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Porosity and free gas estimates from controlled source electromagnetic data at the Scanner Pockmark in the North Sea

Porosity and free gas estimates from controlled source electromagnetic data at the Scanner Pockmark in the North Sea
Porosity and free gas estimates from controlled source electromagnetic data at the Scanner Pockmark in the North Sea

We present porosity and free gas estimates and their uncertainties at an active methane venting site in the UK sector of the North Sea. We performed a multi-disciplinary experiment at the Scanner Pockmark area in about 150 m water depth to investigate the physical properties of fluid flow structures within unconsolidated glaciomarine sediments. Here, we focus on the towed controlled source electromagnetic (CSEM) data analysis with constraints from seismic reflection and core logging data. Inferred background resistivity values vary between 0.6–1 Ωm at the surface and 1.9–2.4 Ωm at 150 mbsf. We calibrate Archie's parameters with measurements on cores, and estimate porosities of about 50±10% at the seafloor decreasing to 25±3% at 150 mbsf which matches variations expected for mechanical compaction of clay rich sediments. High reflectivity in seismic reflection data is consistent with the existence of a gas pocket. A synthetic study of varying gas content in this gas pocket shows that at least 33±8% of free gas is required to cause a distinct CSEM data anomaly. Real data inversions with seismic constraints support the presence of up to 34±14% free gas in a 30–40 m thick gas pocket underneath the pockmark within the stratigraphic highs of a till layer above the glacial unconformity in the Aberdeen Ground Formation.

Electromagnetics, Fluid pathway, North Sea, Physical properties, Resistivity, Seismic reflection
1750-5836
Gehrmann, Romina
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Provenzano, Giuseppe
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Böttner, Christoph
e35e2e9c-3e87-48b2-a5bb-a68cd68266be
Moreno, Hector Marin
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Bayrakci, Gaye
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Tan, Yee Y
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Yilo, Naima Karolina
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Djanni, Axel T.
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Weitemeyer, Karen
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Minshull, Timothy
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Bull, Jonathan
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Karstens, Jens
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Berndt, Christian
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Gehrmann, Romina
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Provenzano, Giuseppe
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Böttner, Christoph
e35e2e9c-3e87-48b2-a5bb-a68cd68266be
Moreno, Hector Marin
3fe1141c-c2f4-475a-89ea-bec8a87388bd
Bayrakci, Gaye
0f436a50-6297-4ed1-9c12-797f16cbc9c4
Tan, Yee Y
7e01f401-e518-4cdd-a123-054d8b9ca5a8
Yilo, Naima Karolina
317f9415-f7ab-4277-acf0-51064751b995
Djanni, Axel T.
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Weitemeyer, Karen
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Minshull, Timothy
bf413fb5-849e-4389-acd7-0cb0d644e6b8
Bull, Jonathan
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Karstens, Jens
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Berndt, Christian
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Gehrmann, Romina, Provenzano, Giuseppe, Böttner, Christoph, Moreno, Hector Marin, Bayrakci, Gaye, Tan, Yee Y, Yilo, Naima Karolina, Djanni, Axel T., Weitemeyer, Karen, Minshull, Timothy, Bull, Jonathan, Karstens, Jens and Berndt, Christian (2021) Porosity and free gas estimates from controlled source electromagnetic data at the Scanner Pockmark in the North Sea. International Journal of Greenhouse Gas Control, 109, [103343]. (doi:10.1016/j.ijggc.2021.103343).

Record type: Article

Abstract

We present porosity and free gas estimates and their uncertainties at an active methane venting site in the UK sector of the North Sea. We performed a multi-disciplinary experiment at the Scanner Pockmark area in about 150 m water depth to investigate the physical properties of fluid flow structures within unconsolidated glaciomarine sediments. Here, we focus on the towed controlled source electromagnetic (CSEM) data analysis with constraints from seismic reflection and core logging data. Inferred background resistivity values vary between 0.6–1 Ωm at the surface and 1.9–2.4 Ωm at 150 mbsf. We calibrate Archie's parameters with measurements on cores, and estimate porosities of about 50±10% at the seafloor decreasing to 25±3% at 150 mbsf which matches variations expected for mechanical compaction of clay rich sediments. High reflectivity in seismic reflection data is consistent with the existence of a gas pocket. A synthetic study of varying gas content in this gas pocket shows that at least 33±8% of free gas is required to cause a distinct CSEM data anomaly. Real data inversions with seismic constraints support the presence of up to 34±14% free gas in a 30–40 m thick gas pocket underneath the pockmark within the stratigraphic highs of a till layer above the glacial unconformity in the Aberdeen Ground Formation.

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Accepted/In Press date: 20 April 2021
e-pub ahead of print date: 12 May 2021
Published date: July 2021
Additional Information: Funding Information: We would like to thank the MSM63 and MSM78 cruise crews and scientific parties, especially the Ocean Bottom Instrument Consortium team and Laurence North for CSEM instrumentation support, the GEOMAR seismic data acquisition and processing team, and the BGS Rock Drill 2 team. We thank the British Ocean Sediment Core Research Facility team, especially Millie Watts, for the core data and the helpful discussions. We are grateful to Steven Constable from Scripps Institution of Oceanography, USA, for advise and lending eight CSEM data loggers for the survey. CSEM processing and inversion input routines are based on scripts by David Myer and Kerry Key. We would like to thank Kerry Key for his advise regarding the phase inversion. We thank two anonymous reviewers for their constructive comments. This work was supported by the European Union's Horizon 2020 research and innovation program under grant agreement no. 654462 and NERC grant NE/N01610/1. We acknowledge the use of the IRIDIS High Performance Computing Facility, and associated support services at the University of Southampton, in the completion of this work. The CSEM and seismic data sets are available at Gehrmann et al. (2020) and B?ttner and Berndt (2019). Funding Information: We would like to thank the MSM63 and MSM78 cruise crews and scientific parties, especially the Ocean Bottom Instrument Consortium team and Laurence North for CSEM instrumentation support, the GEOMAR seismic data acquisition and processing team, and the BGS Rock Drill 2 team. We thank the British Ocean Sediment Core Research Facility team, especially Millie Watts, for the core data and the helpful discussions. We are grateful to Steven Constable from Scripps Institution of Oceanography, USA, for advise and lending eight CSEM data loggers for the survey. CSEM processing and inversion input routines are based on scripts by David Myer and Kerry Key. We would like to thank Kerry Key for his advise regarding the phase inversion. We thank two anonymous reviewers for their constructive comments. This work was supported by the European Union's Horizon 2020 research and innovation program under grant agreement no. 654462 and NERC grant NE/N01610/1 . We acknowledge the use of the IRIDIS High Performance Computing Facility, and associated support services at the University of Southampton, in the completion of this work. The CSEM and seismic data sets are available at Gehrmann et al. (2020) and Böttner and Berndt (2019) . Publisher Copyright: © 2021 Elsevier Ltd
Keywords: Electromagnetics, Fluid pathway, North Sea, Physical properties, Resistivity, Seismic reflection

Identifiers

Local EPrints ID: 449200
URI: http://eprints.soton.ac.uk/id/eprint/449200
ISSN: 1750-5836
PURE UUID: 07806c21-57b8-4fce-a348-2dbbaff78fd8
ORCID for Romina Gehrmann: ORCID iD orcid.org/0000-0002-3099-2771
ORCID for Timothy Minshull: ORCID iD orcid.org/0000-0002-8202-1379
ORCID for Jonathan Bull: ORCID iD orcid.org/0000-0003-3373-5807

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Date deposited: 19 May 2021 18:17
Last modified: 18 Nov 2022 02:44

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Contributors

Author: Romina Gehrmann ORCID iD
Author: Giuseppe Provenzano
Author: Christoph Böttner
Author: Hector Marin Moreno
Author: Gaye Bayrakci
Author: Yee Y Tan
Author: Axel T. Djanni
Author: Karen Weitemeyer
Author: Jonathan Bull ORCID iD
Author: Jens Karstens
Author: Christian Berndt

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