1. Salt imaging in the Nordkapp Basin with electromagnetic data
Classification: Internal Status: Draft
Salt imaging in the Nordkapp Basin with electromagnetic data
Ketil Hokstad*, Eva Andrea Myrlund, Bente Fotland, StatoilHydro Harstad
Harald Westerdahl, StatoilHydro (formerly NGI)
Kai Hogstad, Arild Ingebrigtsen, formerly Statoil
Luca Masnaghetti, Geosystem WG EM
Lina Uri, EMGS
AAPG Moscow 1. October 2009

2. The Nordkapp Basin - Location
Disputed Area
Two wells in the basin.
Bjarmeland Platform
Nordkapp basin North
Hammerfest basin
Nordkapp basin South
Finnmark Platform

3. Main reservoir target- Carnian sst
Quaternary
Tertiary
Cretaceous
Jurassic
Triassic
Approximately 30 salt bodies.
Diapirs in South
Salt walls in North
Main reservoir target- Carnian sst
Permian

4. Uranus pre and post-well interpretations
Max salt interpretation post-well:
Worst case scenario !
Pre-well
Point on 7227/11-1A well path
Depth (m)
TD 7227/11-1A
The seismic imaging problem
Weak primaries
Strong multiples
Diffractions

5. High-resolution image of the sea floor from site survey data
The seismic shadow zone- seafloor
5 km
7227/11-1S
High-resolution image of the sea floor from site survey data
These are present in the whole region.

6. The seismic shadow zone- where is top salt?
Cretaceous sediments
Seafloor
Young sediments
Top diapir
Salt?
Obs this is not the main reason behind the shadow zone, but it contributes a great deal
The top of salt often represents a very strong and undulating reflector resulting in focusing/defocusing energy effects and generating diffractions.

7. The seismic shadow zone - caprock and salt inpurities
Cap rock (Permian)
Pandora
From the original deposition of salt in the evaporittic sequence other sediments such as gypsum, its dewatered equivalent anhydrite, shales muds and carbonates will exist in the matter that makes up the diapir, as the matter moves into the diapir and upwards these layers will be broken up and transported with the lines of flow in the salt. As top of the diapir is exposed to meteoric water during times of terrestrial conditions the halite part of the diapir will be desolved and leaves a caotic, cumulated cap rock layer. These bits and bobs of vareing density and velocity will set up random reflections that adds to the allready difficult seismic image.
(Diapir)

8. Velocity models “Dirty salt”
Model 1: Clean salt model
Model 2: Clean salt model with top diffractors
Model 3: Dirty salt model (pertubations 70% of salt velocity)
Model 4: Dirty salt model with top diffractors(pertubations 70% of salt velocity)
Haugen, Arntsen and Mispel, SEG 2008

9. PSDM with clean-salt velocity model
Diffractors
Diffractors and rugous top salt
Real data
Haugen, Arntsen and Mispel, SEG 2008

10. Can we image the salt-sediment interface with alternative geophysical methods?
Characteristic properties of rock salt:
Low mass density
Gravimetry
High electric resitivity
Magnetotellurics
Controlled-Source EM
Imaging the salt-sediment interface with gravity and EM
Seismic is needed to get the details
Top Salt
Top Diapir

11. Salt imaging with CSEM Phase 1: Proving the concept
Novel idea proposed by Harald Westerdahl, formerly NGI:
Salt has high resistivity (we logged 1000 Ohm-m in Uranus well)
Salt imaging using CSEM should be feasible
Receiver development and field test with NGI in 2006
Receiver sled landing on Uranus seabed, 15. August 2006
Pre-survey modeling
Real data – 100 Hz

12. Feasibility - CSEM Max and min salt models
Salt resistrivity: 1000 Ohmm
Can CSEM discriminate between max and min salt cases?
Can CSEM see the salt stem?
Statoil in-house

13. Real vs 3D synthetic data
Total field amplitude, 1 and 10 Hz
Statoil in-house

14. Min vs max salt interpretation
Relative Magnitude Difference
Phase Difference
Statoil in-house

15. Min salt with and without stem
Relative Magnitude Difference
Phase Difference
Statoil in-house

16. Depth is correct, but lateral resolution is poor
Feasibility - MT
MT is sensitive to thick high-resistive bodies
Model
Depth is correct, but lateral resolution is poor
Inversion
emgs – Lina Uri

17. Feasibility - MT MT is sensitive to thick high-resistive bodies
MT is sensitive to salt shape and depth
Well known from the literature
Model
Inversion
emgs – Lina Uri

18. Salt imaging with CSEM and MMT Phase 2: Acquisition with EMGS April 2007
EM acquisition with emgs, Nordkapp Basin April-May 2007
Same receivers for both CSEM and MT
CSEM: Purpose designed high-frequency source signal
MT: 48 hours nominal listening time

19. CSEM magnitude and phase - Up-going Ex
Magnitude 3 Hz
Phase 3 Hz

20. CSEM salt-flood model and depth migration
StatoilHydro in-house

21. Raw WVSP data; HR-comp (Otnes and Hokstad, EAGE 2009)

22. WVSP after SRME; HR-comp (Otnes and Hokstad, EAGE 2009)

23. WVSP Virtual source image
Uranus S01 (well line) – 3D PSDM
Gravity inversion
WVSP Virtual source image

24. WVSP Virtual source image CSEM BoS interpretation
Uranus S01 (well line) – CSEM migration
Gravity inversion
WVSP Virtual source image
CSEM BoS interpretation

25. WVSP Virtual source image CSEM BoS interpretation
Uranus S01 (well line) – CSEM migration
Gravity inversion
WVSP Virtual source image
CSEM BoS interpretation

26. Magnetotellurics (MT)
Passive exploration method
Naturally occuring geomagnetic variations is the power source

27. Uranus S01 (well line) – MT apparent resistivityTM
mode
MT processing by Geosystem – Luca Masnaghetti

28. Account for static shift in inversion
MT inversion – typical approach
100 s
Account for static shift in inversion 1D 2D 3D
02Rx302
Inversion with smoothness constraints
Account for static in inversion (fixed or free parameter)
Typical periods used: > 1 sec to 200 sec (1 Hz to Hz)
Significance of 3D effects jugded on polarization ellipses or skew
3D near surface effects
3D basement effects

29. CSEM BoS interpretation
Uranus S01 (well line) – MT inversion
CSEM BoS interpretation

30. WVSP Virtual source image CSEM BoS interpretation
Uranus S01 (well line) – MT and CSEM
WVSP Virtual source image
Pick 10 Ohm-m iso-resistivity surface as BoS, concistent with CSEM and gravity
CSEM BoS interpretation

31. Nordkapp Basin - Brief geological history
Early Triassic: Reactive diapirism
Tertiary: Diapir shortening and uplift
Middle Triassic: Differential loading
Tertiary: Erosion
Middle Triassic: Rapid diapir rise and salt extrusion
Nilsen, Vendeville and Johansen (1995)

32. Economic implications of salt body size and shape
Uranus
Large salt – small volumes
Small salt and overhang – large volumes

33. Salt imaging with CSEM and MMT Phase 3: EM campaign in Seismic Area F (PL230)
Layout 4
Layout 3
Layout 2
Layout 1
Uranus ST0750
Mostly Harmless
Jupiter
4 layouts of 48 receivers
13 EM lines
Including mini 3D
Covering 11 diapirs

34. EM campaign – Results
MT inversion
CSEM depth migration

35. Photo: Eva Andrea Myrlund
Conclusions
Base and flanks of salt in NKB can not be properly imaged using seismic alone
MT and CSEM contribute important information, but with lower resolution than seismics
Data integration is important to complete the NKB imaging puzzle (EM, grav, seismic)
Most likely there are no diapirs like the Uranus pre-well model in Southern Nordkapp Basin
Photo: Eva Andrea Myrlund

36. Photo: Eva Andrea Myrlund
Acknowledgements
StatoilHydro GET/R&D
Christopher Stadtler
Kerstin Henneberg
Erik Gundersen
Andreas Becht
Christine Fichler
Alexander Kritski
Lars Løseth
Jon Andre Haugen
Børge Arntsen
Rune Kyrkjebø
Partners:
GdF Suez (PL230)
Chevron and RWE (PL397)
emgs:
Kristoffer Engenes (Aker)
Rune Mittet
Crew of OS Relume
Geosystem WG-EM:
Don Watts
StatoilHydro Management
Geir Richardsen
John Reidar Granli
Tage Røsten
ex-Statoil:
Einar Otnes (WG)
Constantin Gerea (Total)
Photo: Eva Andrea Myrlund