Week 18 Flashcards
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
Primary and secondary shear fractures
R
R’
P
Orientation of σ1 and σ3 in extensional vs compressional regime
Extensional:
σ1 = vertical
σ3 = horizontal
Compressional:
σ1 = horizontal
σ3 = vertical
Tensile joint =
vein
Slip/shear joint =
fault
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
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)
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
What does lithology control?
- FRACTURING
2. FRACTURE DENSITY (FD)
How does lithology control fracturing?
e.g. coal = fine-scale cleats
Although high variability for any given rock type
How does lithology control fracture density?
Coal = ~0.05MPam^(1/2)
Chert
Dolomite
Silica cement
Sandstone
Calcite cement
Limestone
Granite (?!) = 1-3MPam^(1/2)
What else affects fracture density?
Porosity/composition/grain size
e. g. Carbonates
- large grain size and porosity = high FD
- high % dolomite = high FD
What controls fracture height?
Bedding
Thick bed:
= decrease fracture density
= increase fracture spacing
Types of fractures
- OPEN FRACTURES
- FAULTS
- MINERAL-FILLED FRACTURE
- VUGGY FRACTURES
Conductive fault =
open
Faults on folds
Outer arc extension = normal faults
Neutral surface
Inner arc compression = thrust/reverse
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
- Wall-to-wall separation
- average separation - 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
