View
31
Download
0
Category
Preview:
DESCRIPTION
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. Success for a stratigraphic trap. - PowerPoint PPT Presentation
Citation preview
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Nick Loizou
EAGE LondonPESGB North Sea Special Conference Session
12th June 2007
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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.
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Layout
• Analysis of 48 wells
• Key geologic ingredients
• Successes
• Failures
• Interesting leads
• Conclusions
• A message
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Analysis of 48Stratigraphic Wells
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
• Many of the wells did not actually test a valid trap
• Better expectation for future success
But
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Post-drill analysis of ‘stratigraphic’ wells
Each of the wells has been assessed in terms of : -
Trap definition
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Post-drill analysis of ‘stratigraphic’ wells
Each of the wells has been assessed in terms of : -
Trap definition
Reservoir presence and quality
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Key geologicingredients
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Trap Definition
•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
Simplified Paleocene stratigraphic model Laggan Gas Accumulation
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Regional Seal
• 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
Identifying and mapping regional ‘pressure’ seal is fundamental to the building of a robust stratigraphic trap
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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 10-100mD with burial depths below 3 km
• Laggan T35 sandstones are better sorted, with ubiquitous chlorite grain coating
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Successful‘stratigraphic traps’
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Laggan Gas Field, Block 206/1a
• High amplitude Paleocene T35 Vaila sandstones pinchout before the main NE trending growth faults
• Laggan exhibits Class 3 Type AVO response
AVO Cross plot
HydrocarbonTrend
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
204/20a204/25b
204/19a204/24a
4
2
1
Updated: Nov 20060 1kilometres
T35
T34U
T34L
T32
T31
T25
N
204/25b-5
204/24a-5
204/24a-2Y204/24a-2Z
204/24a-4
204/24a-6
204/24-1A
204/24a-3
204/19-9
204/19-3A
204/19-4A
204/24a-8
204/24a-7
204/24a-2
P17P13
P11z
P21
P42
P41
P12
W16
W13
W12
W25
W24
W22
P211
P210
P25
P27
P26
P28
P22
P24
P23
P29
P110
P15
P18
P19
P16
WP14
W14
W11W11Z
W41
W42
W15
P11
4
2
1
PANEL 4
PANEL 3
EAST
PANEL 0
PANEL 2
PANEL 2
PANEL 1
PANEL 1 WING204/20a204/25b
204/19a204/24a
204/20a204/25b
204/19a204/24a
4
2
1
Updated: Nov 20060 1kilometres0 1kilometres
T35
T34U
T34L
T32
T31
T25
T35
T34U
T34L
T32
T31
T25
NN
204/25b-5
204/24a-5
204/24a-2Y204/24a-2Z
204/24a-4
204/24a-6
204/24-1A
204/24a-3
204/19-9
204/19-3A
204/19-4A
204/24a-8
204/24a-7
204/24a-2
204/25b-5
204/24a-5
204/24a-2Y204/24a-2Z
204/24a-4
204/24a-6
204/24-1A
204/24a-3
204/19-9
204/19-3A
204/19-4A
204/24a-8
204/24a-7
204/24a-2
P17P13
P11z
P21
P42
P41
P12
W16
W13
W12
W25
W24
W22
P211
P210
P25
P27
P26
P28
P22
P24
P23
P29
P110
P15
P18
P19
P16
WP14
W14
W11W11Z
W41
W42
W15
P11
P17P17P13P13
P11zP11z
P21P21
P42P42
P41P41
P12P12
W16W16
W13W13
W12W12
W25W25
W24W24
W22W22
P211P211
P210P210
P25P25
P27P27
P26P26
P28P28
P22P22
P24P24
P23P23
P29P29
P110P110
P15P15
P18P18
P19P19
P16P16
WP14WP14
W14W14
W11W11W11ZW11Z
W41W41
W42W42
W15W15
P11P11
4
2
1
PANEL 4
PANEL 3
EAST
PANEL 0
PANEL 2
PANEL 2
PANEL 1
PANEL 1 WING
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Hydrocarbons
Shales
Hydrocarbons
Foinaven AVO analysis (cross plots)
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Examples of failedstratigraphic traps
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Well 204/18-1 Assynt (a 2001 failure)
• 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)
T35 - T36 interval mainly sandstone
& siltstone lithologies
AVO Cross plot
Wet Rock trend
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Well 204/17-1 and relationship to Well 204/18-1
• 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
T36 Kettla Tuff/ base regional seal
T35 - T36 interval ~ mainly
sandstone lithologies
204/17-1
204/18-1
Operator’s AVO
anomalies
Location of seismic line
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
204/17-1 Composite Log over amplitude anomaly
The seismic amplitude anomaly was induced by interface A (characterised mainly by an interbedded succession of sandstones, shales and volcaniclastic tuffs)
Volcaniclastictuffs A
Predominantly sandstones aboveanomaly do not provide a seal
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Wells 204/17-1 & 18-1 AVO analysis
• 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
Dataheavilymuted
204/17-1 204/18-1
204/17-1
204/18-1
“Wet Rock” trend
“Wet Rock” trend
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Interesting prospective leads
Flett Sub-basin
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Lead 214/27A another analogue to Laggan
Seismic Section
• Amplitude anomaly conforms to structure
• Seismic profile smaller but similar to Laggan
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Laggan gas anomaly and nearby analogue
• 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
50 Kilometres 50 Kilometres
Datum – BaseKettla Tuff
0.0 s
1.0 s
0.5 s
Laggan Lead 205/4A
Seismic courtesy of PGS
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Laggan gas anomaly and nearby analogue
•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
50 Kilometres
Datum – BaseKettla Tuff
0.0 s
1.0 s
0.5 s
Laggan Lead 214/27A
Seismic courtesy of PGS
50 Kilometres
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
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
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
Acknowledgments
Many thanks to Ian Andrews and Don Cameron of the BGS, the DTI
and the PESGB
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
A message to all you ‘real’ explorationists
Stairway to SuccessAs 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 showHow the right elements could turn to gold.
And if you’ve listened very hardThe message will come to you at last.
When all are one and one is allSuccess will come to you and more.
+ + +
Improving exploration success for Paleocene Vaila Formation stratigraphic traps west of Shetland
A message to all you ‘real’ explorationists
Stairway to SuccessAs 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 showHow the right elements could turn to gold.
And if you’ve listened very hardThe message will come to you at last.
When all are one and one is allSuccess will come to you and more.
+ + +
Thank You London
Recommended