1. EARS5136slide 1 Introduction to reservoir-scale deformation and structural core description

2. EARS5136slide 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

3. EARS5136slide 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

4. EARS5136slide 4 Core orientation L R Up Core marked to show ‘way-up’

5. EARS5136slide 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

6. EARS5136slide 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

7. EARS5136slide 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

8. EARS5136slide 8 What to record 3 Clast sizes - cataclasite to breccia Distribution with respect to lithology Surface markings – fractography Rubble zones Natural vs. Induced

9. EARS5136slide 9 Recognition of natural fractures Cementation No geometric relationship with core Shear offset Planar Propagation along bedding not down core Multiple sets

10. EARS5136slide 10 Detailed Fault Rock Classification Fisher & Knipe (1998)

11. EARS5136slide 11 Faults in core

12. EARS5136slide 12 Log of deformation features in core

13. EARS5136slide 13 Natural fractures Fracture spacing and layer boundaries in Chalk core

14. EARS5136slide 14 Fracture spacing vs. layer thickness: what is visible in core? Closer than average Wider than average

15. EARS5136slide 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

16. EARS5136slide 16 Core orientation Scribed core Palaeomagnetic Dipmeter Image logs

17. EARS5136slide 17 Orientation of deformation features relative to bedding

18. EARS5136slide 18 Fracture spacing

19. EARS5136slide 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

20. EARS5136slide 20 Fracture surface morphology

21. EARS5136slide 21 Arrest lines indicating Propagation down core

22. EARS5136slide 22 Petal-centreline fractures

23. EARS5136slide 23 Petal-centreline fractures

24. EARS5136slide 24 Scribe knife damage

25. EARS5136slide 25 Scribe knife damage

26. EARS5136slide 26 Core discs

27. EARS5136slide 27 Core discs

28. EARS5136slide 28 Torsional fracturesCore disc

29. EARS5136slide 29 Core spin From Paulsen et al. (2002)

30. EARS5136slide 30 Rubble zones in core Induced Often at base of a core Can develop where lithologies change May correlate with ROP changes

31. EARS5136slide 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

32. EARS5136slide 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

33. EARS5136slide 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.

34. EARS5136slide 34 High resolution image logs allow identification of minor, narrow-aperture fractures when calibrated against core