Overview

1. Porosity or Sidelobes?An Application of Robust Multichannel InversionMaher S. Maklad et al.Presented at the 1993 CSEG Meeting.

2. Overview • Introduce the play • Problem statement • Data preparation • Summary of Robust Multichannel Inversion (Reflect) • Results • Conclusion

3. Introduction • Inversion can provide a fairly confident and reliable mapping of the reservoir before the drilling program commences, and can be a valuable aid to maximum exploitation of the reservoir. It allow the physical characteristics of a reservoir to be defined with satisfactory precision prior to drilling, enabling engineers and geologists to anticipate results, not only outlining the reservoir limits, in order to avoid dry holes, but also enabling optimum well placement to take advantage of the best porosity and reservoir thickness. This is particularly advantageous for offshore discoveries, where a large and very expensive platform must be located before development drilling can commence.

4. Play Concept • Granite Wash (GW) sandstone play in Northern Alberta • Clastic Sand Reservoir overlying basement • Light oil, structurally trapped by Horst blocks Granite Wash is a Devonian clastic oil reservoir in Northern Alberta. The Granite Wash zone is composed of inter-bedded sandstones, saltstones and shales with minor amounts of dolostone and anhydrite. In this area, we have light oil in the Granite Wash structurally trapped on Horst blocks. The paleotopography on the Precambrian erosional surface is extremely irregular throughout Northern Alberta. Exploration for hydrocarbon pools associated with these basement highs is therefore seismically intensive. There is significant risk in this play, particularly in situations where the Granite wash section may be missing due to non deposition or erosion over highs on the precamberian surface and wells are drilled on the flanks of highs in structurally low positions such that they encounter water bearing sandstone.

5. Schematic models of Granite Wash Oil Oil Granite Wash Sandstone Beds water water Precambrian Basement Oil Granite Wash Sandstone Beds water water Precambrian Basement

6. Schematic models of Granite Wash • Schematic models of Granite Wash zone traps, illustrating the various relationships between the Granite Wash sandstone and the underlying structure on the Precambrian basement. This particular basement high extends about one mile across. The distribution of the Granite Wash was strongly influenced by the paleotopography on the Precambrian surface. In some pools the reservoir sandstone laps out against the flank of a Precambrian high and dips away from it due to differential compaction. In other areas the sandstone is continuous over positive features on the basement and the traps are closures formed by differential compaction. The above two trap types are schematically illustrated in the previous slide.

7. Interpretation Method • Slave Point-basement isochron for structural mapping • Character mapping to delineate reservoir interval (though above basement peak) The time interval Slave Point to Precambrian is significant in delineating the structural and stratigraphic configuration of the reservoir type oil traps. It decreases as the Granite wash section thins. An important criterion for the prediction of Granite Wash is to look for a combination of two events, high amplitude trough (anhydrite-sand) over high amplitude Precambrian peak.

8. Problems • Pick basement event accurately • Identify Granite Wash interval and resolve tuning with basement • Data is poor to fair quality and noise filtering removes subtle character changes These events (high amplitude trough over high amplitude peak) are often obscured by multiple energy. A loss in amplitude is observed when the Granite Wash section is missing and the high velocity muskeg formation evaporates directly overlie the high velocity Precambrian basement. As the Granite Wash section thins, both the trough and underlying peak weaken in amplitude. Sometimes a zone of low velocity, weathered Precambrian section located between the Muskeg and the unweathered Precambrian basement can cause similar character (high amplitude trough over high amplitude peak).

9. Approach • Use Resolve to improve seismic resolution and SNR • Use Robust Multichannel Inversion (ROBIN) to resolve the events of interest from the noisy data Reflect uses an input wavelet to reduce a stacked seismic section into a reflectivity section with the locations and amplitudes of the spikes optimally selected to minimize the number of bits required to describe the error and the resulting synthetic model. In order to ensure the robustness of the spike location detection under noisy and band limited wavelet conditions, Reflect uses a multichannel detection scheme. This ensures the continuity of the detected events and guards against detecting false events.

10. Processing Sequence Stacked Seismic Data Dephasing SNR Estimation & Decon of Noisy Data Wavelet Estimation Reflectivity & Pseudo-impedance Section Robust Multichannel Inversion

11. Original Data Slave Point Basement

12. 800 820 840 860 880 900 920 940 960 980 1000 CurrentImpedance OriginalImpedance Phase Correction:Synthetics, Seismic, and Error Seismic LEFT Synthetic Seismic RIGHT Reflectivity 800 820 840 860 880 900 920 940 960 980 1000

13. 1.0 0.8 0.6 0.4 0.2 -0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -30 -10 10 30 50 1.0 100 0.8 90 0.6 0.4 0 0.2 -0.0 -90 0 0 20 40 60 80 100 20 40 60 80 100 Phase Correction: Wavelet Estimation Using Well Control

14. 0 0.1 0.2 0.3 0 0.1 0.2 0.3 Event Detection By Addition and Deletion Input Trace & True R.C.’s Add Best Single Spike Add Best 2nd Spike & Check Significance Add Best 3rd Spike & Check Significance Delete Insignificant Spike Add Best New 3rd Spike 0 0.1 0.2 0.3 0 0.1 0.2 0.3

15. .6 0.7 0.8 0.9 1 Multichannel Weighing For Continuity Weighting Function Central Trace Candidate Event Position

16. Rotated Data Slave Point Basement

17. 0 16 -10 8 -20 0 -30 -40 0 20 40 60 80 100 0 20 40 60 80 100 SNR Spectrum After Decon Power Spectrum After DECON 0 -10 20 -20 10 -30 0 -40 0 20 40 60 80 100 20 40 60 80 100 0 Bandwidth Analysis

18. Rotated Data After Resolve Slave Point Basement

19. 0.8 0.4 0.0 -0.4 -0.8 30 0 20 40 10 100 100 75 75 50 50 25 25 0 0 -25 -25 -50 -50 -75 -75 -100 -100 40 20 -20 0 50 75 100 -40 -25 -100 -75 -50 0 25 Estimated Wavelet At Location Of Interest (Using Seismic Only) 1.0 0.8 0.6 0.4 0.2 0.0 20 40 80 100 0 60 Cepstrum Power Spectrum Computed Wavelet Truncated Wavelet

20. Estimated Reflectivity Slave Point Basement

21. Estimated reflectivity and pseudo-impedance wavelet phase -zero Slave Point Basement

22. Results The salt bearing zone (Muskeg), Granite Wash interval and basement are all detected in inversion with excellent lateral continuity. The slave point, Watt Mountain and Muskeg events have improved in continuity. This result can be used to calculate accurate Slave Point - Basement isochrones and to detect the Granite Wash interval.

23. Conclusions • Reflectis effective on noisy data • Reflectproved useful for exploratory drilling: • Accurate basement pickup • Identification of Granite Wash interval.