Introduction to reservoir-scale deformation and structural core description

1. Introduction toreservoir-scale deformation and structural core description

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

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

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

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

11. Faults in core

12. Log of deformation features in core

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

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

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. Core orientation • Scribed core • Palaeomagnetic • Dipmeter • Image logs

17. Orientation of deformation features relative to bedding

18. Fracture spacing

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. Fracture surface morphology

21. Arrest lines indicating Propagation down core

22. Petal-centreline fractures

23. Petal-centreline fractures

24. Scribe knife damage

25. Scribe knife damage

26. Core discs

27. Core discs

28. Core disc Torsional fractures

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

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

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