spatial variations in microseismic focal mechanisms, yibal field, oman a. al-anboori 1, m. kendall...
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Spatial Variations in Microseismic Focal Mechanisms, Yibal Field,
Oman
A. AL-Anboori1, M. Kendall2, D. Raymer3, R. Jones3 and Q. Fisher1
1 University of Leeds2 Schlumberger Cambridge Research
3 University of Bristol
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1. Introduction
2. Focal mechanisms (FOCMEC)
5. Conclusions
3. Stress inversion (FMSI)
4. Stress magnitudes
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1. Introduction
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N
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shale
carbonate
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Eastern Co-ordinates /m
Nor
ther
n C
o-or
dina
tes
/m
1km
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P
P’
P’P
Station: Orientation available
Station: No Orientation info
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1.1 Event statistics
22 days of data
1) Over 600 located events.
2) Frequency 10-400 Hz.
3) Magnitude (Ml ) -2 to 1
1.2 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
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1.3 Preliminary processing 1.3.2 Rotation to ray frame
Time [s]
Am
plitu
de
E
N
Z
East
Nor
th
horizontal
up
Horizontal Plane Vertical Plane
Before
Time [s]
Am
plitu
de
Sh
Sv
P
After
E
NZ
Sh
PSv
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2. FOCMEC
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FOCMEC (Snoke, 1984)
Uses: - (P,SV,SH) polarities and ratios - ray (azimuth, take off angle)
P
Sh
Sv
+C
B
L
Polarity Amplitude
+11.5
-68.4
- 40.3
Focal mechanism
Assumes: double-couple (pure shear) source
Method: Grid search
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P
P’
P’P
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Compaction?
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3. Stress Inversion
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Uses : -focal mechanisms (FOCMEC output )
FMSI (Gephart & Forsyth, 1984)
(σ1 σ2 σ3) R0 1
σ1 σ2 σ3
R
Assumes: - pure shear-slip earthquakes that occur on pre-existing faults
Directions only
Method : - Grid search
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Fiqa
R=0.70 R=0.70 R=0.90 R=0.80
NatihA Nahr Umr Shuaiba
σ1
σ3
σ2
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NatihA
σ1
σ3
σ2
R=0.70
(Baker Atlas GEOScience, 1999)
σ1
Fracture strike
Nat
ihA
Elsewhere
σ1
σ1
crac
ks
(Al-Anboori et al., 2005)
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5. Stress Magnitudes
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Stress magnitudesassumes: - slip failure along optimally oriented pre-existing faults - p =hydrostatic pressure
- σv =lithostatic pressure - σv = σ1 or σ2 or σ3 NatihA
Shuaiba
σ3
Nahr Umr
Fiqa
σvσ2
σ2
σ1
Model magnitudes(passive basin)
v: poisson ratioConstant v=0.31
real magnitudes
Robs
0 1
σ1 σ2 σ3
R
σ2σ1
σ3
p: pore pressureU=f(): friction angle
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Model magnitudes(passive basin)
0 1
σ1 σ2 σ3
R
NatihA (chalk)
Fiqa (shale)strike
thrust
Shuaiba (chalk)
Nahr Umr (shale)strike
normal
shale chalk
thrustthrust
normalnormal
22
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real magnitudes
=70º v=0.31 =39º v=0.37 NatihA
Compaction?
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5. Conclusions
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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
thrustthrust
normalnormal
12°39°
18°39°
.31v
.37
.31
.31
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Acknowledgements
Petroleum Development Oman (PDO)