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

1. Introduction

2. Focal mechanisms (FOCMEC)

5. Conclusions

3. Stress inversion (FMSI)

4. Stress magnitudes

1. Introduction

N

shale

carbonate

Eastern Co-ordinates /m

Nor

ther

n C

o-or

dina

tes

/m

1km

P

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 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

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

2. FOCMEC

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

P

P’

P’P

Compaction?

3. Stress Inversion

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

Fiqa

R=0.70 R=0.70 R=0.90 R=0.80

NatihA Nahr Umr Shuaiba

σ1

σ3

σ2

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)

5. Stress Magnitudes

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

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

real magnitudes

=70º v=0.31 =39º v=0.37 NatihA

Compaction?

5. Conclusions

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

Acknowledgements

Petroleum Development Oman (PDO)