Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland

1. Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland Nick Loizou EAGE London PESGB North Sea Special Conference Session 12th June 2007

2. Success for a stratigraphic trap It is important to remember that there are now many tools designed to help you interpret and map data. It is the skill of the geoscientist who uses them that may determine, ultimately, the success of a stratigraphic trap. There is no secret formula. Pure stratigraphic traps are relatively rare, as some degree of structural closure is often evident.

3. Layout • Analysis of 48 wells • Key geologic ingredients • Successes • Failures • Interesting leads • Conclusions • A message

4. Analysis of 48Stratigraphic Wells

5. Study of Paleocene ‘stratigraphic’ wells • 48 stratigraphic wells located mainly in the Foinaven and Flett Sub basins • 10 successes38 failures with all successful wells located close to or at the basin margins • A large number of wells drilled on geophysical ‘AVO’ anomalies But • Many of the wells did not actually test a valid trap • Better expectation for future success

6. Post-drill analysis of ‘stratigraphic’ wells Each of the wells has been assessed in terms of : - Trap definition

7. Post-drill analysis of ‘stratigraphic’ wells Each of the wells has been assessed in terms of : - Trap definition Reservoir presence and quality

8. Post-drill analysis of ‘stratigraphic’ wells Each of the wells has been assessed in terms of : - Trap definition Reservoir presence and quality Seal presence and effectiveness

9. Post-drill analysis of ‘stratigraphic’ wells Each of the wells has been assessed in terms of : - Trap definition Reservoir presence and quality Seal presence and effectiveness Source rocks and charge

10. Post-drill analysis of ‘stratigraphic’ wells Each of the wells has been assessed in terms of : - Trap definition Reservoir presence and quality Seal presence and effectiveness Source rocks and charge AVO analysis (considered by some as the joker outside the pack) is actually a key de-risking tool

11. Post-drill analysis of ‘stratigraphic’ wells Simplified Paleocene stratigraphic model The key reason for most failures has been poor trap definition However Many wells failed on a combination of geologic components

12. Key geologicingredients

13. Trap Definition Laggan Gas Accumulation Simplified Paleocene stratigraphic model • The most important prospect specific success factor is for a reliable trap model, particularly the accurate prediction of the pinch-out of reservoir sands • Presence and extent of the Kettla Tuff is shown to have a significant factor in the search for potential hydrocarbon traps

14. Regional Seal Identifying and mapping regional ‘pressure’ seal is fundamental to the building of a robust stratigraphic trap • Hydrocarbon occurrence strongly relates to the T35–T36 regional seal • General increase in aquifer pressure of 350-650 psi is observed across the T31-T35 sequence over most of the Flett Sub-basin • Foinaven Sub-basin similar age sands normally are hydrostatically pressured

15. Regional Seal (Correct log evaluation is essential) • The T36 Kettla Tuff interval in well 205/9-1(BP) is described as a sandstone, however, both the GR and DT log response indicate a tuff • Flett Sub-basin - Kettla Tuff typically 10-50m thick, while the underlying shales add a further 100 - 200m to the seal thickness • Foinaven Sub-basin - the seal is also present, but seismically is less well defined and can be much thinner

16. Reservoir presence and quality • Good reservoir-quality sandstones occur within Vaila Formation • Reservoir quality decreases with burial depth, butcertain sandstones have retained high porosities >20% and permeabilities 10-100mD with burial depths below 3 km • Laggan T35 sandstones are better sorted, with ubiquitous chlorite grain coating

17. Successful‘stratigraphic traps’

18. Success: Laggan Gas Field, Block 206/1a • Excellent analogue of a Paleocene stratigraphic trap Shell discovery 1986 using 2D data • Potentially similar prospects exist along trend over a number of untested blocks • However, high amplitudes are indicative of high porosities, but not always hydrocarbons GWC 3907m Map courtesy of Total

19. Laggan Gas Field, Block 206/1a AVO Cross plot • High amplitude Paleocene T35 Vaila sandstones pinchout before the main NE trending growth faults • Laggan exhibits Class 3 Type AVO response Hydrocarbon Trend

20. Foinaven - Identify and Locate Hydrocarbon-bearing Reservoir • Producing since 1997 • Has produced over 250 mmbbls • Foinaven has seismically- favourable rock properties • Hydrocarbon-bearing sands have low impedance relative to shale, and stand out on conventional full-stack seismic data • It would not be possible to define the extent of the reservoir without seismic data NB Amplitudes shown exclude T35, T25 and T34U to the east of East Foinaven Courtesy of BP 2007

21. Foinaven AVO analysis Combination structural / stratigraphic trap in T31 to T34 Vaila sandstones. Amplitude anomalies generally conform to structure • Hydrocarbon-saturated sands generate strong seismic amplitude anomalies – help to define extent of the pools

22. Foinaven AVO analysis (cross plots) Shales Hydrocarbons Hydrocarbons Clear separation of shales with deeper T32 hydrocarbon bearing Vaila sandstones showing Class 3 AVO Class 3 AVO is mainly due to thin gas cap within T34 Vaila sandstones

23. Examples of failedstratigraphic traps

24. Well 204/18-1 Assynt (a 2001 failure) T35 - T36 interval mainly sandstone & siltstone lithologies AVO Cross plot • Originally interpreted as a significant stratigraphic trap with Class 3 type AVO  • More recent work by DTI/BGS clearly demonstrates a Class 1 type AVO (no hydrocarbons) Wet Rock trend

25. 1990’s success and nearby Assynt 2001 failure • Regional setting shows a fundamental problem particularly with the validity of the trap and sourcing • Assynt was interpreted as a direct fairway analogue to Foinaven - however by comparison - no evidence of true amplitude conformance with structure

26. Well 204/17-1 and relationship to Well 204/18-1 T36 Kettla Tuff/ base regional seal T35 - T36 interval ~ mainly sandstone lithologies 204/17-1 Operator’s AVO anomalies 204/18-1 Location of seismic line • Well 204/17-1 is located 8.8 kms updip of well 204/18-1 • No clear evidence of a reliable trap at the 204/17-1 location

27. 204/17-1 Composite Log over amplitude anomaly Predominantly sandstones above anomaly do not provide a seal Volcaniclastic tuffs A The seismic amplitude anomaly was induced by interface A (characterised mainly by an interbedded succession of sandstones, shales and volcaniclastic tuffs)

28. Wells 204/17-1 & 18-1 AVO analysis 204/17-1 Data heavily muted 204/17-1 204/18-1 “Wet Rock” trend 204/18-1 • Both 204/17-1 and 18-1 wells show decrease of amplitudes with offset • Equally, both wells show Class 1 type AVO • High amplitudes seen predominantly on near offsets mainly due to lithology effects “Wet Rock” trend

29. Interesting prospective leadsFlett Sub-basin

30. Two Paleocene ‘stratigraphic’ leads Lead 205/4A located 12 kms SW of the Laggan gas accumulation Lead 214/27A located 10 kms NE of the Laggan gas accumulation

31. Lead 205/4A - example of a Laggan analogue • Amplitude anomaly generally conforms to structure • Comparable undrilled leads/prospects exist along trend over a number of blocks • Requires further de-risking

32. Lead 214/27A another analogue to Laggan Seismic Section • Amplitude anomaly conforms to structure • Seismic profile smaller but similar to Laggan

33. Laggan gas anomaly and nearby analogue Lead 205/4A Laggan Datum – Base Kettla Tuff 0.0 s 0.5 s 50 Kilometres 50 Kilometres 1.0 s Seismic courtesy of PGS • Flattened on the Base Kettla Member, the anomalies show a consistent amplitude strengthening at the basin margin • Lead 205/4A has distinct similarities with the Laggan gas accumulation

34. Laggan gas anomaly and nearby analogue Lead 214/27A Laggan Datum – Base Kettla Tuff 0.0 s 0.5 s 50 Kilometres 50 Kilometres 1.0 s Seismic courtesy of PGS • Flattened on the Base Kettla Member, the anomalies show a consistent amplitude strengthening at the basin margin • Lead 214/27A although smaller has distinct similarities with the Laggan gas accumulation

35. Conclusions 38 wells failed to find hydrocarbons – more than60% had what I would view as no reliable trap to begin with High success rate for wells positioned on valid traps > 60% Sound interpretation and mapping using high quality seismic data is viewed as the key component to increasing exploration success AVO analysis carried out correctly provides an important element to help further de-risk Paleocene prospects Utilising appropriate data Paleocene Vaila stratigraphic traps including other areas outside this study can be mapped with confidence New technologies including electromagnetic surveying (check resistivity of gas sands) would provide further derisking tools

36. Acknowledgments Many thanks to Ian Andrews and Don Cameron of the BGS, the DTI and the PESGB

37. A message to all you ‘real’ explorationists Stairway to Success As you explore for stratigraphic traps The true geology must be unfold. There walks a man you now all know Who has shown you the light and wants to show How the right elements could turn to gold. And if you’ve listened very hard The message will come to you at last. When all are one and one is all Success will come to you and more. + + +

38. A message to all you ‘real’ explorationists Stairway to Success As you explore for stratigraphic traps The true geology must be unfold. There walks a man you now all know Who has shown you the light and wants to show How the right elements could turn to gold. And if you’ve listened very hard The message will come to you at last. When all are one and one is all Success will come to you and more. + + + Thank You London