improving exploration success for paleocene vaila formation stratigraphic traps west of shetland...

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

Nick Loizou

EAGE LondonPESGB North Sea Special Conference Session

12th June 2007


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.


Layout

• Analysis of 48 wells

• Key geologic ingredients

• Successes

• Failures

• Interesting leads

• Conclusions

• A message


Analysis of 48Stratigraphic Wells


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


Post-drill analysis of ‘stratigraphic’ wells

Each of the wells has been assessed in terms of : -

Trap definition




Trap definition

Reservoir presence and quality




Trap definition


Seal presence and effectiveness




Trap definition



Source rocks and charge




Trap definition



Source rocks and charge

AVO analysis (considered by some as the joker outside the pack) is actually a key de-risking tool



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


Key geologicingredients


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


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


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



• 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


Successful‘stratigraphic traps’


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


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


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


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


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)


Examples of failedstratigraphic traps


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


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


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


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


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


Interesting prospective leads

Flett Sub-basin


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


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


Lead 214/27A another analogue to Laggan

Seismic Section

• Amplitude anomaly conforms to structure

• Seismic profile smaller but similar to Laggan


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


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


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


Acknowledgments

Many thanks to Ian Andrews and Don Cameron of the BGS, the DTI

and the PESGB


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.

+ + +


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

improving exploration success for paleocene vaila formation stratigraphic traps west of shetland...

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