THESIS

Seal Potential of the Zechstein Salt in the Southern
Dutch North Sea, as a Function of the Geometry and
Deformation of the Zechstein 3 Stringer: A Seismic
Scale Interpretation.
Benjamin James Thomas
MESci Exploration and Resource Geology
School of Earth & Ocean Sciences,
Cardiff University,
Main Building,
Park Place,
Cardiff,
CF10 3AT

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“Imagination is more important than knowledge” – Albert Einstein
I dedicate this Thesis to the MESci Elite 2013/14.

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DECLARATION
This work has not previously been accepted in substance for any degree and is not being
concurrently submitted in candidature for any degree.
Signed: ________________________________ (candidate)
Date: __________________________________
STATEMENT
This dissertation is being submitted in partial fulfilment of the requirements for the degree of
Master of Earth Sciences.
Date: __________________________________
STATEMENT
This dissertation is the result of my own independent work, except where otherwise stated.
Date: __________________________________

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ABSTRACT
The 40 m thick, brittle carbonate-anhydrite-clay Zechstein 3 (Z3) stringer exhibits a complex
geometry of boudinaged, folded and stacked high amplitude reflectors in 3D seismic data. In
this study I investigate the regional and local deformation mechanisms responsible for the
complexity of the preserved stringer fragments in the southern Dutch North Sea at a seismic
scale. An extrapolated surface of the uppermost stringer fragments illustrates a broadly
harmonic relationship with the top salt topography, but with local deviations regularly
displayed, particularly in the presence of inverted fault blocks in the Rotliegend Basement. I
compare my observations to those of previous studies of the same area, in order to present a
multiphase deformational model of initial sediment down-building related boudinage of the
Z3 stringer, associated with the Kimmerian extensional tectonic phase during the Late
Triassic and Early Jurassic; followed by multiple phases of intense, complex intra-salt flow
regimes, associated with the compressional tectonics of the Alpine orogenic event during the
Cretaceous. Analysis of the individual stringer fragments in 7 zones of interpreted fluid
migration reveals no evidence of connectivity between the stringers, meaning there is no
verification of a fluid pathway through the salt. A model representing the best scenario for the
formation of a continuous pathway through the Zechstein Group has been produced, which
acts as a fluid conduit for gas to migrate through the salt from the Rotliegend reservoir. This
model is based on observations of this project and previous of works. Ductile formation of the
typically brittle formation is essential, in order for a continuous pathway to be present.

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CONTENTS LIST
CHAPTER 1 - INTRODUCTION 10
1. INTRODUCTION 11
1.1. Rationale 11
1.2. Location of the Study Area 12
1.3. The Zechstein 3 Stringer in the southern Dutch North Sea 13
1.4. Limits to the Sealing Capacity of Rock Salt 14
1.5. Aims and Objectives of this Thesis 15
CHAPTER 2 – GEOLOGICAL SETTING 16
2. GEOLOGICAL SETTING 17
2.1. Tectonic History 17
2.1.1. Paleozoic – Formation of Pangea 18
2.1.2. Mesozoic – Break up of Pangea 20
2.1.3. Cenozoic – Alpine Orogeny and the Formation of the Rhine
Graben
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2.2. Stratigraphy 24
2.2.1. Permian 24
2.2.1.1. Lower Rotliegend 24
2.2.1.2. Upper Rotliegend 24
2.2.1.3. Zechstein Group 25
2.2.2. Triassic 25
2.2.2.1. Lower Germanic Trias Group 26
2.2.2.2. Upper Germanic Trias Group 26
2.2.3. Jurassic 26
2.2.3.1. Altena Group 27
2.2.3.2. Scruff, Niedersachsen and Schieland Groups 27
2.2.4. Cretaceous 27
2.2.4.1. Rijnland Group 28
2.2.4.2. Chalk Group 28
2.2.5. Tertiary 28
2.2.5.1. Lower North Sea Group 29

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2.2.5.2. Middle North Sea Group 29
2.2.5.3 Upper North Sea Group 29
2.3. Geology of the Study Area 29
CHAPTER 3 – DATA COLLECTION and METHODOLOGY 32
3. DATA COLLECTION and METHODOLOGY 33
3.1. 3D Seismic Data Acquisition 33
3.1.1. Marine Acquisition of Three-Dimensional Seismic Data 33
3.1.2. Geological Controls on Seismic Reflection Data 34
3.1.3. Seismic Data Processing 35
3.1.3.1. Seismic migration 35
4.1.3.2. Common-midpoint (CMP) 37
3.2 Interpretation of the 3D Seismic Cube 38
3.2.1. Interpretation of Seismic Horizons 38
3.2.1.1. Zechstein Top 38
3.2.1.2. Zechstein Basement 38
3.2.1.3 Zechstein 3 stringer 39
3.2.2. Continuous Surfaces 40
3.2.3. Thickness Maps 42
3.2.4. Fluid Migration 42
3.2.4.1. Fluid Migration Zones 42
3.2.4.2. Distance from top salt, basement and the angle of dip 43
CHAPTER 4 - RESULTS 44
4. RESULTS 45
4.1 Regional Scale Geological Features 45
4.1.1. Zechstein Base 45
4.1.2. Zechstein Top 50
4.2. Zechstein 3 Stringer 53
4.2.1. Regional Scale Deformation 53
4.2.1.1. Enveloping surface 53
4.2.1.1.1. Enveloping surface position 56

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4.2.1.1.2. Dip angle and azimuth of dip 58
4.2.2. Local Deformation 61
4.2.2.1. Stringer fragments 61
4.2.2.2. Folding and stacking of stringer fragments 61
4.3 Fluid Migration 64
4.3.1. Zone 1 66
4.3.1.1. Stringer geometry 66
4.3.1.2. Dip of stringer fragments 70
4.3.1.3. Stringer distance from Zechstein Top 71
4.3.1.4. Stringer distance from Zechstein Base 72
4.3.2. Zone 2 74
4.3.2.3. Stringer distance from the Zechstein Top 78
4.3.3. Zone 3 82
4.3.2.3. Stringer distance from the Zechstein Top 86
4.3.4. Zone 4 90
4.3.4.3. Stringer distance from Zechstein Top 94
4.3.5. Zone 5 98
4.3.5.1 Stringer geometry 99
4.3.5.3. Distance of stringers from Zechstein Top 102
4.3.5.4. Distance of stringers from Zechstein Base 103
4.3.6. Zone 6 105

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4.3.7. Zone 7 112
4.3.7.2. Dip of stringers 116
CHAPTER 5 – DISCUSSION and INTERPRETATION 120
5. DISCUSSION and INTERPRETATION 121
5.1. Regional Scale Geological Features 121
5.1.1. Upper Rotliegend Basement 121
5.1.2. Zechstein Top Salt 122
5.2. Deformation of the Z3 Stringer 122
5.2.1. Boudinage 125
5.2.2. Folding, Stacking and rotation of Stringer Fragments 131
5.3. Deformation Model 133
5.4. Fluid Migration 136
5.4.1. Seal Potential of Rock Salt 136
5.4.2. Stringer Geometry 137
5.5. Connectivity Model 138
CHAPTER 6 – CONCLUSIONS and FURTHER WORK 140
6. CONCLUSIONS and FURTHER WORK 141
6.1. Conclusions 141
6.1.1. Deformation Mechanisms 141
6.1.2. Gas Migration 142
6.2. Further Work 142
7. REFERENCES 144

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8. ACKNOWLEDGEMENTS 150
9. APPENDICES 151
9.1. Zone 1 151
9.2. Zone 2 154
9.3. Zone 3 157
9.4. Zone 4 160
9.5. Zone 5 163
9.6. Zone 6 166
9.7. Zone 7 168

Chapter 1 - Introduction
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CHAPTER 1 – INTRODUCTION
The introduction firstly identifies the importance of evaporitic deposits and their
relationship with hydrocarbon exploration.
The importance of this study is outlined.
The study area of this thesis is identified.
The geometry and deformation of the Z3 stringer is discussed, with reference to
previous studies.
The South Oman Salt Basin is used as an example of carbonate stringers acting as a
hydrocarbon reservoir.
The quality of a rock salt seal is discussed.
Finally, the aims, objectives and hypotheses of this thesis are outlined.

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1. INTRODUCTION
1.1. Rationale
A large proportion of the world’s hydrocarbon reserves are associated with evaporitic deposits
(Warren, 2006; Van Gent et al., 2011). These reserves include hydrocarbon plays in the
Central European Basin, the Gulf of Mexico, offshore Brazil and basins in the Middle East
(Van Gent et al., 2011). In the South Oman Salt Basin (SOSB) carbonate stringers are known
to act as hydrocarbon reservoirs. The rock salt of the enclosing diapirs acts as the seal of these
hydrocarbon accumulations (Schoenherr et al., 2007). It is essential that the thickness,
geometry, porosity and fluid fill of carbonate-anhydrite stringers be quantified, in order to
determine their economic viability (Van Gent, et al., 2011). These reservoirs are currently
relatively poorly understood; therefore studies into stringers in the Dutch North Sea can
contribute to a better understanding of hydrocarbon-bearing stringers in other parts of the
world, for example in the Ara Salt in Oman (Van Gent et al., 2011).
Carbonate-Anhydrite stringers are also recognized as potential drilling hazards in the North
Sea. This is due to the Carbonate member of the Zechstein 3 (Z3) stringer being significantly
over-pressured and in some cases up to lithostatic (Van Gent et al., 2011). These pressures are
difficult to predict, meaning stringers are therefore avoided when planning drill paths through
the Zechstein Salt, especially when heavily folded and faulted (Van Gent et al., 2011).
As well as implications for the hydrocarbon industry, quantifying the sealing qualities of rock
salt is becoming of increased importance, as the energy sector begins to invest in nuclear
power and assess the validity of Sequestration. Rock salt possesses excellent sealing qualities
(section 1.4), and is therefore the subject of investigations into geologically sound structures,
which can be used for the long-term storage of hazardous material and CO2 (Van Gent et al.,
2011). The identification of continuous, permeable pathways through the Zechstein Salt, with
evidence of fluid flow would have important implications on such storage structures, both in
the North Sea and around the World.

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1.2. Location of Study Area
The 65 km by 25 km 3D seismic survey being analysed in this project is located in the K
block of the North Sea (Figure 1.1). Situated on the Cleaver Bank High (CBH), immediately
north of the Broad Fourteens Basin, the survey area incorporates the entirety of blocks K07
and K08 and the peripherals of surrounding blocks.
Figure 1.1: Map illustrating the location of the study area (highlighted in
red) in relation to the Cleaver Bank High and Broad Fourteens Basin
(highlighted in green) (Modified from Verweij, 2003).

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1.3. The Zechstein 3 stringer in the southern Dutch North Sea
The Southern Permian Basin in NW Europe is a classic area of salt tectonics (Zeigler, 1990;
Taylor, 1998; De Jager, 2007, Geluk et al., 2007; Strozyk et al., 2012). The study area plays
host to the lower four cycles of the Late Permian Zechstein Group (Z1-Z4) (Strozyk et al.,
2012). The Zechstein 3 (Z3) stringer Lliene Formation) is the subject of this study. This unit
consists of a brittle layer of anhydrite, carbonate and clay, which is fully encased in the
surrounding massive halite (Van Gent et al., 2011; Strozyk et al., 2012). The Z3 stringer is
decoupled from the underlying Upper Rotliegend basement and the supra-salt sediments
(Germanic Trias Group), unlike the Z1 and Z2 cycles, which are mechanically coupled to the
basement and the Z4 cycle, which is coupled to the overburden. Salt movement was mainly
accommodated by flows in the Z2 salt, whereas the Z3 and Z4 salts are, on the whole
passively placed, meaning that halokinesis heavily affected the geometry of the Z3 unit
(Taylor, 1998; Strozyk et al., 2012). This deformation is manifested by a variety of
deformation features, including folding and boudinage. Work carried out by Van Gent et al.,
(2011) and Strozyk et al., (2012) has idenitified and quantified these features in studies of 3D
seismic data collected in the same region as the data being analysed in this project.
These studies have led to recognition that the deformed carbonate-anhydrite member is
predominantly situated in the upper third of the salt and remains at a relatively constant
thickness (approximately 40 m), but locally can reach thicknesses of up to 70 m (Strozyk et
al., 2012). The extremity of the folding and the angle of dip of the stringers vary significantly
across the region. In areas where the stringers have a dip of less than 30°, the stringer surface
primarily exhibits gentle, open folds with a wavelength of around 400 m and an amplitude of
less than 200 m (Van Gent et al., 2011). The axes of these folds are curved and show no
preferred orientation. The stringer surface is frequently offset along steep discontinuities (>
35°) with either a symmetrical or monoclinal offset (Van Gent et al., 2011).
The degrees of deformation of the stringer do not always correlate to the local magnitude of
halokinesis (i.e. deformation in large salt features can be more or less complex than the
topography of the top salt surface). This implies that intra-salt deformation is not solely linked
to the deformation of the basement, or indeed the top salt. There are areas that exhibit

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complex, three-dimensional, tight to isoclinal folds. These structures are primarily located in
close proximity to inverted blocks in the basement. Van Gent et al., (2011) separate the folds
into four groups: (1) diapir-scale folds, (2) constriction folds, (3) non- cylindrical folds and
(4) isoclinal folds deep in the salt structures.
Diapir scale folds are interpreted to be broadly concordant with the top salt topography, which
are interpreted to be the product of large-scale flow paths during halokinesis (Strozyk et al.,
2012). The other three types are disharmonic to the top salt and are therefore the result of
local, complex flow regimes. Open folds with sub-horizontal fold axes are interpreted to have
formed from a constrictional process, as the stringer migrated into the salt structures with the
flow of salt (Van Gent et al., 2011).
On a regional scale, Strozyk et al., (2012) conclude that models showing salt flow towards the
salt structures do not correlate to the vertical offset of the stringer fragments. This suggests
that vertical movement in the salt is far greater than depicted in mining literature (e.g,
Bornemann et al., 2008).
1.4. Limits to the Sealing Capacity of Rock Salt
Rock salt (Predominantly halite) is recognized as the best seal for hydrocarbon accumulations
based on three key properties: (1) The near-isotropic stress state of rock salt provides
resistance to hydrofracturing; (2) The in situ permeability and porosity is very high even at
low burial depths (from 70 m) and (3) Plastic deformation of rock salt in nature is ductile and
therefore non-dilatant (Schoenherr et al., 2007). However, under correct conditions, all rocks
can loose their sealing capacity. The geological conditions that result in rock salt loosing its
seal potential are not well understood. Work carried out by Schoenherr et al., (2007) conclude
that the Ara Salt, in the South Oman Salt Basin (SOSB), has in the past, lost its sealing
capacity, resulting in the migration of oil into the surrounding rock salt. Oil initially leaked
into the salt when oil pressure in the reservoir exceeded the near-lithostatic fluid pressures of
the Ara Salt. This was followed by diffuse dilation and marked an increase in permeability
(Schoenherr et al., 2007).

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Microstructures reveal a complex process of dilation and re-sealing of oil with associated
dynamic recrystallization, conversion of oil into solid bitumen and continued salt deformation
(Schoenherr et al., 2007). The maximum past differential stress in the Ara Salt around the
reservoirs was less than 2 MPa. Oil initially escaped into the salt when oil pressure in the
reservoir exceeded the near-lithostatic fluid pressures of the salt by only a few tenths of a
mega-Pascal, to overcome the capillary entry pressure (Schoenherr et al., 2007). This was
followed by diffuse dilation and a marked increase in permeability of the rock salt
(Schoenherr et al., 2007). Oil could flow into the salt as long as the oil pressure remained
larger than the minimum principal stress in the rock salt (Schoenherr et al., 2007).
1.5. Aims and Objectives of this Thesis
The aim of this work is to build upon the limited existing knowledge of the geometry and
deformation of the Z3 stringer in the southern Dutch North Sea and specifically, to identify
the mechanisms that have driven the deformation of the carbonate/anhydrite stringer. The
potential of the stringers to act as fluid conduits and subsequently focus gas flow through the
Zechstein Salt can then be assessed. These aims have lead to the following questions being
posed:
Q1: What are the deformation mechanisms responsible for the complex geometry of
the stringers in the Dutch North Sea?
Q2: Do the deformed stringer fragments manifest themselves in such a way that
allows them to act as a permeable pathway through the impermeable Zechstein Salt?
Q3: Are these features quantifiable through 3D seismic analysis?
I hypothesize that the carbonate-anhydrite Zechstein 3 stringer has been deformed through
multiple phases of deformation, which are strongly linked to the deformation of the top salt
(H1). However, due to the complexity of the deformed stringer and the limitations of 3D
seismic analysis, the potential of the stringer fragments to form pathways through the
Zechstein Salt will not be quantifiable through seismic analysis alone (H2).

Chapter 2 – Geological Setting
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CHAPTER 2 – GEOLOGICAL SETTING
A Description of the tectonic history of the South Permian Basin is given; separated
into three groups: (1) the formation of Pangea during the Paleozoic, (2) the break-up
of Pangea during the Mesozoic and (3) the Alpine orogeny and the formation of the
Rhine Graben during the Cenozoic.
The stratigraphy of the southern North Sea is discussed and split up into the Permian,
Triassic, Jurassic, Cretaceous, Tertiary and Quaternary periods.
A description of the geology of the study area, in particular the Broad Fourteens
Basin and the Cleaver Bank High, is explained.

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2. GEOLOGICAL SETTING
2.1. Tectonic History
The study area of this report is located in the Southern Permian Basin (SPB), of the western
Dutch offshore, in the North Sea (Figure 1.1). Situated on the Cleaver Bank High, directly
north of the Broad Fourteens Basin, the study area has been subjected to multiple phases of
tectonic activity since the Carboniferous (Duin et al., 2006).
These phases have been split up into six separate periods: The Late Carboniferous, Permian,
Triassic, Late Jurassic, Late Cretaceous and Cenozoic. The dominant structural features
preserved in the SPB are Late Jurassic to Early Cretaceous extensional and transtensional rift
basins, including the Broad Fourteens Basin (Duin et al., 2006). De Jager (2007) identifies
four main phases of tectonism that contributed to the complex interaction between platforms,
highs and basins: (1) The formation of the super-continent Pangea during the Paleozoic, as a
result of the Caledonian and Variscan orogenies; (2) The break-up of Pangea during Mesozoic
rifting; (3) The collision of Africa and Europe in the Late Cretaceous to Early Tertiary, during
the Alpine inversion and (4) The development of the Rhine Graben rift system since the
Oligocene. These tectonic events have resulted in multiple reactivation events of the basement
faults within the Rotliegend, which originate from the early stages of the Caledonian
Orogeny. These faults control the fault trends, of later tectonic episodes, independent of stress
direction and tectonic facies (De Jager, 2007).
The deposition of thick Zechstein evaporite group during the Permian further contributed to
the structural complexity within the SPB subsurface (Duin et al., 2006). Faulting of the
Rotliegend sediments, in conjunction with the deposition of the overlying Germanic Trias
Group initiated halokinesis within the salt, which subsequently resulted in transpressional and
extensional faulting of the supra-salt sediments (De Jager, 2007). The Zechstein Unit
decouples supra-salt faults from those in the Rotliegend (De Jager, 2007) and subsequent
periods of post-salt tectonic activity resulted in increased amounts of salt movement
(Remmelts, 1996). Halokinesis increased throughout the Paleozoic and by the Early Triassic
was widespread across the SPB. This resulted in the formation of salt structures, such as salt

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walls, rim synclines and salt pillows, which have been identified in this study. The salt wall
and pillow structures evolved during the deposition of the Triassic Solling, Rot and
Muschelkalk formations (De Jager, 2007). The development of piercing salt domes and
associated rim-synclines occurred during the Early Cretaceous (De Jager, 2007). Halokinesis
continued into the Cenozoic, with large local variations in movement across the basin. The
northern Dutch offshore was subject to large amounts of salt movement, as Duin et al., (2006)
show in thickness maps of the North Sea groups.
2.1.1. Paleozoic – Formation of Pangea
The Caledonian Orogeny commenced with the collision of Baltica and Laurentia in the Early
Ordovician (Figure 2.1). This convergence continued through to the Early Devonian, forming
Laurussia, which then went onto collide with Gondwana in the Variscan Orogeny during the
Late Paleozoic (Figure 2.1) (De Jager, 2007).
As a result of these mountain building events, the area of the Netherlands became landlocked;
bounded by the Variscan mountain range to the south and the Caledonian mountain belt to the
north. These vast mountain ranges acted as a sink of huge amounts of sediments being
deposited into deep forebasins that had developed in front of the mountain belts (Jager et al.,
2007). The compressional tectonics associated with the Variscan orogeny ceased in the region
during the Mid-to-Late Carboniferous and by the end of the Carboniferous, Late Variscan
post-orogenic tectonics were beginning to affect the area of the Netherlands (Ziegler, 1990;
De Jager, 2007).

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A
B
Figure 2.1: A) The closure of the Iapetus Ocean commenced during the Ordovician and
continued through to the Devonian, resulting in the collision of Laurentia, Avalonia and
Baltica in the Caledonian orogeny. B) The closure of the Rheic Ocean during the
Carboniferous resulted in the collision of Laurentia, Eurasia and Gondwana in the
Variscan orogeny, leading to the formation of the super continent Pangea during the
Triassic, as the Tethys Ocean opened to the southeast (www.1).

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During the Mid-to-Late Carboniferous and into the Permian, wrench faulting and thermal
uplift, caused by extensive, deep erosion (Van Buggenum & Jager, 2007) resulted in the
Lower Permian Rotliegend sediments being deposited unconformably on top of the eroded
Carboniferous basement (Duin et al., 2006). This hiatus of 40 to 60 Ma is known as the
‘Base-Permian Unconformity’ (Duin et al., 2006; Ziegler, 2007). The NW-SE trending fault
systems that were initiated during the compression of the Late Carboniferous and Early
Permian were reactivated and a second trend of NE-SW to NNE-SSW conjugate faults
developed (Ziegler, 1990; Duin et al., 2006).
The SPB formed due to thermal subsidence in the forebasin of the Variscan Mountain belt
during the Early Permian (Van Wees et al., 2000; Duin et al, 2006). During this period, the
subsidence rate began to exceed the sedimentation rate as the North Atlantic Rift System
began to evolve and by the end of the Permian, extensive subsidence resulted in saline
seawater flooding the basin. This saline seawater sourced the Zechstein Formation, which is a
halite-dominated sequence of cyclic evaporites that was deposited in arid conditions (Ziegler,
1990; De Jager, 2007).
2.1.2. Mesozoic – Break up of Pangea
The second, extensional phase of tectonics was initiated by the break-up of Pangea during the
Mesozoic (Duin et al., 2006). This break-up saw the area of the Netherlands migrate to sub-
tropical latitudes of the northern hemisphere. Subsidence continued throughout the Triassic
and into the Jurassic, resulting in further sediments being deposited into the basin (De Jager,
2007). There were two phases of extensional tectonics during the Triassic that are associated
with Mesozoic rifting: The Hardegsen Phase, and the Early Kimmerian Phase, which
consisted of five tectonic pulses (Geluk, 2007). These E-W trending, extensional tectonic
phases in the SPB caused minor strike-slip movement along existing faults and triggered the
break-up of the basin into small, complex, fault-bounded basins and platforms, including the
Cleaver Bank High (De Jager, 2007). Tectonic activity in the area was relatively inactive
during the Late Triassic and Early Jurassic (De Jager, 2007).

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A large section of the Dutch offshore was uplifted throughout the Mid-Jurassic, due to the
formation of the Central North Sea Dome (Ziegler, 1990). This Mid-Kimmerian Phase saw a
regional thermal uplift as inversion occurred (Duin et al., 2006). During the Late Jurassic and
Early Cretaceous, the N-S trending North Sea rift system became subject to regional extents
of accelerated rifting as the break-up of Pangea evolved (Figure 2.2).
This increase in tectonic activity also resulted in pre-existing NW-SE fault trends to the south
being reactivated (Duin et al., 2006; De Jager, 2007). Reactivation formed the NW–SE
trending Broad Fourteens and West Netherlands Basins in conjunction with the uplift of
adjacent platforms, resulting in the vast majority of Jurassic and Triassic sediments being
eroded. The erosion of these sediments formed the Late Kimmerian Unconformity (De Jager,
2007; Wong, 2007). The majority of the tectonic features preserved in the Dutch North Sea
originate from this Late Kimmerian stage of extensional tectonic activity, which by the Late
Cretaceous had all but ceased (Ziegler, 1990; De Jager, 2007).
Figure 2.2: Mesozoic rifting occurred in two phases; firstly passive rifting and
lithospheric extension, which later gave way to active, tectonic rifting as the break up of
Pangea took place (www.1).

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2.1.3. Cenozoic – Alpine Orogeny and the Formation of the Rhine Graben
The Alpine Orogeny occurred during the Late Cretaceous, as Africa collided with Eurasia.
This orogenic event resulted in the simultaneous inversion of the Mesozoic basins that
originated from the break-up of Pangea (Ziegler, 1990; De Jager, 2007). Tertiary compression
developed in four main phases: the Subhercynian, Laramide, Pyrenean and Savian phases.
The result of this was the uplift and erosion of Late Cretaceous and Early Tertiary sediments
and in some areas and the truncation of older deposits (De Jager, 2007; Herngreen, 2007). The
presence (or lack of it) of Zechstein Group helped to dictate the nature of inversion that
occurred. In areas with no Zechstein salt, pre-existing, dormant normal faults were reversely
re-activated (e.g. in the West Netherland Basin). In basins, such as the Dutch Central Graben,
which have thick salt deposits, the post-salt sediments were uplifted through halokinesis, as
faults in the pre-and-post salt sediments were detached by the Zechstein Group (Nalpas et al.,
1995; De Jager, 2007).
Situated above the Mesozoic rift structures is a large sag basin, with a N-S trend. This large
Tertiary feature consists of siliclastic deposits and roughly coincides with the current North
Sea Basin (Wong, 2007). During the Eocene and Oligocene, extensive rifting of the Lower
Rhine Graben occurred (Ziegler, 1990; Ziegler, 1994; Duin et al., 2006). The majority of the
North Sea indicates a rapid increase in subsidence in the last few thousand years; however the
southeastern part of the Netherlands is being uplifted by the development of the Rhenish
Massif (De Jager, 2007).

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Figure 2.3: Geological scale of the southern North Sea, including a lithostratigraphic columa,
tectonic deformation phases and lithostratigraphic layers (Modified from Duin et al., 2006).

24
2.2. Stratigraphy
The stratigraphic units in the North Sea are commonly split up into six groups: the Permian,
Triassic, Jurassic, Cretaceous, Tertiary and Quaternary (De Jager, 2007).
2.2.1. Permian
The sediments of the Permian can be separated into three groups (Figure 2.3): the Lower
Rotliegend, the Upper Rotliegend and the Zechstein Salt. The Lower and Upper Rotliegend
are classified as the Lower Permian Formations and the Zechstein is classified as the Upper
Permian Formation (Glennie, 1998). The Rotliegend sedimentary sequence was deposited in
the post-Variscan, South Permian Basin (SPB) that extended 1500 km from eastern England
to the Russian-Polish border (Glennie, 1998). The sandstones of the Upper Rotliegend make
up the most important reservoir rock, which hosts hydrocarbons in the SPB (Glennie, 1998).
2.2.1.1. Lower Rotliegend - The Lower Rotliegend group consists of volcanic and clastic
rocks (Figure 2.3), which accumulated during the Early-Mid Permian and were mainly
deposited in fluvial and lacustrine facies, in a climate that alternated between tropical and
semi-arid, with very little evidence of aeolian deposition (Glennie, 1998). Locally deposited,
the Lower Rotliegend sediments are far less expansive than the Upper Rotliegend. The
volcanic rocks range from rhyolites to ignimbrites and basalts (Glennie, 1998). Lower
Rotliegend sediments were uplifted and eroded, resulting in the formation of the Permian
Base Unconformity between Carboniferous and Upper Rotliegend sediments (Geluk, 2005).
2.2.1.2. Upper Rotliegend - Four distinctive depositional facies have been identified in the
Upper Rotliegend Group: fluvial, aeolian, sabka and lacustrine environments (Glennie, 1998).
These four facies are widespread across the asymmetric SPB. Deposited in a warm, arid
climate, the northern extent of the SPB consists of fine-grained evaporites and clastic
sediments (Figure 2.3). However, in Dutch western offshore areas, sandstones are the
predominant units deposited (Geluk, 2007). Milankovitch cycles and in particular the
eccentricity of Earths orbit, which has a 100-200 ka periodicity, greatly affected the
alternation between wet and dry periods of deposition (Geluk, 2007; De Jager, 2007).

25
2.2.1.3. Zechstein Group - The Zechstein Group consists of cyclic marine evaporites,
carbonates and subordinate clastics, controlled by the initial transgression from the Artic and
evaporation of seawater in the arid Southern Permian Basin (Van Gent et al., 2011). The
sequence is broadly broken down into the lower part (Zechstein 1-3) and the upper part
(Zechstein 4 & 5). The Lower Zechstein deposits display the classic carbonate-evaporite
cycle: Claystone-carbonate-gypsum-halite-potassium salts consecutively (Duin et al., 2006;
De Jager, 2007; Geluk, 2007). In the northern Netherlands, deposition of the Zechstein Group
was fairly continuous, but towards the south, there are parts of the sedimentary sequence that
are completely void of the Zechstein Group (Duin et al., 2006; Van Gent et al., 2011). The
thickness of the Zechstein Salt increases dramatically to the north of the SPB, with the
southern Netherlands only displaying thin deposits due to the relatively shallow nature of the
basin. There are areas where the effects of post-Zechstein tectonics are limited, which has
resulted in the Zechstein Group accumulating to thicknesses of over 900 m (Geluk, 2007).
This is the assumed minimum depositional thickness of the Zechstein Group in the offshore
fields, including the K block. (Geluk, 2005; Geluk, 2007).
The Zechstein 1 (Z1 (Werra Formation)), Zechstein 2 (Z2 (Stassfurt Formation)) and
Zechstein 3 (Z3 (Leine Formation) were deposited under normal marine conditions, with
clastic deposition being restricted to the margins of the basin. The Zechstein 4 (Z4 (Aller
Formation)) and Zechstein 5 (Z5 (Ohre Formation)) do not contain any carbonates and were
deposited during permanent hyper saline conditions (Duin et al., 2006; Geluk, 2007; Van
Gent et al., 2011). The termination of the deposition of the Zechstein Group indicates the
return of continental and humid conditions (Geluk, 2007). The Zechstein Group is well
known for its excellent sealing qualities of the Mesozoic hydrocarbon plays in the southern
North Sea (De Jager, 2007).
2.2.2. Triassic
The Triassic sediments of the Netherlands are split into two groups: the Lower Germanic
Trias and the Upper Germanic Trias. During uplift of the Late Jurassic, the Triassic sediments
were eroded from the highs that formed, resulting in them only being primarily preserved in
the deep, subsiding basins, such as the Broad Fourteens Basin (Duin et al., 2006).

26
2.2.2.1. Lower Germanic Trias Group – The Lower Germanic Group, deposited during the
Late Permian and Early Triassic, consists mainly of fine-grained clastic deposits with
sandstone and oolite intercalations. The majority of the deposits on the margins of the SPB
are sandstones, with the Buntsandstein Group being deposited in relation to high-order
Milankovich cycles (Geluk & Rohling, 1999; Geluk, 2007).
2.2.2.2. Upper Germanic Trias Group - The Upper Germanic Trias Group is comprised of
alternating fine-grained clastics, carbonates and evaporites with subordinate sandstones
deposited in marine and lacustrine environments (Geluk, 2007). The Base Solling
Unconformity forms the boundary between the two subgroups, which is regionally
recognizable (Ziegler, 1990; Geluk & Rohling 1997; 1999).
The Solling Formation consists of basal sandstones, overlain by fine-grained deposits, whilst
the overlying Rot Formation exhibits a lower evaporitic sequence, which was widely
deposited across the area of the Netherlands and an upper sequence, which is comprised of
clays and siltstones (Ziegler, 1990; Geluk, 2007). The sediments of the Muschelkalk
Formation consist of carbonates in the upper and lower stages, with an intercalating evaporitic
formation in between (Geluk, 2007). This formation reaches 250 m in thickness in the Broad
Fourteens Basin and was heavily affected by the Early Kimmerian uplift and subsequent
erosion (Fisher, 1998; Duin et al., 2006). Finally, the Keuper Formation consists of claystones
and alternating evaporites (Geluk, 2007).
2.2.3. Jurassic
The Jurassic units are divided into four groups: the Altena, Schieland, Niedersachsen and
Scruff Groups. Predominantly siliclastic in nature, the latter three units were deposited
simultaneously and are only divided by the area in which they have accumulated. Tectonic
activity during the Middle Jurassic and Early Cretaceous, caused uplift and erosion of the
Jurassic sediments, which resulted in the units only being preserved in the deep basins (Wong,
2007).

27
2.2.3.1. Altena Group - The Altena Group consists mainly of argillaceous sediments, with a
small amount of calcareous intercalations towards the base (Duin et al., 2006; Wong, 2007)
and was deposited during the Late Triassic and Early Jurassic. Erosion of the surrounding
highs resulted in the sediments only being preserved within the Late Jurassic basins (Duin, et
al., 2006).
The Sleen formation was unconformably deposited on top of the Triassic units and is made up
of politic and marine claystones, containing marine and lacustrine fossils (Underhill, 1998).
The Aalburg Formation is consists of uniform claystones with intercalated limestone and
sandstone beds (Underhill, 1998). The Posidonia Shale Formation is the most important oil
source rock in the Broad Fourteens Basin (Duin et al., 2006). It was deposited in a restricted
basin environment, with high levels of anoxia. The Werkendam Formation, made up of silty
mudstones, sandy carbonates and marls was deposited when the basin was again subject to
open ocean currents (Wong, 2007).
2.2.3.2. Scruff, Niedersachsen and Schieland Groups - These groups were deposited during
the Late Jurassic period and into the Early Cretaceous period. Thicknesses of over 1000 m are
identified in the Late Jurassic Broad Fourteens Basin (Duin et al., 2006). The three formations
were deposited in different basins, with the Scruff Formation being deposited in the Central
North Sea Graben, and the Niedersachsen Group deposited in the Lower Saxony Basin (Duin
et al., 2006; De Jager, 2007). The Schieland Group has been divided into the Central Graben
and Delfland Subgroups. The Delfand Subgroup was unconformably deposited in the Broad
Fourteens Basin and consists of alternating sandstones and claystones with subordinate
dolomites and coal beds that were deposited in a coastal-plain environment (Wong, 2007).
2.2.4. Cretaceous
The Cretaceous deposits are divided into the Rijnland Group and the Chalk Group (Lower and
Upper respectively). The Rijland Group is divided further, to include the Vlieland Subgroup
(Oakman, 1998).

28
2.2.4.1. Rijnland Group - The Rijnland Group contains claystones, marls, siltstones and
sandstones and was deposited in the Broad Fourteens Basin and the West Netherlands Basin,
where thicknesses of up to 900 m are evident (Duin et al., 2006). Inversion during the Late
Cretaceous has resulted in these sediments being locally absent or reduced in thickness in the
areas most affected by the inversion (Oakman, 1998).
The Vlieland Sandstone Formation comprises of shallow-marine sandstones, many of which
contain glauconite. The main sedimentary structures are often superimposed by widespread
and intense bioturbation and wave action (Oakman, 1998; Verweij & Simmelink, 2002). The
Vlielend Claystone formation consists of calcareous claystones but in the Broad Fourteens
Basin and the West Netherlands Basin it is very silty and sandy indicating deposition of sands
in storms and periods of low stand (Duin et al., 2006). The Holland Formation contains marls
and marly claystones, which indicate several hiatus events in the form of redish, oxidated
beds.
2.2.4.2. Chalk Group - Comprised mainly of bioclastic and marly limestones, the Chalk
Group is preserved throughout the Netherlands. It is however locally absent due to erosional
events of the Late Cretaceous (Herngreen et al., 2007). Early deposits of the Chalk Group
(known as the Texel Formation) consist of limestones and marly chalks, which develop into
pure limestones further offshore (Oakman & Partington, 1998; Herngreen et al., 2007). Later
sediments (Ommelanden Formation) are typically fine grained and partially argillaceous
limestones that show evidence for multiple erosional episodes. The topmost deposits (Ekofisk
Formation) consist of chalky limestones and alternating thin clay layers.
2.2.5. Tertiary
Tertiary sediments are divided into three main groups: the Lower, Middle and Upper North
Sea Formations. All three are predominantly siliclastic and lie unconformably upon each
other (Bowman, 1998; Wong, 2007).

29
2.2.5.1. Lower North Sea Group - The Lower North Sea Group, which reaches 650 m in
thickness in the North of the Netherlands consists of sands, sandstones, marls and clays,
which are subdivided into the Landen and Dongen Formations (Bowman, 1998; Wong, 2007).
2.2.5.2. Middle North Sea Group - The Middle North Sea Group contains sands, silts and
clays that are unconformably deposited on the Lower North Sea Group. This group is
subdivided into: the Tongeren Formation, which is only preserved in the Zuid-Limburg,
where it reaches a maximum thickness of 50 m; The Rupel Formation, which is preserved
regionally across the Netherlands and reaching thicknesses of 250 m and the Veldhoven
Formation, which is up to 400 m in thickness is predominantly found in the Roer Valley
Graben (Bowman, 1998; Duin et al., 2006; Wong et al., 2007).
2.2.5.3. Upper North Sea Group - Comprised of clays, fine to coarse-grained sands, the Upper
North Sea Group was deposited onto a hiatus surface (Wong et al., 2007). Weerts et al.
(2003) distinguish marine, fluvial, glacial and terrestrial sediments in the sub divisions of the
Group.
2.3. Geology of the Study Area
The 70 km x 25 km study area is located on the perhiperals of the Cleaver Bank High; directly
north of the Broad Fourteens Basin. This Mesozoic basin (120 km x 45 km) trends NW-SE
(Nalpas et al., 1995; Wong, 2007) and has been subject to multiple tectonic events, resulting
in specific sedimentary deposition, which in conjunction with numerous inversions has
resulted in the accumulation of hydrocarbons (Verweij et al., 2003). The Cleaver Bank High
(CBH) has been subject to several periods of extension, uplift, subsidence and compression
since the Carboniferous (Verweij et al., 2003). This platform is approximately 8000 km2
and
due to periods of uplift and erosion, the sediments of the Upper Triassic and Jurassic are not
preserved (Quirk, 1993; De Jager, 2007).
During the Saalian tectonic phase of the Late Carboniferous, the CBH was tectonically
uplifted. Between the Permian and Middle Jurassic the CBH was relatively stable, but during
the Late Jurassic and Early Cretaceous the platform was uplifted and heavily eroded (Duin et

30
al., 2006; Van Gent et al., 2011). However, the study area of this project is situated outside of
the major areas of erosion (Wong, 2007; Van Gent et al., 2011).
Only the lower four of the evopritic cycles of the Zechstein Group are present in the study
area (Z1-Z4). The dominant Z3 cycle (studied in this report) contains a brittle layer of
anhydrite, carbonate and clay (Z3 stringer), which are fully encased in massive halite (Strozyk
et al., 2012). The Zechstein Salt contributed significantly to the development and deformation
of the geology in the region, as a result of its bouyant, ductile nature. The sediments which
overly the Zechstein unit are influenced by this salt movement and display evidence of salt-
induced tectonics (Van Gent et al., 2011; Strozyk et al., 2012).
The study area exibits a high varience in the thicknesses of the Zechstein Group due to the
rising and sinking of salt. The northwest margin of the CBH does not host salt diapirs or
pillows, due to the lack of salt flow in the region (Duin et al., 2006; Van Gent et al., 2011). A
lack of peircing of the overburden through salt movement indicates that the bouyancy of the
salt was not adequate to penetrate the overlying sediments (Strozyk et al., 2012; Verweij &
Simmelink, 2002). Intrusive salt features trend northwest and are influenced by fault patterns
in the original Main Kimmerian rift basins.
Salt tectonics is the main subject of this report and plays a huge role in the structural geology
of the study area. During the Triassic, the Broads Fourteens Basin was acting as a depocentre
of sediments and by the Mid-Jurassic, transtensional development of the basin occurred in a
NW-SE direction (Duin et al., 2006; De Jager, 2007). Tectonic activity of the Late Jurassic
has largely overprinted previous tectonic features due to the wrench-related Late Kimmerian
tectonics that initiated differential basin subsidence and uplift of platforms (Duin et al., 2006).
The Late Jurassic and Early Cretaceous periods saw dextral transtensional movement along
pre-Permian fault systems (Fisher, 1998). The inversion of the Broad Fourteens Basin during
the Cretaceous resulted in immeasurable amounts of crustal shortening, thought to be caused
by transpressive stresses related to the sub-Hercynean tectonic phase (Letsch & Sissingh,
1983; Van Wijhe, 1987; Ziegler, 1990; De Lugt et al., 2003). This compression resulted in the
formation of reverse and listric faults, which detatched within the Zechstein salt (Hayward &
Graham, 1989).

31
Over 1800 m of sediments belonging to the Chalk Group have been preserved on the Jurassic
Highs, such as the CBH, but Late Cretaceous inversion has partly (and locally completely)
removed the sediments across the axial parts of the Broad Fourteens Basin (Duin et al., 2006;
De Jager, 2007). During the Neogene, subsidence of the basin increased. On the northeastern
margin of the Cleaver Bank High, the thickness of Zechstein Group does not exceed 200 m in
thickness and the salt acted as a detachment surface, along which the basin-fill sediments
were thrust onto the platform during crustal shortening (Duin et al., 2006; Nalpas et al., 1995;
Hooper et al., 1995).

Chapter 3 – Data and Methodology
32
CHAPTER 3 – DATA COLLECTION and METHODOLOGY
The processes of collecting marine seismic data and subsequent processing of the data
are briefly outlined.
The methods implemented to interpret the Zechstein Base, Zechstein Top and the Z3
stringers are described.
The method of creating the enveloping surface is described.
The methods used to create continuous surfaces of the interpreted horizons are
explained.
The methods implemented to identify zones of fluid migration into the supra-salt
sediments are identified.
The way the thickness maps and 3D models were created is described.
The way the angles of dip and the azimuth of dip were obtained is explained.

33
3. DATA COLLECTION and METHODOLOGY
The conclusions of this report are based upon interpretations made through the analysis of 3D
seismic data, acquired by Shell in 2012. Signals were sampled every 2 milliseconds, resulting
in a horizontal distance of 25 m per line spacing. The data was then smoothed to a vertical
spacing of 100 m per line spacing to reduce the size of the data file. This has resulted in an
extremely high definition 3D seismic cube.
3.1. 3D Seismic Data Acquisition
Three-dimensional seismic data is the acquisition of very closely spaced seismic reflection
data that produces a detailed horizontal and vertical image of the subsurface (Bacon et al.,
2003). 3D data produces spatially continuous data, which improve interpretations in areas of
complex 3D structural geology compared to that of 2D surveys (Bacon et al., 2003). The
acquisition of seismic data involves the transmission of controlled acoustic energy in to the
subsurface and recording the energy that is reflected back from geological boundaries
(Brown, 2004).
3.1.1. Marine Acquisition of Three-Dimensional Seismic Data
When collecting seismic data at sea, the seismic energy source takes the form of an array of
airguns located just below the sea surface, which are towed behind a vessel and direct
downward energy pulses (Figure 3.1). The airguns are fired at regular intervals (typically 15-
20 seconds) as the vessel moves along a pre-determined survey line (Bacon et al., 2003).
Energy reflected from the subsurface is recorded by hydrophones situated in up to 12 buoyant
streamers that are towed behind the energy source. These streamers are often up to 5 miles in
length (Bacon et al., 2003) (Figure 3.1). A set spacing between the hydrophones is pre-
determined (typically 300 m – 600 m). This results in two-way-time travel data (TWTT)
being gathered in milliseconds of the time it takes for the P-wave energy to travel from the
source and subsequently be recorded by the hydrophones (Figure 3.1). The original seismic
lines, collected through this process are called in-lines in the 3D seismic cube and lines
perpendicular to these are called cross-lines (Bacon et al., 2003; Brown, 2004).

34
Figure 3.1: Cartoon diagram of collection of seismic data in a marine environment;
illustrating the energy source (air gun) being towed behind a vessel, followed by streamers
containing hydrophones, which record the two-way-travel time of the time taken for energy
pulses leaving the gun, to reflect off geological boundaries and subsequently be recorded by
the hydrophones (www.2)
3.1.2. Geological Controls on Seismic Reflection Data
Different stratigraphic units have varying P-wave velocities depending on their composition.
Typically P-wave velocities increase with depth, as the rocks become more compact (Kearey
et al., 2002). Acoustic frequency refers to the number of complete wavelets that are recorded
at one point per second. This frequency decreases with depth, resulting in lower resolution
recordings of the lower rock units in a seismic cube (Bacon et al., 2003; Brown, 2004).
Density and the seismic velocity of the rocks control the strength of the reflections recorded
in seismic data. Positive (peaks) and negative (troughs) amplitudes are represented as distinct

35
colours (black and white respectively) in original seismic data as classified by the ‘SEG
normal polarity’ (Brown, 2004 (Figure 3.2)).
Figure 3.2: A schematic diagram of a simple ‘SEG normal polarity’ wavelet along side an
example of the raw seismic data collected in a seismic survey (www.3).
3.1.3. Seismic Data Processing
There are 3 main methods of processing seismic data: migration, common-midpoint (CMP)
and deconvolution. However, the deconvolution method is very rarely used in the, due to the
noisy, finite bandwidth and discretely sampled datasets (Bacon et al., 2003; Brown, 2004).
3.1.3.1. Seismic migration - The process of seismic migration involves moving seismic events
geometrically in time or space to the position in which they occur in the subsurface, rather
than the location, at which they were recorded at the surface (Brown, 2004 (Figure 3.3)). This
is necessary in areas of complex geology, particularly in the presence of salt, faults and zones
of intense folding. A migrated image has an increased spatial resolution; particularly evident
in areas of complex geology (Bacon et al., 2003). In order to achieve maximum resolution,
the seismic data needs to be cleaned up before migration is carried out, as noise in the data

36
may become smeared during migration and therefore become far more difficult to remove
than if removed before hand (Brown, 2004). There are two types of migration algorithms used
in seismic migration: time migration and depth migration (Figure 3.3).
Time migration is applied in time co-ordinates so does not require a velocity model. This
algorithm accounts for only small amounts of lateral velocity variations, which is not ideal in
areas of complex geology, particularly in the presence of salt. Depth migration is applied in
depth co-ordinates, which are calculated from seismic data in time coordinates. This
algorithm requires a velocity model, which is time consuming, but produces far more accurate
results in areas of large amounts of lateral migration (Bacon et al., 2003).
Figure 3.3: A schematic diagram (left) illustrating the process of seismic migration in seismic
data processing (www.4).

37
3.1.3.2. Common-midpoint (CMP) - The common-midpoint method is the most common
technique used in the oil and gas industry. Used in the acquisition of multichannel seismic
data, a common midpoint is the point on the surface halfway between the source and the
receiver that is shared by a number of source-receiver pairs (Bacon et al., 2003 (Figure 3.4)).
This increases the quality of the seismic data when stacked. The common midpoint is often
located vertically above the common depth point. This method of data collection enhances the
quality of the data by reducing noise (Bacon et al., 2003). CMP can be used in conjunction
with the migration method, which is implemented in the final stage, after ‘normal-move-out’
(NMO) adjustment of the travel times has been carried out. NMO removes the effect of the
slant of individual ray paths and the amplitudes of all the traces at each travel time that have
been stacked (Bacon et al., 2003; Brown, 2004).
Figure 3.4: A schematic diagram illustrating the position of a common midpoint in seismic
data (www.5).

38
3.2. Interpretation of the 3D Seismic Cube
Interpretation of the seismic data in this project was carried out using Schlumberger Petrel
2013. A structured, methodical approach was adopted in order to actively monitor the
progress of interpretation and analysis, which was carried out in the following order:
3.2.1. Interpretation of Seismic Horizons
Three geological horizons were mapped using Petrel 2013: (1) Zechstein Top, (2) Zechstein
Basement and (3) Z3 Stringer. There are 2,800 2D profiles in the cross line of the seismic
cube at spacings of 25 m. The number physically viewed during interpretation varies for the
three horizons, due to their differing geometries.
3.2.1.1. Zechstein Top - The Zechstein Top is interpreted as the distinct high amplitude
reflector (Figure 3.5), situated directly above the low amplitude and extremely chaotic
reflectors within the Zechstein unit (with the exception of the high amplitude reflection of the
Zechstein 3 Stringer). This horizon was interpreted using the 3D seeded auto-tracking tool in
Petrel. Due to the surface’s continuous nature, the confidence of the manually selected seed
was set at 50% and the seismic wavelets were optimized for dipping reflectors. The only areas
of incontinuity occur in the vicinity of large salt structures and in regions of intense faulting
of the supra-salt sediments (Figure 3.5). The horizon was initially picked on the cross-line of
the seismic cube, with a line spacing of 50 (1250 m). This means that 56 seismic profiles were
physically interpreted during the initial interpretation of the top salt surface.
Once completed, the same process was carried out across the in-line. The accuracy of the
interpretation was then assessed by viewing every 10th
profile (250 m) of the cross line (280
seismic profiles). The accuracy was generally extremely high, but where an area of low
accuracy was identified (primarily around large salt structures) the region was re-interpreted
using a 65% seed confidence and a line spacing of 5 (125 m).
3.2.1.2. Zechstein Base – The Zechstein Base is interpreted as the sub-horizontal, continuous,
yet fragmented high amplitude reflector situated below the chaotic reflections of the Zechstein

39
Salt (Figure 3.5). This horizon was interpreted in the same manner as the top salt. However,
due to the broken up nature of the horizon (Figure 3.5), a 60% seed confidence was
implemented and the horizon was interpreted at intervals of 20-line spaces (500 m). This
resulted in 140 seismic profiles being physically interpreted in the cross-line. The seismic
wavelets were also optimized for dipping reflectors. The cross-line of the seismic cube was
initially analysed in this manner, followed by the in-line. Upon completion, the accuracy of
the interpretation was assessed by viewing every 10th
profile (250 m) of the cross-line (280
seismic profiles). Again, the accuracy was extremely high, however areas of inaccuracy were
identified in regions of intense faulting of the Rotliegend Basement. Upon identification,
these regions were re-interpreted by analyzing every 5th
profile, using a seed confidence of
70%.
3.2.1.3 Zechstein 3 stringer - The Z3 stringer is identified as the sole high amplitude reflector
within the Zechstein Group in the seismic cube and exhibits an extremely deformed and
complex geometry (Figure 3.5). Due to the highly fragmented and deformed nature of the Z3
stringer across the study area, the horizon was initially interpreted using the 3D auto-tracking
tool with a seed confidence of 70%, and the wavelets were optimized for dipping reflectors.
Each stringer fragment was interpreted as a separate horizon on the cross-line of the seismic
cube. This was implemented using a line spacing of 4 (100 m), resulting in 700 seismic
profiles being physically interpreted. Particular care was taken when interpreting the stringer
fragments as their highly deformed nature regularly results in dramatic changes in geometry
and position within the salt in the 100 m between each analysed seismic profile.
Interpretation commenced in the west of the study area – migrating eastward. Upon
identification of the first stringer fragment, it was the only horizon interpreted on the
subsequent seismic profiles until the fragment ‘disappeared.’ When this occurred, seismic
interpretation returned to the profile where the first fragment was initially identified and the
second fragment was subsequently highlighted when it was evident in the 2D profiles. This
process was repeated across the entire seismic cube. This method was extremely time
consuming, as the majority of the 700 seismic profiles were viewed multiple times,
interpreting different fragments each time. Such a time consuming method was applied in
order to record the extent of each fragment as accurately as possible. This method was

40
particularly necessary in regions where fragments branched off from each other, because due
to the complex geometries and sudden variations in vertical emplacement, the incorrect
branch could easily be selected in a proceeding profile. This would result in false dramatic
vertical displacements of the stringer fragments. In regions of intense deformity, the number
of profiles viewed was lowered to 2 and in some cases 1-line spacing, in order to be certain
that the correct fragment was being identified.
Upon completing the interpretation of the cross-line, the seismic profiles of the in-line were
checked initially using a line spacing of 10 (250 m). Due to the accurate method of initial
interpretation, there were very few cases of un-identified in the in-line. When encountered,
the un-interpreted stringers were interpreted as a new horizon in the same manner as the
original interpretation. This process resulted in 1334 stringer fragments being interpreted,
with many consisting of two or more branches.
As well as every individual fragment being interpreted as separate horizons, the upper most
stringer fragments across the seismic cube were identified and interpreted as a single horizon,
termed the enveloping surface (Strozyk et al., 2012). This horizon was interpreted using the
same parameters as used when interpreting the individual fragments; using the 3D auto-
tracking tool with a seed confidence of 70% and seismic wavelets optimized for dipping
reflectors. The interpretation was carried out on the cross-line, with a line spacing of four.
3.2.2. Continuous Surfaces
Continuous surfaces of the top salt, basement, stringer fragments and the enveloping surface
were created. This was carried out by using the ‘create quick surface’ function in Petrel. This
tool automatically smooth’s the horizon and interpolates gaps that have been left in the initial
interpretation. Small gaps in the interpreted horizon occur in some of the seismic profiles that
were not analysed (i.e. 3 profiles between the 4 line spacing interpretation). Once created, the
top salt, basement and enveloping surfaces were extrapolated, in order to obtain a continuous
surface across the entirety of the study area.

Chapter3–DataandMethodology
41
Figure3.5:2Dseismicprofilesshowingtheoriginalprofile(top)andthesameprofileafterinterpretationofthehorizonsinPetrelhadtakenplace(Bottom).Theyellowsurfacehighlightsthetop
saltsurface;thegreendepictsthebasementhorizonandthemulti-colouredrepresenttheindividualstringerfragments.Notethecontinuityofthetopsalthorizon,withtheexceptionofinthe
vicinityofthesaltstructureinthemiddleoftheprofile.Thebasementhorizonishighlyfragmentedinthepresenceofinvertedblocks.Alsonotethegapsinthehighlightedstringerfragments.
Theseinterpretationgapsarein-filledduringthecreationofthequicklooksurfaces,asarethegapsinthetopsaltandthebasementhorizons.
Depth (m)Depth (m)
ZechsteinTopHorizon
Zechstein3(Z3)Stringerfragment
ZechsteinBaseHorizon

42
3.2.3. Thickness Maps
Thickness maps were created, displaying the total salt thickness across the seismic cube, the
thickness of salt above the enveloping surface and the thickness below it. These were
produced using the ‘Make Thickness Map’ function in Petrel. In the case of the total salt
thickness and the thickness of salt above the enveloping surface, the extrapolated top salt
surface was used as the top surface, with the base surface being the Zechstein Base and the
enveloping surface respectively.
3.2.4. Fluid Migration
Evidence of fluid migrating through the supra-salt sediments, originating from the Zechstein
Top was identified by firstly analyzing every tenth profile of the cross-line and then the in-
line. The fault interpretation tool was used to highlight areas of washed out and fuzzy seismic
data that surround and enclosed structural features, which may act as fluid migration paths.
These features in the post-salt sediments are mainly faults and gas chimney structures (Figure
3.6).
3.2.4.1. Fluid Migration Zones - Once the identification of fluid migration was completed, the
highlighted areas were separated into seven zones. This was carried out by separating regions
of fluid flow on independent salt structures; both intrusive structures (e.g. diapirs) and
regressive structures (e.g. rim-synclines). Once identified, the cross-line and in-line profiles
were analysed using a line spacing of 10 (250 m). All the stringer fragments in these zones
were identified in the 2D seismic profiles and the respective quick look surfaces were copied
and separated into folders of the corresponding zones. A 3D model of each zone was
subsequently created, displaying the Zechstein Base, the Zechstein Top and the stringer
fragments present in the zone of potential fluid migration. Both the top salt and basement
surfaces of the zones were cut down to the maximum lateral extent of the stringer fragments,
by creating polygons on a copy of the extrapolated surfaces. The surfaces were then
eliminated outside of the polygon, resulting in only the top salt and basement surfaces directly
above and below the zone of fluid escape being displayed.

43
3.2.4.2. Distance from top salt, basement and the angle of dip - Thickness maps for each
stringer fragment in every zone were created, quantifying the distance between the individual
fragment and the top salt and basement surfaces. When creating the thickness map between
the top salt and the stringer, the Zechstein Top was used as the top surface and the stinger
fragment was used as the base and vice versa when creating the thickness between the stringer
and basement (using the basement surface as the base). The angle and azimuth of dip were
identified for each individual stringer using the ‘Dip angle and Azimuth’ function in Petrel.
Figure 3.6: 2D seismic profile illustrating interpreted fluid flow through three faults in the
supra-salt sediments. Note the washed out effect of the seismic horizons in the supra-salt
sediments and the negative flower structure at the cusp of the salt pillow.
Depth(m)

Chapter 4 - Results
44
CHAPTER 4 - RESULTS
Regional scale geological structures are identified in the surfaces of the Zechstein
Base and Zechstein Top.
Regional scale deformation is identified and analysed.
7 Zones of fluid migration into the supra-salt sediments are identified and the local
geometry of the stringer fragments in each area are analysed.
Connectivity of the stringer fragments in each zone is assessed.

Chapter 4 - Results
45
4. RESULTS
The 3D models in this chapter exhibit a vertical exaggeration of 5 and all the 2D maps
represent depth or thickness in metres depth. The interpreted faults in the 2D seismic profiles
are all interpreted on the cross-sections and were not quantified in Petrel.
4.1. Regional Scale Geological Features
The seismic scale interpretation (see Chapter 3) of the Zechstein Top and the Zechstein Base,
carried out in Petrel has identified a host of major geological structures in both horizons.
4.1.1. Zechstein Base
Inverted fault blocks in the Upper Rotliegend basement result in a fragmented topography;
with reflectors of the Rotliegend Group units remaining broadly parallel to the correlating
Zechstein Base reflector. The amplitude of reflections decreases with depth and are disrupted
and become weak and chaotic under large salt structures (Figure 3.5).
On a regional scale across the study area, the depth of Zechstein Base increases from the
highest point in the west at approximately 2,000 mbsf, to 2,800 mbsf in the east, resulting in a
800 m eastward depth increase. The exception to this are the horst structures in the east of the
study area, which are identified in figures 4.1 & 4.2. The horst structure in the northeast
(Figure 4.2) exhibits the shallowest depth of 1,900 mbsf.
Three main fault trends have been identified (Figure 4.1) and highlighted in Figure 4.2. The
predominant trend is NW-SE (black). 3 major structures (red) trending NNE-SSW intersect
this main trend and four large faults trending from WNW-ESE and are all situated on
structural highs of the sub-salt units (Figure 4.1 & 4.2).
Seven major structural features (A-E) have been highlighted in figure 4.2: A large-scale
graben structure in the centre of the study area (A) trends NNE-SSW, but curves eastward to
the north. This feature extends for the entirety of the study area (25 km) and is approximately

Chapter 4 - Results
46
5 km wide, but widens to approximately 8 km, as it turns northeastward (Figures 4.1 and 4.2).
It is intersected by a smaller NW-SE trending graben structure (B) in the north of the survey
area (Figure 4.2). This second structure is approximately 2 km wide and is interpreted to
extend for 38 km to the southeast (Figure 4.2). A large fault zone (C) located in the southwest
of the study area is approximately 20 km in length with a WNW-ESE orientation (Figure 4.2
& 4.3). The western-most extent of this fault zone intersects a similarly extensive fault zone
(D), which trends NNE-SSW (Figures 4.1 & 4.2). Several NNW-SSE trending horst-like
structures (E&F) are interpreted to propagate for approximately 10 km in the northeast and
southwest of the seismic cube (Figures 4.1 & 4.2).
A
A
B
Depth (m)
Figure 4.1: An un-annotated 2D depth map of the Zechstein Base surface, displaying the
geological features described in section 4.1.1. and highlighted in Figure 4.2.
Depth (m)

Chapter 4 - Results
47
Figure4.2:Anun-annotated2DDepthMapoftheZechsteinBasehorizon,highlightingthestructuralfeatures
discussedinsection4.1.1.Thepurplelineslabelled1&2representthe2Dseismicprofilesdisplayedin
Figures4.3and4.4.
Depth(m)

Chapter4-Results
48
E
Figure4.3:2Dseismicprofile(1)highlightedonFigure4.2.Thebottomseismicprofileistheinterpretedversionoftheun-annotatedsectionabove.Thecrosssectioncutsperpendicularlythroughthe
NNE-SSWtrendinggraben(A),thelargeNNE-SSWtrendingfaultzone(D)andincorporatesthehorststructures(E)identifiedinfigure4.1.
Depth (m) Depth (m)
HorstStructureUnder-seating
anIntrusiveSaltStructure
TopographicLowUnder-seating
Rim-synclineinPost-SaltSediments

Chapter 4 - Results
49
Although not quantified in this work, the intensity of the faulting in the sub-salt basement is
highlighted in figures 4.3 and 4.4. Inverted fault blocks are common and strongly dictate the
morphology of the Zechstein Base. Topographic highs are located in areas of inverted fault
blocks and topographic lows, including the two identified grabens are situated where
inversion has had little effect on the original extensional structures (Figures 4.3 & 4.4). On a
regional scale, topographic lows of the sub-salt units correspond to depressions in the supra-
salt sediments. This regional trend is particularly evident in figure 4.3, which shows horst-like
structures (E) being situated beneath large salt structures. However, the NNE-SSW graben
(A) displayed in figure 4.3 is located beneath a small salt pillow, illustrating that this not
always the case. This observation can also be made in Figure 4.4, where the smaller NW-SE
graben (B) is too, situated beneath an intrusive salt structure. Deep-seated faults in the
basement generally produce larger displacements, as displayed in Figures 4.3 & 4.4.
22
Figure 4.4: 2D profile number 2 (highlighted on Figure 4.2). The bottom figure shows the
interpreted version of the plain seismic trace above. This cross section intersects the 2 km
wide, NW-SE trending graben (B) in the northeast of the study area; identified in Figure 4.1.
Depth(m)Depth(m)

Chapter 4 - Results
50
4.1.2. Zechstein Top
The elevation of this surface is extremely variable across the seismic cube, as illustrated in
Figure 4.5. The top salt is deepest in the east of the study area and in particular, the most
northeasterly segment, where it reaches 2,600 mbsf (Figure 4.5). The shallowest areas of the
Zechstein Top are exhibited by the two diapirs in the east of the study area, where the salt
reaches 1,100 mbsf at its shallowest point. These structural highs are flanked by adjacent
topographic lows (Figure 4.5).
Four dominant salt structure types of the Zechstein Unit have been identified on Figure 4.5: A
large NNE-SSW trending salt wall to the west (A); rim-synclines on the flanks of this salt
wall (B); medium scale salt pillows (C) and a large salt Diapir (D).
The salt wall is located directly above the NNE-SSW trending fault zone in the Rotliegend
basement (Figures 4.2 & 4.5). This large salt structure extends for approximately 25 km
across the study area and pinches out to the north and south and, as shown in Figure 4.5 &
4.6A, reaches approximately 1,450 mbsf at its shallowest point and is flanked on both sides
by rim-synclines (Figure 4.5 & 4.6.B), both of which have a surface expression of
approximately 5 km. Salt pillows of different magnitudes occur across the study area (Figure
4.5) and are predominantly situated above structural highs within the sub-salt units (Figure
4.6.C). The largest salt structure in the study area (D) is the salt diapir to the northeast (Figure
4.5). This structure is situated above the tallest horst-like structure of the sub-salt sediments
and at its base is 5 km wide. At its tallest, it reaches an elevation of 1,100 msfb (Figure
4.6.D).
All the large-scale salt structures highlighted in Figure 4.5 host fault systems at the salt cusp
(Figure 4.6). These fault zones have not been interpreted in detail, but possess a range of
different magnitudes and complexity, details of which have not been quantified in this study.
There is little correlation between the vertical displacement of the salt structure and the
complexity of the fault zone situated above it. The most complex system is located above the
salt pillow in the middle of the seismic cube (Figure 4.6.C). However a similar sized
structure, approximately 6 km to the northwest only exhibits a small fault zone.

Chapter 4 - Results
51
Figure4.5:AdepthmapoftheZechsteinTopSalt,showingmajorsaltstructureswithintheZechstein,includingaNNE-SSWtrending
saltwall;Rim-synclines,saltpillowsandsaltdiapirs.TheredlinesrepresentthecrosssectionsdisplayedinFigure4.5.
Depth(m)

Chapter 4 - Results
52
Figure 4.6: Four seismic cross sections, highlighted in Figure 4.4, illustrating the main salt
structures of the Zechstein unit. (1) The NNE-SSW trending salt wall; (2) the salt wall’s
flanking rim-synclines; (3) salt pillows and (4) the large-scale salt diapir in the northeast of
the study area.

Chapter 4 - Results
53
4.2. Zechstein 3 Stringer
The Z3 stringer is identified in the seismic cube, as the sole high amplitude reflector within
the Zechstein Group (see Chapter 3) and exhibits an extremely deformed and complex
geometry, as identified in all the 2D seismic profiles presented in this chapter. All the discrete
values given in terms of the stringers distance from the top salt and the basement and the
angles of dip are acquired from the corresponding appendix in Chapter 9.
4.2.1. Regional Scale Deformation
Intense deformation of the carbonate-anhydrite stringer is regionally exhibited across the
entire study area, as displayed in the 2D seismic profiles in this study. The types of
deformation include: Folding - from open folds to tight, isoclinal folds; vertical stacking of
the stringer and a highly fragmented appearance (Figure 4.7). The creation of the ‘enveloping
surface’ (Strozyk et al., 2012), extrapolates the upper most stringer fragments across the
seismic cube, providing a regional deformation model.
4.2.1.1. Enveloping surface - The geometry of the enveloping surface varies greatly across the
study area in a multitude of ways. These include: the location of the stringer within the
Zechstein Salt, in relation to the Zechstein Top and Zechstein Base; the orientation of dip
direction of the surface and its angle of dip. The variation in the vertical location of the
enveloping surface is exhibited in Figure 4.7 and the magnitudes of the changes in the angle
of dip are highlighted in Figure 4.8. This variation is not completely concordant with either
the top salt or the basement morphologies. On a regional scale, the enveloping surface shares
a closer relationship with the morphology of the Zechstein Top than the basement. This is
illustrates by the surface rising with the topography of the top salt in the large salt structures
(Figure 4.7). Locally however, the stringer deviates regularly from the Zechstein Top surface,
as is displayed in Figure 4.8. Intense deviation for the top salt is particularly evident in areas
of increased salt thickness and at the cusp of the salt features, as shown in the intrusive
structures in Figure 4.7.

Chapter4-Results
54
Depth (m)
Figure4.7:2Dseismicprofile,illustratingthegeometryoftheenvelopingsurface.A)Displaystheraw2Dseismicprofile,withB)presentinganinterpretationversion.Theenvelopingsurfaceexhibits
dramaticchangesinverticalpositionwithinthesaltthatarenotconcordantwiththetopsalttopography,butregionallydisplaysastrongercorrelationtothetopsaltsurfacethanthebasement.Allfaultsare
interpretedsolelyfromtheseismicsection,nofaultanalysiswascarriedoutinPetrel Depth (m)
StrongCorrelationbetweenthe
envelopingsurfaceandthetop
salttopography(Seebelow).Localiseddeviationfromthe
topsaltgeometry(Seebelow).SteepandShallowDipping
Reflectors(seebelow).

Chapter4-Results
55
Figure4.8:3Dmodeloftheenvelopingsurfaceinrelationtothe2Dseismicprofiles,whichillustratethecloseproximityofthesurfacetothetopsaltsurface.Notethatthesurfaceisbroadlyconcordantwith
thehighsandlowsofthetopsalt.However,onalocalscale,thereisagreatvarianceintheverticalpositioningoftheenvelopingsurface.
SaltWall
Rim-Syncline
SaltPillow
SaltDiapir

Chapter 4 - Results
56
4.2.1.1.1. Enveloping surface location – The enveloping surface’s stronger relationship with
the Zechstein Top is emphasized in the thickness maps in Figures 4.9 and 4.10. The distance
from both the top salt and the basement varies between 0 and 500 m. When analyzing the
relationship between the distances from the Zechstein Top and Basement, the total thickness
of the salt must be taken into account (Figure 4.11), as the distances between the surfaces will
be locally distorted in areas of salt thickness extremities. For example, on the southern flank
of the largest salt diapir in the northeast of the study area, the enveloping surface is, in areas
equidistant from the basement and the top salt (approximately 500 m). When compared to an
area of smaller salt accumulation, for example the rim-syncline to the south of the diapir,
where the accumulation of salt is approximately 300 m thick, the enveloping surface is also
equidistant from the top salt and basement at 150 m. If these distances were analysed without
the total thickness of the salt being accounted for, it would be interpreted that the stringer is
dramatically closer to the top salt in the rim-syncline, where as realistically the distances are
proportionally identical.
The enveloping surface is predominantly located between 0 and 300 m from the top salt
across the seismic cube (Figure 4.9). Areas where this distance is between 0 m and 50 m
(primarily in the east of the study area), display smooth fluctuations in distance changes.
However, in the central southern sector of the seismic cube, fluctuations ranging from 100 m-
250 m, are abrupt and clearly visible (Figure 4.9). In both cases, particularly the former, these
variances correlate to corresponding variations in salt thickness displayed in Figure 4.11. The
regions furthest from the top salt (500 m) primarily occur in salt thicknesses of 650 m-850 m
and on the flanks of large salt structures. In zones of salt thicknesses between 0 m-200 m, the
distance from the enveloping surface to the top salt is generally at its lowest (between 0 m-
100 m).
The extrapolated surface is more frequently over 500 m above the basement across a larger
area of the seismic cube (Figure 4.10) than 500 m below the top salt (Figure 4.9). This is
observed primarily along the peak of the salt wall and at the cusp of the two major salt diapirs
in the east, where the Zechstein Group is at its thickest (900 m+ (Figure 4.11)). Areas where
the basement and the enveloping surface are in close proximity occur in the east of the
seismic cube, where the salt accumulation is at its lowest.

Chapter4-Results
57
Figure4.9Figure4.10
Figure4.11Figure4.12
Figures4.9-4.11arethicknessmapsdisplayingthedistancesoftheenvelopingsurfacesfromthetopsalt,thebasementandthetotalsaltthicknessrespectively.Figure4.9showstheenvelopingsurfaceto
haveamoreconstantdistancefromthetopsaltthanthebasement,showninFigure4.10.Themaximumdistancefrombothsurfacesisdisplayedas1,100m.Thisdistancefromthesurfacesisrecordedinthe
sameareainthenortheastofthestudyarea,wherethesaltthicknessisover2,200minthickness,asshowninFigure4.11,meaningthattheenvelopingsurfaceislocatedroughlyinthemiddleofthesaltin
thisarea.ThesaltthicknessdisplayedinFigure4.11mustbetakenintoaccountwhenquantifyingtherelativepositionoftheenvelopingsurfacewithinthesalt.Figure4.12showsthelocationofwherethe
crosssectionsinfigure4.7.
Distance
(m)
Distance
(m)
Depth(m)Thickness
(m)

Chapter 4 - Results
58
4.2.1.1.2. Dip angle and azimuth of dip - As displayed in Figure 4.7, the angle of dip of the
enveloping surface varies significantly. The maximum dip is recorded at 86° and the
minimum is 0° (Figure 4.13). The surface dips more steeply on the flanks of the major salt
structures and in areas of a sudden decrease in the elevation of the Zechstein Top (Figure
4.13). This observation is exaggerated in Figure 4.14, which shows dips of over 45° to be on
the margins of the salt wall, the central salt pillows and the flanks of the two large diapirs.
The azimuth of the dip direction exhibited by the enveloping surface is displayed in Figure
4.15. This shows that the dip direction on the western flank of the NNE trending salt wall
typically ranges between 200 and 300 degrees, with a stronger W-SW trend (between 200 and
250 degrees). The eastern flank of the salt wall displays dip directions of 50-100 degrees; half
the range of the western margin. These two trends are dominant across the seismic cube;
exhibited in NNE-SSW striking, alternating cycles. This highlights the predominant NNE-
SSW axial trend of the salt structures, as fragment dips are predominantly perpendicular to
the strike of the salt structures
The flanks of the salt pillows display very weak 360-degree ranges of dip direction,
particularly on the upper flanks. The zone of minimum salt thickness to the east displays an 8
km x 12 km area, which exhibits a preferred SW dip azimuth. Directly to the north is an area
approximately the same size with a dominant westerly dip direction, but with inclusions of a
northwesterly dipping direction.

Chapter4-Results
59
Figure 4.13: 2D map of the study area displaying the range in dip of the enveloping surface within the seismic cube (0-86°).
Occurrences of steeper dipping areas are located on the flanks the major salt structures in the seismic cube.
Angle of Dip
(Degrees)
Figure 4.14: 2D map showing the range of dip of the enveloping surface between 0-45°. This further highlights the areas of steep
dip to be on the flanks of the major salt features and in zones where an abrupt change in the vertical extent of the Zechstein Top
occurs.
Angle of Dip
(Degrees)

Chapter 4 - Results
60
Figure4.15:2Dmapillustratingthedipazimuthoftheenvelopingsurface.TheNNE-SSWtrendinalternatingeasterlyand
southwesterlydipsshowsverylittlecorrelationtothesaltstructuresoftheZechsteinGroup.Atrendisidentifiedonbothflanks
ofthesaltwallintheeastandveryweaktrendsof360°occuraroundthelargestsaltdiapirinthenortheastoftheseismiccube.
AzimuthofDip
(Degrees)

Chapter 4 - Results
61
4.2.2. Local Deformation
As previously discussed, the enveloping surface exhibits a stronger regional correlation with
the Zechstein Top than the Basement. However, as described and identified in Figures 4.7 and
4.8, this trend is frequently broken on a local scale, where individual (or several) stringer
fragments are situated significantly lower (or less frequently, higher) in the salt than the
dominant trend.
4.2.2.1. Stringer fragments - The most striking geometrical feature of the Z3 stringer is its
broken up nature. Gaps in the carbonate-anhydrite member are frequent and occur in a
multitude of environments; from low to high accumulations of salt and in areas of shallow to
steeply dipping reflectors (Figure 4.16). Fragments that exhibit low angles of dip are
primarily located in regions of medium-to-low accumulations of salt (Figure 4.16). Six of the
ten highlighted areas in Figure 4.16 show fragments exhibiting low angles of dip to be located
beneath topographic lows in the top salt surface. Inverted fault blocks in the basement
frequently correlate to fragments lower in the salt. Gaps between the fragments in the salt
structures are far larger and more frequent (Figure 4.16). The fragments predominantly have a
steeper dip in these locations.
The western most zone, highlighted in Figure 4.16, displays fragmentation of the stringer to
occur between approximately 20-250 m from the Zechstein Base. The third zone from W-E
shows gaps between 350-400 m from the basement. Both of these zones exhibit similar
thickness of salt (approximately 500 m) and are located beneath salt pillows with comparable
geometries. The easternmost zone displays fragments with sub-parallel dips, whereas the
second area consists of more intensely deformed and folded stringers.
4.2.2.2. Folding and stacking of stringer fragments - Folding of stringer fragments occur in a
range of extremities, from open folds, to tight, isoclinal folds. In areas of large accumulations
of salt, where the stringer is located in the upper third of the Zechstein Group, stringer
fragments are generally more severely folded (Figure 4.16). Synclinal folding of the Z3
stringer is very common where the fragments are in close proximity (< 20 m) to the basement,
as displayed in all the seismic cross sections in this chapter (e.g. Figure 4.16). Fragments

Chapter 4 - Results
62
displaying these qualities are typically the lowermost fragments in the region and often occur
adjacent to inverted basement blocks (Figure 4.16). Vertical stacking of these segments is also
very common, with a tight synclinal folded fragment being located directly above the
lowermost segment, which exhibits a shallower dip axis. This is displayed beneath the salt
pillow directly to the east of the salt wall in Figure 4.16. Stacking occurs throughout the
seismic cube, showing a multitude of dip angles and in a wide range of salt thicknesses. This
is exhibited in Figure 4.16, where stacking is seen to occur primarily in the lower half of the
salt and in zones of thin salt accumulation; beneath rim-synclines and basins.
Folding and rotation of the stringer fragments do not exhibit an axial trend in the 2D seismic
profiles and produce a ‘wiggly’ appearance in plan view (Figures 4.13 & 4.14). There is a
very weak perpendicular correlation to the NNE-SSW trend of the stringers. The fold axes of
both anticlines and syncline range in their dip direction and the angle of dip (Figure 4.16).
Folding is not localized to specific environments, but the nature of the folds and the axial
trends differ greatly on a local scale (Figures 4.13; 4.14; 4.15 and 4.16).

Chapter4-Results
63
Figure4.16:Seismiccrosssectionidentifyingareasofboudinagewithintheseismiccube.A)presentstheraw2Dseismicprofile,withB)illustratingtheareasofinterpretedboudinage.Boudinageisseen
toprimarilyoccurinrim-synclinesandontheflanksoftheadjacentsaltstructures.However,boudinageisalsoseentooccurwithinthesaltfeaturesinareasofincreasedsaltthickness.Boudinageisnot
controlledbysaltthickness.Allfaultsareininterpretedonthisseismiccrosssection;nofaultswereinterpretedinPetrel.
Depth (m)Depth (m)
Synclinallyfoldedfragment,adjacent
toinvertedfaultblock(seebelow)
Fragmentsexhibitingdifferent
foldaxes(seebelow)
Stackedstringerfragments
exhibitingshallowanglesof
dip(seebelow)
Steeplydipping,fragmented
stringerfragments(seebelow)
Shallowdipping,
fragmentedstringer
fragments(seebelow)

Chapter 4 - Results
64
4.3 Fluid Migration
Using the fault interpretation tool in Petrel, features including gas chimneys and zones of
fuzzy seismic reflectors originating from the Zechstein Top and propagating through the
supra-salt sediments have been identified and classified as areas of fluid migration, resulting
in a map displaying regions of potential fluid flow from the salt (Figure 4.17). These features
have been separated into seven zones (Figure 4.18). Zones 1-5 incorporate large salt
structures including salt pillows (1, 3 & 5); the salt wall in the west of the study area (2) and
the smaller of the two salt diapirs towards the east of the seismic cube (4). All five zones
represent areas of relatively thick salt accumulation, ranging from approximately 800 m
thickness in the salt wall to 200 m in the central salt pillows (Figure 4.18). Zones 6 and 7 are
located in areas of thin salt deposition of 0-300 m in the east of the study area (Figure 4.18).
The individually interpreted stringer fragments located beneath each zone of fluid migration
have been selected and modelled in order to assess their potential to act as fluid conduits
through the sealing Zechstein Salt.
Figure 4.17: 2D map of the Zechstein basement, with the areas of interpreted fluid
migration from the salt into the supra-salt sediments, displayed as fault traces.
Depth
(m)

Chapter 4 - Results
65
Figure4.18:ThicknessmapoftheZechsteinsalt,withthesevenzonesofgasmigrationhighlighted.Thefivezonesinareasofthick
saltaccumulationarehighlightedinredandthetwozoneslocatedinregionsoflowsaltaccumulationarehighlightedinyellowand
labeledaccordingly.Locationsofthepositionofthefollowingseismiccrosssectionsaremarkedandlabelledappropriately.
Thickness
(m)

Chapter 4 - Results
66
4.3.1. Zone 1
The first zone of fluid migration is located in the NW extent of the study area (Figure 4.19).
Fluid migration occurs through a large normal fault zone at the cusp of a NNW-SSE trending
salt pillow (Figure 4.20). Salt thickness ranges between 500 m and 200 m (Figure 4.19). The
fault system extends along the entirety of the 10 km pillow. Fluid migration is indicated by
the washed out and fuzzy nature of the seismic reflectors along the fault zone, visible and
highlighted in Figure 4.19. As well as the structural high of the salt pillow, Zone 1 is situated
above a number of inverted blocks in the Zechstein base, which are concordant with the
topography of the top salt to northeast (Figure 4.20). 56 stringer fragments are located in the
salt below this zone, which exhibit a range of geometric differences.
4.3.1.1. Stringer geometry - The stringer fragments display a NNW-SSE trend, which is
parallel to the fault zone at the crest of the salt pillow (Figure 4.21). The dip of the stringer
segments increase with the increasing dip of the top salt on the flanks of the structure (Figure
4.22) and the azimuth of dip of the stringers produces a very strong SSW trend (Figure 4.21).
Figure 4.19: 2D Thickness Map showing the thickness of the Zechstein salt and the first
zone of interpreted fluid migration, with the location of the 2D seismic profile.
Thickness
(m)

Chapter 4 - Results
67
This is due to the majority of the fragments being situated on the southwesterly flank of the
salt structure (Figure 4.21). A weaker southeast trend is also identified, which incorporates the
cluster of stringers located on the eastern flank of the pillow structure (Figure 4.21).
The stringers are more concordant the Zechstein Top than the basement. This is highlighted in
Figure 4.22, but the stringer segments have limited connectivity. The highest level of
connectivity is situated on the flanks of the salt pillow. There are two exceptions to this trend;
at the southern and northern extremes of the region (Figure 4.22). In both of these areas, the
stringers deviate from the top salt topography and ‘sink’ within the salt; correlating to
depressions in the basement.

Chapter 4 - Results
68
Figure 4.20: 2D seismic profile through Zone 1, showing fluid migration into the supra-
salt sediments through a large-scale normal fault zone at the peak of the featured salt
pillow. The stringer fragments visible in this 2D section have a strong correlation to the
topography of the Zechstein Top, with the exception of two fragments central in the salt
pillow, which are in close proximity to the basement and the isolated fragment located in
the centre of the salt structure.
Fault, interpreted to be focusing
fluid flow from the Zechstein
Group, note the washed out
seismic reflections

Chapter4-Results
69
N
5 km
Figure 4.21: A 2D map of the stringer fragments overlaying the basement in Zone 1. A strong NNW-SSE trend is clearly visible. The
rose diagram quantifies the mean azimuth of dip of each of the 56 fragments and produces an extremely strong SSW trend.
Depth (m)
Figure 4.22: 3D model of Zone 1, displaying the geometry of the stringer fragments in the salt below the top salt surface (white) and
above the basement (brown).
Depth (m)

Chapter 4 - Results
70
4.3.1.2. Dip of stringer fragments - The mean angle of dip of the 56 stringer fragments present
in Zone 1 ranges from 6° (stringer 67) to 21° (stringers 66 and 57b) (Figure 4.23). This is a
range of 15°. The maximum dip of the stringers ranges from 22° to 81° (stringers 67/84 and
13 respectively). The minimum angle of dip is extremely consistent: 27 of the fragments show
a minimum dip value of 0°, with the highest being exhibited by stinger 66, with a value of 3°.
Stringer 84 has the lowest range of dip angles (21°), with the highest range of 81° being
exhibited by stringer 13. The mean dip of every fragment is consistently situated in the
bottom quarter of its range (Figure 4.23).
Figure 4.23: Graph displaying the mean angle of dip (blue series) of each of the 56 stringer
fragments in Zone 1, with the minimum and maximum dip value represented as the negative
and positive error bars respectively.

Chapter 4 - Results
71
4.3.1.3. Stringer distance from Zechstein Top – The stringer fragments in Zone 1 range from
being relatively concordant with the top salt, to being very un-concordant (Figure 4.24).
Stringer 13c shows the least variation of distance from the Zechstein Top (32 m). This
stringer is 276 m from the top salt at its closest point and 308 m away at its furthest . Stringer
103 also has a small range in distance of 39 m. At the other extreme, stringer 130 exhibits a
distance range of 424 m, from being 142 m from the top salt at its closest, to being 566 m
away at its furthest point. The average range of the distance of the stringers from the top salt
in Zone 1 is 182 m, illustrating an unharmonious relationship with the top salt (Figure 4.24).
Stringer 13 is the closest stringer to the top salt, where it reaches 30 m below the surface. At
its furthest point, this stringer is 420 m away; resulting in an unharmonious range of 390 m.
Stringer 127 is the furthest stringer away from the top salt (588 m). This stringer is 473 m
from the top salt at its closest point, exhibiting a range of 115 m. The mean distance ranges
from 123 m (stringer 84) to 547 m (stringer 127). There is no evidence of any of the stringer
fragments intersecting the Zechstein Top topography (Figure 4.24).
Figure 4.24: A graph illustrating the mean distance (blue series) that each individual
stringer fragment is from the top salt surface. Negative and positive error bars represent
the minimum and maximum distances of each stringer from the top salt respectively.

Chapter 4 - Results
72
There is no relationship between the dip of the stringer fragments and the distance at which
they are from the Zechstein Top (Figure 4.25). The mean angle of dip remains between 6° and
21°, no matter how far the stringer is from the top salt and the maximum dip declines slightly
with increased distance. However, this is a very weak trend across an extremely spread out
data set (Figure 4.25).
4.3.1.4. Stringer distance from Zechstein Base - The stringer fragments display a wide range
of distances from the Zechstein Base (Figure 4.26). Stringer fragment 84 is the most
concordant with the basement, exhibiting a range of 38 m. At its closest, this stringer is 546 m
from the basement and at its furthest point is 585 m away. Stringers 13c and 103 are also
relatively concordant; displaying ranges of 52 m and 45 m. Stringer 13 has the largest range
(388 m). This fragment is 22 m from the basement at its closest point and 411 m when
furthest away. The average range of the individual stringers, is 183 m, showing a non-
concordant relationship (Figure 4.26)
Figure 4.25: The relationship between the mean distance of the stringer fragments from
the top salt and the mean (blue series) and maximum (red series) dip of each stringer.

Chapter 4 - Results
73
There are 5 fragments, which at their closest point come within 25 m from the basement
(stringers 13, 46, 71, 112 and 130). These fragments all exhibit an unharmonious relationship
with the basement with ranges of 388 m, 140 m, 335 m, 203 m and 329 m respectively.
Stringer 52 is the furthest stringer away from the basement at 594 m. This stringer is 313 m
from the basement at its closest point, resulting in a range of 281 m. The mean distance of the
individual stringers ranges from 93 m (stringer 149) to 557 m (stringer 84). This is a range of
464 m. There are no trends in the proximity to the basement, the range of distances, or the
mean distances of any of the fragments and none of the 56 fragments are seen to intersect the
Zechstein Base (Figure 4.26). There is no relationship between the mean or maximum angle
of dip and the distance of the stringer fragments from the basement (Figure 4.27); The
maximum and mean dips exhibit a wide range of values, no matter the distance of the stringer
from the basement.
Figure 4.26: The mean distance (m) (blue series) that each individual stringer fragment is
from the basement. Negative and positive error bars represent the minimum and maximum
distances of each stringer from the basement respectively.

Chapter 4 - Results
74
4.3.2. Zone 2
Fluid migration in Zone 2 occurs through a NNE-SSW trending fault zone located at the cusp
of the salt wall, which runs for its entirety (Figure 4.28). However, fluid migration highlighted
in Figure 4.29, is only identified over a length of 12 km, propagating from the southern most
extent of the salt wall. The salt thickness ranges from 200 m to 750 m and the fault zone
shows a 10 km section of intense faulting below which, the Zechstein Base is relatively flat,
with the exception of a 3 km wide graben feature to the southwest (Figure 4.29). The 57
stringer fragments displayed in Figure 4.31 all exhibit low to medium angles of dip.
Figure 4.27: The relationship between the mean distance of the stringer fragments from
the Zechstein Base and the mean (blue series) and maximum (red series) dip of each
stringer.

Chapter 4 - Results
75
4.3.2.1. Stringer geometry – The stringer fragments broadly trend NNE-SSW, with the
exception of the northern area, where the trend turns to a NNW-SSE direction, as it remains
parallel to the salt wall (Figure 4.30). The dip of the stringers broadly correlates to the
topography of the top salt (Figure 4.31). The predominant dip direction is southwesterly, as
illustrated in the rose diagram in Figure 4.30. This trend accounts for the majority of stringers
being situated on the western, more gently dipping flank of the salt structure. A moderate SE
and NW trend is also identified, shown by the of dip of the fragments on the eastern flank and
the NNW-SSE trending fragments in the north of the region.
Connectivity of the stringer fragments in this zone is limited, as illustrated in Figure 4.31. As
well as being located directly under and on the flanks of the salt wall, fragments are also
situated in the adjacent rim-synclines. These fragments are in closer proximity to the
basement (Figure 4.31). The stringers in the rim-syncline to the west of the salt wall and
fragments at the base of the salt wall flank present the best degrees of connectivity.
Figure 4.28: 2D Map showing the thickness of the Zechstein salt and the second zone of
identified fluid migration, with the location of the 2D seismic profile.
Thickness
(m)

Chapter 4 - Results
76
Figure4.29:SSW-NNE2DseismicprofilethroughZone2,showingfluidmigrationintothesupra-saltsedimentsthroughalarge-scalenormal
faultzoneatthepeakofthesaltwallfeature(cross-sectionparalleltosaltwall).
Depth(m)
Intenselyfaultedregionofthepost-salt
sediments.Profileisparalleltothesaltwall.
Notethewashedoutnatureoftheseismic
reflections.

Chapter4-Results
77
Figure 4.31: 3D model of Zone 2 displaying the NNE-SSW trending salt wall top salt surface (white) and the fault zone in the basement
(brown). Stringer fragments run parallel to the salt wall, showing limited extents of interconnectivity.
N
10 km
Figure 4.30: 2D map of the second zone of gas escape, displaying a NNE-SSW trend of the majority of stringers, which turns NNW-SSE in
the north of the region. The rose diagram portrays a predominant SW trend of dip, with moderate alignments trending SE and NW.
Depth (m)

THESIS

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THESIS