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EARS5136 slide 1
Introduction toreservoir-scale deformation
and structural core description
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EARS5136 slide 2
Reservoir scale deformation
• Small scale faults and fractures plus the internal structure of faults revealed by core and image logs
• Introduce basics of structural core description
• Aim to visit core store later in course
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EARS5136 slide 3
Core basics
• Various diameters: 2” to 6”, 4” (10cm) commonest
• Runs of up to 120 feet per core (30’ to 60’ common)
• ‘Drillers’ depth not measured (log) depth
• Usually slabbed before logging
• Stored in 3ft, 4ft, 1m boxed lengths
• Half cut common
• Resinated ‘museum’ core also common
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EARS5136 slide 4
Core orientationL R
Up
Core marked to show ‘way-up’
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EARS5136 slide 5
Core to log shift
• Core taken whilst drilling
• Logs taken after drilling
• Stretch of log tool cable means that measured depth (log) and driller’s depth (core) do not correspond
• Apply a shift +’ve or –’ve to correlate core and logs
• Core gamma used to pick shifts
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EARS5136 slide 6
What to record?
• Core width
• Continuous core sections
• Fault or fracture length - cuts centreline?
• Fault or fracture width
• Number of tips/terminations: upper or lower
• Layer boundaries?
• Displacement
• Slip sense/direction
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EARS5136 slide 7
What to record 2
• Fracture spacing
• Cross-cutting relationships
• Intersection angle of sets
• Fault rock type: cataclasites/disaggregation, PFFR, clay-smear
• Shale/phyllosilicate smear – abrasion– shear zone– injection
• Cementation: whole or part
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EARS5136 slide 8
What to record 3
• Clast sizes - cataclasite to breccia
• Distribution with respect to lithology
• Surface markings – fractography
• Rubble zones
• Natural vs. Induced
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EARS5136 slide 9
Recognition of natural fractures
• Cementation
• No geometric relationship with core
• Shear offset
• Planar
• Propagation along bedding not down core
• Multiple sets
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EARS5136 slide 10
Detailed Fault Rock Classification
Fisher & Knipe (1998)
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EARS5136 slide 11
Faults in core
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EARS5136 slide 12
Log of deformation features in core
0 4 8 12 16 20
D eform ation features
10000
10020
10040
10060
10080
10100
10120
10140
10160
10180
10200
10220
10240
10260
10280
10300
De
pth
Layer A
Layer B
Layer C
Layer D
W ell nam eFeature 1
Feature 2
Feature 3
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EARS5136 slide 13
Naturalfractures
Fracture spacing and layer boundaries in Chalk core
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EARS5136 slide 14
S p a cin g :th ickn e ss ra tioM axim um S /T = 0 .92
Average S /T = 0.42
M in im um S /T = 0.09
0 0.5 1 1.5Fra ctu re sp a cin g (m )
0
0.5
1
1.5
La
yer
thic
kne
ss (
m)
C ore d iam eter10cm
M axim um layerth ickness 1.22m
Average layerth ickness 0.49m
M inim um layerth ickness 0.16m
Fracture spacing vs. layer thickness: what is visible in core?
Closer than average
Wider than average
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EARS5136 slide 15
Fracture spacing
• Recognition of mechanical layer boundaries
• Fracture spacing/layer thickness relationships
• Comparison with other data and methods– e.g. Average fracture spacing estimated using the technique
of Narr (1996)
Spacing = Core slab surface area Total fracture height in core
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EARS5136 slide 16
Core orientation
• Scribed core
• Palaeomagnetic
• Dipmeter
• Image logs
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EARS5136 slide 17
Orientation of deformation features relative to bedding
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EARS5136 slide 18
Fracture spacing
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EARS5136 slide 19
Coring induced fractures
• Can be mistaken for natural uncemented fractures and so influence identification of productive zones
• Types recognized using characteristic fracture surface morphology or fracture geometry:
– Centreline fractures
– Petal fractures
– Torsional fractures
– Scribe-knife related
– Core-plug related
– Unloading
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EARS5136 slide 20
Fracture surface morphology
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EARS5136 slide 21
Arrest lines indicating Propagation down core
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EARS5136 slide 22
Petal-centreline fractures
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EARS5136 slide 23
Petal-centreline fractures
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EARS5136 slide 24
Scribe knife damage
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EARS5136 slide 25
Scribe knife damage
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EARS5136 slide 26
Core discs
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EARS5136 slide 27
Core discs
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EARS5136 slide 28
Torsional fracturesCore disc
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EARS5136 slide 29
Core spin
From Paulsen et al. (2002)
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EARS5136 slide 30
Rubble zones in core
• Induced
• Often at base of a core
• Can develop where lithologies change
• May correlate with ROP changes
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EARS5136 slide 31
Image logs
• Sonic or resistivity tools
• FMI – Shows a resistivity image of the borehole wall
• UBI/CBIL – Show an acoustic image of the borehole wall
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EARS5136 slide 32
UBI image of open fractures
• Fractures make a sinusoidal trace on the borehole wall
• Data on type and orientation
• Acoustic show open fractures
• Resistivity show open and cemented fractures/faults
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EARS5136 slide 33
Faults on FMI log
• Offsets visible although throw is difficult to measure
• Dip changes may be visible
• Core to log – about 5 times number of features observable in core.
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EARS5136 slide 34
High resolution image logs allow identification of minor, narrow-aperture fractures when calibrated against core