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Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration

Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration
Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration
Fluid-escape structures within sedimentary basins permit pressure-driven focused fluid flow through inter-connected faults, fractures and sediment. Seismically-imaged chimneys are recognised as fluid migration pathways which cross-cut overburden stratigraphy, hydraulically connecting deeper strata with the seafloor. However, the geological processes in the sedimentary overburden which control the mechanisms of genesis and temporal evolution require improved understanding. We integrate high-resolution 2D and 3D seismic reflection data with sediment core data to characterise a natural, active site of seafloor methane venting in the UK North Sea and Witch Ground Basin, the Scanner pockmark complex. A regional assessment of shallow gas distribution presents direct evidence of active and palaeo-fluid migration pathways which terminate at the seabed pockmarks. We show that these pockmarks are fed from a methane gas reservoir located at 70 metres below the seafloor. We find that the shallow reservoir is a glacial outwash fan, that is laterally sealed by glacial tunnel valleys. Overpressure generation leading to chimney and pockmark genesis is directly controlled by the shallow geological and glaciogenic setting. Once formed, pockmarks act as drainage cells for the underlying gas accumulations. Fluid flow occurs through gas chimneys, comprised of a sub-vertical gas-filled fracture zone. Our findings provide an improved understanding of focused fluid flow and pockmark formation within the sediment overburden, which can be applied to subsurface geohazard assessment and geological storage of CO2.
Chimneys, Pipes, Overburden, Pockmarks, Fluid flow, North Sea, CO2 sequestration, Glacial Stratigraphy
0264-8172
Callow, Ben
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Bull, Jonathan
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Provenzano, Giuseppe
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Böttner, Christoph
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Birinci, Hamza
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Robinson, Adam
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Henstock, Timothy
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Minshull, Timothy
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Bayrakci, Gaye
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Lichtschlag, Anna
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Roche, Ben
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Yilo, Naima, Karolina
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Gehrmann, Romina
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Karstens, Jens
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Falcon-Suarez, Ismael Himar
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Berndt, Christian
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Callow, Ben
15166203-d3e6-4b28-8369-e99e1bd00240
Bull, Jonathan
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Provenzano, Giuseppe
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Böttner, Christoph
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Birinci, Hamza
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Robinson, Adam
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Henstock, Timothy
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Minshull, Timothy
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Bayrakci, Gaye
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Lichtschlag, Anna
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Roche, Ben
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Yilo, Naima, Karolina
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Gehrmann, Romina
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Karstens, Jens
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Falcon-Suarez, Ismael Himar
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Berndt, Christian
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Callow, Ben, Bull, Jonathan, Provenzano, Giuseppe, Böttner, Christoph, Birinci, Hamza, Robinson, Adam, Henstock, Timothy, Minshull, Timothy, Bayrakci, Gaye, Lichtschlag, Anna, Roche, Ben, Yilo, Naima, Karolina, Gehrmann, Romina, Karstens, Jens, Falcon-Suarez, Ismael Himar and Berndt, Christian (2021) Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration. Marine and Petroleum Geology. (In Press)

Record type: Article

Abstract

Fluid-escape structures within sedimentary basins permit pressure-driven focused fluid flow through inter-connected faults, fractures and sediment. Seismically-imaged chimneys are recognised as fluid migration pathways which cross-cut overburden stratigraphy, hydraulically connecting deeper strata with the seafloor. However, the geological processes in the sedimentary overburden which control the mechanisms of genesis and temporal evolution require improved understanding. We integrate high-resolution 2D and 3D seismic reflection data with sediment core data to characterise a natural, active site of seafloor methane venting in the UK North Sea and Witch Ground Basin, the Scanner pockmark complex. A regional assessment of shallow gas distribution presents direct evidence of active and palaeo-fluid migration pathways which terminate at the seabed pockmarks. We show that these pockmarks are fed from a methane gas reservoir located at 70 metres below the seafloor. We find that the shallow reservoir is a glacial outwash fan, that is laterally sealed by glacial tunnel valleys. Overpressure generation leading to chimney and pockmark genesis is directly controlled by the shallow geological and glaciogenic setting. Once formed, pockmarks act as drainage cells for the underlying gas accumulations. Fluid flow occurs through gas chimneys, comprised of a sub-vertical gas-filled fracture zone. Our findings provide an improved understanding of focused fluid flow and pockmark formation within the sediment overburden, which can be applied to subsurface geohazard assessment and geological storage of CO2.

Text
Callow_et_al_2021_JMPG-D-21-00551 - Accepted Manuscript
Available under License Creative Commons Attribution.
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Accepted/In Press date: 19 August 2021
Keywords: Chimneys, Pipes, Overburden, Pockmarks, Fluid flow, North Sea, CO2 sequestration, Glacial Stratigraphy

Identifiers

Local EPrints ID: 450937
URI: http://eprints.soton.ac.uk/id/eprint/450937
ISSN: 0264-8172
PURE UUID: 3fbe7880-dd28-40c4-b6bc-c853c80c6ef1
ORCID for Timothy Henstock: ORCID iD orcid.org/0000-0002-2132-2514
ORCID for Timothy Minshull: ORCID iD orcid.org/0000-0002-8202-1379
ORCID for Romina Gehrmann: ORCID iD orcid.org/0000-0002-3099-2771

Catalogue record

Date deposited: 24 Aug 2021 17:03
Last modified: 25 Aug 2021 01:45

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Contributors

Author: Ben Callow
Author: Jonathan Bull
Author: Giuseppe Provenzano
Author: Christoph Böttner
Author: Hamza Birinci
Author: Adam Robinson
Author: Gaye Bayrakci
Author: Anna Lichtschlag
Author: Ben Roche
Author: Romina Gehrmann ORCID iD
Author: Jens Karstens
Author: Ismael Himar Falcon-Suarez
Author: Christian Berndt

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