1. PESGB North Sea Special Conference Session
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 48 Stratigraphic Wells

5. Study of Paleocene ‘stratigraphic’ wells
48 stratigraphic wells located mainly in the Foinaven and Flett Sub basins
10 successes 38 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 geologic ingredients

13. Trap Definition
Simplified Paleocene stratigraphic model
Laggan Gas Accumulation
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 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 m 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, but certain sandstones have retained high porosities >20% and permeabilities mD 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
Class 3 AVO is mainly due to thin gas cap within T34 Vaila sandstones
Clear separation of shales with deeper T32 hydrocarbon bearing Vaila sandstones showing Class 3 AVO

23. Examples of failed stratigraphic 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
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 leads Flett 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
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
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 than 60% 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