GetRichQuick Ltd. A.Anigboro, V. Carter, S. Green, R. Hall, P. Jones, G. Markham, M. Thomas.

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.

2. 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.

3. Fracture density

4. 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.

5. Fracture density

6. 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.

7. 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.

9. 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.

10. 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 10cm 10cm Loc. 1 Loc. 2 N N

11. 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 N N 10cm 10cm Loc. 3 Loc. 4 Faulting increases local fracture density Conjugate fault set intersecting in cliff face

12. Bed thickness – 0.25m • Fracture frequency - 19 • Cumulative fracture length per m2 - 7.95m • Average fracture length – 0.42m • Bed thickness – 0.75m • Fracture frequency - 17 • Cumulative fracture length per m2 - 5.77m • Average fracture length – 0.34m N N 10cm 10cm Loc. 5 Loc. 6

13. 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 frequencyvs. bed thickness (m) • Local fracture densities related to proximity to faulting

14. 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.

15. 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

16. 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 metres

17. 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.

18. 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

19. Throw vs transect length • Clustering of smaller faults around larger faults • Available data suggests larger faults (>15cm) appear approximately every 25m

20. Frequency of fault spacing • Median spacing of faults = 0.5 metres • Trend line fits exponential curve to 94%

21. Fault throw vs cumulative frequency • Higher frequency of small displacement faults • Low frequency of large displacement faults

22. 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

23. 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).

24. 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.

25. 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.