Spatial Variations in Microseismic Focal Mechanisms, Yibal Field, Oman

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 of Bristol
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
8
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
1.3.1 Filtering electric noise
Before
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1.3 Preliminary processing
1.3.2 Rotation to ray frame
Before
Z
N
E
11
1.3 Preliminary processing
1.3.2 Rotation to ray frame (S-wave example)
Before
E
Amplitude
N
Z
Time s
12
2. FOCMEC
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FOCMEC (Snoke, 1984)
Assumes double-couple (pure shear) source
Method Grid search
Uses - (P,SV,SH) polarities and ratios
- ray (azimuth, take off angle)
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2.1 Synthetic seismograms
Event yb021022.0042 (depth1360m)
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Synthetic data
M2
M1
M3
Mechanism
Input
Recovered (Event 42)
Recovered (Event 57)
Attenuative 21-layer Yibal velocity model
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Synthetic data
M2
M1
M3
Mechanism
Input
Recovered (Event 42)
Recovered (Event 57)
Attenuative 21-layer Yibal velocity model
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Real data
Reliable (43)
Bad (32)
18
B
B
Reliable (43)
P
P
T
T
P pressure T tension
19
B
B
Reliable (43)
P
P
T
T
P pressure T tension
20
P
P
21
Compaction?
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B vertical Strike
T vertical Thrust
P vertical Normal
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3. Stress Inversion
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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
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Fiqa NatihA
Nahr Umr
Shuaiba R0.70 R0.70
R0.90
R0.80 F0.4
F3.1 F5.3
F2.8
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Fiqa
R0.70
s1
s3
s2
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1. H-Fiqa (b) H-NatihA (c) G-Nahr Umr
   (d) G-Shuaiba
2. R0.70 R0.70 R0.90
   R0.80
3. F0.4 F3.1
   F5.3 F2.8

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NatihA
R0.70
s1
s3
s2
(Al-Anboori et al., 2005)
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NatihA
Nahr Umr
R0.90
R0.70
s1
s1
s3
s3
s2
s2
30
(No Transcript)
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s1
Nahr Umr
s1
Shuaiba
32
Nahr Umr
Natih
Y402H1 Analysed interval 1386-1501m
Y437H1 Analysed interval 1217-1570m
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5. Stress Magnitudes
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Stress magnitudes
assumes - slip failure along optimally oriented
pre-existing faults - p
hydrostatic pressure - sv lithostatic
pressure - sv s1 or s2 or s3
s3
NatihA
s1
Shuaiba
35
Model magnitudes (passive basin)
36
Robs.7
?70º
Compaction?
37
real magnitudes
?70º v0.31
Compaction?
38
5. Conclusions
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5. Conclusions
?
12
Fiqa (shale)
strike
thrust
thrust
39
NatihA (chalk)
thrust
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).
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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 dip of the maximum stress direction
increases with depth horizontal in Fiqa
Natih-A, sub-horizontal in Nahr Umr, and
sub-vertical in Shuaiba.
The observed stress relative magnitude R
(0.7-0.9) 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.
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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 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.
The stress magnitudes were calculated at each
depth in the four zones with the maximum stress
occurring in Natih A at about 63 MPa.
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Acknowledgements
Petroleum Development Oman (PDO)
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Fiqa
NatihA
Nahr Umr
Shuaiba