1. OPEN FRACTURES 2. FAULTS 3. MINERAL-FILLED FRACTURE 4. VUGGY FRACTURES

Week 18 Flashcards by Elle Kinsman

How do shear fractures form?

Mechanical growth overcome
Sub-critical crack growth
= reactivates/’reshears’ weaknesses at lower differential stress than original rock fracture toughness

Mohr diagrams

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Primary and secondary shear fractures

R
R’
P

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Orientation of σ1 and σ3 in extensional vs compressional regime

Extensional:
σ1 = vertical
σ3 = horizontal

Compressional:
σ1 = horizontal
σ3 = vertical

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Tensile joint =

vein

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Slip/shear joint =

fault

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How does the mode of faulting change with depth, vertically in the same system?

Shallow = tensile fractures

Intermediate = ramp flat geometries

Deeper = shear failures

i. e. as you approach the surface, normal faults steepen upwards to vertical tensile fractures
e. g. Iceland Rift System

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How does fault rock type change with depth?

P/T changes = different rocks

Gouge - cataclasite - mylonite - mylonitic gneiss

15km = boundary between ductile processes (mylonites) and brittle processes (cataclasites)

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Crustal strength profile with increasing depth

0-~15km
Shear strength linearly increases
P-dependent frictional faulting
Discontinuous deformation

~15km onwards
Shear strength (non-linearly) decreases
T-dependent viscous flow mechanisms
Continuous deformation

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What does lithology control?

FRACTURING

2. FRACTURE DENSITY (FD)

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How does lithology control fracturing?

e.g. coal = fine-scale cleats

Although high variability for any given rock type

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How does lithology control fracture density?

Coal = ~0.05MPam^(1/2)

Chert

Dolomite

Silica cement

Sandstone

Calcite cement

Limestone

Granite (?!) = 1-3MPam^(1/2)

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What else affects fracture density?

Porosity/composition/grain size

e. g. Carbonates
- large grain size and porosity = high FD
- high % dolomite = high FD

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What controls fracture height?

Bedding

Thick bed:
= decrease fracture density
= increase fracture spacing

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Types of fractures

OPEN FRACTURES
FAULTS
MINERAL-FILLED FRACTURE
VUGGY FRACTURES

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Conductive fault =

open

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Faults on folds

Outer arc extension = normal faults

Neutral surface

Inner arc compression = thrust/reverse

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Fractures on plunging folds

Fracture distribution varies along fold hinge

Complex strain patterns

OPEN FRACTURES

No mineral fill, usually fluid saturated

Wall mismatch - asperities propping open

High flow // to fracture (flow ∝ aperture^3)

Negligible effect on flow perpendicular to fracture

FAULTS

a) Gouge-filled fractures including deformation bands

b) Slickenside fracture

MINERAL-FILLED FRACTURE

Veins

Original with partial mineral fill/fully mineralised

Vein material fractures more easily than matrix = narrower, open fracture

High flow // to fracture (flow ∝ aperture^3)

Flow perpendicular to fracture ?low due to mineralisation

CAN MEASURE FRACTURE APERTURE

VUGGY FRACTURES

Due to dissolution from groundwater flow

Open fracture walls +/ material fill

Extreme = karst develop near old water table

High flow // to fracture (flow ∝ aperture^3)

Flow perpendicular to fracture ?low due to mineralisation

Fractography =

description, analysis and interpretation of SURFACE features of fractures

In tensile fractures b/c open and stay open, not preserved in shear

Examples of fractography

Twist hackle fringes

Plumose markings

Synthetic =

dip in same direction as main fault

Antithetic =

dip in opposite direction as main fault

Measuring fracture aperture

1. Wall-to-wall separation - average separation 2. Mineral fill - ?fiber/euhedral - ?partial/complete - ?single/multiple stages

Limitations of measuring fracture aperture

Effect of stress Sampling method ?Outcrop data not reflective of sub-surface

Fractured reservoirs and mineral systems

Fractured reservoirs = largest proportion of proven oil reserved Fracture network defines reservoir properties (porosity/permeability/deformability) Tight reservoirs e.g. sands/shales = few natural fractures = for economic production require hydraulic fracturing

Fracture attribute analysis =

key tool to quantify nature fracture systems Fracture damage zones = important conduits

Topology =

whether fractures are: Abutting Trailing Cross-cutting Persistent/inpersistent Straight/curviplanar N/E/S/W trending - invariant with strain - relates to connectivity - dimensionless

Fracture intensity/density =

#/L

Fracture geometry =

measurable elements with units and dimensions

Spacing =

1/density

Frequency (P20) =

#/A

tip/2 | because simple counting overestimates

IYX triangle

``` I = isolated Y = abutting X = cross cutting ``` Closer to YX length = higher connectivity and fluid flow

No. lines =

(N(I) + N(Y))/2

No. branches (between two nodes) =

(N(I) + 3N(Y) + 4N(X))/2

Characteristic branch length Lb =

no. branches/total trace length

Characteristic line length Lc =

total length/no. lines

Dimensionless Fracture Intensity (DFI) =

density x characteristic length

Orientation of tensile fractures

Always // to σ 1 = use to determine orientation of maximum horizontal stress