EXTENSIONAL FAULT
GEOMETRY AND EVOLUTION
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Predict flow patterns and
communication
Fault compartments in the
Sleipner field, Norwegian
North Sea
Different oil-water contacts
Ottesen Ellevset et al. (1998)
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Fault Properties
• Fault Length (strike)
• Fault Throw : Height (Dip)
• Fault Segmentation and Linkage.
• Fault Zone Geometry (individual fault scale)
• Fault Array Geometry (area / basin scale)
• Fault Activity
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Fault Shape & Length:Displacement
Properties
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Normal fault geometry in 3D
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West Africa: lower fault tips & conjugate faults
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Low throw normal faults
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SINGLE FAULTS
Skua field in Timor Sea, NW Australia
• Footwall high
• Systematic heave
polygon shape
– Taper from zero width
at fault tips (low or zero
fault displacement) to a
maximum width near
the fault center.
• Maximum uplift near
center of fault.
Osborne
(1990)
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SINGLE FAULTS
Beatrice Field, N. Sea
Map view
Footwall anticline:
• Cross-section A-A’
shows form of
footwall anticline.
• Maximum uplift near
center of fault.
Transverse section through hanging wall
Hangingwall syncline:
• Maximum structural
low near center of
fault.
Schlische, 1995
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Structure contours around an isolated normal fault
1km
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SINGLE FAULTS: Displacement
Throw and Separation.
Single fault from surface exposure in central Oregon
• Maximum
separation
(throw) near
center of fault.
• Gradual taper of
separation
profile from a
maximum
separation to
zero at the fault
tips.
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SINGLE FAULTS: Displacement
Contours on fault surface of separation measured from 5
horizons intersecting the fault.
Separation varies
across fault surface like
that on an individual
horizon: the maximum
separation occurs near
the center of the fault.
Gulf Coast normal fault
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SINGLE FAULTS: Displacement
• Contours of throw projected onto fault surface.
• Elliptical fault shape most common for buried faults.
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SINGLE FAULTS: Displacement
• Similar
displacement
profiles along dip
and strike profiles.
• Homogeneous,
isotropic material
Higgs and Williams, 1987
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Isolated
North Sea
fault
Throw increases
with depth but only
upper part of fault
mapped
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b
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Fault surface topography
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Fault dimensions
10
Dip dimension (km)
1:1
3:1
1
Derbyshire coalfield
Gulf Coast
Timor Sea
North Sea
0.1
0.1
1
10
Strike dimension (km)
100
From Nicol et al. (1996)
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Fault dimensions
• Aspect ratios average 2:1 but variable
• Linear throw gradients on isolated faults
• Non-linear on restricted faults
• Steeper gradients near overlapping tips
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Fault aspect ratios
Location
Average aspect ratio
Derbyshire coalmines UK
2.3
Timor Sea
2.2
Gulf Coast, USA
1.6
North Sea
2.4
From Nicol et al. (1996)
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Deformation around a fault
• Reverse ‘drag’ profiles generate:
– footwall uplift
– hangingwall subsidence
• Relationship of structure contours to fault vary
with slip direction
• Reverse drag does not imply a listric fault
• Earthquake related elastic strains relax to
permanent bed deformation
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Reverse Drag
Cross-section
Schlische, 1995
• Displacement (structural relief) decreases
asymptotically away from fault in cross-section.
• Footwall high and Hangingwall low.
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Earthquake induced deformation
Strike-slip
< Imperial Valley earthquake 1940
Slip = e-3.5dist – 0.03dist
Strike-slip illustrates offsets
Normal
Borah Peak earthquake 1983 >
Lost River fault, Idaho
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Deformation
around a fault
Radar interferometry image
of ground deformation
induced by Hector Mine
earthquake
Peltzer et al.
http://www-radar.jpl.nasa.gov/sect323/InSar4crust/HME/
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Fault drag profiles
Empirical relationships for:
Single event:
Slip = e-3.5dist – 0.03dist
Multiple event steady state:
Slip = e-5.5dist – 0.004dist
From Gibson et al. (1989)
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Hangingwall & footwall displacement
• Same in hangingwall and footwall for blind faults
• Greater hangingwall subsidence than footwall uplift
for synsedimentary faults
• Percentage contribution of hangingwall
displacement (HW) is given by:
HW = 110 – 2q/3
Where q is fault dip and dip exceeds 30 degrees
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Patterns around synsedimentary faults
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Patterns around synsedimentary faults:
a local example
The Craven fault zone
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Normal drag in footwall to a
6m throw normal fault
Normal drag profiles often
highly localised around fault
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SINGLE FAULTS
• Displacement Summary:
– Maximum separation near center of fault.
– Uniform displacement contour distribution on fault
surface.
– Multiple horizons cut by a single fault:
• greater separation on horizons near center of fault.
• similar separation profile shape.
– In section, the fault drag decreases gradually away
from fault trace.
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MULTIPLE FAULTS
En-echelon Normal Faults.
Schlische, 1995
Faults interact and influence deformation of adjacent faults.
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MULTIPLE FAULTS
Peacock and Sanderson (1991)
Fault overlap in map view and section view.
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MULTIPLE FAULTS
Flamborough Head, UK
Mechanical Stratigraphy:
outcrop
• Fault tip overlap in
cross-section.
Peacock and Zhang, 1993
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Vertical
Segmentation
RHOB-NPHI
1
Seismic
Interpretation.
2
H2b
Faults nucleate
in more brittle
sandstones.
3
4b
From Rives & Benedicto 2000
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Different styles of transfer zones (Morley 1990)
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Relay
Ramps
Shaded
Relief
Time
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Relay ramps – seismic mapping
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Relay ramp
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Geometric Coherence
Structure
map
• Multiple faults act as a composite zone.
• Displacement on composite faults creates
broad footwall uplift.
• Separate fault segments compartmentalize
the trap.
relay
Fault traces
relay
Relays mechanically interconnect to form
longer fault.
Strike Projection of Horizon Throw
• Systematic throw variation.
• Composite throw summed
like a single fault.
Elevation (meters)
2000
1600
Broad Footwall High ("Trap")
splays
graben
splay
hangingwall cut-off
1200
0
relay ramps
10
Length (km)
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30
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Fault shape & linkage
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a
Displacement
pattern – correlated
single fault
2D-seismic data set
– Middle East
b
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Displacement
pattern –
correlated
multiple faults
2D-seismic data
set – Middle East
a
b
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Displacement patterns on
overlapping faults
from Childs et al. (1995)
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Geometric
coherence
A
B
Relay ramp structure and displacement
patterns on overlapping faults. Summed
throws give a coherent pattern.
From Huggins et al. (1995)
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MULTIPLE FAULTS
Segmented Fault Array Nook
Colliery, Lancashire
Throw profiles for main fault segments
Summed throws on fault segments
•
•
•
•
Low gradients at fault zone terminations.
Large gradients in fault overlap.
Summed profile resembles single fault.
Maximum throw near composite fault
center.
Walsh and Watterson (1990)
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Hard linked faults (Krantz 1988)
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Exercise
• Longbranch fault interpretation
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Displacement Variation
Fault Throw Distributions: Northern N. Sea.
Childs et al., 1995
FAULT 1
FAULT 2
Strike
Projection
of Throw
• Asymmetric throw distribution.
• Throw gradient greatest in region of
overlap.
Fault traces: Map
Separation Diagram
FAULT 2
FAULT 1
• Possible breached relay at arrow: hard link.
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Composite Fault Throw
• Symmetric throw distribution.
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Geometric Coherence
Cross-Section
Splay
Distinct anomaly in throw contours
without splay.
Walsh and Watterson, 1991
Crosssection tie
Splay excluded
Splay included
Strike
Projection
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MULTIPLE FAULTS:
Fault laterally
restricted
Presence of
deep shaded
fault restricts
propagation of
contoured fault.
Nicol et al., 1995
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MULTIPLE FAULTS
Map View
Three dimensional geometry
Cross-section
• Multiple fault segments
shallow in the section map as
a single fault segment at
depth.
From Bouvier, 1989
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MULTIPLE FAULTS
• Overlapping en-echelon fault segments comprise larger fault
zone.
• Overlap between segments is small.
• Aggregate slip like that for an isolated fault:
Geometric Coherence
– Maximum slip near center of fault zone.
• Deformation transferred between fault segment across relay
ramp.
• Anomalous patterns in slip indicate perturbations in
deformation, which can indicate closely-spaced faults or fault
connections.
• Echelon faults at one level may link as a single fault at another.
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Geometric coherence
From Walsh et al. (2003)
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Fault
displacement
profiles
From Nicol et al. (1996)
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Restricted faults
10
Dip dimension (km)
1.3
2.5
1
Vertically restricted
Laterally restricted
Unrestricted
0.1
0.1
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1
Strike dimension (km)
100
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Hard-linked splays
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Conjugate faults: field & seismic
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Conjugate faults
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Allan diagrams
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