Faults Fractures
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Transcript of Faults Fractures
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Fractures & Faults
Jan Kees Blom
november 2011
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Faults, joints and fluid flow
Types of fractures Fracture growth and termination Origin of fractures Fractures and fluid flow Deformation bands
Fault (zone) elements Fault shape and slip distribution Fault rocks Faults growth Faults in limestone Fault scaling laws
Faults and fluid flow
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Relevance of faults and fractures
Faults offset layering
(important for mining and water / petroleum industry)
Fractures and faults form mechanical anisotropies
(important for civil engineers, engineering geologists, mining andpetroleum industry)
Fractures and faults can form conduit or baffle for fluidflow
(important for engineering geologists, mining geology, petroleum
geology and petroleum engineering)
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Relevance of fractures and faults 2
Dense networks of sealing faults can makehydrocarbon reservoirs uneconomic
Vertically continuous conductive fracture systems can
influence mechanisms and rate by which oil isrecovered (e.g. not expansion of oil, but gravitydrainage)
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Fractures
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Brittle deformation: Fractures
Fracture is a collective term for discontinuities in rocks Shear fracture vs extension fracture
Here, we only consider extension or opening-modefractures
Also termedjointsor extensionalfractures Conditions favouring tensile fracturing:
Low differential stress
Shallow depth
High fluid pressure
High cohesion
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Orthogonal fracture pattern
View from above on top a bedding surface in
limestone, Lilstock, UK
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Tensile fracture Hybrid fracture
(combination)Shear fracture
discrete break
no evidence for shearing
enhance flow (if not cemented)
Kf = a3
evidence for shearing
offset markers
enhance/impede flow
evidence for shearing
offset markers
enhance/impede flow
Basic fracture types
Can you draw these on a Mohr-Coulomb failure plot ofshear stress versus normal stress?
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Fracture type vs P,T conditions
Extension fractures only in upper part of crust
(low P, T)
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Fractures and failure criteria
a. tensile fractures
b. hybrid fractures
c. shear fractures
d. semi-ductileshear bands
e. plasticdeformation
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Griffith
Small defectsweaken rocks
Why not incompressive
regime?
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Shear fracture growth
Shear fractures do not propagate in their own plane, but
form by connecting and forming new extension fractures attheir tips
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Joint surface structures Plumose structures and hesitation
marks See progressive downwards
propagation from top layer, andending at lower layer
One fracture influences / triggers the
next
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Plumose structures
Common in fine-grainedrocks due to formation
of hackles
Hackles form when fastdeformation causes stressadjustments
Edge of bed has different stressorientation => hackle fringes
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Fracture termination
Different structures may occur at tip of fracture,dependent open stresses and material
If fracture continues to grow, these structures mayalso be found along the middle of the fracture:damage zone.
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Origin of tensile fractures
Orientation #sets areal distribution vertical distr. Cooling -- -- 3 homogen. Layer
Regional stress // h-max 1 homogen. layer / lith
Uplift // h-max 1-2 homogen. layer / lith
Fault-related 60oto faults >1 heterogen. layer / lith
Fold-related Variable >1 heterogen. lith / struct
Remember: fractures are embedded in sediments.Mechanical stratigraphy (lithology, rock stiffness,
distribution and size of weak shales etc.) is very important
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Cooling joints
Form regularhexagons.Hexagonshape relatedto way heatdiffuses duringcooling.
Common inbasalts
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Cooling joints
Can give spectacular landscapes
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Regionally consistent joints
Example fromAppalachians, see noteson in-situ stress
Consistent pattern ofparallel joint zones
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RegionallyconsistentjointsArches National park,
Utah
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Fault-related fractures
Clustered in fault vicinity, dependent on lithology,
rate of faulting, etc.
Local and large scale variations occur due togeometrical irregularities and stress release on fault
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Fracture clusters around faults
Peaks of high fracture density near fault(zone)s In-between faults, no or few fractures
0
5
10
15
20
1400 1500 1600 1700 1800 1900 2000 2100
Along Hole depth (m)
#Fractu
res/10m
Fracture
density
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Large-scale joint pattern Weak faults can carry no or limited shear stress. Thus
principal stresses (near) perpendicular to faults.Therefore joints often curve in fault vicinity, becomingparallel or perpendicular to fault
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Asymmetry across fault
In fault tip region, joint patterns on either side of the
fault can be very different, due to differences in stressstate on either side
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Fold-related fractures
Fold-related fracturing is favoured by:
High bed curvature
Large bed thickness
Fracture porosity is proportional to strain
Caveat: Vertical distribution of slip planes (e.g. shales) play a role
Lateral terminations of potential slip planes may lead to fractureclusters
Consider stratigraphy and factors such as stylolites
Well documented examples are rare
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Fold-related fractures
Theoretical model, actually (very) hard to find in nature
Joints parallel or perpendicular to fold axis. Why? Shear fractures oblique to fold axis. Why?
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Oil Mountain anticline
Fracture prediction can be done using
various alternative folding mechanisms
Bending
Flexural slip
Strain
Etc. Calibrate model by comparing
observations to prediction
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Fractures & Fluid Flow
Fracture permeability = aperture3
Most often aperture < 1mm, unless substantial leaching Aperture very hard to reliably estimate:
Thin sections of partially cemented fractures
Cores which were cemented in-site by human
intervention after mud-losses
Derived from well-test data
Rough estimates of effective permeability (requiresknowledge matrix K and fracture spacing)
Fracture spacing depends on strain
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Fracture spacing depends on strainand bed thickness
Low strain: irregular network with great
variation in spacings High strain and well-bedded rocks:
saturated, regular network with nearlyconstant spacings related to bedthickness
Bai et al, 2000
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Fractures and layering
Fracture pattern can vary from one layer to the next,
especially if the brittle layers are separated by thick,weaker layers. Usually, the fracture systems in thevarious layers have the same symmetry axis
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Naturally fractured reservoirs
Reservoir whereby natural fractures:
Provide reservoir porosity & permeability
Provide only reservoir permeability
Assist permeability in an already producible reservoir
Enhances matrix porosity & permeability
Create baffles to fluid flow and reservoir compartments
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Examples of naturally fractured
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Examples of naturally fracturedreservoirs
Carbonates
Sandstones
Granite
Cherts
Siltstones
Cretaceous: Oman, Qatar, Abu Dhabi
Oligo-Miocene: Iran, Iraq
Venezuela: Cretaceous of Lake Maracaibo
North Sea: Cretaceous chalks
Netherlands: Zechstein
Rotliegendes in NW Europe
North Sea: Horizon field (Broad 14s)
Venezuela (Mara La Paz)
Egypt (Zeit Bay)
Texas (Amarillo)
California (Monterey, Santa Maria)
Texas (Permian Spraberry)
Often very high production rates (up to 100.000 b/d), but also very
difficult to manage in terms of producing water or gas instead of oil, andin terms of leaving as little oil as possible behind
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Oil recovery mechanisms
Imbibition
Matrix sucks up waterspontaneously,expelling oil
Gas oil gravity drainage
Gas-filled fracturessurrounding oil-filledmatrix blocks. Densitydifference leads topressure differential.
Pressure differentialdrives out the oil
WOC in fractures
Matrix
layers
Pressure
Depth
z
ZGOC
gas gradient
oil gradient
fractures
matrix
GOC in fractures
GOC in matrix
P
Z1
Fragmentation processes in porous
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Fragmentation processes in porousrocks
Depends on nature of rock at time of deformation as well as
on rate of faulting, pressure and temperature (I.e. a functionof depth)
E.g in sandstones get rolling / translation of grains at shallowdepths and breaking / crushing of grains at greater depth.
This leads to so-called deformation bands
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Deformation bands:tectonic settings
Deformation bands canbe found in nearly alltectonic settings
Fossen et al, 2007
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Types of deformationbands
Based on deformation mechanism (right)
Based on kinematics (below)
Fossen et al, 2007
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Deformation bands in sandstone
Compaction bands overprint disaggregation bands
in Navajo Sst, Utah Note diminishment of porosity and dissolution and
fracturing in deformation band thin section
Fossen et al, 2007
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Influence on fluid flow
Deformation bands may lead to a
reduction of permeability of severalorder of magnitude, but may alsoform conduits for fluid flow (above)
Fossen et al, 2007
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Type of deformation band
Dependant on depth and clay content
Faults
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Faults
Strike slip fault in Kaynasli, Turkey, 2000
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Effects of faults in layering Faults offset layering in cross section and in map view
Faults form discontinuities in contours of layer depth
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Faults and the Mohr circle
a. tensile fractures
b. hybrid faults
c. faults
d. semi-ductileshear zones
e. plasticdeformation
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Fault types Normal
Strike slip Reverse
End members of the total
spectrum of oblique faults Not necessarily flat
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Gullfaks Faults curved in
map view,
flat in section
Horst, graben, half grabenSynthetic and antithetic faults
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Fault anatomy
Main displacement along
fault core Minor displacement
along damage and dragzones
Cores are mostly (but
not always) baffles toflow
Damage zones aremostly (but not always)conduits for fluid flow
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Fault localization
Fault core: narrow zone in which bulk of offset is accommodated.
Rock is crushed (fine-grained gouge or mylonite) or smeared-out Fault damage zone: zone of secondary deformation (e.g small
faults, fractures and veins, folds etc.).
Fault zone character in terms of core and damage zone dependson pre-faulting rock character and on T, p and speed of fault slip
((a)seismic)
Caine et al. 1996
Fault damage zone: shapes
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Fault damage zone: shapes
Damage zone widthand character can
change dramaticallyalong the fault.
Major factors are faultgeometry (bends,splays) and lithology
(brittle versus ductilelayers)
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Damage zone
Wide damage zone due to fold bend, Sinai
Fault rocks
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Fault rocks
Fault displacement variations
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Fault displacement variations
Fault displacement (slip orthrow) is NOT constant.
Otherwise faults in Holland wouldre-appear in New Zealand.
Rocks are compressible. Faultdisplacement varies from somemaximum near centre of the faultsurface to zero at the fault tip-line
Fault shape & slip distributions
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Fault shape & slip distributions
Many single isolated faults have an elliptical tip-lineand a slip maximum somewhere near the centre of thefault
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Mechanics of faults and fractures
Both faults and fractures are
discontinuities in the rock. Their kinematics are described by
the sense of wall rock movementrelative to the plane and the tipline(the line where the discontinuityends)
What modes do we find at a fault?
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Observing faults in 3D
Best data come from coal mines in UK and open-cast
pits in Germany
3D seismic data also provides unique opportunity toobserve entire fault planes at km-scale
However, seismic cannot map the near-tip region as
lowest reflector (layer) offsets visible are around 10-20m
Sideviewonto fault
plane
Apparent tip line Real tip line
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Fault tip porous sandstones
Deformation bands are formed in the process zone ahead
of the fault tip, and may be several 100s of meters long Should be considered when modeling fluid flow in faulted
reservoirs
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Example discontinuous faults
Fault is not one singleplane, but series ofobliquely arranged sub-
faults or fault segments Oblique arrangement is
called en echelon
Each segment has its
own slip distribution
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Effects along discontinuous faults
Many faults are not one single plane. Many faults are
discontinuous or segmented
Discontinuities influence slip distribution
Total slip along all segments is similar to that of singlefault
i l l i
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Displacement accumulation
Large displacementsform by multiple smallevents (earthquakes)
Relation Displacementvs Fault Length followslogaritmic scale
F l li k
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Fault linkage
As faults grow, they may link up
with other faults Where they do not line up, relay
ramps may form, which eventuallybreak up.
R l
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Relay ramps
They form an intermediatestage in the growth of largefault, but may be preserved.
Viking
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VikingGraben
Normalfaults oneast flank of
VikingGraben
Note variousfault linkage
L th di l t ti
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Length displacement ratio
Displacement influenced by bedding
Rule of thumb: L = 25-100*D
F lt li
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Fault scaling
Long faults generally havegreater offsets
There are only few largefaults (either in terms oflength or offset)
There are numerous smallfaults
It can be important topredict the density and
occurrence of small-scalefaults
F t l M d l f f lti
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Fractal Model for faulting
Fractal fragmentation model: scale independence
Ratio between fault slip and length is constant overmany length scales
S ll f lt di ti
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Small fault prediction
Use observations on seismic(km-scale faults with > 10 moffset) to predict small-scalefaults (cm offsets)
Rounded curves caused bylimits of resolution: seismiccan hardly see faults withoffsets < 10 m, therefore toofew are observed, giving
down-ward curve
Li t f lt 50 mm
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Limestone faults
There are often no
grains to roll, translate,rotate, break or crush.
These faults form initiallyby opening-modefracturing, followed by
slip along pressuresolution surfaces(stylolites)
Thus are theyconduits??
50 mm N
tail tail
(e) initiation of pull-aparts
50 mm
tail
tail
width of
pull-apart
50 mm
N
(d) slip along solution seams
slip surfacesolution seamcalcite fill
(f) widening of pull-aparts
E ample of limestone fa lt
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Example of limestone fault
Fault tip region near Lilstock, UK.
Faults: offsets & fluid flow
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Offset alone influences fluid flow across the fault
This purely geometrical juxtaposition effect is always present,and simple to quantify (how?).
Juxtaposition sealing can be modified further by flow properties ofthe fault (zone) itself or presence of relay ramps
Faults: localization & fluid flow
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Some faults have coresonly, others consistprimarily of damage zones.
Most faults are combinedconduit - barrier systems:they are a barrier foracross-fault flow, andconduit for along-fault flow
Modified from Caine et al. 1996
Faults: core gouge and fluid flow
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Mechanical processes (e.g. breaking of the rock, injection ofclay etc.) cause changes in particle size
This causes a change in permeability
This causes a change in capillary properties
Antonellini 1995
Fault sealing:
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Fault sealing:for clean sands
Dependson depthof burialand initialmatrixporosity
Fault sealing:
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Fault sealing:fault gouge
Fault gouge is crushedand ground-up rockproduced by frictionbetween the two sides
when a fault moves. Very fine grained, not very
permeable
Other fault seals
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Other fault seals
In alternating layers
of sand and shale,the lithologies mayget mechanicallymixed up inside thefault core
Type of seal will varyalong fault
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Juxtaposition seal/diagram
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Juxtaposition seal/diagram
In a triangle diagram, we can illustrate the
juxtaposition of different rocktypes across the fault. Areas of sand-sand and sand-clay contact can easily
be found
SGR values can also be added (right, for Brent group)
Questions?
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Questions?
Types of fracturesFracture growth and termination Chapter 7
Origin of fracturesFractures and fluid flowDeformation bands
Fault (zone) elementsFault shape and slip distribution
Fault rocksFaults growth Chapter 8Faults in limestoneFault scaling lawsFaults and fluid flow
Next week: Folds & Contractional regimes
Chapters 11, 12 & 16