can be found at https://
doi.org/10.1016/j.jsg.2018.08.012.
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