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Spatial Variations in Microseismic Focal Mechanisms, Yibal Field, Oman

Spatial Variations in Microseismic Focal Mechanisms, Yibal Field, Oman. A. AL-Anboori 1 , M. Kendall 2 , D. Raymer 3 , R. Jones 3 and Q. Fisher 1. 1 University of Leeds 2 Schlumberger Cambridge Research 3 University of Bristol. Outline. 1. Introduction. 2. Focal mechanisms (FOCMEC).

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Spatial Variations in Microseismic Focal Mechanisms, Yibal Field, Oman

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  1. Spatial Variations in Microseismic Focal Mechanisms, Yibal Field, Oman A. AL-Anboori1, M. Kendall2, D. Raymer3, R. Jones3 and Q. Fisher1 1 University of Leeds 2 Schlumberger Cambridge Research 3 University ofBristol

  2. Outline 1. Introduction 2. Focal mechanisms (FOCMEC) 3. Stress inversion (FMSI) 4. Stress magnitudes 5. Conclusions

  3. 1. Introduction

  4. N

  5. shale carbonate

  6. Northern Co-ordinates /m 1km Eastern Co-ordinates /m

  7. P P’ Station: Orientation available Station:No Orientation info P P’

  8. 1.2 Aims • Determine fault regime using FOCMEC. • Estimate directional stress field using FMSI. • Compute full stress tensor (magnitudes) from a friction model 1.1 Event statistics 22 days of data June,Aug,Sep,Oct02 1) Over 600 located events. 2) Frequency 10-400 Hz. 3) Magnitude (Ml) -2 to 1

  9. 1.3 Preliminary processing Sv E P Amplitude Sh N Horizontal Plane Vertical Plane Z North up Time [s] After Sh Amplitude Sv East horizontal P Time [s] 1.3.2 Rotation to ray frame Before Z N E

  10. 2. FOCMEC

  11. Polarity Amplitude Sh L - 40.3 Sv B -68.4 Focal mechanism + P C +11.5 FOCMEC (Snoke, 1984) Assumes: double-couple (pure shear) source Method: Grid search Uses: - (P,SV,SH) polarities and ratios - ray (azimuth, take off angle)

  12. P P’ P P’

  13. Compaction?

  14. 3. Stress Inversion

  15. σ2 σ3 σ1 σ2 (σ1 σ3) R 0 R 1 FMSI (Gephart & Forsyth, 1984) Assumes: - pure shear-slip earthquakes that occur on pre-existing faults Method : - Grid search Uses : -focal mechanisms (FOCMEC output ) Directions only

  16. NatihA Nahr Umr Shuaiba R=0.90 R=0.80 R=0.70 Fiqa R=0.70 σ1 σ3 σ2

  17. σ1 (Baker Atlas GEOScience, 1999) NatihA σ1 Elsewhere cracks σ1 Fracture strike NatihA R=0.70 σ1 σ3 σ2 (Al-Anboori et al., 2005)

  18. 5. Stress Magnitudes

  19. σv σ2 Fiqa σ1 σ2 σ2 p: pore pressure U=f() : friction angle Nahr Umr σ3  real magnitudes Model magnitudes (passive basin) Robs v: poisson ratio Constant v=0.31 σ2 σ3 σ1 0 R 1 Stress magnitudes assumes: - slip failure along optimally oriented pre-existing faults - p =hydrostatic pressure - σv =lithostatic pressure - σv = σ1orσ2orσ3 σ3 NatihA σ1 Shuaiba

  20. Fiqa (shale) strike thrust thrust thrust NatihA (chalk) Nahr Umr (shale) strike normal normal normal Shuaiba (chalk) chalk shale 22 σ2 σ3 σ1 0 R 1 Model magnitudes (passive basin)

  21. =39º v=0.37 NatihA real magnitudes =70ºv=0.31 Compaction?

  22. 5. Conclusions

  23. 5. Conclusions v .31 .37 .31 .31  12° Fiqa (shale) strike thrust thrust 39° thrust NatihA (chalk) 18° Nahr Umr (shale) strike normal normal 39° normal Shuaiba (chalk) The deduced stress field is consistent with the fracture strike inferred from shear-wave splitting measurements. The deduced stress field in the Natih reservoir also agrees closely with the in-situ stress inferred from wellbore breakouts (Baker Atlas GEOScience, 1999).

  24. Acknowledgements Petroleum Development Oman (PDO)

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