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Structural > Week 18 > Flashcards

Week 18 Flashcards

1
Q

How do shear fractures form?

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

Mohr diagrams

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2
Q

Primary and secondary shear fractures

A

R
R’
P

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3
Q

Orientation of σ1 and σ3 in extensional vs compressional regime

A

Extensional:
σ1 = vertical
σ3 = horizontal

Compressional:
σ1 = horizontal
σ3 = vertical

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4
Q

Tensile joint =

A

vein

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5
Q

Slip/shear joint =

A

fault

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6
Q

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

A

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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7
Q

How does fault rock type change with depth?

A

P/T changes = different rocks

Gouge - cataclasite - mylonite - mylonitic gneiss

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

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8
Q

Crustal strength profile with increasing depth

A

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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9
Q

What does lithology control?

A
  1. FRACTURING

2. FRACTURE DENSITY (FD)

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10
Q

How does lithology control fracturing?

A

e.g. coal = fine-scale cleats

Although high variability for any given rock type

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11
Q

How does lithology control fracture density?

A

Coal = ~0.05MPam^(1/2)

Chert

Dolomite

Silica cement

Sandstone

Calcite cement

Limestone

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

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12
Q

What else affects fracture density?

A

Porosity/composition/grain size

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

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13
Q

What controls fracture height?

A

Bedding

Thick bed:
= decrease fracture density
= increase fracture spacing

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14
Q

Types of fractures

A
  1. OPEN FRACTURES
  2. FAULTS
  3. MINERAL-FILLED FRACTURE
  4. VUGGY FRACTURES
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15
Q

Conductive fault =

A

open

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16
Q

Faults on folds

A

Outer arc extension = normal faults

Neutral surface

Inner arc compression = thrust/reverse

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17
Q

Fractures on plunging folds

A

Fracture distribution varies along fold hinge

Complex strain patterns

18
Q
  1. OPEN FRACTURES
A

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

19
Q
  1. FAULTS
A

a) Gouge-filled fractures including deformation bands

b) Slickenside fracture

20
Q
  1. MINERAL-FILLED FRACTURE
A

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

21
Q
  1. VUGGY FRACTURES
A

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

22
Q

Fractography =

A

description, analysis and interpretation of SURFACE features of fractures

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

23
Q

Examples of fractography

A

Twist hackle fringes

Plumose markings

24
Q

Synthetic =

A

dip in same direction as main fault

25
Q

Antithetic =

A

dip in opposite direction as main fault

26
Q

Measuring fracture aperture

A
  1. Wall-to-wall separation
    - average separation
  2. Mineral fill
    - ?fiber/euhedral
    - ?partial/complete
    - ?single/multiple stages
27
Q

Limitations of measuring fracture aperture

A

Effect of stress

Sampling method

?Outcrop data not reflective of sub-surface

28
Q

Fractured reservoirs and mineral systems

A

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

29
Q

Fracture attribute analysis =

A

key tool to quantify nature fracture systems

Fracture damage zones = important conduits

30
Q

Topology =

A

whether fractures are:

Abutting

Trailing

Cross-cutting

Persistent/inpersistent

Straight/curviplanar

N/E/S/W trending

  • invariant with strain
  • relates to connectivity
  • dimensionless
31
Q

Fracture intensity/density =

A

/L

32
Q

Fracture geometry =

A

measurable elements with units and dimensions

33
Q

Spacing =

A

1/density

34
Q

Frequency (P20) =

A

/A

35
Q

=

A

tip/2

because simple counting overestimates

36
Q

IYX triangle

A 
I = isolated
Y = abutting
X = cross cutting

Closer to YX length = higher connectivity and fluid flow

37
Q

No. lines =

A

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

38
Q

No. branches (between two nodes) =

A

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

39
Q

Characteristic branch length Lb =

A

no. branches/total trace length

40
Q

Characteristic line length Lc =

A

total length/no. lines

41
Q

Dimensionless Fracture Intensity (DFI) =

A

density x characteristic length

42
Q

Orientation of tensile fractures

A

Always // to σ 1

= use to determine orientation of maximum horizontal stress

Structural (17 decks)

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