grq ltd. – oct. 2003 - dti spindrift bidding. slide 1 investigation of fracture & fault...

$: GRQ Ltd. – Oct. 2003 - DTI Spindrift bidding. slide 1 Investigation of fracture & fault populations in analogue outcrops for use in the Spindrift subsurface$
GRQ Ltd. – Oct. 2003 - DTI Spindrift bidding. slide 1

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.


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.


Fracture density


Fracture density

• As bed thickness increases fracture spacing increases.

• In smaller beds (<15cm) fracture spacing rarely exceeds 20cm.

• In larger beds (>30cm and especially >50cm) 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 doesn’t account for fracture clustering around faults.


Fracture density

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 0.1 0.2 0.3 0.4 0.5 0.6

Bed Thickness (m)

Fra

ctur

e Sp

acin

g (m

)

Bed 1 Bed 2 Bed 3 Bed 4 Bed 5


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.


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.


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.


Loc. 1 Loc. 2

NN

NN

10cm10cm 10cm10cm

• Bed thickness – 0.25m

• Fracture frequency - 53

• Cumulative fracture length per m2 - 11.27m

• Average fracture length – 0.21m






Loc. 3 Loc. 4

NN

NN

10cm10cm10cm10cm









Faulting increases Faulting increases local fracture local fracture

densitydensity

Conjugate fault set Conjugate fault set intersecting in cliff intersecting in cliff

faceface


Loc. 5 Loc. 6

10cm10cm10cm10cm

NN

NN










Bed thickness vs. Plan Fracture properties

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Thickness (m)

Fra

ctu

re f

req

uen

cy

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Thickness (m)

Cu

mlu

ativ

e le

ng

th (

m)

• Weak correlation between measures of plan fracture density and bed thickness;– Limited data set

– Difficult to assess bed thickness

• Local fracture densities related to proximity to faulting

Cumulative length (m) per mCumulative length (m) per m2 2 vs. bed thickness (m)vs. bed thickness (m)

Fracture frequencyFracture frequency vs. bed thickness (m)vs. bed thickness (m)


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.


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


Stratigraphic control of faulting

• 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

4 metres4 metres


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.


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


Throw vs transect length

• Clustering of smaller faults around larger faults

• Available data suggests larger faults (>15cm) appear approximately every 25m

Throw with distance along traverse

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61

Distance along traverse (M)

Th

row

of

Fau

lts

(M)


Frequency of fault spacing

• Median spacing of faults = 0.5 metres

• Trend line fits exponential curve to 94%

Frequency of Fault spacings.

0

10

20

30

40

50

60

70

0 0.5 1 1.5 2 2.5 3distance (M) between Faults

Fre

qu

ency


Fault throw vs cumulative frequency

• Higher frequency of small displacement faults

• Low frequency of large displacement faults

Cumulative frequency vs. Throw

1

10

100

0.1 1 10 100

Throw (cm)

Cu

mu

lati

ve f

req

uen

cy


Large scale faulting – examples of damage zone

Main fault damage Main fault damage zonezone

Rotated, dragged & Rotated, dragged & thrusted beddingthrusted bedding

Complex filled veins Complex filled veins & fractures& fractures

Calcite filled Calcite filled fractures/veins fractures/veins (mm-dm width) (mm-dm width) within the damage within the damage zonezone

Significant Significant reduction if fracture reduction if fracture permeabilitypermeability

Barrier to fluid flowBarrier to fluid flow


Prediction of fracture & fault permeability

• Little vertical connectivity of fractures (strata-bound >90%),

• 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).


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.


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.

grq ltd. – oct. 2003 - dti spindrift bidding. slide 1 investigation of fracture & fault...

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