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AAPG Bulletin
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AAPG Bulletin, V.
2014. The American Association of Petroleum Geologists. All rights reserved.
DOI: 10.1306/04301413177
Fault
interactions and reactivation within a normal-
fault
network at Milne Point, Alaska




Casey W. Nixon,1 David J. Sanderson,2 Stephen J. Dee,3 Jonathan M. Bull,4 Robert J. Humphreys,5 and Mark H. Swanson6
1University of Southampton, Ocean and Earth Science, National Oceanography Centre Southampton, SO14 3ZH, United Kingdom; c.w.nixon@noc.soton.ac.uk
2University of Southampton, Ocean and Earth Science, National Oceanography Centre Southampton, SO14 3ZH, United Kingdom; University of Southampton, Faculty of Engineering and the Environment, SO17 1BJ, United Kingdom; BP Exploration Operating Company Limited. Chertsey Road, Sunbury-on-Thames, TW16 7BP, United Kingdom d.j.sanderson@soton.ac.uk
3BP Exploration Operating Company Limited. Chertsey Road, Sunbury-on-Thames, TW16 7BP, United Kingdom; Stephen.Dee@uk.bp.com
4University of Southampton, Ocean and Earth Science, National Oceanography Centre Southampton, SO14 3ZH, United Kingdom; bull@noc.soton.ac.uk
5BP Exploration (Alaska), Inc., 900 East Benson Boulevard, Anchorage, Alaska 99508-4254; robert.humphreys@uk.bp.com
6BP Exploration (Alaska), Inc., 900 East Benson Boulevard, Anchorage, Alaska 99508-4254; Mark.Swanson@bp.com
ABSTRACT
A normal-fault
network from Milne Point, Alaska, is investigated focusing on characterizing geometry, displacement, strain, and different
fault
interactions. The network, constrained from three-dimensional seismic reflection data, comprises two generations of faults: Cenozoic north-northeast–trending faults and Jurassic west-northwest–trending faults, which highly compartmentalize Upper Triassic to Lower Cretaceous reservoirs. The west-northwest–trending faults are influenced by a similarly oriented underlying structural grain. This influence is characterized by increases in throw on several faults, strain localization, reorientation of faults and an increase in linkage maturity.
Reconstructing fault
plane
geometries and mapping spatial variations in throw identified key characteristic features in their interactions and reactivation of pre-existing structures. Faults are divided into isolated, abutting, and splaying faults. Isolated faults exhibit a range of displacement profiles depending on the degree of restriction at
fault
tips.
Fault
splays accommodate step-like decreases in throw along larger main faults with a throw maximum at the intersection with the main
fault
. Throw profiles of abutting faults are divided into two groups: early stage abutting faults with throw minima at both the isolated and abutting tips, and developed abutting faults with throw maxima near the abutting tip.
Developed abutting faults accumulate throw after initial abutment, locally reactivating and transferring throw onto the pre-existing fault
. Two abutting faults can link kinematically by reactivating a segment of the pre-existing
fault
forming a trailing
fault
. The motion sense of the trailing
fault
can be synthetic or antithetic to the reactivated pre-existing
fault
, producing increases or decreases in the throw of the pre-existing
fault
, respectively.
INTRODUCTION
The major aim of this paper is to analyze the deformation within a fault
network formed by more than one generation of faults, focusing on the way that the different faults interact within the network. Reconstruction of
fault
plane
geometries at Milne Point, Alaska, and mapping their spatial variations in throw, allows us to recognize key features in their interactions including splaying, abutting, and reactivating.
Many fault
networks consist of more than one
fault
set. These can either be conjugate
fault
sets (e.g., Zhao and Johnson, 1991; Nicol et al., 1995; Ferrill et al., 2009; Nixon et al., 2011) that formed in the same stress system, or multiple
fault
sets that form from the overprinting of two or more stress systems (Davatzes et al., 2003; Bailey et al., 2005). The latter can form new faults with different orientations and/or cause reactivation of pre-existing faults (e.g., Kim et al., 2001), which can also have a strong influence on the development of later
fault
sets (e.g., Segall and Pollard, 1983; Bailey et al., 2005). Hence, complex cross-cutting relationships and interactions can form between
fault
sets.
Understanding the relationships between different fault
sets within a network is important as interconnected faults can provide pathways for fluids, allowing the migration and entrapment of hydrocarbons (Aydin, 2000). They can also act as fluid barriers compartmentalizing reservoirs (Bouvier et al., 1989; Leveille et al., 1997), which is a major uncertainty in the qualitative and quantitative assessment of a reservoir in the hydrocarbon industry (Jolley et al., 2010). Furthermore,
fault
interaction and reactivation are important when assessing reservoir quality and heterogeneity, as they can contribute to damage zones as well as cause variations in bed thinning, attenuation and trap integrity (i.e., Fossen et al., 2005; Gartrell et al., 2006; Ferrill et al., 2009). Such affects often occur around
fault
intersection lines or branch lines, hence, being able to identify and characterize different
fault
interactions, within a network, is essential when interpreting reservoir deformation and evaluating communication between
fault
bound compartments.
In this paper, we address this problem through the analysis of a three-dimensional (3-D) seismic reflection dataset that images a normal-fault
network at Milne Point, Alaska. To better understand the behavior of the
fault
network, we characterize the geometry, throws, and strain distribution within the
fault
network, and the relationships among different
fault
sets. Most interpretation of faults from a 3-D seismic volume focuses on picking individual faults and linking these from line to line. However, to investigate the different interactions within the
fault
network we focus on establishing the branch lines between different
fault
planes and consider the displacement variation around these. As well as characterizing the interactions within the
fault
network, we also investigate the effects that pre-existing structures can have on displacement distributions and
fault
network development. Ultimately, we aim to provide a thorough description and classification system that will allow these numerous
fault
interactions to be easily identified by petroleum geoscientists during seismic interpretation.
GEOLOGICAL SETTING
Milne Point is located on the northern edge of the Alaska North Slope approximately 450 km (280 mi) north of the Arctic Circle and 40 km (25 mi) northwest of Prudhoe Bay (Figure 1A). The region is of particular interest because of the presence of numerous major gas and oil fields including the Prudhoe Bay, Milne Point, and Kuparuk River oil fields (Carman and Hardwick, 1983; Collett, 1993; Bird, 1999; Boswell et al., 2011). Large quantities of oil have been produced, since the discovery of the first field in 1968, from complex structural/stratigraphic traps within Permian to Cenozoic reservoirs at production depths >2000 m (>6562 ft) (Boswell et al., 2011).
Figure 1.
(A) Location map showing the key structural features of the Alaska North Slope (ANS) and the position of Milne Point. (B) Summary of the ages of the stratigraphic sequences and the formation lithologies that were seismically imaged. The Shublik Formation shale, Kingak Shale, and Pebble shale are highlighted as important source rock formations for the Prudhoe Bay, Milne Point, and Kuparuk River oil fields. The KUP and SAG horizons are indicated as the bottom and top of the Kuparuk River and Sag River reservoir sandstone formations, respectively.
The principal structural features of the region (Figure 1A) are the Barrow Arch to the north, and the Colville Basin and Brooks Range to the south (Carman and Hardwick, 1983; Bird, 1999; Boswell et al., 2011). The Barrow Arch is an east–west–trending rift shoulder and the Brooks Range is a fold and thrust mountain belt related to continent–continent collision (Bird, 1999). Together, these structural highs provided source material that filled the Colville (foreland) Basin, which has an east–west axial trend (Figure 1A) (Carmen and Hardwick, 1983; Bird, 1999).
The sedimentary rocks of the Alaska North Slope consist of south-dipping passive continental margin deposits of late Paleozoic and Mesozoic age, overlain by north-dipping foreland basin deposits in the Mesozoic and Cenozoic (Collett, 1993; Bird, 1999). These deposits have been divided into three main tectono-stratigraphic sequences (Figure 1B) that are described in detail by Bird (1999). The earlier passive-margin deposits are termed the Ellesmerian sequence. These consist of clastic and carbonate strata of middle Devonian to Triassic age, that onlap onto a stable south-facing continental margin (Collett, 1993; Bird, 1999). The Ellesmerian was followed by the Beaufortian sequence that was deposited during a period of continental rifting in the Jurassic and Early Cretaceous (Bird, 1999). The rifting is characterized by south-dipping normal faulting in the Jurassic followed by north-dipping normal faulting in the Early Cretaceous (Hubbard et al., 1987). This rift formed the paleo-high of the Barrow Arch. Then during the Cretaceous and Cenozoic, continent–continent collision caused uplift to the south, that is, the Brooks Range, and subsidence to the north producing the Colville Basin and the foreland basin deposits of the Brookian sequence (Carmen and Hardwick, 1983; Collett 1993; Bird, 1999). The Brookian sequence is also extensively faulted in the Milne Point region by north-northeast-striking normal faults (Boswell et al., 2011; Lorenson et al., 2011). The Jurassic–Cretaceous rift faults and later Brookian faults are likely to have initiated at different burial depths.
The Milne Point Field (Figure 1A) produces from three separate reservoirs: the Schrader Bluff, Kuparuk, and Sag River Formations. The Schrader Bluff is a shallow, unconsolidated viscous oil reservoir. The Sag River is a deeper, Upper Triassic sandstone reservoir with light oil occurring within the Ellesmerian sequence. It is also a reservoir elsewhere in the Prudhoe Bay area and contains gas in the Kavik field southeast of Prudhoe Bay (Bird, 1999). The Kuparuk Formation forms the main reservoir (Figure 1B). This is a Lower Cretaceous, shallow-marine sandstone that hosts several oil reservoirs in northern Alaska, including Milne Point and the neighboring Kuparuk River field (Bird, 1999). Both these accumulations occur in combination structural stratigraphic traps. According to Dzou (2010), the Kuparuk River reservoir was charged from deeper, Shublik Shale source rocks with some gas contribution from Kekiktuk coals. Published resources for Milne Point’s light (American Petroleum Institute [API] 22) oil Kuparuk reservoir are approximately 920 MM (million) stock tank barrels original oil-in-place (Ning and McGuire, 2004). Water-flood assisted production began in 1985.
The fault
network studied in this paper affects the Triassic to Lower Cretaceous rocks (Figures 1B, 2). We concentrate on analyzing the network at two stratigraphic horizons within the 3-D seismic data: the younger horizon that follows the bottom of the Kuparuk Formation (KUP horizon; Figure 1B) and an older horizon that follows the top of the Sag River Formation (SAG horizon; Figure 1B). The orthogonal
fault
network cuts both of these reservoir horizons, frequently with displacements exceeding reservoir interval thicknesses. This leads to a range of variable oil–water contacts and closely spaced compartmentalization. Understanding the controls on fluid flow of such a
fault
network clearly impacts efficient reserve recovery and well planning.
Figure 2.
Seismic reflection images of (A) a northwest–southeast trending crossline and (B) a northeast–southwest trending in line. Red represents the west-northwest–trending
fault
set that was picked on the in lines and blue represents the north-northeast–trending
fault
set that was picked on the crosslines. Dashed lines are the faults that were not picked on the in line or crossline but have been projected onto the seismic section. (C) Location map showing the orientation of the in lines and crosslines.
METHODS
Data Acquisition and Interpretation
The 3-D seismic reflection data was acquired using Vibroseis in 2007. The data are of high quality, cover an onshore area at Milne Point of (
) (Figure 1A), and are 120-fold containing frequencies between 6 and 96 Hz. The 3-D migrated seismic volume comprises 1238 inlines bearing N045°E and 897 crosslines bearing N135°E, each with a spacing of 16.8 m (55 ft) (Figure 2). Interval velocities of 3050 m/s (10,007 ft/s) and 4100 m/s (13,451 ft/s) were calculated for the KUP horizon and the SAG horizon, respectively, using true vertical depth subsea (TVDSS) and two-way time (TWT) values taken from geophysical wireline-log well data.
The fault
network was interpreted for a sequence of sedimentary rocks, ∼650 m (2133 ft) thick at a depth greater than 2000 m (6562 ft) (Figure 2). Faults were identified and picked on every tenth inline section (N045°E) and crossline section (N135°E) from offsets of key seismic reflectors. In general, the seismic data imaged faults with >10 m (>32.8 ft) displacement. Using TrapTester, a seismic interpretation and seismic modeling software developed by Badley Geoscience Limited, an interconnected 3-D
fault
model was produced. This involved identifying
fault
intersections from displaced raw horizon data for multiple horizons that were then validated using coherency time slices. Branch lines were then created to connect the intersecting
fault
planes.
Hanging wall and footwall cutoffs of the KUP and SAG horizons were projected from raw horizon data onto the modeled fault
surfaces. To correct for local effects, such as
fault
drag around
fault
surfaces, the raw horizon data that were within 75 m (246 ft) of each
fault
were trimmed and a 100 m (328 ft) wide patch of horizon data was used to calculate and project each horizon surface onto the
fault
surface. The interval velocities and the TWT of each hanging wall and footwall cutoff were then used to measure numerous
fault
-attribute data (such as displacement, throw, heave, dip, azimuth, and strike) at 100 m (328 ft) intervals along the plan view length of each interpreted
fault
surface.
Network Analysis
The fault
attribute data were extracted from TrapTester software, with associated
and
coordinates. The data were imported into ArcGIS as point data and used to digitize
fault
traces to produce
fault
maps for both the KUP and SAG horizons. Each
fault
trace was split into shorter segments (∼100 m [328 ft] in length) at each
fault
attribute data point. Average throws and segment azimuths were calculated allowing the network to be displayed by
fault
trend and
fault
throw. The
fault
maps combined with length-weighted rose diagrams,
fault
length vs.
fault
throw plots and
fault
throw profiles were used to investigate the geometry, kinematics, and interactions within the
fault
network.
In addition, 3-D strain was calculated to assess the partitioning of deformation within the fault
network. This uses the
fault
orientation and dip separation to construct the Lagrangian strain tensor, as described by Peacock and Sanderson (1993) and Nixon et al. (2011). This involves calculating the eigenvalues and eigenvectors of the Lagrangian strain tensor (
) when sampling faults from a
plane
:
in which is the sample area,
is the
fault
trace-length, and
is the displacement tensor, and
is the transpose. Although interactions within normal-
fault
networks often produce complex 3-D strains, which are supported by simple geometric models (e.g., Ferrill and Morris, 2001) as well as numerical models (e.g., Imber et al., 2004; Goteti et al., 2013), no slip direction data exists for the normal faults observed in the 3-D seismic reflection data at Milne Point. Therefore, as these are normal faults, and we are considering the entirety of the
fault
network, we assume dip-slip displacement and apply a weighting factor (
) defined by Peacock and Sanderson (1993) to the displacement tensor, which corrects for the orientation bias between the sample
plane
and the dip angle (θ) of the faults, hence:
in which is the displacement and unit vectors
and
are normal to the
fault
plane
and parallel to the slip direction, respectively. As we assume dip-slip movement on these faults, in which faults have a dip angle (θ) and a dip direction (Φ), then:
FAULT
NETWORK CHARACTERISTICS
General Structural Trends and Relationships
The study area has an underlying structural grain trending northwest-southeast which forms broad-scale graben and horst structures on both horizons (Figure 3A). These are particularly well defined in the deeper SAG horizon and coincide with an overall deepening to the east-northeast of ∼490 m (1608 ft) in the KUP horizon and ∼615 m (2018 ft) in the SAG horizon. The fault
network overprints this structural grain and has two sets of normal faults, a north-northeast-trending set and a west-northwest–trending set (Figures 3B, 4).
Figure 3.
Fault
maps of the KUP horizon on the left and the SAG horizon on the right: (A) Surface horizon maps showing the topography of the horizons; (B)
fault
map color-coded by azimuth with red generally representing west-northwest–faults and blue generally representing north-northeast-faults; (C)
fault
map color-coded by throw with blue and orange representing low- and high-throw values, respectively. The location of specific
fault
maps and 3-D diagrams used in later figures are also shown.
Figure 4.
Length-weighted rose diagrams and an equal-angle stereographic projection of poles to
fault
segments for each
fault
set in the KUP horizon (top) and SAG horizon (bottom).
The north-northeast–trending faults are regularly spaced (1–2 km [0.6–1.2 mi]) and most down throw to the southeast (Figures 2A, 4) with constant dips of ∼50–60°. They displace both the KUP and SAG horizons by similar amounts (Figures 2A, 5A); therefore, these are post-depositional, as indicated by the constant thickness of stratigraphy across each fault
(Figure 5A).
Figure 5.
Interpreted seismic sections showing the stratigraphic thickness in relation to (A) the north-northeast–trending faults and (B) the west-northwest–trending faults. (C) An isochore map showing the variation in stratigraphic thickness (two-way time [TWT]) between the KUP and SAG horizons. The black patches cover the areas that have been tectonically thinned by throughgoing faults.
The majority of the faults in the west-northwest–trending fault
set dip to the southwest (Figures 2B, 4). Unlike the north-northeast–trending faults these are not regularly spaced and many of the larger faults become steeper at depth with dips increasing from ∼40–50° to ∼70–80° (Figures 2B, 5B). Furthermore, the west-northwest–trending faults often displace the SAG horizon more than the KUP horizon (Figures 2B, 5B) and, hence, have a component of syndepositional movement associated with them (i.e., thickening of stratigraphic sequence 4; Figure 5B). Overall the stratigraphic thickness between the KUP and SAG horizons increases from a minimum of ∼325 ms (TWT) in the north to a maximum of ∼425 ms (TWT) in the south of the study area and is not related to syndepositional sedimentation associated with either
fault
trend (Figure 5C).
Because of the downward increase in throw and resultant hanging-wall thickening (Figure 5B), the west-northwest–trending faults are suggested to be associated with rifting during the deposition of the Beaufortian sequence (Jurassic to Lower Cretaceous). The majority of these faults dip south, consistent with the polarity of Jurassic rifting observed elsewhere on the Alaska North Slope (Hubbard et al., 1987; Bird, 1999). Abutting and cross-cutting relationships indicate that the north-northeast–trending faults postdate many of the west-northwest–trending faults (Figure 3), which is supported by the north-northeast–trending faults consistent with those that cut the Brookian sequence in the Cenozoic (Boswell et al., 2011), which have been shown to cut the top of the Kuparuk Formation (Masterson et al., 2001).
Later local reactivation of the west-northwest–trending faults, related to changes in regional and/or local stress orientations, is supported by the fact that some of the north-northeast–trending faults are abutted by some small west-northwest–trending faults (Figure 3). This is also consistent with the presence of small faults and fault
splays that trend approximately northwest–southeast in the Eocene Sagavanirktok Formation (Boswell et al., 2011; Lorenson et al., 2011), which are thought to be genetically linked to the underlying faults (Collett et al., 2011). Furthermore, the present-day stress regime of the area favors the reactivation of the west-northwest–trending faults (Zoback, 1992; Heidbach et al., 2010).
Organization of Faulting and Throw Distribution
Many of the large west-northwest–trending faults are aligned with or influenced by the underlying northwest–southeast structural grain. This is more obvious in the deeper SAG horizon where large faults from the west-northwest–trending fault
set bound many of the graben structures (Figure 3A). In addition, en echelon arrays of smaller west-northwest–trending faults form above the underlying northwest-southeast structures and often splay from/rotate into the larger graben-bounding faults (Figure 3B). This influence becomes more emphatic with depth where the trend of the west-northwest–trending faults is oriented ∼10° farther clockwise in the deeper SAG horizon, which can be seen clearly in the length-weighted rose diagrams (Figure 4C, F).
The west-northwest–trending faults also become more pervasive with depth increasing in density from (
) in the KUP to
(
) in the SAG horizon (Table 1). In contrast, the north-northeast–trending faults have similar
fault
density values for each horizon (i.e.,
[
] at the KUP horizon and
[
] at the SAG horizon; Table 1). As a result, the KUP horizon has approximately equal proportions of north-northeast–trending and west-northwest–trending faults, whereas the SAG horizon has an increased proportion of west-northwest–trending faults (∼64%; Table 1).
The largest faults in the network trend west-northwest with maximum throws of up to 138 m (453 ft) in the KUP horizon and 332 m (1089 ft) in the SAG horizon (Table 1; Figure 3C). In general, the maximum throw of faults increases from the shallower KUP horizon to the deeper SAG horizon (Figure 6A). This increase is seen mainly in the west-northwest–trending faults (Figure 6C) as indicated by the average maximum throw values for each fault
set (Table 1).
Figure 6.
Logarithmic plots of
fault
length versus maximum throw for (A) all faults, (B) the KUP horizon, and (C) the SAG horizon. Note the significantly greater throws for some west-northwest–trending faults in the SAG horizon.
In summary, the north-northeast–trending faults are evenly distributed both in space and with depth showing similar orientations, fault
densities, and throws for each horizon. In contrast, the west-northwest–trending faults increase in density, size, and dip with depth. They also become more parallel to the underlying structural grain, suggesting influence of deeper pre-existing structures.
Strain Analysis
Each fault
trend has a narrow range of strike orientations and shows negligible amounts of extension for the intermediate strain component (Figure 7A, B; Table 2). The north-northeast–trending faults accommodate similar amounts of extension in each horizon, whereas the maximum extension that is accommodated by the west-northwest–trending faults increases in the SAG horizon (Table 2). Hence, in the SAG horizon the majority of the strain (69%) is accommodated by the west-northwest–trending faults.
Figure 7.
Equal-angle stereographic projection of poles to
fault
segments showing the principal strain orientations for (A) the north-northeast–trending faults, (B) the west-northwest–trending faults, and (C) all faults.
Like the strain accommodated by the individual fault
trends, the overall composite strain of the
fault
network indicates subhorizontal extension and subvertical shortening at each horizon (Figure 7; Table 2). The maximum extension accommodated by the
fault
network increases from 2.4% oriented at N70°E for the KUP horizon to 3.7% oriented at N225°E for the SAG horizon (Table 2), which is accommodated entirely by the west-northwest–trending faults. As the strain accommodated by the
fault
network is a superposition of the two strains accommodated by each
fault
trend, which are orthogonal to one another, an intermediate strain component also exists with an extension of ∼1.5% oriented at N340°E and N135°E for the KUP and SAG horizons, respectively. In the KUP horizon, the total strain is accommodated equally between the two
fault
trends, as indicated by the minimum extension percentages (Table 2), resulting in the maximum extension direction of the network approximately bisecting the angle of intersection between the two
fault
trends (east-northeast–west-southwest; Figure 7C). However, in the SAG horizon the network’s maximum extension direction is rotated 25° counterclockwise to approximately northeast–southwest (Table 2; Figure 7C), as more of the total extension is produced by the west-northwest–trending faults.
In general, the strain analysis shows an increase in strain with depth because of more strain being localized onto the west-northwest–trending faults in the deeper SAG horizon. The contrast between the behavior of the two fault
sets indicates that they are independent
fault
sets. In addition, localization of strain onto the west-northwest–trending faults further suggests that these faults are being influenced by the pre-existing structures that form the northwest–southeast underlying structural grain.
INDIVIDUAL FAULTS AND SPLAYS
Isolated Faults
Very few isolated faults exist within the fault
network at Milne Point, and these are mostly small faults with lengths ranging from approximately 400 to 1700 m (1312 to 5577 ft) accumulating maximum throws of less than 50 m (164 ft). Throw profiles of the isolated faults can be divided into three main groups: Unrestricted, single-tip restricted, and double-tip restricted (Figure 8) (compare Nicol et al., 1996; Manighetti et al., 2001; Soliva and Benedicto, 2005).
Figure 8.
Normalized
fault
profiles for isolated faults from both the KUP and SAG horizons with length/maximum length (
) along the x axis versus throw/maximum throw (
) on the y axis. (A) Isolated faults with unrestricted tips; (B) isolated faults with a single tip restricted; (C) isolated faults with both tips restricted. The graphs on the right side are cartoon representations of each
profile
.
The examples in Figure 8A are symmetrical profiles with the maximum throw located near the center of the fault
. These either match that of an ideal elastic
profile
, as modeled for fractures in a homogenous material by Pollard and Segall (1987), or a symmetrical cone-shaped
profile
, as described by Muraoka and Kamata (1983) for faults that form in incompetent layers. Such profiles have been shown to be characteristic of faults with unrestricted tips (compare Nicol et al., 1996, 2010; Manighetti et al., 2001) and are the smallest isolated faults within the network as indicated by their average maximum throw and average length (Figure 8A).
Other profiles are asymmetrical with the maximum throw located closer to one of the fault
tips producing a tip with a steep throw-length gradient (Figure 8B). These profiles match the single-tip and half-restricted
fault
-displacement profiles described in Manighetti et al. (2001). These are not caused by
fault
abutments but either by lithological barriers (such as changes in competency) or by soft linkage with nearby faults that restrict the propagation rate of a
fault
tip indicating kinematic interaction between faults (e.g., relay ramps) (Peacock and Sanderson, 1996; Schlagenhauf et al., 2008; Nicol et al., 2010).
The majority of isolated faults at Milne Point produce a symmetrical profile
with a flat top and steep gradients at each
fault
tip (Figure 8C). Such a shape in
fault
-displacement profiles has been described in numerous studies (Muraoka and Kamata, 1983; Peacock and Sanderson, 1991; Manighetti et al., 2001; Nicol et al., 2010). Muraoka and Kamata (1983) describe these as a mesa-shaped
profile
for faults that form with tips that terminate in strain-absorbing incompetent stratigraphic layering. Hence, these are double-tip restricted-
fault
profiles and have the largest average maximum throw and average-length values of the isolated faults.
In summary, the isolated faults within the network have isolated tips and produce common throw-length profiles the shape of which depends on the restriction of the fault
tips (Muraoka and Kamata, 1983; Pollard and Segall, 1987; Nicol et al., 1996, 2010; Manighetti et al., 2001; Schlagenhauf et al., 2008). In general,
fault
-tip restriction is characterized by a high throw gradient at the restricted tip.
Individual Faults
Although the isolated faults have short lengths (less than 2000 m [6562 ft]), many of the faults within the network have longer fault
lengths (up to 9000 m [29,528 ft]) and accumulate much larger throws (Figures 3, 6). These longer faults are often segmented by cross-cutting faults or have numerous faults that abut them (Figure 3). Even though these long faults are segmented by many intersecting faults, the displacement variations along their
fault
planes are consistent with each other (Figure 9A). Hence, their throw profiles are often symmetrical with maximum throws near the center of the
fault
plane
and minimum throws at their tips (Figure 9), which is similar to throw profiles of isolated faults.
As each segment has a displacement profile
that is consistent with its adjacent segment, these can be considered as coherent structures and not isolated
fault
segments that have aligned and linked (compare Walsh et al., 2003). Therefore, despite interactions with other
fault
sets the larger and longer faults still act as individual isolated faults. This can be identified for both
fault
sets and indicates that the faults in both sets originally developed as individual faults rather than simultaneously.
Figure 9.
Fault
-throw profiles of long individual faults (>2000 m [>6562 ft] length) which have numerous intersecting and abutting faults. (A) A 3-D diagram of an individual west-northwest–trending
fault
plane
(
fault
11-KUP) in the KUP horizon with throw contoured onto the
fault
plane
; (B) a throw
profile
along the length of
fault
11-KUP (see Figure 3C for location within the
fault
network); and (C) normalized throw profiles of numerous long individual faults within the network. Examples are taken from both the KUP and SAG horizons. Note the similarity to isolated-
fault
throw-length profiles.
Splays
Fault
splays often occur near the tips of faults and involve a smaller
fault
that splays away from a larger
fault
. The smaller splay
fault
has a
fault
plane
that is obliquely oriented to the larger main
fault
plane
and has a displacement maximum along the line of intersection (Figure 10A). The displacement distribution on the
fault
plane
of the main
fault
shows an abrupt drop in displacement at the line of intersection with the splay
fault
(Figure 10A).
Figure 10.
(A) A 3-D diagram showing the distribution of throw on the
fault
planes of a splay
fault
(
fault
100-KUP) and its associated main
fault
(
fault
99-KUP). Panels (B) and (C) are
fault
profiles of a main
fault
and a splay
fault
showing their variations in throw along distance X, which increases to the east. To the right of each graph are plan-view
fault
maps of the interacting faults. See Figure 3C for the locations of these faults within the
fault
network.
Fault
-throw profiles indicate that the decrease in displacement is accommodated by the splay
fault
. For example, Figure 10B and C show the throw profiles of two main faults (99-KUP and 57-SAG) that have corresponding splays (faults 100-KUP and 177-SAG) at intersection points A and B, respectively. Both of the main faults show a step-like decrease in throw at the intersection with the splay faults in the direction of the acute angle of intersection. This step down in throw approximately matches the throw of the respective splay faults near the point of intersection (Figure 10). After the point of intersection both the main
fault
and splay
fault
steadily decrease in throw before reaching null values at their isolated
fault
tips. This is consistent with results of Maerten et al. (1999), who observe and model similar throw profiles for splays along normal faults in both plan view and cross section.
Overall, the splay faults are characterized by a throw maximum at the point of intersection, with the throw gradually decreasing toward their tips, and they accommodate decreases in throw along a larger main fault
with which they share an intersection line (Figure 10). Nixon et al. (2011) describe
fault
splays in strike-slip faults as synthetic interactions that also accommodate a decrease in displacement on a larger main
fault
. The
fault
splays identified in the normal-
fault
network at Milne Point accommodate similar decreases in
fault
throw (Figure 10) and have the same motion sense (i.e., downthrown on the same side) as their corresponding main faults. Hence, they are called synthetic interactions.
ABUTTING FAULTS AND TRAILING
Abutments
When a fault
network has two or more
fault
sets, the tip of one
fault
often abuts and terminates against another. This produces a Y- or T-shaped intersection (Figure 11) in which the abutting
fault
becomes pinned and can only propagate away from its abutted tip. Manighetti et al. (2001) describe these faults as single-tip restricted or half-tip restricted; however, we consider abutting faults to be separate from faults with restricted tips. This is because an abutting tip is actually pinned and cannot propagate any further, whereas a restricted-
fault
tip can still propagate at low propagation rates.
Figure 11.
Three-dimensional diagrams of
fault
planes that form abutting interactions: (A) an example of an abutting
fault
that shares a footwall block with the main
fault
at the SAG horizon; (B) an example of an abutting
fault
that shares a hanging-wall block with the main
fault
at the KUP horizon. Throws are contoured onto each
fault
plane
showing displacement transfer from the abutting
fault
to the main
fault
. See Figure 3C for the locations of these faults within the
fault
network.
There are two geometrical relationships that abutting faults form with the earlier abutted fault
(Figure 11). They can either form in the footwall (Figure 11A) or the hanging wall (Figure 11B) of the earlier
fault
sharing a footwall or hanging-wall block, respectively. Abutting faults also have the possibility of interacting and transferring displacement onto the earlier
fault
, thus allowing displacement to build up at the abutting tip (e.g., Maerten, 2000; Maerten et al., 2001). This is accommodated by local reactivation of the earlier
fault
and can cause local variations in throw adjacent to the intersection line (Figure 11). In general, the earlier
fault
will locally increase in throw where it shares a
fault
block with the abutting
fault
(Figure 11).
The abutting faults can either be single-tip abutting (Figure 12) or double-tip abutting (Figure 13). Within the fault
network at Milne Point, these are small faults with lengths less than 2000 m (6562 ft). In general, single-tip abutting faults can be divided into two groups, which are shown in Figure 12. Group 1 has minimum throws at its isolated and abutting tips, suggesting that these faults abutted at a late stage of their development. This is supported by the average
fault
lengths, which are longer than other
profile
types for single-tip abutting faults (Figure 12). Therefore,
profile
types 1A and 1B are abutting faults that have preserved their isolated
fault
throw profiles for unrestricted and single-tip restricted faults, respectively (Figure 12A, B).
Figure 12.
Normalized
fault
profiles of length/maximum length (
) against throw/maximum throw (
) for single-tip abutting faults taken from both the KUP and SAG horizons with no intersections with other faults. Five
profile
types are identified and divided into two groups. The right graph for each
profile
type is a cartoon representation. See text for discussion.
Figure 13.
Normalized
fault
profiles of length/maximum length (
) against throw/maximum throw (
) for double-tip abutting faults taken from both the KUP and SAG horizons with no intersections with other faults. Four
profile
types are identified and divided into two groups. The right graph for each
profile
type is a cartoon representation. See text for discussion.
Group 2 has shorter average lengths than Group 1 with maximum throws at the abutting tips. This indicates that the faults have grown in size while being pinned by their abutments, thus interacting with the earlier fault
.
Profile
type 2A is thought to represent a
fault
at an intermediate stage of development as it still inherits parts of a previous isolated-
fault
profile
(Figure 12C). However, types 2B and 2C are abutting faults with a restricted tip (flat top; Figure 12D) and unrestricted tip (linear; Figure 12E), respectively, that have grown and propagated since abutting another
fault
.
Double-tip-abutting faults also display two groups of fault
-throw
profile
(Figure 13). Group 1 preserves the throw
profile
of an isolated
fault
with both abutting tips recording minimum throws (Figures 13A, B). This suggests that these faults abutted at the late stages of
fault
development. Group 2 represents a slightly more developed double-tip-abutting
fault
that has accumulated throws while being pinned at each abutting tip. Hence, these show a maximum throw either at one abutting tip (Figure 13C) or at both
fault
tips (Figure 13D). The asymmetry of the throw profiles could be caused by the abutting tip with the largest throw value having abutted first.
Overall, the profiles of abutting faults can indicate the relative time of abutment during the faults’ growth and development. Using these numerous throw profiles identified for abutting faults, we show the evolution of the different stages of growth in Figure 14 for abutting faults with an unrestricted tip and a restricted tip (identified by high throw gradients). In general, an abutting fault
evolves from an isolated
fault
that has grown in length to abut and terminate at an earlier
fault
(stage 1). Therefore, early-stage abutting faults have throw minimums at both the abutting tip and isolated tip with a maximum throw near the middle of the
fault
(stage 2; Figure 14). If the abutting
fault
continues to grow, displacement can accumulate and increase at the pinned tip transferring displacement and locally reactivating the abutted
fault
(Figures 11, 14) (compare Maerten et al., 2001). They then increase in throw until a throw maximum is reached at the abutting tip and a throw minimum at the isolated tip (stages 3 and 4; Figure 14). Each stage is analogous to different stages of
fault
growth by segment linkage in the sense that the throw
profile
changes from an individual
fault
at stage 1, to a geometrically linked
fault
at stages 2 and 3, to a kinematically linked abutting
fault
at stage 4 (compare Soliva and Benedicto, 2004).
Figure 14.
Schematic diagram of throw profiles for abutting faults at different stages of development. Stage 1 is an isolated
fault
profile
. Stage 2 is an early-stage abutment with throw minima at the tips of the faults; Stage 3 is an intermediate stage with throw increasing at the abutting tip; and Stage 4 is a fully developed abutting
fault
with a maximum throw at the abutting tip. Panels (A) and (B) represent abutting faults with an unrestricted and restricted tip, respectively. (C) Three-dimensional cartoon illustrating a developing abutting
fault
shaded. The shading represents the displacement distribution of the abutting
fault
. See text for discussion.
Trailing Faults
Although Figure 9 indicates that many long faults in the fault
network are acting as isolated individual faults, increases and decreases often occur in some of their throw profiles. These usually coincide with abutments and interactions with other faults of the opposite
fault
set causing local reactivation of the pre-existing abutted-
fault
plane
(compare Figure 11). Sometimes a section of a
fault
plane
between two abutting faults is reactivated. This can be seen particularly well for longer west-northwest–trending faults the
fault
planes for which show a change in displacement between the intersections with two abutting north-northeast–trending faults (Figure 15). This indicates trailing of displacement from the abutting faults onto the original pre-existing abutted
fault
.
Figure 15.
Three-dimensional diagram of north-northeast–trending
fault
planes that abut and locally reactivate west-northwest–trending
fault
planes and form a trailing
fault
segment that links two abutting faults. The distribution of throw is contoured onto each
fault
plane
and shows increases in throw at the trailing
fault
segments. This example is taken from the SAG horizon. See Figure 3C for the locations of these faults within the
fault
network. TWT = two-way time.
For example, the west-northwest–trending fault
207-SAG, seen in Figure 16, is abutted by two north-northeast–trending faults at intersections A (
fault
240-SAG) and B (
fault
121-SAG). The two abutting faults have very similar throw values near the points of intersection (Figure 16B), whereas the segment AB of the west-northwest–trending
fault
(
fault
207-SAG) shows a marked increase in accumulated throw between the two abutting faults (Figure 16A). A reconstruction of the original throw profiles (Figure 16) indicates that the increase in throw along segment AB (35–40 m [115–131 ft]) is broadly similar to the throw values of the two abutting faults at their points of intersection (35–40 m [115–131 ft]). This suggests that the movement of the two abutting north-northeast-faults (faults 240-SAG and 121-SAG) has reactivated segment AB producing a trailing-
fault
segment. This links the two abutting faults to form a trailing
fault
. The increase in throw along the trailing-
fault
segment AB is because it shares the same kinematic motion sense (i.e., downthrown to the east) as the two abutting faults. Therefore, this may be regarded as a synthetic trailing interaction.
Figure 16.
Fault
-throw profiles showing examples of trailing
fault
interactions: (A), (B), and (C) show a synthetic trailing
fault
interaction in which two faults abut and reactivate a portion of another
fault
that shares the same motion sense. (A) Plan-view map of the
fault
interaction. In this case,
fault
207 in (B) is reactivated between intersection points A and B, increasing in throw, because of the abutting interactions of faults 121 and 240 shown in (C); (D), (E), and (F) show an example of an antithetic trailing
fault
in which faults abut and reactivate a portion of another
fault
that has the opposite-motion sense. (D) Plan-view map of the
fault
interaction. In this case,
fault
199 in (E) is reactivated between intersection points C and E, decreasing in throw, because of interactions with abutting faults 5, 249, and 72 shown in (F). The dashed lines are an estimated reconstruction of the original
fault
-throw profiles before interaction. See Figure 3C for the locations of these faults within the
fault
network.
In addition, examples of antithetic trailing interactions exist between the two fault
sets (Figure 16D, E, F). These are produced when the trailing segment does not share the same motion sense as the abutting faults. For example,
fault
199-SAG is a west-northwest–trending
fault
that is downthrown to the west, whereas the abutting faults (faults 240-SAG, 72-SAG, and 5-SAG) are all downthrown to the east (Figure 16F). As a result, marked drops occur in the throw
profile
of
fault
199-SAG at intersection point A and between intersection points B and C. The reconstructed throw
profile
of
fault
199-SAG (dashed line; Figure 16E) indicates that these decreases in throw match the throw values of the three abutting faults at the points of intersection. Furthermore, the throw profiles of the abutting faults are broadly coherent on either side of
fault
199-SAG (Figure 16E). This indicates that segment AC on
fault
199-SAG inversely reactivated and interacted with the three abutting faults producing a kinematic and geometric link between them.
Thus, two types of trailing interactions occur between different fault
sets. A synthetic trailing interaction produces a trailing
fault
with segments that have the same motion sense (Figure 16). These cause an increase in throw along the trailing segment. Whereas, an antithetic trailing interaction involves abutting faults that have the opposite motion sense to the trailing segment causing inversion of the reactivated trailing segment and a decrease in throw (Figure 16).
In summary, the trailing fault
interactions involve faults sets that are at a high angle (i.e., orthogonal) to one another. As a result, the pre-existing
fault
is only locally reactivated and acts as a transfer
fault
between the abutting faults, with the reactivated segment being analogous to a linking
fault
that may breach a relay ramp between two parallel
fault
segments. This provides a kinematic link as well as a geometric link between the two abutting faults (Figure 15). Maerten (2000) describes similar trailing interactions between faults from the Chimney Rock
fault
array in central Utah. These include the Bluebell
fault
which has an increase in displacement on a segment between two abutting faults. The segment also has slickensides with different pitch orientations to the rest of the
fault
, indicating reactivation and a kinematic link between the two abutting faults (Maerten, 2000).
EFFECTS OF PRE-EXISTING STRUCTURES AT DEPTH
Analysis of the fault
network at Milne Point has shown that the deeper SAG horizon has higher
fault
densities and accommodates larger strains in comparison with the KUP horizon. These changes are attributed to an increase in the number and size of the earlier west-northwest–trending faults as the later north-northeast–trending faults have similar density and strain values at each horizon (Table 1). We have suggested that the west-northwest–trending faults are influenced by the pre-existing underlying structural grain that trends northwest–southeast and bounds broad-scale horst and graben structures. This affects the largest faults within the network, which become steeper with depth (Figure 2), increase in throw in the SAG horizon (Figure 6), and are often oriented northwest–southeast matching the underlying structural grain (Figure 3).
To investigate this influence further, Figure 17 shows the throw profiles for a group of four large west-northwest–trending faults (faults i, ii, iii, and iv) at the KUP and SAG horizons. All of these faults downthrow to the southwest, including smaller splay faults and breach faults in relays. In plan view, only faults ii (pink) and iii (orange) are geometrically linked, and these share relay ramps with fault
i (blue) to the northwest and
fault
iv (turquoise) to the southeast (Figure 14). This suggests that these are interacting
fault
segments at different stages of linkage.
Figure 17.
Fault
-throw profiles from (A) the KUP horizon and (B) the SAG horizon of four large west-northwest–trending faults (Faults i, ii, iii, and iv) that share relay ramps and interact with each other with some associated splay faults. The plots show variations in throw for each
fault
along distance X, which increases to the east, indicating an increase in interaction, linkage, and a clockwise rotation with depth. To the right are plan-view maps of the interacting faults. See Figure 3C for the locations of these faults within the
fault
network.
A comparison of the plan-view geometries of the four fault
segments at each horizon indicates that faults i, iii, and iv are re-oriented further clockwise in the deeper SAG horizon and become aligned with the northwest–southeast structural grain (Figure 17). In addition, the relay ramps between each
fault
segment decrease from ∼750 m (∼2461 ft) at the shallower KUP horizon to ∼250 m (∼820 ft) at the deeper SAG horizon (Figure 17). This indicates that the
fault
segments are becoming more geometrically linked with depth. The geometrical relationship between
fault
iii and the other faults further supports this. In the KUP horizon,
fault
iii (orange) is linked to
fault
ii (pink) by a breach
fault
between intersection points A and B but becomes a throughgoing
fault
in the SAG horizon that spans the distance between faults i (blue) and iv (turquoise), whereas
fault
ii becomes a splay
fault
at intersection point E (Figure 17). Furthermore, in the SAG horizon a breach
fault
forms at intersection point D in the relay between faults i and iii (Figure 17B).
The throw profiles of each fault
indicate that they accumulate more throw with depth and become more developed. For example, in the KUP horizon
fault
iii (orange) has an incongruent
fault
profile
with a splay
fault
(intersection point C) and a breach
fault
(intersection point B) causing large step-like variations in its throw
profile
(Figure 17A). This matches the splay faults described in Figure 10. However, in the deeper SAG horizon,
fault
iii has a more congruent throw
profile
along its strike (Figure 17B), indicating that it has a coherent
fault
plane
. The throw profiles also indicate that the faults are more kinematically linked in the SAG horizon. For example, the throw
profile
of
fault
iv (turquoise) has a symmetrical throw
profile
at the KUP horizon (Figure 17A) but an asymmetrical throw
profile
in the SAG horizon with a very steep throw gradient at its northwest tip, indicating interaction with
fault
iii (Figure 17B).
Overall, Figure 17 further supports that the development of west-northwest–trending faults was influenced by deeper pre-existing structures. The effect of these underlying structures is characterized by several changes with depth:
1. Clockwise rotation of west-northwest–trending faults with depth as they align themselves with the underlying structural grain (Figure 17);
2. Increase in fault
dip of larger west-northwest–trending faults (Figures 2, 5);
3. Increase in throw and strain localization onto west-northwest–trending faults (Table 1);
4. Better linkage between large west-northwest–trending faults (Figure 17).
Reactivation of pre-existing structures can often produce and affect new fault
sets in the overlying stratigraphy (Bailey et al., 2005; Frankowicz and McClay, 2010). For example, Bailey et al. (2005) see similar changes in the spatial development of two normal-
fault
sets in the East Pennines Coalfield (United Kingdom) caused by reactivation of underlying basement faults causing strain localization onto one
fault
set.
As the west-northwest–trending faults are obliquely oriented to the northwest-trending underlying structural grain, it is possible that these were driven by left-lateral transtension along the previous structures. This is supported by the strain orientation of the west-northwest-trending fault
set, the steepening of the larger faults with depth, and the splaying and rotation of faults into the northwest-trending structural grain, which resembles the organization of faults in upward-splaying flower structures and bifurcating up-tips above left-lateral strike-slip faults (compare McGrath and Davison, 1995; Kim et al., 2004). Furthermore, Giba et al. (2012) show similar characteristics for an obliquely reactivated normal
fault
in the Taranaki Basin, New Zealand, with
fault
splays propagating upward from the reactivated
fault
and rotating to align with the regional stress field.
SUMMARY AND CONCLUSIONS
A normal-fault
network from onshore Milne Point, Alaska has been analyzed using 3-D seismic reflection data. The network comprises north-northeast–trending and west-northwest–trending
fault
sets, which were analyzed at two stratigraphic horizons: the Kuparuk River and the Sag River formations, which are both hydrocarbon reservoir formations. Analysis shows that the following.
1. Both fault
orientations are different generations of faulting. We suggest a Jurassic age for the west-northwest–trending faults and a Cenozoic age for the north-northeast–trending faults. It is also probable that there has been later reactivation of parts of the west-northwest–trending faults both during formation of the north-northeast–trending faults and subsequent stress.
2. The north-northeast–trending faults generally dip to the southeast producing a strain with a maximum extension orientation of ∼N103°E. These are consistently developed in both horizons with similar fault
densities,
fault
sizes, and strains. In contrast, the majority of west-northwest–trending faults dip to the southwest and have a strain tensor with a maximum extension orientation of ∼N30°E. They show variation with depth increasing in size, number, and density, hence accommodating greater strains. The overall strain accommodated by the
fault
network is a superposition of the two strain tensors.
Mapping variations in throw along fault
planes and around intersections lines (branch lines) allowed the identification of numerous interactions within the
fault
network. Faults can be divided into isolated, abutting, and splaying faults. In general, abutments and cross-cutting faults involve faults from different
fault
sets, whereas splay faults are from the same
fault
set. The different interactions can be characterized by their throw distributions along
fault
planes (summarized in Table 3):
3. Isolated faults produce symmetrical or asymmetrical fault
profiles depending on the degree of restriction at
fault
tips, which can be identified by high-throw gradient at the restricted tip.
4. Splay faults have a throw maximum at the line of intersection and steadily decrease in throw toward their tip. They accommodate decreases in throw along a larger main fault
.
5. Abutting faults form a range of throw profiles depending on the timing of abutment during fault
development. There are two main groups: early-stage abutting faults with throw minima at the abutting and isolated tips and late-stage abutting faults that have grown and accumulated a throw maximum at the abutting tip.
Developing faults within the network also interact with pre-existing structures. These can be oriented at either a high angle or a low angle to each other and have different characteristics (summarized in Table 3):
6. Abutting faults form at high angles to the pre-existing fault
. As a result, when they grow and accumulate displacement after initial abutment, they locally reactivate the pre-existing
fault
through the transfer of displacement. This can produce a trailing
fault
that links two abutting faults through the reactivated segment of the pre-existing
fault
. The motion sense of the trailing
fault
can either be synthetic or antithetic to the reactivated pre-existing
fault
producing an increase or decrease in throw, respectively.
7. The west-northwest–trending faults have interacted with the similarly oriented underlying northwest–southeast structural grain. This has resulted in reactivation of these pre-existing structures and has influenced the development of the west-northwest–trending faults. These influences are characterized by increases in dip and throw on several faults, strain localization, clockwise rotation of faults, and an increase in linkage maturity.
Overall, this paper provides a robust example of a network analysis applied to a normal-fault
network with cross-cutting relationships and multiple generations of faulting. Using throw distributions has identified and characterized numerous
fault
interactions as well as the influence of pre-existing structures on network development. It has also characterized different types of reactivation (i.e., local and regional) within
fault
networks. Identification of such interactions is important as it furthers our understanding of the kinematic behavior of faults within a
fault
network.
The geometry of the two generations of faulting means that the two reservoir horizons are at least geometrically compartmentalized by the faults. Therefore, this study has important implications for hydrocarbon exploration in northern Alaska. The identification of areas of local reactivation along fault
planes, caused by abutting faults or trailing faults, could be particularly useful as this may produce an increased amount of
fault
damage around intersection lines. Small faults and fractures associated with such damage could provide a fluid pathway across a previously sealing
fault
plane
. Hence, this could affect reservoir connectivity and/or provide communication between compartments. Therefore, being able to identify such interactions may influence the location of boreholes/wells into a compartmentalized reservoir.
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AUTHORS
Casey Nixon received his B.Sc. Honors degree in geology from the University of Edinburgh and his Ph.D. in structural geology from the University of Southampton. He is presently working as a post-doctoral research and teaching fellow at the University of Southampton. His current research interests are fault
and fracture network analysis, basin dynamics, active rifting, and seismic hazard assessment.
David Sanderson is professor of tectonics and geomechanics at the University of Southampton and a consultant to BP. His main research interests are faulting, fracturing, and fluid flow, with applications in the hydrocarbon, mineral, and engineering industries. He has published over 130 scientific articles and a recent book on distinct element modeling of deformation and fluid flow in fractured rock.
Stephen Dee is a senior structural geologist at BP, working in the Integrated Subsurface Description and Modelling team and consulting on structural and geomechanics projects. He has a Ph.D. from the University of Southampton and specializes in fault
and fracture interpretation, geomechanical modeling, and seal analysis.
Jonathan Bull is professor of geophysics at the University of Southampton with main research interests in marine seismology and structural geology. He has published more than 70 peer-reviewed publications and has developed and licensed technology related to 3-D marine imaging.
Robert Humphreys received his M.A. degree in natural sciences from Cambridge University, and Ph.D. in structural geology from Cardiff University (UK). He has worked in exploration, production and structural geology technology teams for BP and is currently assisting Alaskan North Slope development. One of his particular interests is developing structural guidelines for seismic interpretation.
Mark Swanson received his M.S. degree in geology from the University of Toledo, Ohio, and has been further trained in geophysics. He has worked in both exploration and production and is presently working on the Milne Point Reservoir Management Team trying to better understand the compartmentalization of this highly faulted field.
ACKNOWLEDGEMENTS
The research presented in this paper was completed as part of a Ph.D. by C. W. Nixon supported financially from a NERC case studentship (NE/H524922/1) with BP. We thank the Milne Point Reservoir Management Team, BP Exploration (Alaska) for provision of the 3-D seismic reflection data used in this article. In particular, we thank Kip Cerveny (team leader, Milne Point Reservoir Management Team) and Rebecca Bailey (geologist, Milne Point) for their input and support. We would also like to thank the Integrated Subsurface Description and Modelling team, BP Exploration (Sunbury) for their logistical and technical support throughout.
The AAPG Editor thanks Senior Associate Editor Richard H. Groshong and the following reviewers for their work on this paper: Daniel Ciulavu, J. Steven Davis, and an anonymous reviewer.
EDITOR’S NOTE
Color versions of Figures 1, 14, and 16 may be seen in the online Open Access version of this paper.