Investigation of fracture

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Investigation of fracture fault populations in
analogue outcrops for use in the Spindrift
subsurface reservoir/fluid flow model.

• GetRichQuick Ltd.
• A.Anigboro, V. Carter, S. Green, R. Hall, P.
  Jones, G. Markham, M. Thomas.
• MSc. Structural Geology with Geophysics,
• Dept. Earth Sciences, University of Leeds.

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Objective

• Use of analogue data collected from outcrops at
  Flamborough Head for input into the Spindrift
  prospect subsurface fluid flow model.

Aims

• Analysis of collected data in terms of
• Relationship of fracture spacing/density to bed
  thickness vertical connectivity,
• Lateral connectivity and orientation of
  fractures,
• Stratigraphic controls on fault geometries
  fault rock properties,
• Fault throw, orientation, clustering
  relationships,
• Assessment of all data in terms of predictability
  of fault fracture populations permeability.

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Fracture density
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Fracture density

• As bed thickness increases fracture spacing
  increases.
• In smaller beds (lt15cm) fracture spacing rarely
  exceeds 20cm.
• In larger beds (gt30cm and especially gt50cm)
  fracture spacing reaches as high as 90cm.
• The greater thickness gives the bed a higher
  competence, which results in the stress needed
  to form fractures being greater.
• Data doesnt account for fracture clustering
  around faults.

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Fracture density
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Fracture density

• Trend visible suggesting most fractures fit a
  general rule.
• 2/3 Bed Thickness 20cm
• Data set is not large enough for a definitive
  equation.
• Data also suggests that larger beds show more
  fractures above the general trend.

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Vertical Connectivity

• Fractures do not show a tendency to cross from
  one bed to another.
• Fractures that do cross from one bed to another
  are associated with faults.
• Most beds show well developed Stylolites.
• Stylolites appear to facilitate more pervasive
  fracturing.
• Stylolites were formed before the vertical
  fractures.
• Beds show well developed clay layers on their
  tops, which act as an inhibitor to vertical
  pervasiveness.

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Plan Fracture connectivity

• 6 x 1m2 quadrant samples taken from exposed
  bedding surfaces of several different units.
• Digital photo mapping field based measuring
  implemented in tandem.
• Orientation, length density (cumulative length
  per m2), average fracture length, bedding
  thickness recorded.
• Impact of faulting on fracture populations
  investigated.

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Loc. 1 Loc. 2

• Bed thickness 0.25m
• Fracture frequency - 53
• Cumulative fracture length per m2 - 11.27m
• Average fracture length 0.21m

• Bed thickness 0.35m
• Fracture frequency - 245
• Cumulative fracture length per m2 - 21.74m
• Average fracture length 0.09m

N
N
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Loc. 3 Loc. 4

• Bed thickness 0.18m
• Fracture frequency - 128
• Cumulative fracture length per m2 - 14.96m
• Average fracture length 0.11m

• Bed thickness 0.30m
• Fracture frequency - 227
• Cumulative fracture length per m2 - 21.53m
• Average fracture length 0.09m

Faulting increases local fracture density
Conjugate fault set intersecting in cliff face
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Loc. 5 Loc. 6

• Bed thickness 0.75m
• Fracture frequency - 17
• Cumulative fracture length per m2 - 5.77m
• Average fracture length 0.34m

• Bed thickness 0.25m
• Fracture frequency - 19
• Cumulative fracture length per m2 - 7.95m
• Average fracture length 0.42m

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Bed thickness vs. Plan Fracture properties
Cumulative length (m) per m2 vs. bed thickness
(m)

• Weak correlation between measures of plan
  fracture density and bed thickness
• Limited data set
• Difficult to assess bed thickness

Fracture frequency vs. bed thickness (m)

• Local fracture densities related to proximity to
  faulting

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Plan Fracture orientations

• Data collected from 6 x 1 m2 quadrants (700
  fractures)
• Wide spread of fracture strike orientations, with
  335-155 and 260-080 exhibiting dominant trends
• Local fault orientations influence fracture
  density orientations.

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Observations from Plan fractures

• Near 100 connectivity of joints/fractures
• Connectivity independent of density of
  fractures/faulting
• Increased local density of fracturing around
  faults
• Density of fracturing is related to bed
  thickness, data collected from foreshore
  difficult to relate to bed thickness.
• Plan densities should be correlated with
  cross-sectional data
• Dominant trends of fractures related to mean
  fault orientations
• Need to be correlated with fault orientations

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Stratigraphic control of faulting
4 metres

• Strain taken up by weaker Marl beds.
• Which often mark the tip of faults
• Here they also provide a weak medium for fault
  propagation and linkage.
• Some fault planes contain breccia and clay smears

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Fault geometry

• Fault geometry is strongly linked to fracture
  orientation.
• Flat geometry causes heavy fracturing, mostly in
  the Hanging-wall
• This leads to fracturing along strike of the
  fault orientation.

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Fault relationship with jointing /orientation

• Fault orientation.
• Poles to planes and average great circle
• Synthetic (left). Antithetic (right).

Mean fault planes 332 / 53 North-east
Mean fault planes 241 / 64 South-west
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Throw vs transect length

• Clustering of smaller faults around larger faults
• Available data suggests larger faults (gt15cm)
  appear approximately every 25m

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Frequency of fault spacing

• Median spacing of faults 0.5 metres
• Trend line fits exponential curve to 94

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Fault throw vs cumulative frequency

• Higher frequency of small displacement faults
• Low frequency of large displacement faults

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Large scale faulting examples of damage zone
Main fault damage zone
Calcite filled fractures/veins (mm-dm width)
within the damage zone Significant reduction if
fracture permeability Barrier to fluid flow
Rotated, dragged thrusted bedding
Complex filled veins fractures
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Prediction of fracture fault permeability

• Little vertical connectivity of fractures
  (strata-bound gt90),
• High degree of lateral connectivity along beds,
• Higher density of fractures within thinner beds,
• Small offset faults may provide vertical
  connectivity,
• Larger offset faults may produce fault seal
  gouges/smears leading to potential
  compartmentalisation.
• Large offset faults are likely to have a wide,
  complex damage zone
• High density of damage around faults (eg.
  Compressional over steps/damage zones).

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Uncertainty analysis

• Data collection
• Limited sample size
• More data required over larger area
• Measurement errors
• Orientation of sample lines relative to trends of
  features
• Upscaling
• Do relationships found occur at all scales?
• Use of analogue data set
• Uplift induced fracturing, jointing faulting
• How closed are fractures under subsurface
  pressure conditions.

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Implications for reservoir production/development

• Analogue data collection allows for greater
  understanding of potential reservoir production
  issues, ie fluid flow during production.
• Interaction of fractures small offset faulting
  creates high lateral permeability allowing
  efficient drainage of beds.
• Very High fracture permeability parallel to small
  offset faults
• Vertical restriction of fracture permeability
  presence of marl units may prevent excessive
  water cut in wells.
• Larger offset faults, if open may encourage water
  production, however complex low perm damage zone
  fault gouge likely to create sealing faults.
• Evaluation of seismic structure understanding
  of sub-seismic features populations is key to
  successful well planning development.