1. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved1 Post-Processing Geostatistical Realisations - the Spillpoint ashley.francis@sorviodvnvm.co.uk http://www.sorviodvnvm.co.uk Presented at SEG Development & Production Forum, 11-16 th July 1999, Kananaskis, Calgary, Canada

2. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved2 Prospect Generation Interpret 2D seismic - horizon and faults Map TWT Map Stacking Velocity Depth Convert Find lowest closing contour Report volumes Propose well location

3. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved3 Variograms

4. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved4 Observed Variograms Time Velocity Depth X=

5. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved5 Observed Variograms Prospect Analysis Spillpoint :1844 m Volume :882.2 x 10 6 m 3 Area :34.7 x 10 6 m 2 Thickness:25.4 m Prognosis :1775 m Connection:100%?

6. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved6 Observed Variograms Conditional Simulation/Spillpoint

7. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved7 Observed Variograms Isoprobability Map/GRV Curve Spillpoint :1834 m Volume :717.3 x 10 6 m 3 Area :23.1 x 10 6 m 2 Thickness:30.4 m Prognosis :1776 m Connection:79%

8. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved8 Observed Variograms Isoprobability Map/GRV Curve (all) Spillpoint :1834 m Volume :791.5 x 10 6 m 3 Area :25.8 x 10 6 m 2 Thickness:30.1 m Prognosis :1776 m Connection:79%

9. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved9 Variogram Problems Horizon time data - Gaussian model variogram? – Parabolic variogram behaviour at origin – Horizon times continuous surfaces? – Fresnel zone spatial averaging - “support effect” Stacking velocity data is noisy – Large nugget component on variogram – Expect Stacking Velocities to be noisy – Nugget inferred to be random error – Filter nugget during kriging or simulation

10. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved10 DEM DATA for SE England (Data courtesy Nigel Press Associates. EuroDEM ©NPA 1997)

11. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved11 DEM Variogram

12. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved12 DEM Variogram (Detail)

13. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved13 DEM Smoothing Correction

14. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved14 DEM Smoothing Correction (detail)

15. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved15 Fresnel Zone Support Effect

16. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved16 “Support” Effect Support is a sample volume concept Variance decreases with increasing sample volume “Regularisation” - no overlap between samples Fresnel zone/Vertical resolution is spatial mixing Not strictly “support”.

17. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved17 Time Variogram Corrected Time Velocity Depth X=

18. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved18 Time Variogram Corrected Prospect Analysis Spillpoint :1844 m Volume :886.6 x 10 6 m 3 Area :34.7 x 10 6 m 2 Thickness:25.5 m Prognosis :1775 m Connection:100%?

19. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved19 Time Variogram Corrected Conditional Simulation/Spillpoint

20. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved20 Time Variogram Corrected Isoprobability Map/GRV Curve Spillpoint :1834 m Volume :654.8 x 10 6 m 3 Area :20.6 x 10 6 m 2 Thickness:31.4 m Prognosis :1774 m Connection:68%

21. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved21 Time Variogram Corrected Isoprobability Map/GRV Curve (All) Spillpoint :1834 m Volume :802.0 x 10 6 m 3 Area :25.5 x 10 6 m 2 Thickness:30.8 m Prognosis :1774 m Connection:68%

22. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved22 Time/velocity Variograms Corrected Time Velocity Depth X=

23. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved23 Time/velocity Variograms Corrected Prospect Analysis Spillpoint :1843 m Volume :849.5 x 10 6 m 3 Area :33.8 x 10 6 m 2 Thickness:25.1 m Prognosis :1772 m Connection:100%?

24. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved24 Time/velocity Variograms Corrected Conditional Simulation/Spillpoint

25. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved25 Time/velocity Variograms Corrected Isoprobability Map/GRV Curve Spillpoint :1839 m Volume :807.8 x 10 6 m 3 Area :28.9 x 10 6 m 2 Thickness:27.5 m Prognosis :1774 m Connection:92%

26. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved26 Time/velocity Variograms Corrected Isoprobability Map/GRV Curve (All) Spillpoint :1939 m Volume :844.2 x 10 6 m 3 Area :30.3 x 10 6 m 2 Thickness:27.5 m Prognosis :1774 m Connection:92%

27. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved27 Volume Summary

28. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved28 Recovering the Variogram Smoothing will be a function of: – Frequency/Bandwidth – Migration (Collapse of Fresnel zone) Smoothing effect is “locked in” to data set. Correction is not a nugget - additional variance is spatially correlated Effect of smoothing is quite small in this example Effect of spillpoint criteria is more significant

29. 11-16 July, 1999© 2001 Earthworks Environment & Resources Ltd. All Rights Reserved29 How can we condition to a known hydrocarbon contact? Discard realisations with “wrong” spillpoint? – Discard all realisations? (Precision) Discard realisations with too deep spillpoint? Discard realisations with too shallow spillpoint? Condition to a spillpoint level? How do we know the control on spillpoint?