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Fault intersections as critical hydrocarbon leakage zones: integrated
field study and numerical modelling of an example
from the Timor Sea, Australia
Anthony Gartrella,*, Yanhua Zhangb, Mark Liska, David Dewhursta
aCSIRO Division of Petroleum Resources, Australian Petroleum Co-operative Research Centre, ARRC,
26 Dick Perry Avenue, Technology Park, Kensington, WA 6151, AustraliabCSIRO Division of Exploration and Mining, ARRC, 26 Dick Perry Avenue, Technology Park, Kensington, WA 6151, Australia
Received 8 April 2003; received in revised form 2 August 2004; accepted 5 August 2004
Abstract
Fault intersections are identified as important sites for hydrocarbon leakage from the Skua oil field in the Timor Sea, Australia. Integrated
structural and fluid history data sets suggest that these fault intersections may be efficient and long-lived fluid conduits. Three-dimensional
(3D) numerical modelling, based on fault patterns observed in the Skua Field, generated zones of high dilation in the vicinity of fault
intersections during contraction, even at low bulk strain values. In nature, these dilational zones are likely to be sites of high structural
permeability containing concentrated open fracture networks ideal for high fluid flux. The potential for fluid leakage from these zones may be
further enhanced where low shear strain occurs due to mechanical locking at the fault intersection. Although not tested in the numerical
experiments, fault gouge development is likely to be less extensive in these zones of low shear strain, reducing the probability of forming
membrane seals. The modelling results support previously published charge and leakage history studies of the Skua Field and highlight the
potential for large volumes of hydrocarbons to be lost where fault intersection zones breach the top seal. Fault intersections may therefore
play a significant role in influencing trap integrity conditions in other areas.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Fault intersections; Trap integrity; Numerical modelling; Hydrocarbon leakage
1. Introduction
Fault intersections are recognised as highly efficient and
focussed fluid conduits important for forming mineral
deposits (Betts & Lister, 2002; Craw, 2000; Sibson, 1996;
Tripp & Vearncombe, 2004). The importance of fault
intersections in earthquake behaviour has also been
recognised, where they provide locations for the initiation
and cessation of ruptures, concentrate stress and generate
earthquakes, control earthquake sequences by loading or
unloading stresses on adjacent faults, promote fluid flow and
provide localised weak spots (Talwani, 1999). However,
0264-8172/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.marpetgeo.2004.08.001
* Corresponding author. Tel.: C61-8-6436-8742; fax: C61-9-6436-
8555.
E-mail address: [email protected] (A. Gartrell).
their role in petroleum systems has not been well
documented.
High trap failure rates in the Timor Sea region (Fig. 1)
have been attributed to leakage of hydrocarbons due to post-
rift fault reactivation (e.g. Lisk, Brincat, Eadington, &
O’Brien, 1998; O’Brien et al., 1999; O’Brien & Woods,
1995). The Skua oil field represents one of only three
commercial hydrocarbon discoveries within the Vulcan
Sub-basin, southeastern Timor Sea (Fig. 2), although
evidence for significant leakage from this field also exists.
Cowley and O’Brien (2000) suggested that zones of
preferential fluid leakage may have occurred at the
intersection of NNW–SSE trending basement faults with
younger fault systems at Skua, based on the location of
seismic velocity anomalies (hydrocarbon related diagenetic
zones). Gartrell, Lisk and Undershultz (2002) provided a
more detailed analysis of the fluid-flow history of the field
Marine and Petroleum Geology 21 (2004) 1165–1179
www.elsevier.com/locate/marpetgeo
Fig. 1. Tectonic elements map indicating the location of the Timor Sea off
the northwest coast of Australia.
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–11791166
by combining 3D structural restoration techniques with fluid
inclusion analysis (GOIe) and hydrodynamic evaluation.
This integrated analysis significantly refined the under-
standing of the role of key fault intersections in the charge
and leakage history of the Skua Field. Subsequently,
Gartrell, Zhang, Lisk and Dewhurst (2003) used 3D
numerical modelling in an attempt to better understand the
mechanics of a simple fault triple junction.
The aim of the current paper is to present the results of
some additional numerical modelling designed to more
closely represent the fault intersection geometry at Skua and
to draw together and expand on the recent work looking at
the trap integrity of the Skua Field. It is hoped that the work
helps to shed light on the role of fault intersections as
potentially critical fluid pathways in a petroleum system
context.
2. Structural setting and hydrocarbon habitat
of the Skua field
The primary structural traps in the Skua area consist of
NE–SW and ENE–WSW trending tilted fault blocks formed
during Late Jurassic rifting (Figs. 2, 4 and 5). Hydrocarbons
are reservoired within Early Jurassic sandstones of the
Plover Formation and are sealed by Early Cretaceous
calcareous shales (Figs. 2 and 3, Osborne, 1990; Pattillo &
Nicholls, 1990; Woods, 1994). A series of NNW–SSE and
N–S oriented faults, which are thought to represent
reactivated Late Proterozoic basement faults (Cowley &
O’Brien, 2000; O’Brien, 1993), trend at high angles to the
younger NE–SW trending rift faults resulting in a network
of fault intersections (Fig. 4). The basement structures
are best observed on maps derived from the base of
the Cretaceous seal unit (Base Seal) seismic horizon
(Fig. 4A), but are poorly imaged in seismic cross-section
(Fig. 6A). Based on subtle offsets and seismic amplitude
cut-offs, we interpret these faults to be sub-vertical
structures that have propagated through the Early Cretac-
eous sealing units (Fig. 6A).
A number of deformation events occurred during the
post-rift history of the region in association with plate-scale
reorganisations and collision on the northern margin of the
Australian Plate (Etheridge, Mcqueen, & Lambeck, 1991;
Gartrell et al., 2002; O’Brien et al., 1998; O’Brien &
Woods, 1995; Veevers, Powell, & Roots, 1991; Woods,
1992, 1994). Pulses of folding and uplift throughout the Late
Cretaceous and into the early part of the Tertiary are
observed along the length of the Skua Fault. Fault
reactivation and inversion at the bend in the Skua Fault
towards the southwestern end of the field resulted in the
development of an eastward verging asymmetric fault-
related fold (Fig. 5A). Sediment onlap and growth on the
eastern side of the fold indicate that the main phase of fold
development was initiated after the Campanian. Growth on
the fold continued in pulses throughout the Paleocene and
early Eocene. Further north, where the Skua Fault bends
back in an east-north-east orientation, Late Cretaceous to
middle Eocene post-rift deformation is characterised by
normal faults that cut into the top reservoir close to the main
rift-phase fault (Fig. 5B). The structural associations at Skua
developed during the Late Cretaceous to early Tertiary are
consistent with that of a dextral wrench system associated
with E–W bulk contraction. The southwestern end of the
Skua Fault apparently acted as a constraining bend and the
kink to the north acted as a releasing bend. A critical change
in plate spreading pattern occurred in the Cenemonian
(96 Ma) when a jump in the position of the spreading ridge
between Greater India and Australia occurred (Veevers
et al., 1991). Ridge push forces generated from subsequent
rapid spreading between Australia and Greater India may
have caused the observed inversion at Skua. Greater India
continued to move rapidly to the north in the early Tertiary,
until it collided with Asia sometime in the middle Eocene
(40–45 Ma; Veevers et al., 1991). The collision event
caused a major change in global plate kinematics (Etheridge
et al., 1991), which may have also impacted on the Timor
Sea region.
A series of en echelon normal faults, formed above the
Skua Fault, terminate just above Early Miocene level
(Fig. 4). Most of the early Miocene normal faults detach
within the Early Cretaceous shales and marls. However, a
series of early Miocene faults cut the top reservoir level in
the southwestern end of the field, separating Skua-3, Skua-8
and Skua-2 wells. These faults are poorly imaged in the
seismic data due to the small displacements, however, their
presence is supported by hydrodynamics data that shows
pressure breaks across these structures (Gartrell et al.,
2002). The early Miocene deformation correlates with the
initial phase of collision on the northern Australian passive
Fig. 2. Regional setting of the Skua Field. (A) Simplified structural elements map of the southern Timor Sea region. (B) Schematic cross-section through the
Rowan, Skua and Swift fault blocks (modified from Fittall & Cowley, 1992).
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–1179 1167
margin (Etheridge et al., 1991). The Solomon Sea plate was
obducted onto the Australian plate at Papua New Guinea
during this event (Smith, 1990).
Convergence and collision between the Australian Plate
and Banda Arc (Fig. 1) in the Late Miocene and Early
Pliocene (Mio-Pliocene) resulted in increased rates of
subsidence, as well as wide-spread fault reactivation
observed throughout the Timor Sea region (e.g. O’Brien
& Woods, 1995; Woods, 1992). However, Mio-Pliocene
normal faulting in the Skua region is localised above the
Rowan Fault, with no seismic evidence for reactivation of
the Skua Fault during this time (Fig. 5B). Strong
Mio-Pliocene extensional reactivation of the nearby
Rowan Fault may have partitioned strain in the Skua area
so that the Skua Fault appears to have been largely
unaffected by this event (Gartrell et al., 2002).
3. Trap integrity and the role of fault
intersections in the Skua region
3.1. Hydrocarbon leakage indication
Trap integrity issues were highlighted during appraisal of
the Skua Field with the extent of the accumulation
turning out to be significantly smaller than that suggest by
Fig. 3. Stratigraphic column for the Vulcan Sub-basin, including timing of
tectonic events (modified from Osborne, 1990).
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–11791168
the mapped closure and with the identification of residual oil
shows below the current oil–water-contact (OWC) in most
wells (Osborne, 1990). Subsequently, evidence for vertical
leakage from the field has been based on three key data sets:
1.
Remotely sensed hydrocarbon seepage2.
Hydrocarbon related diagenesis3.
Mapping of palaeo-oil zones using fluid inclusion dataA variety of remote sensing techniques have been used at
Skua to define leakage from the underlying Jurassic traps
into the water column. Airborne laser fluorosensor (ALF)
data has been collected over large areas of the Timor Sea
(e.g. O’Brien et al., 1998). Hydrocarbon slicks on the water
surface associated with present day leakage are detected by
response to an ultra-violet laser fired from a low flying
aircraft. The main ALF anomalies in the Skua region are
roughly aligned in a N–S orientation on the eastern side of
the field and tend to cluster above fault intersections
(Fig. 4C). Several strong methane and ethane anomalies
have also been detected within the water column using
water bottom geochemical sniffer equipment (O’Brien et al
1998.). This data set shows a less obvious relationship to the
position of fault intersections. Present day seepage detected
in the region using these techniques is thought to be unlikely
to represent volumetrically significant leakage. Rather, the
seepage anomalies are probably related to hydrocarbons
currently being generated and migrating through faults to
the surface (O’Brien et al., 1998).
Features known as hydrocarbon related diagentic zones
(HRDZs) are commonly observed throughout the Timor Sea
region and are suggested to have formed when hydro-
carbons leaked from the Mesozoic traps and migrated
upwards into Eocene aquifer sands. Carbon isotope analysis
on samples taken from these zones (e.g. at Skua-3) suggest
that biological oxidation of the hydrocarbons produced
intense, localised carbonate cementation (O’Brien et al.,
1998; O’Brien & Woods, 1995). This cementation produces
sufficient acoustic impedance contrast to cause a strong
seismic response (Fig. 6B), allowing the HRDZs to be
mapped from seismic data (Fig. 4C). Several HRDZs have
been identified in the Skua/Swift region (Cowley &
O’Brien, 2000), most of which are located above intersec-
tions between Jurassic rift faults and cross-trending base-
ment faults (Fig. 4C). The three HRDZs located over the
Skua Field itself provide a good indication of the lateral
extent of the field (Cowley & O’Brien, 2000). Furthermore,
the depth and position of the fault intersections that underlie
the HRDZs at the northeastern and southwestern ends of the
field correspond with the location and depth of the present
day OWC (Fig. 4B). This observation suggests that these
fault intersections may limit the size of the field at the
current day. These HRDZs are suggested to essentially
emanate from point sources; however, they show some
alignment with the basement post-rift faults. The fault
intersection apparently associated with the HRDZ located
towards the middle of the field forms a relatively circular
anomaly and is located shallower than the OWC (Fig. 2A
and B).
3.2. 3D Restoration of palaeo-oil water contacts
Gartrell et al. (2002) described a methodology that
combines 3D structural restoration techniques with analysis
of palaeo-oil–water contacts using a fluid inclusion technique
known as GOIe. The GOI technique measures the
abundance of oil-bearing inclusions in a sandstone sample.
An empirical data base of over 300 wells indicates that GOI
values O5% (indicated by vertical dashed line in Fig. 7) are
typical of oil accumulations, whereas water zones tend to
have GOI values !1% (Eadington, Lisk, & Krieger, 1996;
Lisk et al., 1998; Lisk & Eadington, 1994). This relationship
allows palaeo-oil–water contacts to be picked with greater
reliability than using conventional show data alone. As the
GOI technique is thought to record the initial reservoir filling
history (Gartrell et al., 2002; George, Lisk, Eadington, &
Quezada, 1998), a horizontal palaeo-oil–water contact
Fig. 4. Fault patterns (map view) at base Cretaceous seal (Base Seal) level. (A) Two way time structure map for the Base Seal horizon showing fault trends. (B)
Interpretation of fault patterns observed at Base Seal level. A network of intersecting faults comprises a set of NE–SW trending Jurassic rift faults (thick solid
black lines) and a set of NNW–SSE and N–S trending basement faults (dashed lines). An array of ENE–WSW trending post-rift faults are observed above the
Jurassic fault blocks. The position of the present day OWC corresponds with location of fault intersections at the northeastern and southwestern ends of the
Skua Field. (C) Relationship between fault patterns, field extent and direct leakage indicators in the Skua/Swift area. Seismic velocity anomalies (HRDZs)
typically located above fault intersections. Airborne laser fluorosensor (ALF) anomalies show a general N–S alignment and also correlate with the position of
fault intersections in some cases.
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–1179 1169
Fig. 5. Seismic sections through the Skua Field (see Fig. 4A for locations). (A) Seismic section through southern end of the Skua Field showing Late Cretaceous
to Early Tertiary inversion anticline above the main trap bounding Skua Fault. (B) Seismic section through the northern end of the field showing Late
Cretaceous to Early Tertiary extensional faulting above the Skua Fault. Late Miocene extensional faulting is localised above the Rowan Fault.
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–11791170
Fig. 6. (A) Seismic cross-section showing an interpretation of the cross-
trending basement faults (see Fig. 4A for location). These structures are
interpreted to be near vertical and to penetrate the Cretaceous seal rocks.
(B) Seismic cross-section showing three hydrocarbon related diagenetic
zones (HRDZs) situated over the Skua Field (see Fig. 4A for location).
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–1179 1171
(palaeo-OWC) is assumed to be the most likely orientation at
the time of initial oil charge. A tilted contact due to
hydrodynamic gradients is considered to be unlikely in this
case, as minimal surface topography exists in the field area.
Palaeo-OWCs, derived from GOI analysis, were used as
stratigraphically-constrained markers that were tracked
throughout 3D restoration of the field. Restoration of a trap
to the time when the palaeo-OWCs were located at a common
depth is considered to yield constraints on the timing of initial
oil charge and the geometry of the accumulation at this time.
Comparison between the palaeo-field and present-day field
geometries with structural architecture and fluid flow
indicators (both past and present) allows hydrocarbon charge
and preservation histories to be further assessed.
GOI analysis of the Skua–3, Skua–4, Skua–8 and
Skua–9 wells suggests southwesterly dipping palaeo-OWC
exists in the Skua Field (Fig. 7). GOI values from samples
obtained for Skua 6 were all below the 5% threshold,
implying that the oil accumulation did not extend to this
location even though it lies well within structural closure
(Fig. 7). It was found that the variation in the depth of the
palaeo-oil–water contacts in the Skua Field can be
reconciled by restoring the combined effects of post-rift
tilting and Late Eocene to Early Miocene faulting (Fig. 8).
Restoration further back to the Late Palaeocene horizon
results in the palaeo-OWCs deviating from a common
depth. This result is interpreted to indicate that initial oil
charge occurred sometime between Late Palaeocene and
Early Miocene. Reconstruction of the palaeo-field by
mapping palaeo-OWC depth contour on the restored base
seal map indicated that the field was about 10% (by bulk
rock volume) larger than at the present day. It also
showed that the depth of the main fault intersection at the
NE end of the field coincides with the position and depth
of the palaeo-oil–water contact (Figs. 8 and 9). Agreement
between the location of the palaeo-OWC (1699 mSS) with
the restored location of the main fault intersection at the
time of charge implies that this fault intersection may
have not only acted as a control on the present day extent
of the oil field, but also may have acted as the primary
control on the extent of the field at the time of initial oil
charge (Figs. 8 and 9). Furthermore, the consistently low
GOI values recorded at Skua 6 indicate that oil
accumulation has never extended to this location since
the time of initial charge (mid-Eocene; Fig. 9). Therefore,
the main fault intersection appears to have acted as an
efficient and long-lived (w30 Ma) leak zone beyond
which hydrocarbons have not been able to accumulate.
The location of the peak of the largest and most
prominent of HRDZs observed over the field directly
above the main fault intersection is consistent with large
volumes of hydrocarbon being lost through this zone
(Figs. 4B and 6B). Lower volumes of hydrocarbons
probably leaked from the fault intersections associated
with the smaller HRDZs situated in the middle and
southeastern end of the field, categorising them as
secondary leakage features. The apparent control of the
key fault intersection on the extent of the field at the time
of initial oil charge also implies that the top seal was
compromised at this location prior to, or synchronous
with, initial charge. Hence, reactivation of the intersecting
faults during Late Cretaceous to Early Miocene
deformation is suggested to have been responsible for
the development of across-seal structural permeability
(Fig. 9).
Fig. 7. Summary of GOI fluid inclusion data from the Skua Field. Palaeo-oil–water contacts determined using the GOI technique show increasing depth to the
southwest.
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–11791172
4. Numerical modelling methods
The numerical code FLAC3D (Finite Langrangian
Analysis of Continua, Cundall & Board, 1988) was used
for the modelling performed in this study. It treats rock as
a continuum, represented by average values of mechanical,
fluid flow and heat transport properties. However, only the
mechanical component of the modelling is described here.
FLAC3D allows the user to create interfaces, which join
various 3D meshes. The interfaces can be assigned properties
of friction, cohesion, normal and shear stiffness, and tensile
strength in order to represent faults on which sliding occurs.
Two 3D models were constructed, both comprising a 1 km
thick sandstone layer overlain by a 500 m thick shale
(Fig. 10), representing the basic stratigraphy of the Skua
Field at the time of initial reactivation in the Late Cretaceous
(Gartrell et al., 2002). During deformation, a constant
displacement rate is applied to two opposing vertical sides
of the model (depending on the shortening direction), while
the other vertical edges are supported by crustal stress (so
they do not collapse under gravity). The bottom boundary of
the model can move freely in the horizontal direction, but is
not allowed to move in the vertical direction. Mechanical
deformation of the models was governed by Mohr-Coulomb
elastic–plastic rheology, where rocks initially deform
elastically, but continue to deform plastically to large strain
once the maximum shear stress reaches the yield stress (e.g.
Ord, 1991; Zhang, Hobbs, Ord, & Muhlhaus, 1995).
The geomechanical parameters used for the model
components were taken from Turcotte and Schubert (1982)
and are listed in Table 1.
The first model incorporated a simple triple junction fault
geometry made up of three vertical faults that intersect at 1208
to each other (Fig. 10A). The second model was designed to
more closely imitate the structural architecture of the critical
fault intersection at the northeastern end of the Skua Field
(Fig. 10B). A relatively complex fault geometry was
constructed comprising a vertical fault (representing the
basement fault) that changes strike by 108 at the fault
intersection and a fault dipping at 608 (representing the rift
fault). Each model was subjected to 3% shortening in a
number of directions (E–W, N–S, NE–SW, NW–SE). Only
results from the experiments with E–W contraction are
illustrated here, as this orientation is thought to best represent
the situation at Skua during Late Cretaceous to early Tertiary
deformation (Gartrell et al., 2002).
The parameters calculated during the numerical exper-
iments were displacement, shear strain, volumetric strain
(dilation), the orientation and magnitude of the maximum
(s1) and minimum principle stresses (s3), and the differential
stress (s1Ks3). The differential stress was used to map the
likely fracture failure mode within the seal layer for
experiments with E–W shortening. Following Sibson
(1996) we assume a composite Griffith-Coulomb failure
envelope so that if (s1Ks3)!4T (where T is tensile strength)
then tensile failure is predicted, if 4T!(s1Ks3)!6T then
hybrid tensile-shear failure is predicted, and if (s1Ks3)O6T
then shear failure is predicted.
Fig. 8. Schematic diagram illustrating the effects of 3D restoration on the
trap geometry and the location of palaeo-OWC marker points. (a) Strike-
section through the field at the present day. The palaeo-OWC markers at
Skua 3, Skua 4, Skua 8 and Skua 9 are located at different depths and a
small Early Miocene fault is located between Skua 3 and Skua 9. (b)
Restoration to Late Miocene horizon rotates the SW end of the trap
upwards, which enhances the structural relief on the trap and decreases the
depth difference of the palaeo-OWC markers. (c) The palaeo-OWC
markers are brought to equal depths (1699 mSS) as a result of a small
amount of additional rotation and removal of fault displacement during
restoration to the Early Miocene horizon. (d) Restoration to the Late
Paleocene horizon flattens the trap due to unfolding and causes the palaeo-
OWC markers to diverge. Thick black lines represent the top of the
reservoir. Thin black line represents the base of the reservoir. Black dots
represent the stratigraphic location of palaeo-OWC markers defined from
GOI data (from Gartrell et al., 2002).
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–1179 1173
5. Numerical modelling results
5.1. Model 1
For all shortening directions tested on the simple fault
model, deformation rapidly caused the opening of a
triangular zone of high dilation in the vicinity of the
fault intersection within the shale layer (Fig. 11). This zone
resulted from two of the blocks rotating away from the
intersection in order to accommodate the wedge shape of
the third block as it driven in towards the intersection
(e.g. Fig. 11A and B). Dilation also occurred on
the activated fault planes at higher strain, but with a
volumetric strain increment several times lower than in
vicinity of the fault intersection. In contrast, the shear strain
on the activated faults is generally high relative to that
calculated in the triangular zone of high dilation
(e.g. compare Fig. 11D and E).
The orientation of the principal stress axes is shown to
rotate in the vicinity of the less dilatant faults, with some
focussing towards the fault intersection (Fig. 11C). Stress
magnitudes in the strongly dilating fault zone are relatively
low compared to the rest of the model. Mapping the
differential stress on the shale layer indicates that tensile
failure mode is likely in the vicinity of the strongly dilating
fault with surrounding hybrid tensile-shear fractures
(Fig. 11F). Hybrid tensile-shear failure mode is also
predicted in the vicinity of the other two faults.
5.2. Model 2
The results for the complex fault model showed
increased complexity and variability between experiments
due to the asymmetry in the model (Fig. 12). Reverse
movement was generated due to contractional reactivation
of the dipping fault, which has strikes at a high angle to the
shortening direction (Fig. 12A). However, zones of high
dilational strain, similar to those formed in the simple fault
model, occur at the fault intersection in all the complex fault
models tested. The relationship between these zones of
high dilation and shear strain showed more variability than
in the simple fault model, so that in some cases dilational
zones overlap with high shear zones. However, in the case
which most closely represents the deformation (i.e. E–W
contraction) the zone of high dilation is developed in
association with low shear strain (compare Fig. 12D and E).
Principle stress orientations and magnitudes vary dra-
matically within the E–W contractional model, due to the
more complex fault movement patterns (Fig. 12C). Tensile
stress is predicted in a zone around the fault intersection and
along the leading edge of the footwall side of the dipping
fault (Fig. 12C). Tensile failure mode is predicted in this
zone using the criteria described above (Fig. 12F). Hybrid
tensile-shear failure is predicted adjacent to the northern-
most fault segment and in a rim around the tensile failure
zone. A lobe of tensile-shear failure mode is also shown to
propagate into one of the blocks. Tensile areas at the edge of
the models are regarded as boundary effects (Fig. 12F).
6. Discussion
Fault zones can form fluid conduits if connected open
fracture networks are present within a rock mass. The
highest fluid flux potential will occur where and when
fracture apertures, density and connectivity are greatest
(Cox, Knackstedt, & Braun, 2001; Sibson, 1996). Alter-
natively, fault zones can form fluid barriers where
the impermeable fault gouge forms during the shearing
process (e.g. shale gouge, shale smear, cataclasis) or as a
result of post deformational cementation (e.g. Knipe, 1992;
Sibson, 1996).
Fig. 9. Integrated structural and charge history schematic model for the Skua Field (not to scale). (a) Prior to reactivation and hydrocarbon charge. (b) Late
Cretaceous to early Tertiary reactivation of basement fault and subordinate rift fault causes intense seal damage at the intersection of the faults. (c) Initial oil
charge around the mid-Eocene. Extent of the palaeo-field is controlled by the location of the fault intersection (leak zone). (d) Palaeo-OWC is tilted due to post
rift subsidence and faulted due to Early Miocene deformation. Tilting of the trap also causes hydrocarbons to flow up-dip towards the NE to continually feed the
leak zone. Note: leakage of hydrocarbons is into overlying strata and not directly to the seafloor (modified from Gartrell et al., 2002).
Fig. 10. Perspective views of the two fault models used in the numerical
experiments. (A) Model 1 is a simple fault system with vertical faults
intersecting at 1208. (B) Model 2 is a more complex fault system designed
to simulate the critical fault geometry in the Skua Field. Dark grey layer
represents a 500 m thick shale layer. Light grey layer represents a 1 km
thick sandstone layer.
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–11791174
Strongly dilational zones in the vicinity of the modelled
fault intersections were created with only a minor amount of
bulk deformation during the numerical experiments
(Figs. 11D and 12D). This result is consistent with
numerical modelling performed by Sanderson and Zhang
(1999) and Zhang and Sanderson (2001), which showed that
relatively large fracture apertures, and hence highly
localised and enhanced fluid flow, can develop at intersec-
tions in fracture systems during reactivation. Geometrical
analysis by Andrews (1989) and McKenzie and Morgan
(1969) showed that fault–fault–fault triple junctions are
unstable (cannot maintain their geometry) during defor-
mation (Fig. 13). When slip occurs, a void must open in the
case where the angles between the faults are all less than
1808 (Fig. 13A and B), whereas material overlap must occur
if one of the angles is greater than 1808 (Fig. 13C and D).
The former applies to the fault geometries modelled here.
Lithostatic pressure at depth in the Earth’s crust will resist
void opening so that finite deformation is likely to be
accommodated on a fractal array of faults and fractures
around the intersection (Andrews, 1989; King, 1983).
Although the numerical modelling package used here is
not able to explicitly demonstrate fracture development,
fracture mode analysis predicts that this focussing of
dilational strain would probably generate a concentrated
network of interlinking tensile and tensile-shear fractures
capable of providing high permeability fluid conduits in
Table 1
Geomechanical parameters taken from Turcotte and Schubert (1982) used for the models
Density
(kg mK3)
Young’s
modulus
(GPa)
Poisson’s
ratio
Bulk mod-
ulus (GPa)
Shear mod-
ulus (GPa)
Cohesion
(MPa)
Tensile
strength
(MPa)
Friction
angle (8)
Dilation
angle (8)
Seal (shale) 2400 20 0.2 11 8.3 10 5 30 2
Sandstone 2450 35 0.25 23 14 15 7.5 30 2
Fault 2300 10 0.15 48 43.5 1 0.5 10 2
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–1179 1175
natural rocks (e.g. Sibson, 1996). A direct analogy would be
the efficient fluid flow systems associated with dilational
fault jogs described by Sibson (1985).
The fault geometries modelled here are examples of
locked fault intersections, where the slip vectors of
intersecting faults have non-parallel or opposing directions
(Fig. 13A–D, Curewitz & Karson, 1997). This locking
geometry resulted in relatively low shear strain occurring at
the intersections. Hence, it seems reasonable to predict that
the opportunity for fault rock membrane seals to develop in
the vicinity of the modelled fault intersections would be low
relative to the fault planes segments away from the
intersection where slip is greater. In natural systems, this
may further enhance the fluid flow efficiency of such fault
intersections relative to the surrounding faults. However,
Fig. 11. Results from the simple fault model (Model 1) with E–W contraction. (A
model after 3% contraction with arrows indicating displacement vectors. (C) Stress
Colour contour of volumetric strain increment (dilation) after 1% contraction. (E)
contour of differential stress distribution (s1–s3) calculated on the surface of the
fracture mode (if failure criteria reached) distinguished (see text).
the development of fault gouge was not modelled during the
numerical experiments.
Non-locking fault intersections may also exist where no
kinematic incompatibility develops at the point of intersec-
tion during deformation (Fig. 13E, Curewitz & Karson,
1997). Previous numerical modelling has shown that in this
case, fault intersections can be zones of relatively high shear
strain (Maerten, Willemse, Pollard, & Rawnsley, 1999).
These fault systems are likely to behave differently to
locked systems with respect to fluid flow, highlighting the
importance of the fault intersection configuration.
In the Skua Field, the main fault intersection appears to
have been transmissive to fluids from the time of initial oil
charge (Mid-Eocene) to the present day. Furthermore, the
low GOI values obtained at Skua-6 suggest that
) Perspective view of the model after 3% contraction. (B) Plan view of the
distribution calculated on the surface of the model after 1% contraction. (D)
Colour contour of shear strain increment after 1% contraction. (F) Colour
model after 1% contraction. Areas of tensile, hybrid tensile-shear and shear
Fig. 12. Results from the relatively complex fault model (Model 2) with E–W contraction. (A) Perspective view of the model after 3% contraction. (B) Plan
view of the model after 3% contraction with arrows indicating displacement vectors. (C) Stress distribution calculated on the surface of the model after 1%
contraction. (D) Colour contour of volumetric strain increment (dilation) after 1% contraction. (E) Colour contour of shear strain increment after 1%
contraction. (F) Colour contour of differential stress distribution (s1Ks3) calculated on the surface of the model after 1% contraction. Areas of tensile, hybrid
tensile-shear and shear fracture mode (if failure criteria reached) distinguished (see text).
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–11791176
accumulation did not extend beyond the main fault
intersection at the north eastern end of the field, and
therefore, hydrocarbon leakage from this zone appears to
have been both efficient and long-lived. Similarly, studies
on hot springs show that significant fluid flow tends to focus
at dilational sites, such as fault intersections and fault tips,
and that high permeability and hydrothermal fluid circula-
tion can be long-lived in these zones (Curewitz & Karson,
1997). These observations suggest that open fracture
network can be maintained by stress focussing on the
complex fracture system likely to develop at the fault
intersection, providing many fracture orientations suscep-
tible to reactivation. Alternatively, fault and fracture zones
may not necessarily need to be mechanically active in order
to transmit fluids in all cases (cf fault-valve model; Sibson,
1996). At depth in the Earth’s crust, and especially at
elevated temperatures in active hydrothermal systems,
porosity destruction (such as healing and sealing of fractures
by cementation) can cause permeability to decrease on
timescales that are relatively short (Cox et al., 2001).
Further deformation is then required to reopen the fractures
to allow further fluid movement. Crack-seal microstructures
in veins indicate that macroscopic veins in some deep
hydrothermal regimes can open and seal up to several
thousand times (Cox, 1995; Cox et al.; Ramsay, 1980).
However, these porosity destruction processes are likely to
be less effective at depths of investigation relevant to
hydrocarbon exploration (!4 km), where lower tempera-
tures occur and hydrothermal processes are not as active
(Cox et al.).
The presence of hydrocarbons in the system may also
help to maintain structural porosity and permeability.
Interactions between faults at the intersection create
porosity and permeability by concentrating stresses and
fracturing. The observations made at the Skua Field
suggest that this localised zone of enhanced permeability
leads to focussed hydrocarbon fluid flow. High concen-
trations of hydrocarbons, at the expense of hydrothermal
fluids, in the localised fracture system may reduce the
potential for mineral precipitation and fracture blockage to
occur. Experiments show that, in the absence of
cementation, faults and fractures are very difficult to
close to fluids due to natural fracture surface roughness
(Gutierrez, Oino, & Nygard, 2000). In addition, partial
filling of fractures by mineral cements can actually act to
maintain structural permeability by holding open the
fractures (e.g. Stowell, Laubach, & Olson, 2001). Fluid
focussing at the fault intersection may also contribute to
increased pore pressures in the fracture zone, which may
enhance fracture activity.
Fig. 13. Examples of locking and non-locking fault intersection behaviour
during slip (after Andrews, 1989; Curewitz & Karson, 1997). (A) A locking
fault triple junction composed of faults F1, F2 and F3, in which the opposite
angles a, b, and g are all less than 1808. (B) Rigid body displacement on the
fault geometry in (A) causes a void to open at the intersection. (C) A
locking fault triple junction in which one of the opposite angles, g, is
greater than 1808. (D) Rigid body displacement of the fault geometry in (C)
causes material overlap at the intersection. (E) An example of a non-locking
fault intersection geometry in which slip can occur without changing the
geometry of the fault system.
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–1179 1177
Given the appropriate conditions, the long-lived and
efficient nature of leakage zones developed at fault
intersections make them prime sites for losing large
volumes of hydrocarbons, particularly if they are located
in a position to accept continual supply of hydrocarbons
(e.g. at the crest of a structure). In contrast, leakage due to
reactivation of individual fault planes may be more sporadic
and possibly inhibited by fault membrane seal processes
(associated with higher shear strains). For example, the
main trap bounding Skua fault does not appear to have been
associated with significant hydrocarbon leakage, even
though it was reactivated (Late Eocene). Therefore, under-
standing the likely distribution and behaviour of enhanced
zones of structural permeability, where high dilational strain
occurs in conjunction with low shear strains, may be a key
factor in predicting leakage patterns associated with fault
reactivation. Other important structural sites likely to be
associated with high dilation—low shear zones are
dilational jogs, fault bends, fault relays and fault tips.
The modelling performed for this study demonstrates
how reactivation of fault intersections can lead to the
development of critical hydrocarbon leakage sites
and supports previously published charge and leakage
history models for the Skua Field (Gartrell et al., 2002).
Numerical modelling may help to better assess the risk of
seal breach due to the development of enhance zones of
structural permeability, as the formation of these structural
sites is likely to exist below seismic resolution. However,
several important factors were not considered in the
modelling, such as multiple phase fluid flow, fracture
density, fracture connectivity, fault gouge processes, and
diagenetic processes. Some of these parameters may be
incorporated with further development of the numerical
techniques, whereas others may be derived from compari-
sons with field examples and geomechanical experiments.
7. Conclusions
A fault intersection is identified as the primary control on
hydrocarbon leakage from the Skua oil field, Timor Sea.
Integrated fluid inclusion, hydrodynamic and structural data
sets suggest that the leak zone was generated during Late
Cretaceous to early Tertiary contraction and that it was both
an efficient and long-lived fluid conduit.
Numerical modelling, using FLAC3D, of two fault triple
junction geometries (similar to that at Skua) within a
sandstone-shale sequence demonstrates that zones of high
dilation can be generated at fault intersections during
contraction. Only a small amount of deformation was
required to initiate the dilational zones. In a natural system,
these zones would probably contain high concentrations of
open fractures, providing effective fluid conduits.
The effectiveness of the dilational zones may be enhanced
by attendant relatively low shear strain, which may
otherwise generate fluid barriers from shale gouge
processes.
Predicting and/or identifying enhanced zones of struc-
tural permeability at sites of high dilation and low shear
(e.g. fault intersections, dilational jogs, fault relays, fault
tips) may be critical to trap integrity assessments, as
relatively high volumes of hydrocarbons may be lost at
these sites. Numerical modelling of the type used here may
lead to more sophisticated and accurate prediction of seal
breach due to fault reactivation when combined with stress
and fluid history data. However, further development of the
numerical technique is required to address some of its
current limitations.
Acknowledgements
The results and discussions presented in this paper
could not have been achieved without the funding support
of company sponsors to the APCRC Seals Consortium.
Structural restoration software (2DMove and 3DMove)
was generously provided by Midland Valley Exploration
Ltd. Our gratitude also goes to Schlumberger Oilfield
A. Gartrell et al. / Marine and Petroleum Geology 21 (2004) 1165–11791178
Australia Pty Ltd for the use of GeoFramee software,
which was used exclusively for interpretation and depth
conversion of seismic data in this project. Comments by
an anonymous reviewer helped to improve the original
manuscript. Thanks also to Travis Naughton and Luke
Johnson for his help with drafting some of the figures.
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