Structural core description

Report
Introduction to
reservoir-scale deformation
and structural core description
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slide 1
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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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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Core orientation
L R
Up
Core marked to show ‘way-up’
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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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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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What to record 2
• Fracture spacing
• Cross-cutting relationships
• Intersection angle of sets
• Fault rock type: cataclasites/disaggregation, PFFR, claysmear
• Shale/phyllosilicate smear
– abrasion
– shear zone
– injection
• Cementation: whole or part
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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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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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Detailed Fault Rock Classification
Fisher & Knipe (1998)
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slide 10
Faults in core
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slide 11
Deformation features
Log of deformation
features in core
0
4
8
12
16
20
10000
Well name
Feature 1
Feature 2
Feature 3
10020
10040
Layer A
10060
10080
10100
10120
Layer B
Depth
10140
10160
10180
Layer C
10200
10220
10240
10260
Layer D
10280
10300
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slide 12
Natural
fractures
Fracture spacing
and layer
boundaries in
Chalk core
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Fracture spacing vs. layer thickness:
what is visible in core?
1.5
Closer than
average
Wider than
average
Layer thickness (m)
Maximum layer
thickness 1.22m
Spacing:thickness ratio
Maximum S/T = 0.92
Average S/T = 0.42
Minimum S/T = 0.09
1
Average layer
thickness 0.49m
0.5
Core diameter
10cm
Minimum layer
thickness 0.16m
0
0
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0.5
1
Fracture spacing (m)
1.5
slide 14
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 =
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Core slab surface area
Total fracture height in core
slide 15
Core orientation
• Scribed core
• Palaeomagnetic
• Dipmeter
• Image logs
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Orientation of deformation features relative to bedding
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Fracture
spacing
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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:
–
–
–
–
–
–
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Centreline fractures
Petal fractures
Torsional fractures
Scribe-knife related
Core-plug related
Unloading
slide 19
Fracture surface morphology
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Arrest lines indicating
Propagation down core
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Petal-centreline fractures
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Petal-centreline fractures
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Scribe knife damage
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Scribe knife damage
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Core discs
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Core discs
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Core disc
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Torsional fractures
slide 28
Core spin
From Paulsen et al. (2002)
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slide 29
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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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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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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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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slide 33
High resolution image logs allow identification of minor,
narrow-aperture fractures when calibrated against core
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slide 34

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