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

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

1. Introduction

N

shale carbonate

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

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

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

1.3 Preliminary processing Filtering electric noise Before After

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

1.3 Preliminary processing Rotation to ray frame (S-wave example) Time [s] Amplitude E N Z Before Time [s] Amplitude Sh Sv P After

2. FOCMEC

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

2.1 Synthetic seismograms Half-space model Non-attenuative -recovered with negligible (0.08) ratio error Attenuative - (Bef Qcorr) recovered with -0.4 ratio error - (Aft Qcorr) recovered with negligible ratio error Model Focal mechanism (Using only 3 wells) Event yb (depth=1360m) Yibal 21-layer model Non-attenuative -recovered with -0.3 ratio error Attenuative - (Bef Qcorr) recovered with ratio error - (Aft Qcorr) recovered with -0.3 ratio error Model Focal mechanism (Using only 3 wells) Compression Dilatation Attenuative - (Bef Qcorr) recovered with ratio error

Synthetic data compression dilatation amplitude ratio Input Recovered (Event 42) Mechanism Attenuative 21-layer Yibal velocity model Recovered (Event 57) M=1 M=2 M=3

Synthetic data compression dilatation amplitude ratio Input Recovered (Event 42) Mechanism Attenuative 21-layer Yibal velocity model Recovered (Event 57) M=1 M=2 M=3

Reliable (43) Bad (32) Real data compression dilatation amplitude ratio

Reliable (43) T B P T P B P: pressure T: tension compression dilatation amplitude ratio

Reliable (43) T B P Uncertainties T P B P: pressure T: tension compression dilatation amplitude ratio

P P’ P

Compaction?

B vertical Strike P vertical Normal T vertical Thrust

3. Stress Inversion

Uses : -focal mechanisms (FOCMEC output ) FMSI (Gephart & Forsyth, 1984) (σ1(σ1 σ2σ2 σ3)σ3) R 01 σ1σ1 σ2σ2 σ3σ3 R Assumes: - pure shear-slip earthquakes that occur on pre-existing faults Directions only Method : - Grid search

Fiqa NatihA Nahr Umr Shuaiba R=0.70 R=0.70 R=0.90 R=0.80 F=0.4° F=3.1 ° F=5.3 ° F=2.8 °

Fiqa R=0.70 R=0.90R=0.80 NatihA Nahr Umr Shuaiba σ1σ1 σ3σ3 σ2σ2

(a)H-Fiqa (b) H-NatihA (c) G-Nahr Umr (d) G-Shuaiba (b)R=0.70 R=0.70 R=0.90 R=0.80 (c)F=0.4 F=3.1 F=5.3 F=2.8

NatihA σ1σ1 σ3σ3 σ2σ2 R=0.70 (Baker Atlas GEOScience, 1999) σ1σ1 Fracture strike NatihA Elsewhere σ1σ1 σ1σ1 cracks (Al-Anboori et al., 2005)

R=0.90 NatihA σ1σ1 σ3σ3 σ2σ2 Nahr Umr σ1σ1 σ3σ3 σ2σ2 R=0.70 (Baker Atlas GEOScience, 1999) σ1σ1 σ1σ1 Fracture strike (Al-Anboori et al., 2005) NatihA Elsewhere σ1σ1 σ1σ1 cracks

Shuaiba Nahr Umr σ1σ1 σ1σ1

Y402H1 Analysed interval m Nahr UmrNatih Y437H1 Analysed interval m

5. Stress Magnitudes

Stress magnitudes assumes: - slip failure along optimally oriented pre-existing faults - p =hydrostatic pressure - σ v =lithostatic pressure - σ v = σ 1 or σ 2 or σ 3 NatihA Shuaiba σ3σ3 Nahr Umr Fiqa σvσv σ2σ2 σ2σ2 σ1σ1 Model magnitudes (passive basin) v: poisson ratio Constant v=0.31 real magnitudes R obs 01 σ1σ1 σ2σ2 σ3σ3 R σ2σ2 σ1σ1 σ3σ3 p: pore pressure U=f(  )  : friction angle 

Model magnitudes (passive basin) 01 σ1σ1 σ2σ2 σ3σ3 R NatihA (chalk) Fiqa (shale) strike thrust Shuaiba (chalk) Nahr Umr (shale) strike normal shale chalk thrust normal 22

real magnitudes Model magnitudes (passive basin) thrust normal v: poisson ratio=0.31 R obs =0.7,  =39º v =0.37 NatihA Compaction? R obs =.7  =70º

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

5. Conclusions

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). NatihA (chalk) Fiqa (shale) strike thrust Shuaiba (chalk) Nahr Umr (shale) strike normal thrust normal 12° 39° 18° 39° .31 v.37.31

5. Conclusions Fault Regime : Strike-slip movements in Fiqa and Nahr Umr shale cap rocks. Thrust faulting in the gas carbonate Natih-A reservoir. Normal faulting in the oil-bearing carbonate Shuaiba reservoir. Stress Inversion: The deduced stress field is consistent with the fracture strike inferred from shear-wave splitting measurements (Al-Anboori et al., 2005). The deduced stress field in the Natih reservoir also agrees closely with the in-situ stress inferred from wellbore breakouts (Baker Atlas GEOScience, 1999). The observed stress relative magnitude R ( ) concludes that stresses are flattening rather than constricting. No distinct changes of fault mechanisms with magnitude or time is established suggesting a stationary stress at least for the investigated period. σ1σ1 σ2σ2 σ3σ3 The dip of the maximum stress direction increases with depth: horizontal in Fiqa & Natih-A, sub-horizontal in Nahr Umr, and sub-vertical in Shuaiba.

5. Conclusions (Contd.) Stress magnitudes (friction model): The observed relative magnitude in Natih A suggests a positive anomaly in poisson ratio (increasing by about 0.06) which is consistent with the undergoing compaction in this unit. The stress magnitudes were calculated at each depth in the four zones with the maximum stress occurring in Natih A at about 63 MPa. The modelling shows only one acceptable scenario which is an exerted regional thrusting system in the top reservoir and its cap rock and normal system in the bottom reservoir and its cap rock. The transition at each shale cap rock/carbonate reservoir could be formed by variation in friction angles across the interface. The resulted best fit friction angles in shale (12° in Fiqa and 18° in Nahr Umr) and chalk (39° in Natih A and Shuaiba) closely agree with the reported values in the literature.

Acknowledgements Petroleum Development Oman (PDO)

FiqaNatihA Shuaiba Nahr Umr