gartrell 2004 marine

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

http://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).


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 seepage
2.
Hydrocarbon related diagenesis
3.
Mapping of palaeo-oil zones using fluid inclusion data
A 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.


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.


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).


(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.


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).


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.


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


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).


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.


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


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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