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Seabed Fluid Flow
Impact of geology, biology and the
marine environment
Alan Judd and Martin Hovland
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Contents
Tables ...................................................................................................................................................ix
Figures .................................................................................................................................................ix
Accompanying CD .................................................................................................................................. x
Figures ................................................................................................................................................. x
Maps .................................................................................................................................................... x
Contributed Presentations .....................................................................................................................xi
Foreword ............................................................................................................................................... xii
Note on the accompanying CD ............................................................................................................ xii
Acknowledgements .............................................................................................................................. xiii
SEABED FLUID FLOW INTRODUCTION .......................................................... 1
POCKMARKS, SHALLOW GAS AND SEEPS: AN INITIAL APPRAISAL ......... 6
2.1 The Scotian Shelf: the early years ............................................................................................. 6
2.2 North Sea pockmarks ................................................................................................................. 7
2.2.1 History of Discovery ................................................................................................................ 7
2.2.2 Pockmark Distribution ............................................................................................................. 8
2.2.3 Pockmark size and density........................................................................................................ 9
2.2.4 Pockmark morphology ............................................................................................................. 9
2.2.5 Evidence of Gas ..................................................................................................................... 12
2.3 Detailed surveys of North Sea pockmarks and seeps .............................................................. 12
2.3.1 The South Fladen Pockmark Study Area ................................................................................ 13
2.3.2 Tommeliten: Norwegian Block 1/9 ......................................................................................... 15
2.3.3 Norwegian Block 25/7 ........................................................................................................... 17
2.3.4 The Holene: Norwegian Block 24/9 ....................................................................................... 18
2.3.5 The Norwegian Trench .......................................................................................................... 18
2.3.6 Gullfaks ................................................................................................................................. 19
2.3.7 Giant pockmarks: UK Block 15/25......................................................................................... 21
2.4 Conclusions............................................................................................................................... 22
SEABED FLUID FLOW AROUND THE WORLD ............................................. 24
3.1 Introduction ............................................................................................................................. 24
3.2 The Eastern Arctic ................................................................................................................... 24
3.2.1 The Barents Sea ..................................................................................................................... 24
3.2.2 Håkon Mosby Mud Volcano .................................................................................................. 25
3.3 Scandinavia .............................................................................................................................. 26
3.3.1 Fjords in northern Norway ..................................................................................................... 26
3.3.2 The Norwegian Sea ................................................................................................................ 26
3.3.3 The Skagerrak........................................................................................................................ 27
3.3.4 The Kattegat .......................................................................................................................... 28
3.4 The Baltic Sea........................................................................................................................... 28
3.4.1 Eckernförde Bay .................................................................................................................... 28
3.4.2 Stockholm Archipelago, Sweden ............................................................................................ 29
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3.5 Around the British Isles ........................................................................................................... 31
3.5.1 Pockmarks, domes and seeps ................................................................................................. 32
3.5.2 ‘Freak’ sandwaves.................................................................................................................. 32
3.5.3 MDAC ................................................................................................................................... 33
3.5.4 The Atlantic Margin ............................................................................................................... 33
3.6 Iberia ........................................................................................................................................ 35
3.6.1 The Rías of Galicia, NW Spain ............................................................................................... 35
3.6.2 Gulf of Cadiz ......................................................................................................................... 36
3.6.3 Ibiza ....................................................................................................................................... 38
3.7 Africa ........................................................................................................................................ 38
3.7.1 The Niger Delta and Fan .................................................................................................... 38
3.7.2 The Continental Slope of West Africa .................................................................................... 39
3.8 The Mid-Atlantic Ridge ........................................................................................................... 40
3.9 The Adriatic Sea....................................................................................................................... 40
3.9.1 Seeps and carbonates of the Northern Adriatic ....................................................................... 40
3.9.2 Pockmarks, seeps and mud diapirs in the Central Adriatic ....................................................... 40
3.10 The Eastern Mediterranean..................................................................................................... 41
3.10.1 Offshore Greece ................................................................................................................. 42
3.10.2 Mediterranean Ridge .......................................................................................................... 43
3.10.3 The Anaximander Mountains ............................................................................................. 45
3.10.4 Erastosthenes Seamount .................................................................................................... 45
3.10.5 Nile Delta and Fan ............................................................................................................. 45
3.11 The Black Sea ........................................................................................................................... 46
3.11.1 Turkish Coast .................................................................................................................... 47
3.11.2 Offshore Bulgaria............................................................................................................... 47
3.11.3 North-western Black Sea ................................................................................................... 47
3.11.4 Central and Northern Black Sea ......................................................................................... 48
3.11.5 The 'Underwater Swamps' of the East Black Sea abyssal plain ............................................ 49
3.11.6 Offshore Georgia ............................................................................................................... 49
3.12 Inland Seas of Eurasia ............................................................................................................. 49
3.12.1 The Caspian Sea ................................................................................................................ 49
3.12.2 Lake Baikal........................................................................................................................ 50
3.13 The Red Sea.............................................................................................................................. 51
3.14 The Arabian Gulf ..................................................................................................................... 52
3.14.1 Setting ............................................................................................................................... 52
3.14.2 Seabed features .................................................................................................................. 53
3.14.3 Strait of Hormuz ................................................................................................................ 54
3.15 The Indian sub-continent ........................................................................................................ 54
3.15.1 The Makran Coast ............................................................................................................. 54
3.15.2 Western coast of India ....................................................................................................... 55
3.15.3 The eastern coast of the sub-continent................................................................................ 55
3.15.4 Indian Ocean vent fauna ..................................................................................................... 55
3.16 South China Sea ....................................................................................................................... 56
3.16.1 Offshore Brunei ................................................................................................................. 56
3.16.2 Offshore Vietnam............................................................................................................... 56
3.16.3 Hong Kong ........................................................................................................................ 56
3.16.4 Taiwan............................................................................................................................... 56
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3.17 Australasia ................................................................................................................................ 57
3.17.1 Sawu Sea ........................................................................................................................... 57
3.17.2 Timor Sea .......................................................................................................................... 57
3.17.3 New Britain and the Manus Basins ..................................................................................... 58
3.17.4 New Zealand...................................................................................................................... 59
3.18 Western Pacific......................................................................................................................... 61
3.18.1 Silicic dome volcanism in the Mariana Trough back-arc basin ............................................. 61
3.18.2 Serpentine mud volcanoes near the Mariana Trench ........................................................... 61
3.18.3 The Yellow and East China Seas ........................................................................................ 62
3.18.4 Offshore Korea .................................................................................................................. 62
3.18.5 Japan ................................................................................................................................. 62
3.18.6 Sea of Okhotsk .................................................................................................................. 64
3.18.7 Piip submarine volcano, East of Kamchatka ....................................................................... 66
3.19 Offshore Alaska ........................................................................................................................ 67
3.19.1 Bering Sea ......................................................................................................................... 67
3.19.2 Gulf of Alaska.................................................................................................................... 69
3.19.3 The Aleutian Subduction Zone ........................................................................................... 70
3.20 British Columbia ...................................................................................................................... 71
3.20.1 Queen Charlotte Sound ...................................................................................................... 71
3.20.2 The Fraser Delta ................................................................................................................ 71
3.21 Juan de Fuca ............................................................................................................................ 73
3.21.1 Hydrate Ridge.................................................................................................................... 73
3.21.2 Axial Seamount ................................................................................................................. 74
3.22 California .................................................................................................................................. 75
3.22.1 Northern California ............................................................................................................ 75
3.22.2 Monterey Bay .................................................................................................................... 77
3.22.3 Big Sur .............................................................................................................................. 79
3.22.4 Santa Barbara Channel ....................................................................................................... 79
3.22.5 Malibu Point ...................................................................................................................... 81
3.23 Ocean Spreading Centres of the East Pacific .......................................................................... 81
3.23.1 Guaymas Basin, Gulf of California ..................................................................................... 81
3.24 Central and South America ..................................................................................................... 82
3.24.1 Costa Rica ......................................................................................................................... 82
3.24.2 Peru ................................................................................................................................... 84
3.24.3 The Argentine Basin .......................................................................................................... 84
3.24.4 The Mouth of the Amazon ................................................................................................. 85
3.25 The Caribbean ......................................................................................................................... 85
3.25.1 Barbados Accretionary Prism ............................................................................................. 85
3.25.2 Birth of Chatham Island, Trinidad ...................................................................................... 87
3.26 Gulf of Mexico .......................................................................................................................... 87
3.27 The Eastern Seaboard, USA .................................................................................................... 91
3.27.1 Cape Lookout Bight .......................................................................................................... 91
3.27.2 Atlantic Continental Margin ............................................................................................... 91
3.27.3 Chesapeake Bay ................................................................................................................. 93
3.27.4 Active pockmarks, Gulf of Maine ....................................................................................... 94
3.28 The Great Lakes ....................................................................................................................... 94
3.28.1 Ring-shaped depressions, Lake Superior ............................................................................ 94
3.28.2 Pockmark-like depressions, Lake Michigan ........................................................................ 94
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3.29 Eastern Canada ........................................................................................................................ 95
3.29.1 The Scotian and Labrador Shelves, and the Grand Banks ................................................... 95
3.29.2 The Laurentian Fan ............................................................................................................ 96
3.29.3 The Baffin Shelf ................................................................................................................. 96
3.30 Finale ........................................................................................................................................ 97
THE CONTEXTS OF SEABED FLUID FLOW ................................................. 98
4.1 Introduction ............................................................................................................................. 98
4.2 Oceanographic settings ............................................................................................................ 98
4.2.1 Coastal settings ...................................................................................................................... 98
4.2.2 Continental shelves................................................................................................................100
4.2.3 Continental slopes and rises ...................................................................................................100
4.2.4 Abyssal plains .......................................................................................................................100
4.3 Plate Tectonics Settings ..........................................................................................................101
4.3.1 Divergent (Constructive) plate boundaries .............................................................................101
4.3.2 Convergent (destructive) plate boundaries .............................................................................101
4.3.3 Transform plate boundaries ...................................................................................................103
4.3.4 Intra-plate igneous activity ....................................................................................................103
4.3.5 Serpentinite Seamounts .........................................................................................................105
4.4 Conclusion ...............................................................................................................................106
THE NATURE AND ORIGINS OF FLOWING FLUIDS ................................... 107
5.1 Introduction ............................................................................................................................107
5.2 Hot fluids .................................................................................................................................107
5.2.1 Magma and volcanic fluids ....................................................................................................107
5.2.2 Geothermal systems ..............................................................................................................108
5.2.3 Hydrothermal circulation systems ..........................................................................................108
5.2.4 Exothermic hydrothermal systems .........................................................................................112
5.3 Water flows..............................................................................................................................113
5.3.1 Submarine Groundwater Discharge (SGD) ............................................................................113
5.3.2 Expelled Porewater ...............................................................................................................113
5.4 Petroleum Fluids .....................................................................................................................114
5.4.1 Organic origins......................................................................................................................114
5.4.2 Microbial methane .................................................................................................................115
5.4.3 Thermogenic hydrocarbons ...................................................................................................117
5.4.4 Hydrothermal and abiogenic petroleum .................................................................................119
5.5 Discriminating between the origins ........................................................................................124
SHALLOW GAS AND GAS HYDRATES ........................................................ 125
6.1 Introduction ............................................................................................................................125
6.1.1 The character and formation of gas bubbles ...........................................................................126
6.2 Geophysical Indicators of Shallow Gas ..................................................................................127
6.2.1 The acoustic response of gas bubbles ....................................................................................127
6.2.2 Seismic evidence of gassy sediments ......................................................................................128
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6.2.3 Novel gas detection and mapping ..........................................................................................132
6.2.4 Seasonal shallow gas depth variations....................................................................................133
6.3 Gas hydrates – a special type of accumulation ......................................................................133
6.3.1 Nature and formation ............................................................................................................133
6.3.2 Gas hydrates and fluid flow ...................................................................................................136
6.3.3 The BSR ...............................................................................................................................137
6.3.4 Other hydrate indicators ........................................................................................................140
6.3.5 Dissociation ..........................................................................................................................141
MIGRATION AND SEABED FEATURES ....................................................... 142
7.1 Introduction ............................................................................................................................142
7.2 Pockmarks and related features .............................................................................................143
7.2.1 Distribution ...........................................................................................................................144
7.2.2 Pockmarks and fluid flow ......................................................................................................144
7.2.3 Pockmark activity .................................................................................................................147
7.3 Mud volcanoes and Mud Diapirs ...........................................................................................148
7.3.1 The distribution of mud volcanoes and mud diapirs ...............................................................149
7.3.2 Mud volcano morphology .....................................................................................................150
7.3.3 Mud Volcano Emission Products ..........................................................................................152
7.3.4 Mud Volcano Activity ...........................................................................................................153
7.4 Related Features ......................................................................................................................156
7.4.1 Seabed doming ......................................................................................................................157
7.4.2 Collapse depressions .............................................................................................................157
7.4.3 Freak sand waves ..................................................................................................................157
7.4.4 Shallow mud diapirs and mud volcanoes ................................................................................157
7.4.5 Red Sea diapirs .....................................................................................................................159
7.4.6 Diatremes..............................................................................................................................159
7.4.7 Sand intrusions and extrusions...............................................................................................160
7.4.8 Polygonal faults.....................................................................................................................161
7.4.9 Genetic relationships .............................................................................................................161
7.5 Movers and shakers: influential factors .................................................................................162
7.5.1 The deep environment ...........................................................................................................163
7.5.2 Driving Forces ......................................................................................................................165
7.5.3 Fluid migration ......................................................................................................................166
7.5.4 Modelling the processes ........................................................................................................176
7.5.5 Triggering events ..................................................................................................................178
7.5.6 Ice-related influences.............................................................................................................187
7.6 A unified explanation ..............................................................................................................189
7.6.1 Fundamental principles ..........................................................................................................189
7.6.2 Explaining seeps ....................................................................................................................190
7.6.3 The formation of pockmarks and related seabed features .......................................................192
7.6.4 Mud volcanoes and diapirism ................................................................................................195
7.6.5 Alternative explanations ........................................................................................................196
7.7 Fossil features ..........................................................................................................................197
7.8 Related features - looking further afield ................................................................................198
SEABED FLUID FLOW AND BIOLOGY ........................................................ 199
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8.1 Seabed fluid flow habitats .......................................................................................................199
8.1.1 Cold seeps on continental shelves ..........................................................................................199
8.1.2 Deep water cold seeps ...........................................................................................................203
8.1.3 The link between hydrocarbons and cold seep communities ...................................................205
8.1.4 Shallow groundwater discharge sites .....................................................................................205
8.1.5 Deep-water groundwater discharge sites ...............................................................................206
8.1.6 Coral reefs and seabed fluid flow ...........................................................................................206
8.1.7 Hydrothermal vents ...............................................................................................................210
8.2 Fauna and seabed fluid flow ...................................................................................................213
8.2.1 Microbes – where it all begins ...............................................................................................213
8.2.2 Living together: symbiosis and seeps .....................................................................................216
8.2.3 Non-symbiotic seep fauna .....................................................................................................220
8.3 Seeps and marine ecology .......................................................................................................223
8.3.1 Geographical distribution ......................................................................................................225
8.3.2 Communities as indicators of seep activity and maturity.........................................................226
8.3.3 Do shallow water cold seeps support chemosynthetic communities? ......................................228
8.3.4 Do seeps contribute to the marine food web? ........................................................................231
8.3.5 Is Fluid Flow relevant to Global Biodiversity? .......................................................................234
8.3.6 The 'Deep Biosphere' and the origins of life on Earth .............................................................236
8.4 A glimpse into the past ............................................................................................................237
8.4.1 Fossil cold seep communities .................................................................................................237
SEABED FLUID FLOW AND MINERAL PRECIPITATION ............................ 239
9.1 Introduction ............................................................................................................................239
9.2 Methane–derived authigenic carbonates ...............................................................................239
9.2.1 North Sea ‘pockmark carbonates’ .........................................................................................239
9.2.2 ‘Bubbling reefs’ in the Kattegat .............................................................................................240
9.2.3 Carbonate mineralogy ...........................................................................................................241
9.2.4 Other modern authigenic carbonates ......................................................................................242
9.2.5 Isotopic indications of origin .................................................................................................243
9.2.6 MDAC formation mechanism ................................................................................................244
9.2.7 Associated minerals ...............................................................................................................246
9.2.8 MDAC chimneys ...................................................................................................................248
9.2.9 Self-sealing seeps ..................................................................................................................249
9.2.10 MDAC: block formation ..................................................................................................250
9.2.11 Carbonate mounds ............................................................................................................250
9.2.12 Fossil seep carbonates .......................................................................................................252
9.2.13 Summary of MDAC occurrences ......................................................................................255
9.3 Other fluid flow-related carbonates .......................................................................................255
9.3.1 Microbialites and stromatolites ..............................................................................................255
9.3.2 Ikaite ....................................................................................................................................258
9.3.3 Whitings ...............................................................................................................................258
9.3.4 Carbonates and Serpentinites .................................................................................................259
9.4 Hydrothermal seeps and mineralization ................................................................................260
9.4.1 Sediment-filtered hydrothermal fluid flow ..............................................................................261
9.4.2 Anhydrite mounds .................................................................................................................262
9.4.3 Hydrothermal salt stocks .......................................................................................................263
9.5 Other mineral precipitates ......................................................................................................265
9.5.1 Iron from submarine groundwater discharge ..........................................................................265
9.5.2 Phosphates on seamounts, guyots and atolls ..........................................................................265
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9.6 Ferromanganese nodules.........................................................................................................266
9.7 Final thoughts .........................................................................................................................268
IMPACTS ON THE HYDROSPHERE AND ATMOSPHERE .......................... 269
10.1 Introduction ............................................................................................................................269
10.2 Hydrothermal vents and plumes ............................................................................................269
10.2.1 Plumes ..............................................................................................................................270
10.2.2 Plume composition............................................................................................................271
10.2.3 Plumes and the composition of the oceans .........................................................................272
10.2.4 Heating the oceans ............................................................................................................274
10.3 Submarine Groundwater Discharge .......................................................................................274
10.3.1 Detection and quantification ..............................................................................................275
10.3.2 Water quality ....................................................................................................................276
10.4 Seeps ........................................................................................................................................276
10.4.1 Identifying seeps ...............................................................................................................277
10.4.2 Eruptions and blowouts ....................................................................................................279
10.4.3 Quantifying seeps ..............................................................................................................280
10.4.4 The fate of the seabed flux ................................................................................................283
10.5 Methane in the 'normal' ocean ...............................................................................................287
10.5.1 Rivers, estuaries and lagoons ............................................................................................287
10.5.2 The open ocean.................................................................................................................288
10.5.3 The influence of seabed methane sources ..........................................................................290
10.6 Emissions to the atmosphere...................................................................................................291
10.6.1 Methane emissions from the oceans...................................................................................291
10.6.2 Seabed Sources of Atmospheric Methane ..........................................................................292
10.7 Global Carbon Cycle ...............................................................................................................295
10.8 Limiting Global Climate Change ...........................................................................................296
10.8.1 Quaternary Ice Ages .........................................................................................................296
10.8.2 Earlier events ....................................................................................................................298
10.9 Afterword ................................................................................................................................298
IMPLICATIONS FOR MAN ............................................................................ 300
11.1 Introduction ............................................................................................................................300
11.2 Seabed slope instability ...........................................................................................................300
11.2.1 Gas-related slope failures: case studies ..............................................................................301
11.2.2 Associated tsunamis ..........................................................................................................303
11.2.3 Why do submarine slopes fail? ..........................................................................................303
11.2.4 Predicting slope stability ...................................................................................................305
11.2.5 Impacts of slope failures on offshore operations ................................................................306
11.3 Drilling hazards .......................................................................................................................306
11.3.1 Blowouts ..........................................................................................................................306
11.3.2 Hydrogen sulphide (H2S) ..................................................................................................310
11.3.3 Drilling and gas hydrates ...................................................................................................310
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11.4 Hazards to seabed installations...............................................................................................313
11.4.1 Pockmarks as seabed obstacles .........................................................................................313
11.4.2 Trenching through MDAC ................................................................................................314
11.4.3 Foundation problems ........................................................................................................314
11.4.4 Effects of gas hydrates ......................................................................................................315
11.5 Eruptions and natural blowouts .............................................................................................316
11.5.1 Gas-induced buoyancy loss ...............................................................................................316
11.6 Benefits ....................................................................................................................................318
11.6.1 Metallic ore deposits .........................................................................................................318
11.6.2 Exploiting gas seeps ..........................................................................................................319
11.6.3 Gas hydrates – fuel of the future? ......................................................................................319
11.6.4 Exploration for hydrocarbons ...........................................................................................321
11.6.5 Benefits to fishing?............................................................................................................324
11.6.6 Seeps, vents and biotechnology .........................................................................................325
11.7 Impacts of human activities on seabed fluid flow and associated features ...........................325
11.7.1 Potential triggers ...............................................................................................................325
11.7.2 Environmental Protection ..................................................................................................326
References ............................................................................................................................................330
Index .....................................................................................................................................................395
Tables
8.1 Possible microbial metabolic processes at hydrothermal vents
9.1 Biomarker evidence of fossil seeps
10.1 Indicative composition of mud volcano gases
1.1 Causes of submarine slope failure
Figures
see separate file
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Accompanying CD
The accompanying CD has been prepared and reproduced by Statoil.
Figures
as in text, but those marked * are in colour
Maps
1 Index map (see Figure 3.1)
2 Barents Sea
3 Northwest Europe
4 United Kingdom
5 Iberia
6 West Africa
7 Adriatic
8 East Mediterranean.
9 Black Sea
10 Caspian Sea
11 Lake Baikal
12 Red Sea
13 Arabian Gulf
14 Makran Coast of Pakistan
15 The Indian Subcontinent
16 South China Sea
17 Timor Sea
18 New Britain and the Manus Basins
19 New Zealand
20 West Pacific
21 Yellow Sea and Japan
22 Sea of Okhotsk
23 Alaska
24 British Columbia
25 Juan de Fuca Ridge
26 California
27 Gulf of California
28 South America
29 Southeast Caribbean
30 Gulf of Mexico
31 Eastern USA
32 Eastern Canada
33 Mud volcanoes (worldwide)
34 Hydrothermal vents and submarine volcanoes (worldwide)
35 Gas hydrates (worldwide)
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Contributed Presentations
Introduction
Contribution 1, D.C.Kim, Korea: High resolution profiles of gassy sediment in the southeastern
shelf of Korea.
Contribution 2, G. Bohrmann, Germany: Mud volcanoes and gas hydrates in the Black Sea – an
important linkage to the methane cycle: The Dvurechenskii mud volcano, initial results
from M52/1 MARGASCH.
Contribution 3, I.W. Aiello & R.E. Garrison, USA: Subsurface plumbing and three-dimensional
geometry in Miocene fossil cold seep fields, Coastal California.
Contribution 4, F. Abegg, Germany: Structure and distribution of gas hydrates in marine
sediments.
Contribution 5, I.R. MacDonald, USA: Stability and change in Gulf of Mexico chemospheric
communities.
Contribution 6, I. Guliyev, Azerbaijan: South Caspian Basin, seeps, mud volcanoes.
Contribution 7, S. Garcia-Gil, Spain: A natural laboratory for shallow gas: The Rías Baixas
(Spain).
Contribution 8, T. Treude & A. Boetius, Germany: Anaerobic oxidation of methane (AOM) in
marine sediments.
Contribution 9, A. Mazzini et al., UK: Methane derived carbonates in seafloor sediments.
Contribution 10, G. Papatheodorou et al., Greece: Gas charged sediments and associated seabed
morphological features in the Aegean and Ionian Seas, Greece.
Contribution 11, L. Dimitrov, Bulgaria: Black Sea methane hydrate stability zone.
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Foreword
‘Seabed fluid flow’ encompasses a wide range of fluids (gases and liquids) that pass from
sediments to seawater, involving natural processes that modern science would pigeon-hole into a
wide range of disciplines, mainly in the geosciences, biosciences, chemical sciences,
environmental sciences and ocean sciences; they also impinge on (or are affected by) human
activities. With our background, it is inevitable that the most prominent fluid in this book is
methane. There is a vast literature on hydrothermal vents, and a growing interest in submarine
groundwater discharge with which we do not wish to compete. However, we recognise the
importance of considering all forms of seabed fluid flow so that similarities and differences in
the processes may be considered. We have attempted to assimilate all forms, manifestations, and
consequences of seabed fluid flow of whatever origin.
It is impossible, in a single volume, to do justice to such a multi-disciplinary subject. The pace
of research has progressively increased since our own interests in pockmarks and seeps began.
Of particular significance is the move of the petroleum industry from the continental shelves into
the deeper waters of the continental slope and rise; this has rejuvenated research in deep-seabed
processes, and has resulted in re-thinking many old ideas – not least because of the discovery of
many deep-water features associated with seabed fluid flow.
The blossoming of interest in this specialised topic is demonstrated by the number of
conferences, workshops, and meetings dedicated to it. Interest has grown not only because more
people from an increasing number of countries are involved, but also because of the variety of
related phenomena identified and described. Now the tide is in full flow and it is impossible to
keep up with new literature coming from all over the world. The task of pulling together all the
threads to make a coherent and comprehensive synthesis is impossible. We synthesise current
understanding of seabed fluid flow, and to demonstrate the interactions between processes often
considered separately. We encourage others to think beyond their own specialism, and accept
that ‘seabed fluid flow’ is far more than a mere geological curiosity.
Note on the accompanying CD
The figures presented in this book are in black and white. They are all reproduced on the
accompanying CD; some of them (whose number have the suffix ‘*’) are in colour. Having
made the decision to include a CD we have used the available space to include Powerpoint
presentations contributed (by invitation) by various colleagues to supplement the text with
additional detail, and more images. We thank these colleagues for their efforts which, we think,
are a valuable asset.
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Acknowledgements
We acknowledge Den norske stats oljeselskap a.s. (Statoil of Norway) to whom we are especially
grateful for making it possible to include a CD with this book. Statoil assisted with the
preparation of the CD and reproduced it.
Many individuals, particularly Keith Kvenvolden, Jean Whelan, and the late Gabriel Ginsburg,
have encouraged us to complete this new book. Fellow researchers with whom we have worked
or met at conferences etc. have provided invaluable discussions, helping us (perhaps unwittingly)
to formulate the ideas we present here, and / or providing us with information, data, figures,
comments on sections of text, etc.. These include:
Adel Aliyev, Alan Williams, Andy Hill, Antje Boetius, Bahman Tohidi, Ben Clennell,
Ben de Mol, Beth Orcutt, Bo Barker Jørgensen, Björn Linberg, Daniel Belknap, Dave
Long, Derek Moore, Eric Cauquil, Fritz Abegg, Geoff Lawrence, Geoff O’Brien,
Gerhard Bohrmann, Gert Wendt, Giovanni Martinelli, Giuseppe Etiope, Graham
Westbrook, Gunay Çifçi, Günther Uher, Helge Løseth, Ian MacDonald, Ibrahim Guliev, Ira
Leifer, Irina Popescu, Jean-Paul Foucher, Jeff Ellis ,Jens Greinert, John Woodside, Jon Ottar
Henden, Lori Bruhwiler, Louise Tizzard, Luis Pinheiro, Lyoubomir Dimitrov, Mandy Joye, Mike
Leddra, Mike Sweeney, Nils-Martin Hanken, Peter Croker, Raquel Díez Arenas, Richard
Salisbury, Roar Heggland, Rob Sim, Rob Upstill-Goddard, Rolf Birger Pedersen, Ruth Durán
Gallego, Soledad García-Gil, Steve McGiveron, Tim Francis, Troels Laier, Valery Soloviev, Vas
Kitidis, Veronica Jukes, Vitor Magalhães, Wolfgang Bach.
While acknowledging the help of all these people we are responsible for any omissions,
oversights or errors.
We also acknowledge the various publishers, organisations, and individuals who have granted us
permission to reproduce figures. The maps presented on the CD prepared using the on-line
mapping system ’GeoMapApp’ [http://www.marine-geo.org/geomapapp/] of the Marine
Geoscience Data Management System, Lamont-Doherty Earth Observatory.
Finally we thank our wives (Laraine and Målfrid) and families (children and grandchildren) who
have patiently put up with us over the years during which we have been working on this book.
Laraine has also provided invaluable assistance with the preparation of the text – it could not
have been completed without your support.
Alan Judd Martin Hovland
High Mickley Sola
Northumberland Stavanger
UK Norway
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Chapter 1
Seabed Fluid Flow Introduction
Discovery commences with the awareness of anomaly, i.e. with the recognition that nature has somehow violated the
paradigm-induced expectations that govern normal science.
Kuhn, 1970
This chapter introduces the concept that seabed fluid flow is a widespread and
important natural process. It has important consequences for sub-seabed and seabed
geological features, and also for marine biological processes, and the composition of
the oceans. Seabed fluid flow provides both hazards and benefits for human activities,
and it is recognised that some sites are precious and need protection.
Earth scientists remember the 1960s as the decade of the Plate Tectonics 'revolution'. In the same
decade, the discovery of two remarkable seabed features; hydrothermal vents and pockmarks,
provided evidence of extensive emissions of fluids from the seabed. Since then there has been a
growing awareness that dynamic geological processes, driving the exchange of fluids across the
seabed-seawater interface, are of fundamental importance to the nature and composition of the
'marine system'; not only to marine geology, but also to the chemical and biological composition
of the oceans. Today as in the geological past, seabed fluid exchange is as important as the
interactions between the oceans and atmosphere. Gas bubble streams and columns of coloured
or shimmering water, mineral crusts and chimneys, and biological communities that thrive
without the aid of sunlight are all evidence of ‘Seabed Fluid Flow’.
Spectacular discoveries made during investigations of ocean spreading centres like the East
Pacific Rise and the Mid-Atlantic Ridge have turned upside-down concepts of how our planet
works. On the continental shelves, and more recently in the deeper waters of the slope and rise,
the oil industry has not only driven technological advances, but has also been responsible for an
increasing awareness of the fundamental role of fluids in sedimentary processes. Tryon, et al.,
2001) pointed out that: "Subsurface fluid flow is a key area of earth science research, because
fluids affect almost every physical, chemical, mechanical, and thermal property of the upper
crust." They went on by saying that research in the deep biosphere, gas hydrates, subduction
zone fluxes, seismogenic zone processes, and hydrothermal systems all are “directly impacted by
the transport of mass, heat, nutrients, and other chemical species in hydrogeological systems."
Mankind’s activities, particularly during the last century, have resulted in increasingly serious
pollution of the marine environment. Some of the principal causes relate to the petroleum
industry, yet natural processes have been responsible for petroleum ‘pollution’ for a far greater
period of time. In the Bible God instructed Noah to make an ark and “coat it inside and out with
pitch” (Genesis 6:14). Indigenous populations from parts of the world where seeps occur have
made good use of the special properties of natural petroleum products; Native Americans in
California used 'asphaltum' to caulk their canoes, hold together hunting weapons and baskets, for
face paints, and even chewing gum (USGS, 2000). The 'eternal flames' of natural gas seeps in
Azerbaijan are central to the Zoroastrian faith.
Such seepages gave the first indications of the presence of petroleum in most of the world’s
petroleum-producing regions (Link, 1952). Indeed, Link considered that at least half the reserves
proved by 1952 were discovered by drilling on or near seeps. But petroleum seeps are not
confined to the land. Great lumps of floating tar, such as that illustrated in Fig. 1.1, caused the
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Romans to call the Dead Sea Mare Asphalticum, and early navigators of the Gulf of Suez, the
Gulf of Mexico, the Californian coast, and many other parts of the world’s oceans discovered oil
slicks and tar-polluted beaches centuries before the modern oil industry was founded and oil-
powered ships and tankers were introduced (Soley, 1910; MacDonald, 1998). Kvenvolden and
Cooper (2003) reported that natural seepage introduces between 0.2 and 2.0 x 106 (best estimate
0.6 x 106) tonnes of crude oil per year into the marine environment. This is about 47% of all the
crude oil currently entering the marine environment; mankind is responsible for the rest.
Hornafius, et al., 1999) estimated that the present-day natural hydrocarbon seeps in Santa
Barbara Channel, California are a significant source of air pollution, the flux being "twice the
emission rate from all the on-road vehicle traffic in Santa Barbara County".
Petroleum seeps are not the only form of seabed fluid flow that has been known for thousands of
years. Taniguchi, et al., 2002) identified the following ancient reports of submarine groundwater
discharge:
the Roman geographer, Strabo, who lived from 63 BC to AD 21, mentioned a submarine
spring (fresh groundwater) 2.5 miles offshore from Latakia, Syria (Mediterranean), near the
island of Aradus. Water from this spring was collected from a boat, utilizing a lead funnel
and leather tube, and transported to the city as a source of fresh water.
Pliny the Elder (1st century AD) reported submarine “springs bubbling fresh water as if from
pipes” along the Black Sea coast.
Pausanius (2nd century AD) told of Etruscan citizens using coastal springs for ‘hot baths’.
Historical accounts tell of water vendors in Bahrain collecting potable water from offshore
submarine springs for shipboard and land use.
Considering the long history of knowledge of petroleum and freshwater seeps, it is perhaps
remarkable that hydrothermal vents and chemosynthetic biological communities have been
discovered so recently. However, they are not the only features hidden in the ocean depths, out
of reach of all but the most recent technology. Vogt, et al., (1999) made a comparison,
highlighting the progress made in one decade between the contents of The Nordic Sea, a
synthesis published by Springer-Verlag, New York, in 1986, and current understanding. They
noted that: “Two thirds of that 777-page volume was devoted to topography and geology… …
yet the words 'methane', 'hydrate', 'pockmark', 'gas vent or seep', 'chemosynthesis', and 'mud
volcano' do not appear even once in the 42-page subject index".
The development in the mid-1960s of the side-can sonar and towed photographic cameras made
widespread high-resolution seabed mapping possible, while the parallel development of high-
resolution seismic profilers extended this mapping to include the sub-seabed sediments and
rocks. More recently multi-beam echo sounders (MBES), manned and remotely operated
submersibles (ROVs), autonomous underwater vehicles (AUVs), and many more sophisticated
instruments have enabled more rapid and detailed inspection of survey areas and individual
seabed locations. These developments have enabled the pace of discovery to increase
progressively. Features that were only recently regarded as geological curiosities are now known
to be widely distributed geographically, from the coasts to the ocean depths, and through
geological time. It is amazing how far knowledge of the seabed has advanced in little more than
60 years since the following words were written:
In 1911, Fessenden made the first attempts to determine depths by sonic methods,
and from about 1920 sonic depth finders have been in use with which soundings can
be taken in a few seconds from a vessel running full speed. This new method has in
a few years completely altered our concept of the topography of the ocean bottom.
Basins and ridges, troughs and peaks have been discovered, and in many areas a
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bottom topography has been found as rugged as the topography of any mountain
landscape. Sverdrup, et al., 1942.
Today's technology facilitates not only detailed, 3D mapping, but also sampling and visual
inspection, revealing features Sverdrup could not have dreamed of. This technology now permits
an appreciation of how widespread emissions of water, petroleum fluids, and hydrothermal fluids
are; it also enables associated features such as mineralised chimneys and chemosynthetic
biological communities to be sampled and investigated. Only now is the importance of the
natural processes responsible for them being realised in marine science.
It was through our curiosity towards pockmarks that we became aware of the importance of
‘seabed fluid flow’. An initial appraisal of pockmarks, in Chapter 2, is an account of the
pockmark investigations of the Scotian Shelf and the North Sea that provided us with a
preliminary insight into seabed fluid flow. This research, undertaken in the 1970s and 1980s, led
us to realise the significance of pockmarks as indicators of fluid flow, and documented North Sea
gas seeps (Fig. 1.2*) and the associated carbonates and benthic fauna for the first time. Chapter
3 (supported by maps provided on the CD) is a review of some key sites around the world that
have provided evidence critical to the development of our present understanding of seabed fluid
flow. It emphasises the relationship between the natural processes (geological, biological,
physical, and chemical) involved, and shows that the study of this topic is not possible without
crossing traditional scientific discipline boundaries. It is clear from Chapter 3 that seabed fluid
flow is widespread, and that various types of fluid are involved. This book is concerned with
three main types of fluid
1
: hydrothermal fluids generated by the circulation of seawater through
the cooling igneous rocks of ocean spreading centres and submarine volcanoes; gases,
particularly methane, generated in marine sediments; and groundwater flowing from catchment
areas on land. Perhaps it is normal to deal with each of these fluid types separately. But,
although major differences, for example in temperature and chemical composition, result in
contrasting behaviour, many processes and associated features are either common, or so closely
related that it is hard to consider one without mentioning the others. So, we consider the cycles
of generation, migration, and utilisation or escape of these three fluid types, pointing out the
similarities and contrasts between them, and the overall significance of seabed fluid flow. Our
objective is to be inclusive rather than selective.
It is remarkable how common seabed fluid flow is. As we show in Chapter 4, the examples
described in Chapter 3 come from every seabed environment from coastal waters down to the
deep ocean trenches. Also, seabed fluid flow is integral to every marine plate tectonics setting:
hydrothermal venting is part of the system that cools igneous rocks at plate boundaries; mud
volcanoes and seeps permit the compaction of sediments trapped in the accretionary prisms of
convergent boundaries; buoyant hydrocarbon fluids escape from intra-plate sedimentary basins
through seeps. The nature and origins of the various types of fluid (discussed in Chapter 5) are
largely a function of these contexts, and the geological and biological processes operating in
them. So, where igneous processes dominate, hydrothermal fluids are formed by the interactions
between pore fluids and hot rocks. In sedimentary basins the most significant fluids are
hydrocarbons, particularly methane, formed by the degradation of organic matter held within the
sediments.
At this point it is appropriate to clarify some terminology. The word 'biogenic' is commonly
used, particularly by geoscientists, when referring to methane that has been derived by the
activity of micro-organisms, as opposed to 'thermogenic' methane, derived from processes
1
It is not uncommon in the literature to find "gas and fluids" mentioned as if they are separate phases. They are not.
The Oxford English dictionary defines a fluid as "a substance that is able to flow freely, not solid or rigid", and
specifically states that this includes both liquids and gases. So, throughout this book when we refer to fluids we mean
both gases and liquids. However, direct quotes do not necessarily conform to this standard.
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occurring deeper within the sediments. However, in the biological sciences 'thermogenic'
methane is also regarded as being 'biogenic' because the source materials are of organic origin;
thus 'biogenic' is distinct from 'abiogenic', formed without the involvement of living organisms.
We will avoid this confusion by avoiding the word 'biogenic' altogether. Instead we distinguish
between 'thermogenic' and 'microbial' methane. This also avoids the use of 'bacterial methane',
which is generally incorrect as microbes, ‘minute living beings’, which generate methane are
actually archaea, not bacteria. However, although these are definitions we stick to, quotations
from other authors may imply something different; we do not wish to modify other people's
words.
Methane, formed during sediment burial, is buoyant and therefore inclined to migrate towards the
surface. Although seepage is a natural result of this migration, geological conditions often result
in the formation of accumulations. In deep water, temperature and pressure conditions favour the
formation of gas hydrates that also inhibit migration. In order to understand the distribution of
seeps in both space and time it is essential to appreciate how and why these accumulations form,
and how to identify them. We address these issues in Chapter 6. Diatremes, mud diapirs, gas
chimneys, and mud volcanoes form as a result of the pressure that builds up in some sub-seabed
gas accumulations. However, the nature of the migration mechanism is dependent on the stress
environment within the sediments. In some places migration is a much more gentle process, and
the plumbing system may lead to pockmarks, or to seeps with no associated seabed
morphological features at all. As we discuss in Chapter 7, the style of migration and seabed
escape is determined by interactions between many factors. Fluid flow is clearly a dynamic
process.
Perhaps the most amazing biological discovery of the twentieth century was made in 1977 when
deep-ocean chemosynthetic communities were found at the Galapagos Rift. Until then it was
inconceivable that life could exist without benefiting from the Sun's energy. Although such
communities are probably rare, they are clearly widespread and, as we discuss in Chapter 8, they
are not confined to ocean spreading centres or to hydrothermal vents. The principal effect of
petroleum seeps, particularly those of the shelf seas, might be expected to be the pollution of the
seabed sediments and the overlying waters. This is not the case. Similarities between hot vent
and cold seep communities are remarkable, as is the suggestion that the first life on Earth may
have relied on chemosynthesis. Is photosynthesis a relatively recent adaptation?
Some of the most spectacular seabed scenery is associated with seabed fluid flow. At some
locations the scenery is provided by carbonate chimneys associated with methane seeps, at others
by chimneys of hydrothermal metal sulphides. Exceptional examples stand metres tall; Godzilla,
a structure on the Juan de Fuca Ridge, towers 45 m above the seabed, belching black smoke from
its chimneys (Fig. 1.3). Mineral precipitation, the subject of Chapter 9, results in changes to the
composition of flowing fluids, whether by microbial utilisation, as in the case of methane-
derived authigenic carbonate ('MDAC'), or precipitation as a result of a sudden change in
temperature (as at hydrothermal vents). As we see in Chapter 10, the fluids that escapes
contribute to the composition of the overlying water column, adding heat as well as metals or
hydrocarbons; nutrients and substrates that can be oxidised by microbes in the water also
contribute to biological productivity. If seabed fluid flow were a rare phenomenon, then these
contributions would be of little consequence. But, given the widespread distribution shown in
Chapters 3 and 4, perhaps the composition of the oceans has been significantly influenced by
geological contributions. Seeps and mud volcanoes may also influence atmospheric
concentrations of methane, particularly in shallow water where gas bubbles can survive a journey
to the sea surface. Vast volumes of methane are sequestered by seabed gas hydrates during
interglacial periods, and may be released during glaciations, so it seems possible that variations in
the seabed flux of geological methane moderates the extremes of global climate change.
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In the final chapter we discuss both the implications of seabed fluid flow for mankind, and the
effects of offshore activities on seabed fluid flow. Marine geohazards include slope failures and
drilling hazards associated with shallow gas, and the possible implications of seabed eruptions
for seabed installations and shipping. However, seabed fluid flow offers benefits too. The
mining of metals from hydrothermal ore deposits on land is a major industry, and active
hydrothermal vents provide useful information for mining, as well as having future potential as a
resource. The energy potential of gas hydrates has encouraged significant research programmes
in several countries, and the oil industry makes use of seeps in petroleum exploration. A more
recent concern to marine science is the vulnerability of benthic ecosystems associated with
seabed fluid flow. International legislation is now affording some protection, for example, the
European Union's Habitats Directive has identified “sub-marine structures made by leaking gas”
as a habitat worth protecting.
In this book we suggest that seabed fluid flow is of fundamental importance to the marine
environment and the working of our planet. It is widespread, dynamic, and influential. Although
it is essentially a geological process, it affects marine ecology, ocean chemistry, and the
composition of the atmosphere. The seabed does not mark the limit of the marine system.
Fluids flowing out of the seabed contribute to and, we argue, play a significant role in ocean
processes and the Global Carbon Cycle.
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Chapter 2
Pockmarks, shallow gas and
seeps: an initial appraisal
The North Sea's fattyness is, after its saltiness, a peculiar property, . . . It should be assumed here that in the ocean
as on land there exists, here and there, seepages of running oily liquids or streams of petroleum, naptha, sulphur,
coal-oils and other bituminous liquids. Erich Pontoppidan, 1752.
This chapter begins with a review of the pioneering work undertaken on the Scotian
Shelf, off eastern Canada, by L.H. King and his colleagues at the Bedford Institute of
Oceanography. However, having 'cut our teeth' in the North Sea, the pockmarks and
seeps here have become the standard against which we compare those of other areas.
Consequently it is appropriate to review our early studies of North Sea pockmarks. This
provides a historical perspective on pockmark research, and indicates how this early
work led us to the conclusions that pockmarks and seabed seeps are important
geological phenomena and indicators of processes associated with seabed fluid flow.
In some cases the sites we visited early on have been the subjects of further work.
This is also reviewed here.
By the end of this chapter it becomes clear that seeps and pockmarks, along with the
associated carbonates and biological communities, are components of the important
hydrocarbon cycle.
2.1 The Scotian Shelf: the early years
Pockmarks were first described on the continental shelf offshore Nova Scotia, Canada by King
and MacLean (1970). Subsequent work in this area was reported by Josenhans et al. (1978).
Pockmarks were found to be present over an area of 3,000 - 4,000 km2 in the Roseway, LaHave
and Emerald Basins, and two smaller basins. From echo sounder and side-scan sonar records,
and from visual observations made from the manned submersible ‘Shelfdiver’, the features were
described as cone-shaped seabed depressions that bottomed at a well-defined point. In plan,
most are elongate with a preferred orientation that, on average, is north - south. No raised rims
were present, but the pockmark edges were found to be sharply defined, the slope changing from
horizontal to an estimated 30° within a distance of only 0.5 m.
The surficial sediments of the Scotian Shelf range in thickness from a few metres to over 200 m.
They consist of five formations of which the oldest, the Scotian Shelf Drift, is mainly glacial till.
The basins are infilled with Emerald Silt, a fine-grained, muddy sediment¸ predominantly silt but
locally sandy and containing some gravel. This is overlain by the mainly-Holocene LaHave
Clay, comprising homogeneous, loosely compacted marine silty clay that locally grades to clayey
silt. These three sediment units are illustrated on the seismic profile (Fig. 2.1), where it can be
seen that the younger two thicken towards the deeper parts of the basin. King and MacLean
(1970) found that the pockmark distribution is related to the distribution of the LaHave Clay.
However, pockmarks are not found throughout this area, neither are they restricted to this
sediment type. Some are found in the Emerald Silt and a few small, isolated pockmarks have
been reported in the Sambro Sand (medium to fine grained sand, moderate to well sorted, with
up to 20% silt and clay-sized material) near the edge of the Emerald Basin (Josenhans et al.,
1978). Pockmarks are not found in the intervening Sambro and Roseway Banks, and, with the
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exception of a slight overlap in the Roseway Basin, they overlie the coastal plain sediments.
These are a thick sequence of well-stratified, seaward-dipping Tertiary and Cretaceous sediments
that wedge out against the basement rocks along a line sub-parallel to the coast. The basement
rocks comprise folded Cambro-Ordovician metasediments and granitic intrusions of Devonian
age.
The pockmarks in the three basins are similar, but King and MacLean found those of the
Roseway Basin to be more numerous (200 per km2) and smaller (15-30 m across and 3-6 m
deep) than those of the Emerald and LaHave Basins (45 per km2, 30-60 m in diameter and 6-9 m
deep). A more detailed study of a small (150 km2) area in the Emerald Basin (Josenhans et al.,
1978) indicated that pockmark density and size were related to the surficial sediment type and
thickness, more but smaller pockmarks occurring in the silts, fewer and larger pockmarks in the
clays. The largest pockmark they recorded lay in the LaHave Clay and measured 300 m long,
150 m wide and 15 m deep.
King and MacLean (1970) considered that “the crater-like nature of the pockmarks strongly
suggests that they are erosional features”. After discussing various possible mechanisms, they
concluded that the association with the underlying coastal plain sediments suggested a link and
surmised that water or gas rising from these sediments (or underlying coal-bearing Upper
Carboniferous strata) to the seabed was the most likely cause or agent. Although the currently
known petroleum fields on the Scotian Shelf lie further seaward, considerable up-dip migration
cannot be ruled out. It was further envisaged that water currents would disperse suspended
sediment, and that the pockmark walls would slump, enlarging the feature, until a stable slope
developed. The preference of pockmarks for fine-grained sediments was considered to reflect
the inability of escaping fluids to percolate through such sediments without disturbing them. In
contrast, percolation could occur in areas such as the Roseway and Sambro Banks where coarse
sediments are present. Josenhans et al. (1978) observed that elongate pockmarks are aligned
with their long axes parallel to the dominant tidal flow, which has an oscillating tidal component
of 10 cm s-1 with a major axis oriented north to northwest, and a residual current flow of 3 cm s-1
from the north.
Although Josenhans et al. (1978) favoured gas escape as the pockmark-forming process, they
could find insufficient evidence to support present-day gas escape from shallow seismic
reflection profiles, echo sounder profiles, side-scan sonar records, the analysis of piston core
samples (reported by Vilks and Rashid, 1975), or hydrocarbon sniffer data. This led them to
conclude that the Scotian Shelf pockmarks are largely relict features.
2.2 North Sea pockmarks
The first North Sea pockmarks were discovered in 1970 by Decca Surveys during a rig-site
survey in preparation for exploration drilling at BP’s Forties field. The following year they were
found off the Norwegian coast during a research survey (van Weering et al., 1973). Indications
of gas seeps from pockmarks were also recorded in the early 1970s (Fig. 2.2), but it was not until
1983 that positive proof of gas seepage was obtained (Hovland et al., 1985; 1987).
2.2.1 History of Discovery
In 1971 the Netherlands Institute of Sea Research (NIOZ) conducted a survey in the Norwegian
Trench between Oslo and Bergen, using a hull-mounted 3.5 kHz sub-bottom profiler. The main
objective was to map the thickness of the surficial sediments. Side-scan sonar was not used, but
the seabed notches were correctly interpreted as pockmarks by comparing them to the pockmarks
of the Scotian Shelf. From this survey it was evident that there are pockmarks along most of the
Norwegian Trench, including some parts of the Skagerrak. This conclusion has been confirmed
by subsequent work. Indeed, it is now known that pockmarks are present throughout most of the
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area covered by the youngest sediments, the Kleppe Senior Formation, although they are
generally most common along the Western Slope of the Norwegian Trench. NIOZ also
discovered that pockmarks are extensive in the Witch Ground Basin of the UK sector of the
North Sea (Jansen, 1976).
During the period 1974-78 the British Geological Survey (BGS) undertook a research
programme to find out more about pockmarks in an area that was then attracting increasing
attention from the oil industry. This programme was concentrated in the South Fladen area,
northwest of the Forties field in blocks UK15/28 and 21/3, on the southern side of the Witch
Ground Basin. Ten investigations were undertaken. They included gravity and vibrocore
sampling, drilling, visual inspection using the unmanned submersible (ROV) Consub, in situ
geophysical (seismic velocity and electrical resistivity) measurements, geophysical surveys (side-
scan sonar and seismic profiling) and geochemical studies of core samples and seawater. The
results were summarized by McQuillin et al. (1979) and referred to by Fannin (1979), and
McQuillin and Fannin (1979). Subsequent analyses of the data acquired during some of the
surveys were undertaken by Judd (1982a, b). The results of this and some subsequent work
(including the regional mapping of the UK continental shelf by the BGS) are reviewed in Section
2.3.1.
These early surveys were intended to obtain basic information to delimit the area in which
pockmarks occur, and to give some indication of the mode of formation. In particular it was felt
necessary to establish whether or not the process of pockmark formation might be hazardous to
offshore installations. During these surveys a range of features were identified, including
evidence of the presence and migration of gas. Many of them had not been recognised before, so
a terminology was developed to describe them (see Sections 2.2.4 (pockmarks) and 6.2.2 (gas)).
Because pockmarks occur over such wide areas of the northern North Sea they have been a
source of considerable interest to the oil industry. In the UK sector, many producing petroleum
fields (e.g. Balmoral, Britannia, Forties, Ivanhoe, Piper, and Tartan) lie within the Witch Ground
Basin, and several pipeline systems cross this area. Several fields (e.g. Troll, Veslefrikk, Snorre,
and part of Gullfaks) are located at pockmarked sites in the deeper waters of the Norwegian
Trench, and several pipelines (e.g. the Statpipe, Zeepipe, and Europipe systems) also cross the
Trench. The work involved in site and route planning surveys for these installations has provided
a considerable volume of data about pockmarks. Unfortunately, the operators retain much of this
in confidence. Of the oil company data that have been released, Statoil produced the
overwhelming majority. These include echo sounder, side-scan sonar records, shallow seismic
reflection profiles, and sedimentological data from coring. Also, ROVs have been utilized for
seabed inspections.
The vast majority of the survey data acquired prior to 1983 represents a form of remote sensing,
using side-scan sonar, shallow seismics etc.. As with all remote sensing, a proper interpretation
cannot be made without ground-truthing. During two research cruises in 1983 and 1985, Statoil
conducted detailed inspections of pockmarks using ROVs. The results of these, and some
subsequent surveys, are discussed in Section 2.3.
2.2.2 Pockmark Distribution
The North Sea can be subdivided into three bathymetric zones: the southern and northern North
Sea (separated by the Dogger Bank), and the Norwegian Trench. For a long time no pockmarks
had been located south of the 56º parallel. This was assumed to be a function of the seabed
sediment types rather than being due to the absence of gas seepages. However, careful analysis
of MBES and shallow seismic data from the Zeepipe pipeline route data has revealed pockmarks
in areas of sandy sediments and sandwaves. These are discussed in Section 3.5.2.
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The seabed of the northern North Sea is a gently inclined plateau; water depths gradually
increasing from about 60 m in the south to about 250 m in the far north, at the edge of the
continental shelf. The largest of several basins within the plateau is the Witch Ground Basin.
Here, water depths increase to more than 150 m. Many of the smaller basins are actually
channels cut into the plateau sediments during the late Pleistocene and subsequently partially
infilled. Sediment types on the plateau vary, but stiff glacial clays covered by varying layers of
sand predominate. In contrast, the basins and channels tend to be characterized by soft, muddy
sediments (Andrews et al., 1990; Johnson et al., 1993; Gatcliff et al., 1994). The origin of the
Norwegian Trench, where waters are as much as 700 m deep (in the Skagerrak), has long been
debated. However, it is now generally believed to originally have been cut fluvially in the late
Tertiary and subsequently deepened by glaciers and ice-sheets during the Pleistocene. It is an
asymmetric trough in form. The Western Slope is smooth, whereas the landward side is steeper
and frequently rugged (Holtedahl, 1994).
Most pockmarks are found in the three muddy sediment formations in the northern North Sea:
the Witch Ground Formation, in the Witch Ground Basin, the Flags Formation of the smaller
basins further north, and the Kleppe Senior Formation that occupies the floor of the Norwegian
Trench. There are also pockmarks in equivalent sediments that infill or partially infill channels
cut into the stiffer clays of the plateau. These sediments are all post-glacial and are similar in
most respects. Indeed, Hovland et al. (1984) noted that both the Witch Ground and Kleppe
Senior Formations are remarkably similar to the Emerald Silt - LaHave Clay sequence of the
Scotian Shelf. This comparison is valid in respect of their lithological characteristics,
seismostratigraphic character, and depositional environment. Also, sedimentation has all but
ceased in the basins of the Scotian Shelf, as it has in the northern North Sea.
2.2.3 Pockmark size and density
The density of pockmarks varies from area to area both within the North Sea and within the
individual pockmarked areas in the North Sea. In the Norwegian Trench the density varies from
0 to about 60 per km2 (counting only those that are more than 10 m across); the most densely
pockmarked area of substantial size lies over the Troll gas field. The sizes of individual
pockmarks in any given area are varied, but the only change in the range of sizes within the
Trench is associated with the Western Slope, which is the only area in which elongated
pockmarks are found. In contrast, the size and density of the pockmarks in the Witch Ground
Basin vary, apparently in response to variations in the thickness and lithology of the seabed
sediments (Long, 1986). In general, pockmarks are between 50 and 100 m in diameter with
depths in the range 1-3 m. The highest densities (>40 km-2) occur within bathymetric hollows
characterized by sandy muds, but here sizes rarely exceed 50 m. In the deepest parts of the basin,
where seabed sediments are pure mud, the density is 10-15 per km2, but sizes are much larger
(100-150 m). Both pockmark density and size tend to decrease towards the edges of the basin
beyond the outcrop of the Witch Member and particularly where the Fladen Member becomes
thinner and coarser. At the basin edge where the underlying Coal Pit and Swatchway Formations
approach the seabed, there are pockmarks, but they are few and far between and very small in
size.
2.2.4 Pockmark morphology
Although early investigations concluded that pockmarks are approximately circular in plan, it is
clear that there is a considerable variety of shape. Size also varies considerably both between
areas and within an individual area. The following descriptions are based mainly on seismic
(mainly deep-towed boomer) profiles and side-scan sonar data from surveys undertaken in the
1970s and 80s in the South Fladen area (Witch Ground Basin) and the Norwegian Trench, but we
have used MBES images as illustrations where appropriate.
Standard circular and elliptical pockmarks are perhaps the most common (Fig. 2.3*).
Length:breadth ratios vary considerably from 1 (circular) to 1.25 or more in the South Fladen
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area. On the Western Slope of the Norwegian Trench the axes are generally aligned parallel to
the slope suggesting a relationship between the slope and pockmark shape. In the South Fladen
area, where seabed slope is less pronounced, there is a preferred orientation that apparently
conforms to the dominant tidal current; slope-normal bottom currents may also explain
pockmark orientation on the Western Slope. On detailed inspection individual standard
pockmarks are found to be discrete depressions whose perimeters are complicated by
indentations and lobes (Fig. 2.4). They are not as ‘regular’ in shape as is commonly supposed.
The pockmark floors tend to be undulating rather than smooth.
Composite pockmarks occur where individual standard pockmarks merge with one another. In
some instances groups of pockmarks are found together or merging (Fig. 2.5), while in other the
merger has proceeded to the extent where a single feature with a complex shape has been
produced.
Asymmetric pockmarks are best imaged by MBES (Fig. 2.6*). On side-scan sonar records they
appear to have a distinct and often quite lengthy ‘tail’ and a strong backwall reflection on one
side only. On seismic sections it can be seen that the lack of back-wall reflection occurs where
the slope up to seabed level is long and gentle. Stoker (1981) suggested that asymmetrical
pockmarks are more common than regular pockmarks in the Witch Ground Basin. Surprisingly,
the orientation of the asymmetry, although uniform in individual areas, varies considerably from
one part of the Witch Ground Basin to another.
Pockmark strings comprise individual pockmarks, commonly symmetrical, shallow and 10-15
m in diameter, arranged in strings or chains (Figs. 2.7 and 2.8*) that often extend for several
hundred metres, but more normally to 100-150 m. A space approximately equivalent to the
pockmark diameter occurs between the pockmarks. Many strings end in a single standard
pockmark much larger than those forming the string. In some cases the strings radiate out in
several directions from a single large pockmark. In the Norwegian Trench the most common
orientation (42% of those measured by Hovland, 1981) of these features is NW-SE, parallel to
the bottom currents. The remainder are aligned NNW-SSE (30%) or N-S (24%), with only 4%
aligned in other directions. Similar alignments were reported from the Troll field (Green et al.,
1985). Some long, thin pockmarks may have been formed by the growth and merger of the
pockmarks in a string, into a single feature.
Elongated pockmarks and troughs; along some mid-depth sections of the Western Slope of the
Norwegian Trench the pockmarks tend to be elongated to the extent that they resemble gullies or
troughs rather than standard (circular or oval) pockmarks. Inside these troughs the topmost
sediment layers are absent and older sediments are exposed at the seabed (Hovland, 1983). The
Norwegian Geological Survey (NGU) mapped similar features in the southern portion of the
Norwegian Trench (Bøe et al., 1998).
Near the foot of the Western Slope, there is a single trough more than 1 km long and about 200
m wide. It is composed of a series of large interlinked pockmarks aligned approximately north-
south (Fig. 2.9). Shallow seismic sections crossing the channel indicate that it corresponds to a
furrow in the surface of the Norwegian Trench Formation, although the axes of the two features
do not coincide exactly. The furrow has the appearance of an ice-ploughed mark cut by either
the keel of a large iceberg or the irregular underside of an ice-sheet. Although subsequently
deposited sediments have smoothed out most irregularities in the surface of the Norwegian
Trench Formation and its equivalents, this one has been maintained, probably by a secondary
process during and after the deposition of the Kleppe Senior Formation.
Unit pockmarks are very small (<5 m) seabed depressions found in isolation, in groups and in
association with larger pockmarks (Fig. 2.4). Harrington (1985) referred to these features as
‘pits’ and ‘pit clusters’. He also described areas of “disturbed seabed” where individual pits
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were apparently merging to form a single nascent pockmark. He suggested a genetic relationship
between these features, there being a progression from a single unit pockmark or pit to a
pockmark proper.
Giant pockmarks are anomalously large when compared to other pockmarks in the vicinity. For
example, whereas most pockmarks in the Witch Ground Basin are <150 m in diameter and <5 m
deep, the well-studied active pockmarks in UK Block 15/25 (see Section 2.3.7) are about 500 m
across and >20 m deep. These, and other giant pockmarks from the central Barents Sea, are
discussed in more detail in Section 7.5.7.
Erosive nature
Shallow seismic records from both the Norwegian Trench and the Witch Ground Basin indicate
the erosive nature of pockmarks. The internal reflectors of the uppermost sediments are
truncated by the V-shaped incisions of the pockmarks, as can be seen in Fig. 2.10. However, the
pockmarks are not simple inverted cones; some have smooth, relatively flat bottoms while in
others the bottom is mounded. Pockmark sides are rarely smooth, but commonly show several
breaks in slope or changes in the angle of slope. Stoker (1981) suggested that the combination of
shallow tensional depressions close to the edge of these pockmarks and a hummocky floor may
be an indication of slumping. In certain cases the profile shape suggests that rotational shear has
occurred. Frequently the pockmarks are asymmetric, with one side considerably steeper than the
other. In many instances, one side of the pockmark does not regain seabed level for some
considerable distance beyond what appears at first sight to be the edge of the pockmark. This is
probably caused by the restriction of slumping to one side of the feature. These are the same
asymmetric features described above from side-scan sonar records.
Partial infilling
BGS data from the South Fladen area have shown that the pockmarks sampled by gravity core
contained 2 m or more silt compared to about 30 cm on the surrounding seabed. Statoil cores
from pockmarks in blocks NO24/9 and NO25/7 also showed a greater thickness of the topmost
sediment, compared to outside the pockmarks. This 'infill' may have been derived by one or
more of three mechanisms: winnowing of the original sediment to leave a coarse 'lag' deposit (the
fines having been removed and dispersed), sidewall slumping, or infilling by sedimentation
during a period of inactivity since formation.
Particle-size distributions of the BGS samples showed that the sediments inside the pockmarks
are identical to those of the Glenn Member outside. Andrews et al. (1990) attributed the Glenn
Member of the Witch Ground Basin to "pockmark re-working of underlying sediments". This
suggests that pockmark activity has played a significant role in the generation of fine sediments
in this area, and possibly beyond.
Associated features
Close inspection of high-resolution shallow seismic data has resulted in the identification of a
range of intra-sedimentary features.
Buried ('fossil') pockmarks
Pockmarks are not found exclusively on the seabed. Buried or ‘fossil’ pockmarks occur at
various horizons within the Kleppe Senior Formation and the Witch Ground Formation (Fig.
2.11). Long (1992) described them as "pockmarks that have ceased venting and have
subsequently been covered by sediments". When these features are seen only on two-
dimensional seismic sections it is possible to confuse them with linear features such as ice scour
marks (as in Fig 2.12). However, in areas for which adequate seismic coverage is available, there
can be no such mistake.
Domes
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Localized doming of the seabed (Fig. 2.13) has been recorded in a few places in the Norwegian
Trench, in block NO25/7, and the Witch Ground Basin. The domes are isolated circular or
elliptical positive topographic features, generally about 100 m in diameter and standing some 1-2
m above the surrounding seabed. They are very unspectacular seabed features with side slopes of
<2° (Stoker, 1981). If it were not for the vertical scale enhancement and their rarity on echo
sounder and seismic records, they might easily be overlooked.
2.2.5 Evidence of Gas
In the original pockmark paper King and MacLean (1970) suggested: "the main agent
responsible for the formation of pockmarks is either ascending gas or water". Evidence of gas
above and below the seabed takes many forms. The following indicators were found on shallow
seismic profiles from North Sea pockmark areas: acoustic turbidity, enhanced reflections,
columnar disturbances, and intra-sedimentary doming (Figs. 2.10 and 2.13). These terms are
explained and illustrated in Section 6.2.2. Various other features not uncommonly found on
shallow seismic profiles in association with pockmarks, include hyperbolic reflections and
sloping reflections (illustrated in Fig. 2.10). These are most probably artefacts produced by the
interference of reflections from the pockmark walls beside the path of the seismic equipment,
although in some instances hyperbolic reflections are caused by point-source targets, such as
drop-stones dropped by melting icebergs, and methane-derived authigenic carbonate (described
below). Drop-stones are not confined to pockmarks.
There have been several reports of shallow cores from pockmark areas in the North Sea
expanding on removal from the core barrel, or even extruding themselves. Such cores contain
cavities and often they smell of hydrogen sulphide, and analyses have shown elevated
concentrations of methane. The expansion and extrusion, and the cavities are caused by gas
expanding with the release of pressure as the cores are brought to the sea surface. Before the
removal of the sediment from the seabed the gas may have been in solution or in bubbles.
Correlations between this evidence and seismic evidence (acoustic turbidity and bright spots)
were first made in the Forties field, where Lucas (1974) considered that bright spots represented
discontinuous lenses of gas-bearing sand. This interpretation was supported by Caston (1977),
who reported that it correlated with high-pressure gas at a depth of 540 m in well UK21/10-3.
The correlation has been demonstrated many times since.
During the many years of North Sea oil and gas exploration every well has been continuously
monitored for gas, once the first casing has been set. The results of these investigations are not
normally released into the public domain. However, it is certain that there is a considerable body
of evidence of gas in the shallow sediments (Tertiary peat, for example) in various parts of the
North Sea.
The extent of shallow gas
Seismic evidence of the presence of gas has been recorded from every North Sea area in which
pockmarks are known to exist. Gas is not ubiquitous, neither is it always found in close
association with pockmarks. Yet, the presence of acoustic turbidity in pockmark areas is
sufficiently common to be significant. It is also significant that evidence of gas is not restricted
to pockmark areas (see Section 3.5). Regional geochemical studies of the northern North Sea
(Faber and Stahl, 1984; Gervitz et al., 1985; Brekke et al., 1997), in which analyses were
undertaken on gases from seabed cores, have suggested that gas is quite widespread, although
there is some disagreement as to whether the gas is of thermogenic (Gervitz et al.) or microbial
(Brekke et al.) origin.
2.3 Detailed surveys of North Sea pockmarks and
seeps
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In this section we review the results of some detailed surveys conducted in the North Sea. The
locations are shown in Fig. 2.14*.
2.3.1 The South Fladen Pockmark Study Area
In order to understand more about pockmarks, the BGS undertook detailed studies of an area
within the Witch Ground Basin during the 1970s (McQuillin et al., 1979); some of this work was
co-funded by BP. This South Fladen Pockmark Study Area, Blocks UK15/28, 21/3 and 21/4,
was selected because the pockmark density is high. Three principal avenues were followed: a
study of pockmark morphology and distribution, investigations to see if the geotechnical
properties of the sediments varied within and outside pockmarks, and an examination of the
possible relationship between pockmarks and gas within the sediments. In both 1977 and 1978
Sonarmarine Ltd provided complete side-scan sonar coverage of a 3 x 19 km area, and ran
shallow seismic (boomer) lines at 150 m intervals. In 1975, BGS drilled two boreholes using the
dynamically positioned drillship Wimpey Sealab (later named the Pholas). Borehole 75/33 was
sited within a pockmark and 75/34 was some 200 m away on ‘normal’ seabed as a control.
Borehole 75/33 penetrated to 180 m; however, poor recovery from the top 20 m necessitated the
drilling of an additional borehole (75/33a) close by. The following account describes the results
of these surveys, and some more recent work.
Pockmark Morphology and Distribution
Side-scan sonar mapping (Fig. 2.15) allowed the sizes, shapes and distribution of the pockmarks
to be analyzed. However, a clearer image (Fig. 2.16*) was obtained in 2001 with MBES. Most
of the pockmarks are elliptical in shape, rather than circular, and the long axes are generally
aligned with the dominant tidal current. They are <130 m long (mean 55 m), <3.5 m deep (mean
1.4 m), <5,000 m2 in area (mean about 1,000m2), and occupy <5% of the seabed. The overall
pockmark density is 33 km-2, but the distribution is clearly not uniform; there are fewer in the
south. Furthermore, the pockmarks appear to be preferentially arranged in lineations, or even
rings. Statistically the distribution is non-random, and the lineations did not occur by chance
(Judd, 1982a). It seems probable that there must be some control exercised by the subseabed
sediments to account for these lineations.
Evidence of gas
Early surveys produced evidence of gas within the Quaternary sediments of parts of the South
Fladen area. Bright spots recorded during a 1975 site survey for Total Oil Marine were thought
to indicate gas at a depth of 320 to 380 m (this was confirmed by data acquired in 1987; see
Judd, 1990), and Holmes (1977) reported bright spots covering about 46% of a 44 km2 area
centred on BGS borehole 75/33. The sediments of the Ling Bank Formation at a depth of 59 m
in this borehole were described as ”stiff sandy clay: honeycombed with ovoid gas cavities”
Holmes (1977 - see Fig. 2.17); gas voids were also found within stiff, silty clays of the Aberdeen
Ground Formation at a depth of 160 m in the same borehole. However, no obvious differences
between any of the measured geotechnical properties of the sediments beneath and away from the
pockmark were found - at least not in the Witch Ground Formation and the underlying
Swatchway Formation (borehole 75/34 went no deeper). Holmes also reported that acoustic
turbidity within the Swatchway Formation occupied 10% of the same area. Acoustic turbidity
within the Witch Ground Formation is confined to the southeast corner of the area, around the
unusually large pockmark known as the Witch's Hole. Columnar disturbances and enhanced
reflections are present within the Witch Ground Formation throughout the area, indicating the
presence of gas or its previous passage through the sediments.
The presence of hydrocarbon gases within the water column was demonstrated by a survey
undertaken in 1975 as a collaborative venture between BGS and BP. This survey, one of the first
such surveys to be performed in the North Sea, was done with the Interocean Sniffer. The main
survey covered an area of 340 km2. A total of 700 line kilometres were run at approximately 0.5
km intervals. Three detailed study areas were also surveyed, two within the main area and the
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third further south, to provide coverage of the location of the (then proposed) borehole, 75/33,
and two places where BP had previously reported gas bubbles coming from the seabed. The
average total hydrocarbon concentration was approximately 0.5 ppm, but values of up to 0.9
ppm were recorded. The majority (approximately 98%) of detected hydrocarbon was methane,
but the higher hydrocarbons were all found to be present. C1: (C2 + C3) ratios were generally in
the range 80 to 110.
These reports confirm the presence of gas, and it is tempting to suggest that they represent stages
in the vertical migration of gas from underlying Tertiary and Mesozoic sediments to the seabed
sediments, and then into the water column.
Evidence of pockmark growth and activity
In order to see if new pockmarks were formed, or existing pockmarks grew during the period
between the 1977 and 1978 surveys, detailed comparisons were made of pockmarks in part of
the South Fladen study area. This was done by first identifying the best possible side-scan record
of each individual pockmark from each year, and then digitizing the pockmark outlines. No new
pockmarks were found, but the results suggested a slight increase in the total area occupied by
pockmarks. Comparisons between the new (2001) and old (1978) maps were attempted, but
proved not as straightforward as might be expected because of discrepancies between position
fixing systems, acoustic geometry etc.. Although no quantitative comparisons could be made, it
seems that there were no major changes to the pockmark distribution over the 23 years between
surveys. However, there is a hint that gas beneath the seabed is mobile. Judd (1990) found
minor discrepancies between the 1975 and 1978 extents of the acoustic turbidity near the Witch's
Hole.
A second kind of evidence of pockmark activity was reported by McQuillin et al. (1979). The
following quotation describes what occurred on 30th July 1978 during one of the geophysical
surveys:
At 07:36 hours survey commenced of a line across the main survey area. Large dark
clouds were observed on the sonar record which were interpreted as being due to
suspended material in the water mass. These clouds are not similar to any
previously detected fish shoal. The boomer records showed evidence of a thin layer
of recently deposited material at seabed. Over the period 07:36 hours to 15:30
hours, this cloud was seen on a number of records to disperse within the eastern
part of the area being studied. A closely spaced group of E-W lines were being
surveyed. At 13:00 hours a new event occurred to the west of the earlier event. On
the line being surveyed, the sonar record showed an even stronger reflection from a
concentrated cloud of material in the water mass [shown here as Fig. 2.18b] and
the boomer record showed plumes of material in the water elevated to about 10 m
above seabed [Fig. 2.18a]. This same event was again detected on a parallel line
150 m to the south at 14:40 hours though now smaller and less elevated.
The sonar record on this line indicated some dispersion of the main cloud though
the boomer record indicated close association between the smaller plume and a
particular pockmark. This association, however, may not be significant. At 15:44
hours, a line was surveyed across the same course as that which had first detected
the plume (at 13:00 hours) and crossed the plume site at 15:54 hours. By this time
all evidence of the plume had disappeared and there was little evidence of
suspended sediment on the sonar record. McQuillin et al., 1979.
It was concluded that neither gas nor fish produced these features. Fish shoals were frequently
observed during this survey on side-scan sonar (40 kHz) and echo sounder (100 kHz) records, yet
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they are seldom recorded by boomer (1-6 kHz). The clouds seen on sonar records are larger, and
denser than those normally associated with fish shoals. Had the clouds been caused by gas they
would have dispersed rapidly and upwards, but the recordings showed that the clouds dispersed
over a period of a few hours, and settled gradually to the seabed. The conclusion drawn was that
two separate disturbances occurred at or near the seabed, probably over a relatively long time
period, resulting in large masses of fine sediment being lifted into suspension to a height of at
least 10 m above the seabed, and that the sediment gradually settled back to the seabed. These
events occurred on a calm day, although there had been gales two days previously. A similar
cloud was recorded on a single boomer record during the 1977 survey. McQuillin et al. surmised
that ‘the disturbances recorded on 30th July were associated with pockmark growth though not
necessarily with the initial development of any new pockmarks’.
This evidence suggests that the pockmarks in this study area include some features that are at
least periodically active.
Post-glacial pockmark activity
The persistence of pockmark formation throughout the post-glacial period represented by the
Witch Ground Formation is indicated by the presence of buried pockmarks at several layers.
Long (1992) reported that there have been several periods of non-deposition over this time, each
represented by a pockmarked former seabed surface. He suggested that the pockmark density
was generally proportional to the time period over which an individual seabed surface had been
preserved. However, he concluded that a high density of buried pockmarks at the Witch/Fladen
Member boundary about 13,000 years BP was consistent with an increase in gas escape activity.
This coincided with a period of rapid climatic amelioration and "the degradation of subseabed
permafrost ice lenses", which had previously trapped gas.
The Witch's Hole
Fig. 2.19*, a seismic profile across the Witch's Hole not only indicates that gas lies close beneath
the seabed, but also that there is a 'plume' rising from the pockmark itself (the shadow of this
'plume' can also be seen on Fig. 2.20). The acoustic turbidity, the unusual ('fresh') appearance of
the pockmark, the Sniffer evidence of methane within the seawater, and this plume were all taken
as evidence that this pockmark was actively seeping gas. However, a side-scan sonar survey
undertaken in 1987 by Total Oil Marine produced a surprising result. The 'plume' is in fact a
shipwreck (discussed further in Section 11.5.1)!
2.3.2 Tommeliten: Norwegian Block 1/9
This location lies in the Greater Ekofisk area over the Tommeliten Delta salt diapir. Tommeliten
was a small gas-condensate field associated with three salt diapirs: Alpha, Gamma, and Delta.
The total net hydrocarbon pore volume for the Alpha and Gamma reservoirs was estimated as
between 120 and 148 x 106 m3 (D’Heur and Pekot, 1987), but the Delta structure, which has
domed and pierced the enclosing sedimentary rocks, lacks a proper seal. Well number 1/9-5,
which was drilled on this structure, turned out to be dry.
There are seismic ‘chimneys’ (chaotic or turbid zones) above all three Tommeliten salt diapir
structures (D’Heur and Pekot, 1987). The strongest may be seen above the Delta structure (Fig.
2.21*), where gas seeps were recorded during a routine site survey in 1978. The seabed at this
site appears flat and featureless on echo sounder records. Water depths range between 74 and
75.5 m, with a very slight slope towards the southwest.
Geophysical reconnaissance and sediment sampling
Shallow seismic (3.5 kHz pinger) profiles indicate that acoustic turbidity (indicating gas) is
present in the surficial sediments over an area of approximately 120,000 m2. Water column
targets visible on pinger and echo sounder records were located in various parts of this area, but
most were concentrated in the 6,500 m2 area indicated on Fig. 2.22.
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Side-scan sonar records showed that the seabed was essentially flat and featureless, but a few
areas of high reflectivity were recorded. The sizes of the patches ranged from 100 m2 upwards;
the larger ones are shown in Fig. 2.22. During a subsequent site survey, small (about 5 m
diameter) oval to circular shallow depressions noted on short-range side-scan sonar records were
found to be distributed with a spacing of between 50 and 500 m. Each depression was found to
include a patch of highly reflective material, so they were called ‘eyed pockmarks’.
Piston core samples demonstrated that the seabed sediments consist of fine- to medium-grained
sands and silty sands that frequently contain an abundance of shell material. In some cores the
lower sections of this sand were partially cemented by a calcareous cement. This topmost
sediment was found to be about 30 cm thick in most places, but near the centre of the area the
underlying stiff clay was exposed at the surface. Interstitial sediment gases were taken from the
piston cores and analysed for hydrocarbons (see below).
ROV surveys
The first objective of the ROV surveys was to locate and identify the water column targets. It
was found that these were gas seeps. It was estimated that there were about 120 individual seeps,
all but a few of which are located within the area of gas-charged sediments. Most commonly
each seep occurred as a single stream of bubbles, about 1 cm in diameter, rising from a discrete
hole of the same size within the sandy seabed. These holes were generally located within a
shallow funnel-shaped depression approximately 20 cm in diameter (Fig. 1.2*). As an
experiment one of these small craters was filled with sand. After about 1.5 minutes the bubble
stream was re-established from the same vent, eroding a similar small crater with an initial burst
of a few large bubbles. It was concluded that the gas was emanating from a single conduit from
the underlying clay and that new escape routes were not easily formed, it was estimated that, on
average, each seep produced one bubble every 6 seconds, and that the total gas production from
the main seepage area was about 24 m3 per day (at ambient pressure, 75 m water depth).
Samples of gas were collected from two vents by filling partially evacuated flasks via a funnel.
All the seeps observed from the ROV were located on a s