Faults Fractures

8/10/2019 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

Faults Fractures

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