Content uploaded by Joanna Garland
Author content
All content in this area was uploaded by Joanna Garland on Jul 05, 2023
Content may be subject to copyright.
Content uploaded by Joanna Garland
Author content
All content in this area was uploaded by Joanna Garland on Jul 05, 2023
Content may be subject to copyright.
1
Journal of Petroleum Geology, Vol. 46 (3), July 2023, pp 1-32
SEDIMENTOLOGY, PALAEOGEOGRAPHY AND
DIAGENESIS OF THE UPPER PERMIAN (Z2)
HAUPTDOLOMIT FORMATION ON THE
SOUTHERN MARGIN OF THE MID NORTH SEA
HIGH AND IMPLICATIONS FOR RESERVOIR
PROSPECTIVITY
Jo Garland1*, Colin Tiltman2 and Callum Inglis2
This paper provides an updated understanding of the reservoir stratigraphy, sedimentology,
palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the
Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the
Mid North Sea High. The paper is based on the examination and description of core and
cuttings data from 25 wells which were integrated with observations based on existing and
new 3D seismic.
Based on thin-section petrography of cuttings and core from the wells studied, it is
evident that Hauptdolomit microfacies are distributed in a relatively predictable way, and
well-dened platform interior, platform margin, slope and basin settings can be distinguished.
Platform margins are typically characterised by the development of ooid shoals and, to a
lesser-extent, by microbial build-ups. High-energy back-shoal settings are characterised by a
more complex combination of peloid grainstones, thrombolitic and microbial build-ups, and
ne crystalline dolomites. Lower energy lagoons which developed further behind the platform
margin are characterised by a variety of microfacies types; ne crystalline dolomites are
common in this setting as well as peloidal facies and local microbial build-ups. Intertidal and
supratidal settings are typied by increased proportions of anhydrite and the development
of laminated microbial bindstones (stromatolites). Platform margins are in general relatively
steep and pass into slope and basinal settings. Only a few wells have penetrated Hauptdolomit
successions deposited in a slope setting, and these successions are characterised by a range
of resedimented shallow-water facies together with low-energy laminated dolomicrites and
ne crystalline dolomites. Slope zones in the study area are interpreted from seismic data
to be typically 1-1.5 km in width. Basinal Hauptdolomit deposits have been strongly affected
by post-depositional diagenesis and are dedolomitised to variable degrees. The original
depositional facies are rarely preserved.
Diagenetic studies show that dolomitisation has affected almost the entire Hauptdolomit
Formation throughout the study area in both basinal and platform settings. The dolomite is
considered to result from seepage-reux processes and is an early diagenetic phase. Mouldic
porosity is present in many facies types as a result of dissolution, especially in ooid grainstones,
1Cambridge Carbonates Ltd, PMJ House, Highlands Rd,
Solihull, B90 4ND.
2 Spirit Energy, IQ Building, 15 Justice Mill Lane, Aberdeen,
AB11 6EQ.
*correspondence:
JoGarland@cambridgecarbonates.co.uk
Key words: Hauptdolomit Formation, Mid North
Sea High, Permian, Zechstein, dolomite, dedolomite,
polyhalite, dolomite seismic response, seismic impedance,
carbonate seismic geometry, reservoir prospectivity.
© 2023 The Authors. Journal of Petroleum Geology © 2023 Scientic Press Ltd
2 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
thrombolitic build-ups and peloidal facies. The dissolution
cannot be associated with any one diagenetic phase
but was most likely a result of the dolomitisation
process itself. Stable isotope analyses indicate that all
dolomites were precipitated from Permian marine-
derived pore uids. Fluid inclusion analyses of dolomite
cements indicate that cementation continued into the
burial realm. Anhydrite cementation occurs in two
phases: early anhydrite precipitation was associated
with dolomitisation, and can be distinguished from a
later, pore-lling cement which is highly detrimental to
reservoir quality.
The Hauptdolomit succession in basinal wells (and
in some slope wells) in the study area has undergone
signicant dedolomitisation. Dedolomitisation was a
shallow burial process which affected precursor dolomites,
whereby excess calcium from the transition of gypsum to
anhydrite during burial combined with CO2 and organic
acids derived from basinal sediments. The process
was triggered by excess calcium reacting with excess
carbonate ions from dissolution.
3D seismic volumes supplemented by numerous
2D lines were available in the study area and allowed
an interpretation to be made of Hauptdolomit gross
depositional settings; platform margins and base-
of-slope polygons were mapped, with the greatest
condence in areas of 3D seismic . The basin, slope
and platform settings were distinguished using seismic
data integrated with the results of micro-facies analysis
and incorporating seismic-to-well ties. The data shows
that large parts of the study area are characterised
by the presence of polyhalites within the overlying
(Z2) Stassfurt Halite Formation, which may create
particular seismic geometries at the Hauptdolomit
slope. These are interpreted to be intra-Stassfurt Halite
features, providing an alternative model to the thickened,
prograded Hauptdolomit which has been suggested in
previous publications.
Because few wells drilled in the study area had the
Hauptdolomit as the primary target, cores were limited
but signicant data was obtained from cuttings analyses.
More than 400 thin sections were evaluated, allowing
depositional models based on microfacies observations
to be developed, verifying the seismic-scale observations.
INTRODUCTION
The purpose of this paper is to provide a regional
evaluation of the palaeogeography, sedimentology
and diagenesis of the Z2 Hauptdolomit Formation
of the Upper Permian Zechstein Supergroup
(“Hauptdolomit”) in the southern margin of the Mid
North Sea High (MNSH). The study area extends
from UK Quadrants 35 and 41 in the west to the A
and E Blocks in the oshore Netherlands (Fig. 1).
Previous regional-scale work by Patruno et al. (2018)
and Mulholland et al. (2018) provide an excellent
starting point for the study but were based on 2D and
vintage 3D seismic and did not incorporate detailed
sedimentological and diagenetic evaluations of well
samples. Alongside Peeters et al. (2023, this issue)
and Browning-Stamp et al. (2023, this issue), this
paper is one of the few publications that details the
sedimentology and diagenesis of subsurface samples
within the extensive MNSH area. The interpretation of
the 2022 3D seismic survey by TGS in the Mid North
Sea High, which was acquired with the Hauptdolomit as
a primary target, resulted in an improved understanding
of Hauptdolomit palaeogeography. Integration with
detailed sedimentological and diagenetic observations
from 340 cuttings sample thin sections and 103 core
sample thin sections from 25 wells in the study area
has enabled comprehensive depositional models and
diagenetic histories to be established.
The Hauptdolomit is a relatively underexplored
play in the UK. However, recent discoveries most
notably at Ossian (42/04-1, 1z), West Newton
(PEDL183 licence), Crosgan (Blocks 42/10 and 42/15),
and Pensacola (41/05a-2), have resulted in renewed
interest in it. The play is, however, well-established
in mainland Europe with production taking place
in Poland (e.g. at the LMG and BMB elds), in the
onshore Netherlands (i.e. at Schoonebeek) and in NW
Germany (i.e. at South Oldenburg) (Peryt et al., 2010).
Regional setting
The structural and tectonic history of the northern
Southern North Sea (SNS) is long and complex.
Structures inherited from Caledonide and later events
have been reactivated many times, most notably
by Variscan compression and more recent Triassic
and Tertiary extension (Grant et al., 2019). There
is a dramatic change in structural style between the
pre- and post-Zechstein successions. Pre-Zechstein
sediments largely show evidence of extension and
varying degrees of inversion, and the Carboniferous
sediments beneath the Variscan unconformity (Fig. 2)
show evidence of compression in the form of large
open folds with a variety of orientations. These folds
occur both on- and oshore and create pre-Permian
prospectivity in the region.
The area of study is situated in the Southern
Permian Basin between the stratigraphic pinch-out
of the Rotliegend succession and the Mid North Sea
High, where the Upper Permian Zechstein succession
rests directly on truncated Carboniferous strata (Fig.
2). In this area, Namurian to middle Dinantian strata
sub-crop the base-Permian unconformity (Patruno et
al., 2018; Mulholland et al., 2018). This unconformity
is folded and faulted, and perhaps was not completely
peneplained during the Zechstein transgression,
3
J. Garland et al.
Fig. 1. Map of the study area in the southern margin of the Mid North Sea High showing the general depositional settings of the Hauptdolomit Formation (platform,
slope and basin). Platform margins and toe-of-slopes are mappable on 3D seismic and gross depositional environments were veried by cuttings analyses. Wells
discussed in this paper are located by red dots. Selected seismic lines: A = Fig. 8c; B = Fig. 8c; C = Fig. 9c; D = Fig. 11c; E = Fig. 11c; F = Fig. 12; G = Fig. 13b; H = Fig. 13b.
43/06-1
42/10a-1
43/16-2
42/15a-2
42/09-1
42/04-1
41/24a-2
41/20-2
41/20-1
41/24a-1
41/18-1
41/15-1
41/10-1
41/05-1
41/01-1
36/26-1
36/23-1
36/13-1
36/15-1
37/12-1
37/23-1
37/25-1
43/03-1
43/02-1
43/05-1
44/06-1
44/07-1
38/29-1
38/24-1
38/18-1
38/16-1
38/25-1
42/10b-2
42/15a-3
E02-02
44/02-1
41/08-1
41/08-2
F
A
BE
D
H
G
Seismically defined Hauptdolomit platform
Wells discussed in paper
Wells studied as part of larger sedimentological study
Additional wells that penetrate Zechstein
C
CROSGAN
Seismically defined base of slope
Quad 36
Quad 37
Quad 38
Quad 43
Quad 42
Quad 44
FIGURE 1
0° 1°E 2°E 3°E
55°N
4 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
maintaining some palaeo-relief during deposition of
the subsequent Permian.
The Southern Permian Basin developed in the
foreland of the Variscan foldbelt and was bounded
and dened by massifs including local highs such as
the Mid North Sea High (Doornenbal et al., 2019).
The basin was episodically ooded by marine waters
from the north which resulted in cycles of carbonate
and evaporite deposition (Fyfe and Underhill, 2023
this issue; Tucker, 1991; Peryt et al., 2010) After the
initial transgression at the beginning of each Zechstein
cycle, carbonates such as the Z2 Hauptdolomit
Formation were deposited before the water body
became increasingly restricted and more saline,
leading to evaporite deposition. Across the basin, up
to seven Zechstein cycles are recorded although only
ve are observed within the study area (Fig. 2). The
increased evaporation during later cycles resulted in
Fig. 2. Stratigraphic column for the southern margin of the Mid North Sea High adapted from Archer et al.
(2022) and Patruno et al. (2017). In this area, the Upper Permian Zechstein Supergroup rests directly on
truncated Carboniferous strata and the Rotliegend is absent.
Polyhalite
wedge
Litho-Stra�graphy
Chrono-strat
Group Forma�on/Mb
Lithologies Seismic
markers
Tectonic
events
Paleo.
Cretaceous
Triassic
Permian
Carboniferous
North Sea
Chalk
Haisborough
Bacton
Zechstein
Farne
Bunter Sandstone
Bunter Shale
Undierenated
Undierenated
Roet Halite
Yoredale
Fell Sandstone
Cementstones
Scremerston
Lower Limestone
Whitby Sandstone
Z1
Z2
Z3
Z4
Z5
Shelf polyhalite
Cimmerian & Base Cretaceous Unconformity
Variscan & Saalian Unconformity
mid-Miocene Unconformity
Top Zechstein
Top Hauptdolomit
Top Werraanhydrit
BPU
Hauptdolomit
Werraanhydrit
Stassfurt Halite
Kupferschiefer
Zechsteinkalk
Basalanhydrit
Grauer Salzton
Deckanhydrit
Plaendolomit
Hauptanhydrit
Leine Halite
Roter Salzton
Pegmatanhydrit
Aller Halite
Grenzanhydrit
Basinal polyhalite
Jurassic
Visean
Nam
Gua
dal
up
Lopingian Lower
Mid
Upper
West
Steph
Cis
Lower
Mid
Upper
Upper
Lower
Penarth
Lias Oa
Comer Knoll Speeton Clay
Neog.
5
J. Garland et al.
the deposition of more complex salts, for example
carnallite, sylvite and potash.
The rst two Zechstein cycles (Z1 and Z2) are
the focus of this study. The development of sulphate
platforms during the first Zechstein cycle (Z1)
was inuenced by the antecedent topography; the
sulphate platforms subsequently formed seed points
for the deposition of the Hauptdolomit carbonates
during the second cycle (Z2). After the deposition of
the Hauptdolomit, there was a basin-wide return to
restricted, evaporitic conditions with the deposition
of the overlying Z2 Basalanhydrit and Stassfurt
Halite Formations; the latter includes both halite and
polyhalite within the study area. Younger Z3 (Leine)
and Z4 (Aller) cycle deposits, and the Grenzanhydrit
from the Z5 cycle, are present in the study area but
were not investigated.
Regional extension in an intracontinental setting
continued in the Mid North Sea High area throughout
the Triassic, and sedimentation in Triassic grabens
initiated a rst phase of salt movement; the mobilisation
of Zechstein evaporites and the development of salt
pillows took place in the Late Triassic (Stewart and
Coward, 1995). Much of the subsequent Jurassic
sedimentation was removed by Mid to Late Jurassic
“Cimmerian” uplift which produced a prominent
angular unconformity at the base of the Cretaceous (Fig
2) (Duguid and Underhill, 2010). Rapid subsidence
resumed in the Early Cretaceous with a rise in global
sea-level, leading to widespread deposition of the
Chalk Group (Grant et al., 2019).
Early Tertiary sedimentation marked a change from
carbonate to predominantly clastic deposition which
continued until the Miocene. Alpine movements in
the Oligo-Miocene produced the nal phase of basin
inversion, resulting in a continued reversal of faults
within the pre-Zechstein section and stimulating
renewed halokinesis involving Zechstein evaporites
(Grant et al., 2019).
Hauptdolomit facies evolution and
palaeogeography
The Z2 Hauptdolomit interval is characterised by
diverse depositional settings in the study area, but
overall by the development of shallow-water carbonate
platforms with complex embayments and irregular
margins (Fig. 1). In addition, isolated platforms and
pinnacles also developed here locally (Fig. 1).
A fundamental control on the location and
development of shallow-water Hauptdolomit platforms
was the presence or absence of a sulphate platform
formed by the underlying Z1 Werraanhydrit Formation.
The Werraanhydrit was deposited during a period of
sea level lowstand and drawdown which promoted
the precipitation of primary evaporites such as
gypsum. Gypsum precipitation was focussed around
depositional highs, and pre-existing structure on the
base-Permian Unconformity had a signicant inuence
on the initiation of Werraanhydrit platform development
(Fig. 3). Evaporation in shallow-water environments
resulted in relatively rapid nucleation and growth of
gypsum crystals, and precipitation and accumulation
rates were high (van der Baan, 1990). Indeed, where
precipitation rates of gypsum exceeded rates of
subsidence and sea-level rise, gypsum accumulations
rapidly built up to sea level (Van de Sande et al., 1996;
also see gure 17 in Grant et al., 2019). However in
deeper-water areas, gypsum saturations were lower and
thus precipitation rates were reduced. This variation
in gypsum saturations may have been intensified
according to the model proposed by Van de Sande et
al. (1996) (Fig. 3), whereby sulphate-reducing bacteria
v
v
v
v
v
v
v
vv v
v
v
v
v
v
v
v
v
v
OXIC ZONE
CHEMOCLINE (abundant sulphate reducing bacteria)
ANOXIC ZONE
LOW RATES OF
GYPSUM
PRECIPITATION
HIGH RATES OF
GYPSUM PRECIPITATION
v
vv
v
v v vv v
v
v
v
EVAPORATION
BASIN PLATFORMSLOPE
Rapid accumulation of gypsum Little or no accumulation of
gypsum
v
Fig. 3. Model for the deposition of gypsum in the Z1 Werraanhydrit Formation and the development of a
sulphate platform. Modied after Van de Sande et al. (1996).
6 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
caused dissolution of gypsum at, and below, the
chemocline (oxic/anoxic boundary). Basinal settings
were therefore characterised by low sedimentation
rates and the deposition of carbonate mudstones or
carbonate/sulphate laminations (varves). Dierences
in evaporite deposition rates between shallow and
deep waters were fundamental to the development of
sulphate platforms. The Z1 Werraanhydrit sulphate
platforms range from 100-200 m in thickness on the
southern margins of the Mid North Sea High, based
on the studied dataset.
During the sea level transgression and highstand
following Werraanhydrit deposition, waters freshened
and Z2 (Hauptdolomit) carbonate deposition became
established both on the shallow-water sulphate
platforms and in slope and basinal settings. It should be
noted, however, that the lack of a diverse macro- and
microbiota in the Hauptdolomit carbonates suggests
that environmental conditions were still stressed
although microbial communities often flourish in
these settings.
DATASETS AND METHODS
Well data
Twenty-ve wells were studied as part of a larger-scale
investigation, of which eight wells were selected to
demonstrate the key facies variations and diagenetic
processes aecting the Hauptdolomit in the study area
(Table 1) (36/26-1, 38/18-1, 38/26-1, 38/29-1, 41/08-1,
42/04-1, 43/02-1, 43/05-1, 44/07-1; locations in Fig.
1). Two of these eight wells (38/29-1 and 44/07-1) had
short cores through the Hauptdolomit interval. The
cores were logged at a 1:50 scale using the slabbed
cut held at the BGS store at Keyworth, Nottingham.
Selected core samples were chosen from core for
petrographic analysis.
For the uncored wells, washed and dried cuttings
were sampled at the BGS core store. Approximately
5 g of cuttings sample was collected for each depth
(generally every 10 ft/ 3m) where sample availability
allowed. Thin sections were prepared from both core
samples and cuttings. The thin sections were treated
with Alizarin Red S and potassium ferricyanide stain
and cover-slipped.
Cuttings were evaluated using a standardised work-
ow. In each sample, all microfacies were recorded
and described, and porosity was noted. A visual
estimate of the proportion of each microfacies was
made. Macropore indicators such as euhedral crystals
together with lost circulation material (mica, wood
and nut shells) were also recorded. Relative abundance
charts were created, per well, to display the relative
proportion of each cuttings category (Fig. 4). It should
be noted that the proportions were estimates whose aim
was to highlight broad vertical changes in microfacies
relative abundances.
A detailed diagenetic study was also undertaken of
Hauptdolomit samples from selected wells, incorporating
techniques including cathodoluminescence (carried
out at Cambridge Carbonates by Francis Witkowski),
O and C stable isotope analyses (University of
Liverpool), BSEM/EDX (Heriot Watt University),
and uid inclusion analyses (Jon Bouch, Pore Scale
Solutions).
Seismic Database
The Spirit Energy exploration licences mainly focused
on the western part of the Mid North Sea High and
covered parts of Quadrants 36, 41 and 42. During a
nine-year period, Spirit Energy evaluated these licence
areas using a number of 3D seismic datasets (Fig. 5),
including the TGS North Breagh PSTM survey (2013),
the PGS MC2013 (2013), the CGG Darach survey
(2010), and the TGS MNSH 3D (2022). Each of these
four surveys cover the Hauptdolomit discovery wells
at Ossian/Darach 42/04-1 and 42/04-1Z (2019). The
Dogger Bank survey covers the Crosgan discovery,
and the Lytham PSDM survey covers the area around
wells 41/05 and 41/10, covering the southern part of
the Pensacola discovery (41/05a-2, December 2022
well spud). The area around the platform-margin well
44/02-1 was also covered by 3D seismic data, the PGS
Cygnet (2012) survey, with coverage obtained from a
merge of this survey with the CGG Loadstone Phase 3
(2013) and Cygnus Loadstone/Greater Cygnus North
Well
Interpreted GDE
position
Basis of evaluation
43/05-1
Platform interior
cuttings
42/04-1
Platform interior
cuttings
41/08-1
Platform margin
cuttings
38/29-1
Platform margin
core and cuttings
43/02-1
Slope
cuttings
36/26-1
Slope
cuttings
44/07-1
Basin
core and cuttings
38/18-1
Basin
-
TABLE 1
Page 57 of 52
For review only
Journal of Petroleum Geology
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. List of the eight wells in the Mid North Sea High from which samples of Hauptdolomit facies were
recovered. Well locations in Fig. 1.
7
J. Garland et al.
(2019) surveys. The TGS surveys extend beyond the
areas of the Spirit Energy exploration licences but
where Spirit did not have access to 3D coverage, 2D
seismic, of poorer resolution compared to the 3D, was
used (Fig. 5: i.e. through wells 38/29-1, 43/02-1 and
43/05-1 (SNR18), 38/24-1 and 38/29-1 (G92) and
38/18-1 (WG15)).
Some of the seismic surveys have been reprocessed,
including gather conditioning and Pre-Stack Depth
Migration. Seismic attributes, including eXchroma
and frequency decomposition blends, seismic inversion
attributes (three term simultaneous inversion of ve
angle stacks), and seismic forward modelling were also
successfully used to help interpret seismic geometries
and to conrm interpretation strategies.
Seismic interpretation and well-to-seismic ties/
synthetics were undertaken using Schlumberger Petrel
software. RokDoc was used for quantitative seismic
attributes and forward seismic modelling, and GeoTeric
for seismic attributes and seismic data conditioning
(structurally oriented noise cancellation and spectral
enhancement). Seismic attributes were calculated in
both Petrel and Geoteric.
Seismic interpretation strategy
The study area for this paper covers a large part of
the Mid North Sea High totalling around 20,000 km2
(Fig. 1). The integration of microfacies analysis from
25 wells with the generation of synthetic seismic
data for well-to-seismic ties, together with seismic
interpretation of key events, has allowed a mapping
methodology to be established for the areas of 3D
coverage centred on the areas where Spirit Energy held
exploration licences (Fig. 1). Learnings from areas
covered only by 2D seismic were also incorporated,
as were regional analogues and publications from the
wider Southern Permian Basin including areas in the
Netherlands, Germany and Poland.
The seismic signature of the Hauptdolomit is
affected both by the overlying stratigraphy (i.e.
the Basalanhydrit/Stassfurt Halite) and also by the
underlying stratigraphy (the Werraanhydrit and
Zechsteinkalk), as well as by thickness and lithological
variations within these intervals. As the Hauptdolomit
is the reservoir target, variations in reservoir quality
also contribute to the seismic signature. In each of the
broad depositional environments which are detailed
in the following sections, the seismic signature of
two example wells will be discussed together with
references to local variations. The synthetic seismic
and seismic signatures of the key wells are summarised
in Fig. 6.
For all seismic datasets discussed, the polarity
convention used is described as North Sea Normal
(European Normal), where a decrease in acoustic
impedance (AI) is represented by a peak (soft event)
coloured blue, and an increase in acoustic impedance is
represented by a trough (hard event), coloured red. The
Fig. 4. Workow for cuttings evaluation. (a) At each cuttings depth, a visual estimate of the relative abundance
of microfacies was assessed and plotted. (b) The cuttings microfacies was plotted alongside well logs to establish
relationships between log response (i.e. porosity) and dominant microfacies.
Cuttings depth (ft)
38/16-1
MD
(m)
GR
(API)
0 100
Lithology
Sonic (us/ft)
Porosity
(%)
40 140
Density (g/cm3)
2.95 1.95 30 0
Cuttings
microfacies
Depositional
setting
(a) (b)
8 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
synthetic seismic displays (Fig. 6) were generated from
sonic and density logs using a 25Hz Ricker wavelet
for consistency across the wells.
Seismic mapping has allowed the broad-scale
palaeogeography of the Hauptdolomit platforms in the
study area to be established (Fig. 1) (see Browning-
Stamp et al. (2023 this issue) for a slightly modied
interpretation). Isolated platforms and attached
platforms/peninsulas can be distinguished based on
the seismically-dened platform geometries.
Approximately 30 wells penetrate the Hauptdolomit
in the Mid North Sea High area. Many of these wells
were drilled to test Carboniferous structures and the
Hauptdolomit was not the main target. Consequently,
few of the wells recovered core from the Hauptdolomit
and cuttings were therefore the main source of
sedimentological, petrographic and diagenetic data.
RESULTS
Detailed cuttings microfacies evaluations were used
to investigate the facies variations and diagenetic
evolution of the Hauptdolomit succession. Table 2
provides a summary of the key microfacies recognised
in the Hauptdolomit in the studied wells. Interpretation
of depositional environments from the cuttings
microfacies trends in 25 wells (340 cuttings samples;
103 core samples) integrated with well logs and
seismic observations, has enabled detailed conceptual
depositional models to be established. Four gross
depositional environments (GDEs) were recognised
from the depositional models: platform interior,
platform margin, slope and basin. The key seismic and
sedimentological characteristics of the Hauptdolomit
in these settings, using the selected wells in Fig. 7, are
discussed in the following sections.
Platform Interior facies
Two wells have been chosen to represent the variability
of facies in the Hauptdolomit platform interior:
43/05-1 and 42/04-1 (Fig. 7; locations in Fig. 1). The
interior setting occupies a large proportion of the
Hauptdolomit platform in the study area (Fig. 1), and
there are therefore local variations in depofacies. The
Hauptdolomit in the platform interior is generally
expressed on seismic by a peak, although the amplitude
is commonly weaker than the response along the
platform margin due to the lower average porosities
(Fig. 8c). The platform area around well 42/04-1 has
a low level of seismic reectivity, especially at the
top-Werraanhydrit (Fig. 8c, right-hand image). It has
also been observed that widespread rafting of the
Z3 Hauptanhydrit/Plattendolomit succession which
overlies the Hauptdolomit in this area may inuence
amplitude responses and reector continuity.
In the area around well 43/05-1, there is good
continuity of the Hauptdolomit and Werraanhydrit
seismic events. There is less rafting of the Hauptanhydrit/
Plattendolomit in this area, and there is a zone of
increased porosity at the base of the Hauptdolomit
Fig. 5. Map of the study area showing the 3D seismic database available for this paper. The seismically dened
margin of the Hauptdolomit platform and the toe-of-slope are also shown, together with the wells and seismic
lines discussed in the paper (from both 3D and 2D seismic datasets).
43/06-1
42/10a-1
43/16-2
42/15a-2
42/10b-2
42/15a-3
42/09-1
42/04-1
41/24a-2
41/20-2
41/20-1
41/24a-1
41/18-1
41/15-1
41/10-1
41/08-1
41/08-2
41/05-1
41/01-1
36/26-1
36/23-1
36/13-1 36/15-1
37/12-1
37/23-1
37/25-1
43/03-1
43/02-1
43/05-1
44/06-1
44/07-1
44/02-1
38/29-1
38/24-1
38/18-1
38/16-1
38/25-1
E02-02
DOGGER BANK
SEISMIC CROSGAN
TGS MNSH (2022)
FULL EXTENT
TGS MNSH PRIME
PSDM PHASE 1
PGS MC2013
(2013)
LYTHAM
PGS CYGNET
(2012)
CYGNUS
LOADSTONE
(2019)
CGG LOADSTONE
PHASE 3 (2013)
SHELL PENSACOLA
BLUEWATER
PROPRIETARY
TGS MNSH
PHASE 2 (2022)
TGS N BREAGH
PSTM (2013)
TGS N BREAGH FULL EXTENT
TGS MC3D
ZCD3D-19
G92-120 2D
SNR18
SNR18
SNR18
Seismically defined Hauptdolomit platform
Seismically defined base of slope
Z3FUG2012A
(2022)
WG15 (OGA)
CGG
DARACH
9
J. Garland et al.
Fig. 6. Composite correlation prole between key wells in the study area (well locations in Fig. 1); the wells are located in their correct relative depositional settings,
and the prole illustrates the transition from the Hauptdolomit platform interior (wells 43/05-1 and 42/04-1), via the platform margin and slope, to the adjacent basin.
The seismic response is represented by both the synthetic seismic calculated with a 25Hz Ricker wavelet, and a sample of the actual seismic at the well location
(except 41/08-1, where the seismic is offset from the well by nearly 1000 m).
Haupdolomit
Werraanhydrit
BPU
38/18-1
GR DT
Lithology
Cuttings
microfacies
Synthetic
Density
SNP
Seismic
GR
DT
Lithology
Cuttings
microfacies
Synthetic
Seismic
GR DT
Lithology
Cuttings
microfacies
Synthetic
Neutron
Seismic
Porosity
Resistivity
GR
Lithology
Cuttings
microfacies
Synthetic
Seismic
Porosity
Resistivity
DT
GR
DT
Lithology
Cuttings
microfacies
Synthetic
Neutron
Seismic
Porosity
Resistivity
GR
DT
Lithology
Cuttings
microfacies
Synthetic
Neutron
Seismic
Porosity
Resistivity
GR
Lithology
Cuttings
microfacies
Synthetic
Seismic
Porosity
Resistivity
GR
Lithology
Cuttings
microfacies
Synthetic
Neutron
Seismic
Porosity
Resistivity
DT
Density
DT
Density
GR
Lithology
Cuttings
microfacies
Synthetic
Neutron
Seismic
Porosity
Resistivity
DT
Density
BASIN
SLOPE
PLATFORM
MARGIN
PLATFORM
INTERIOR
44/07-1
36/23-1 36/26-1
43/02-1
38/29-1
41/08-1
42/04-1 43/05-1
0
100m
MD
0
44ms
TWT
Basin wells
without polyhalite
(i.e. 38/24-1;
37/23-1)
Basin wells with polyhalite
(i.e. 41/05-1; 41/10-1;
42/09-1; 42/10-1; 43/06-1;
Great Hatfield-1)
Slope wells
(i.e. West Newton B1)
Platform margin wells
(i.e. West Newton B1Z;
42/15a-3; 44/02-1; E02-02)
Platform interior wells
(i.e. 42/10b-2; 42/15a-2)
Basin Polyhalite
Shelf Polyhalite
Halite wedge
Wedge Geometry
10 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
which has resulted in a greater AI contrast. Anhydrite
of the Z2 Basalanhydrit typically overlies the
Hauptdolomit in platform wells in the study area (Fig.
2). Well 43/05-1, however, is an exception in that
halite rather than anhydrite is present in the overlying
Stassfurt Halite and forms a strong peak.
Sedimentologically, the Hauptdolomit in wells
43/05-1 and 42/04-1 is dominated by ne crystalline
dolomites (Fig. 7; Fig. 8a, photomicrographs 2,
5 and 6; Fig. 8b, photomicrographs 1, 2, 5 and 6)
which are in general tight and locally cemented by
anhydrite. Preservation of depositional textures or
fauna is rare. The ne dolomites most likely replaced
matrix-supported carbonate mudstones which were
deposited in low energy settings within a platform
interior lagoon.
Although both wells are in general dominated by
ne crystalline dolomite, they exhibit some variability
within the Hauptdolomit interval, perhaps as a result
of smaller scale cyclicity or autocyclic processes in the
platform interior setting. In 43/05-1, the Hauptdolomit
includes a 10-15 m thick package of probable micro-
ooid grainstones (Fig. 8a, photomicrographs 3 & 4).
The ooids are very small, typically <100 µm, and only
preserve a maximum of one concentric oolitic coating.
The grainstones are in general cemented by coarse
anhydrite or dolomite. This microfacies is conned
to this platform interior location, and it is dicult
to establish its depositional character from cuttings.
Comparisons can, however, be drawn to similar facies
which have been reported from the Hauptdolomit
equivalent in NW Germany, where Steinhoff and
Strohmenger (1996) suggested that small micro-ooids
are indicative of deposition in pelletal tidal at settings.
Cuttings recovery in well 42/04-1 was poor
due to significant well-control issues through the
Hauptdolomit interval. However, re-evaluation of the
cuttings indicates that the Hauptdolomit here is also
in general dominated by ne crystalline dolomites,
but there are clear indications of intervals composed
of oolitic grainstone and microbial bindstone (Fig.
8b, photomicrographs 3 and 4). The ooids are larger
in size compared to those in the Hauptdolomit in well
43/05-1 (generally 250-500 µm); the grains contain
multiple oolitic coatings suggesting either a more
persistent high-energy setting, or more likely that
they have been reworked during storm or washover
events from the platform margin into the platform
interior. A characteristic feature is that the coated
grains/ooids commonly have an early circumgranular
cement. Microbial facies are also present and typically
have a lamentous or micropeloidal texture (Fig. 8b,
photomicrograph 4).
In both wells, there is a clear upwards transition
from the top of the Hauptdolomit into the anhydrites
of the Basalanhydrit Formation (Fig. 8).
Platform Margin facies
Wells 38/29-1 and 41/08-1 have been selected to
demonstrate representative facies and stratigraphic
MICROFACIES GROUP MICROFACIES COLOUR CODE
Dolomicrite
Fine crystalline dolomite
Medium crystalline dolomite
Acicular dolomite
PELOIDAL Dolomised peloidal grainstone
Dolomised ooid grainstone
Dolomised coated-grain grainstone
Dolomised thrombolic grainstone/boundstone
Dolomised grainstone
Dolomised, peloidal, cloed microbial pack-grainstone
Dolomised micro-ooid grainstone/ Dolomised algal framestone
Laminated microbial dolomicrite
Dolomised laminated microbial bindstone
Anhydrite
Nodular anhydrite
Halite
Polyhalite/gypsum
Dark argillaceous organic-rich shale
Silty reddened claystone
Coarse crystalline dedolomised calcite
Dedolomised microspar calcite
Caved limestone cungs
Laminated coarse pseudospar/microspar
Quartz grains addives
LCM/ Introduced cement
NON-GEOLOGICAL
OOLITIC
MICROBIAL
SULPHATES/HALIDES
CLASTICS
LIMESTONES
CRYSTALLINE DOLOMITE
Table 2 . Hauptdolomit microfacies and microfacies groups in the studied wells (interpretations based mainly on
cuttings descriptions), together with colour key for well tracks (e.g. Fig. 4 and later gures).
11
J. Garland et al.
evolution of the Hauptdolomit platform margin in the
study area (Fig. 7; locations in Fig. 1). These wells
are important for reservoir characterisation purposes
as the Hauptdolomit facies in this setting has the best
reservoir properties.
The Hauptdolomit in platform margin settings is
typically around 50-60 m thick although there are
signicant thickness variations. It should be noted,
however, that because the Hauptdolomit was rarely
a primary target in wells drilled to-date, a well truly
representative of the platform margin has probably
not yet been drilled. Indeed, the two wells selected are
in general located slightly inboard of the seismically
mapped shelf margin, where thicker platform-margin
sections may occur.
On seismic, the Hauptdolomit in both attached and
isolated platforms is generally a peak, as noted above.
In wells in which the Hauptdolomit has high porosities
(i.e 38/29-1), there is generally a strong amplitude
response (Fig. 9c). Areas in which the Hauptdolomit
platform is best developed in general show the greatest
thickness of the underlying Z1 Werraanhydrit, but
well and seismic data indicate that the thickness
is variable and had a significant influence on the
Supratidal
Intertidal low energy
Intertidal high energy
Lagoonal
Oolitic shoals
Oolitic bars
Washover
Open marine
low energy
Grainshoal
SUPRATIDAL
SUPRATIDAL
SUPRATIDAL
INTERTIDAL
BAR
SHOAL
GRAINSHOAL
WASHOVER
PLATFORM MARGIN
SLOPE
BASIN
PLATFORM
INTERIOR
WERRAANHYDRIT
PLATFORM
LAGOON
38/29-1
42/04-1
43/05-1
44/07-1
38/18-1
36/26-1
43/02-1
36/13-1
38/25-1
44/02-1 42/09-1
36/23-1
41/18-1
SCALABLE
SLUMPS/
TURBIDITES
BAR
SHOAL
SUPRATIDAL
PLATFORM MARGIN
SLOPE
BASIN
41/08-1
WERRAANHYDRIT
PLATFORM
41/08-2
42/15a-3
E02-02
PLATFORM
INTERIOR
41/01-1
41/05-1
41/10-1
36/15-1
42/15a-2
38/16-1
38/22-1
FIGURE 7
Fig. 7. Depositional models for Hauptdolomit platforms in the study area; well locations in Fig. 1:
(above) large-scale attached platform; (below) isolated platform. Note that platform interior facies are more
areally important in attached platforms. The models were built from core and cuttings microfacies observations
from 25 wells. Wells marked in red are key wells discussed in the text; wells marked in grey are additional wells
which were evaluated in order to build the models.
12 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
Fig. 8. Platform Interior setting:
(a) Well 43/05-1: (1) anhydrite microfacies with patchy ne crystalline dolomite; (2, 5, 6) ne crystalline
dolomite microfacies; (3, 4) supercial ooid, or possible algal grainstones – note that the cores of the grains are
commonly dissolved and partially or totally cemented by later anhydrite.
(b) Well 42/04-1, with cased-hole GR, sonic and derived-density well logs, and calculated porosity logs:
(1, 2, 5, 6) ne crystalline dolomite microfacies; (3) dolomitised ooid grainstone microfacies – note the subtle
concentric laminae and the well-developed circumgranular cements; the cores of the ooids have been dissolved
and cemented by anhydrite; minor interparticle porosity is present. (4) Dolomitised microbial bindstone
microfacies; the dolomite is inclusion-rich, very ne crystalline (aphanocrystalline) and replaces textures
which have a microbial, lamentous and micropeloidal texture. NB. Well 42/04-1 took signicant losses in the
Hauptdolomit interval, suggesting it is strongly fractured. Cuttings recovery from this well were poor, there is
therefore a high proportion of additives and squeezed cement in the cuttings. The unadjusted microfacies track
indicates the proportion of additives (black colour), and the adjusted microfacies track shows the microfacies
abundance of the actual rock component. For both wells the key to the cuttings microfacies track is in Table 2.
All photomicrographs are from cuttings.
(c) These platform interior wells have a peak seismic event that can be mapped to represent the Hauptdolomit,
with variable continuity and reector strength. Around well 42/04-1, the Werraanhydrit has a weak trough
expression, but a more continuous trough can be mapped around well 43/05-1. The Plattendolomit rafting, and
wing-geometries at the slope, are prominent in these areas. Seismic courtesy of TGS (42/04-1) and GeoPartners
Lts and Seabird (43/05-1).
(a)
(b)
21
34
56
2
1
3
4
56
42/04-1
0 100
40 140
2.95 1.95
30 0
Unadjusted Adjusted
1 2
3
4
5
6
LagoonalLagoonal Washover
grainstones
(c)
43/05-1
42/04-1
1
2
3
4
5
6
43/05-1
MD
(m)
GR
(API)
0 100
Lithology
Sonic
(us/ft) Porosity
(%)
40 140
Density
(g/cm3)
2.95 1.95
30 0
Cuttings
microfacies
Depositional
setting
LagoonalLagoonal Supratidal
Hauptdolomit
Werraanhydrit
Hauptdolomit
Werraanhydrit
2.5km
NE
1100
1000
1200
1300
1400
1500
1600
1700
1800
1900
SW
ms
Top Zechstein
Hauptdolomit
Werraanhydrit
BPU
EW
Top Zechstein
Hauptdolomit
Werraanhydrit
BPU
2000
2100
1400
1500
1600
1700
1800
1900
ms
2.5km
MD
(m)
GR
(API)
Lithology
Sonic
(us/ft) Porosity
(%)
Density
(g/cm3)
Cuttings
microfacies
Depositional
setting
Wing geometry at slope
Plattendolomit rafting
FIGURE 8
High energy
intertidal
13
J. Garland et al.
(a)
(b)
21
34
56
2
1
34
56
1
2
3
4
5
6
41/08-1
MD
(m)
GR
(API)
0 100
Lithology
Sonic
(us/ft) Porosity
(%)
40 140
30 0
Cuttings
microfacies
Depositional
setting
1
2
3
4
5
6
Hauptdolomit
Werraanhydrit
38/29-1
MD
(m)
GR
(API)
0 100
Lithology
Porosity
(%)
30 0
Cuttings
microfacies
Depositional
setting
Ooid barGrainshoal Intertidal
CORE
Ooid bar
Lagoonal High energy
lagoon/ intertidal
Hauptdolomit
Werraanhydrit
(c)
38/29-1 E
W
Top Zechstein
Hauptdolomit
Werraanhydrit
BPU
2100
2000
1500
1600
1700
1800
1900
ms
1.25km
Plattendolomit rafting
FIGURE 9
Fig. 9. Platform margin setting:
(a) Well 41/08-1: (1, 2) dolomitised peloidal grainstone microfacies; (3) dolomitised oolitic grainstone
microfacies; note that the ooids have a well-developed circumgranular cement, and that the cores of the ooids
are commonly dissolved and subsequently (partially) cemented by anhydrite. (4, 5) Often these are preserved as
loose cuttings of ooids, suggesting that the oolitic grainstones are poorly consolidated; note that the ooids are
generally >1000 µm in size. (6) Fine crystalline dolomite microfacies.
(b) Well 38/29-1: (1) possible tubular foraminifera forming encrustations; (2) dolomitised ooid grainstone
microfacies; the interparticle pore space is cemented by anhydrite. (3) Note that the cores of some ooids has
been locally dissolved; some ooid/pisoids are very large with subtle but well-developed and regular concentric
laminae. (4) Laminated microbial bindstone microfacies with well-preserved lamentous microbial laminae.
(5) Dolomitised thrombolitic boundstone microfacies with a peloidal internal texture. (6) Fine crystalline
dolomite microfacies. For both wells, the key to the cuttings microfacies track is in Table 2. All
photomicrographs are from cuttings.
(c) The platform margin well 38/29-1 has a peak seismic event that can be mapped to represent the
Hauptdolomit, with good continuity and reector strength. The amplitude of the Hauptdolomit is greater in the
northeast which may indicate improved primary porosities. The Werraanhydrit has a trough expression, with
variable continuity and reector strength. Plattendolomit rafting is also evident in this area. Seismic courtesy
of GeoPartners Ltd and Seabird.
14 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
palaeotopography. The thickness and lithology of the
overlying stratigraphy also vary between Hauptdolomit
platform, slope and basin settings. Thus the overlying
Z2 Basalanhydrit is thicker above platforms (80-130 m)
compared to slope (~10 m) and basin (10-40 m) settings
(Fig. 6). The Basalanhydrit is suciently thick to have
inuenced the seismic response and contributes to a
trough immediately above the Hauptdolomit peak, but
also has internal reectivity. Conversely, the Stassfurt
Halite is much thinner above Hauptdolomit platforms
(10-20 m) than it is in slope and basin settings. In wells
38/29-1 and 42/04-1, it is composed predominantly of
anhydrite with occasional traces of halite.
Almost all platform margin wells in the study
area display an upwards evolution in Hauptdolomit
facies which is interpreted to represent progressive
shallowing, although this may be overprinted by subtle
cycles. The GR log commonly has a blocky, low API
character although there is some variability, again
perhaps reecting smaller-scale cyclicity (Fig. 9).
The entire Hauptdolomit interval in each platform
margin well has been dolomitised. The base of the
Hauptdolomit interval marks a transition from thick
anhydrites of the underlying Z1 Werraanhydrit
platform and in the two wells selected is represented by
a 10-15 m thick package of ne or medium crystalline
dolomites (Fig. 9a, photomicrograph 6; Fig. 9b,
photomicrograph 6). Although there are no diagnostic
faunas or sedimentary structures, the ne crystalline
dolomites most likely represent deposition below wave
base in either an open-marine setting or a protected
lagoon. The medium crystalline dolomites in well
38/29-1 locally appear “grainy” and commonly have
a bimodal distribution, with coarser dolomite crystals
distributed within a very ne crystalline matrix. It is
possible that these dolomites are replacing peloidal
facies which were most likely deposited within a
protected lagoon.
In well 38/29-1, the Hauptdolomit becomes
increasingly microbial upwards, with common
thrombolitic textures in the cuttings (Fig. 9b,
photomicrograph 5). A core which was taken near the
middle of the Hauptdolomit interval is characterised by
a 5m thick interval of laminated microbial bindstones
(stromatolites) interbedded with thrombolitic/
oncoidal carbonates. The stromatolitic intervals are
characterised by packages of sub-horizontal, crinkly
laminae which have a distinct microbial nature and
common fenestrae. The laminae are well developed,
and lamentous microbial laminae are well preserved.
However, there are intervals in which the laminae are
disturbed, either by coarse, in situ breccias (composed
of early-lithied microbial mats which were broken
up as a result of storm activity); or by large-scale
intersecting desiccation cracks. Laminae appear
to dip more steeply towards the top of the cored
interval, suggesting a possible domal geometry to the
bindstones.
A thrombolitic build-up is present at the base of the
cored interval (Fig. 10) and, although not fully cored,
has a height of around 20 cm. The thrombolitic textures
are dominated by mesoclots with irregular shapes and
contain oncoidal and coated grains. The thrombolites
are closely associated with stromatolitic textures
which appear to coat and overlie the build-ups. The
thrombolitic build-ups are typical of shallow subtidal
FIGURE 10
Fig. 10. Core photograph of thrombolite build-ups (Th) in the Hauptdolomit from well 38/29-1 (7562.33 ft),
which is coated by laminated microbial stromatolites. Thin section scan = 24 mm. All photographs are PPL, with
porosity denoted by blue-stained epoxy resin.
15
J. Garland et al.
to intertidal settings, and the absence of micrite within
the framework pores supports a generally high-energy
setting in which micritic sediment was winnowed or
was absent. Laminoid microbial bindstones are more
typical of intertidal settings, and the steepened laminae
may indicate a domal or linked-column geometry rather
than at mats.
In both wells 38/29-1 and 41/08-1, the main
Hauptdolomit reservoir facies consists of dolomitised
oolitic and coated-grain grainstones (Fig. 9) which
form packages about 15-20 m thick. Ooid grainstones
were not cored in either well; however, cuttings allow
a good description of the microfacies to be made.
Ooids commonly occur as loose cuttings, suggesting
that they are locally poorly consolidated. Although
ooids are dominant in the grainstones, other coated
grains such as oncoids or composite ooids are also
present, and grains are in general well-sorted and of
medium size. However in cuttings from some wells
(including 38/29-1 and 41/08-1), the ooids and coated
grains are sometimes poorly sorted and may be of
large size; thus it is not unusual for ooids to be >1000
µm (Fig. 9a, photomicrographs 3, 4, 5; and Fig. 9b,
photomicrograph 3) and some are in excess of 3000
µm. The concentric laminae around ooids and coated
grains are locally well preserved, but are on occasion
dicult to distinguish because of variable preservation.
Many ooids have rims of isopachous cement which
has subsequently been dolomitised. The cores of
ooids are commonly dissolved and locally cemented
by anhydrite.
The ooid and coated-grain grainstones in general
represent deposition in shallow-water, platform-margin
shoals (Fig. 7). Where the grainstones are well sorted
and grains are of medium size, deposition is interpreted
to have occurred in shoals where persistently high
energy resulted in consistent grain sorting. The coarser
grainstones may represent deposition in protected but
still high-energy back-shoal settings or more likely
tidal bars. Comparisons can be drawn to almost
identical grain assemblages which are reported from
the Hauptdolomit in NW Germany. Steinho and
Strohmenger (1996) described these subfacies as
having complex fabrics, with very large ooids up to
1250 µm in size and a variety of other coated grains
such as deformed ooids, snouted ooids, oncoids and
stretched or notched ooids. They suggested that these
grain types indicate sediments deposited in ooid bars.
Overall, wells located in Hauptdolomit platform-
margin settings therefore indicate very shallow-water
conditions which were periodically within the intertidal
zone. It is probable that the ooid-shoals and bars and
microbial build-ups occupied similar locations on the
platform margin, although the microbial build-ups
perhaps represent a more back-shoal, high energy
lagoonal setting (Fig. 7). Above the intervals of ooid
shoals, the upper Hauptdolomit succession in the
platform-margin wells studied is characterised by ne
crystalline dolomites (most likely replacing carbonate
mudstones), dolomitised peloidal grainstones, and
microbial textures (Fig. 9). These most likely represent
deposition within a protected lagoonal setting. Cuttings
from the uppermost Hauptdolomit include increased
proportions of anhydrite, and the interval passes into
the sulphates of the overlying Basalanhydrit Formation
(Fig. 9).
Slope facies
The nature of Hauptdolomit slope facies depends
on several variables including the steepness of the
underlying sulphate platform margin, the nature of
the platform margin facies (e.g. ooidal or microbial
framestones), the subsidence rate, the nature of local
tectonics and the palaeo wind direction.
In the western part of the Mid North Sea High,
two wells (43/02-1 and 36/26-1: Fig. 1) penetrate
Hauptdolomit slope facies. The Hauptdolomit is thin
(15-20 m) in both locations, and overlies a slightly
thickened Werraanhydrit and is overlain by a thin
Basalanhydrit (Fig. 6). Together the amalgamated
Werraanhydrit, Hauptdolomit and Basalanhydrit form
a bright trough on seismic proles that is characteristic
of the slope facies location and is often mappable on
3D and 2D seismic data, allowing platform areas to be
identied (Fig. 11c; Fig. 6).
The internal reectivity of the Stassfurt Halite has
a strong impact on the seismic geometries observed
above the thin Hauptdolomit. In slope locations,
polyhalites in the Stassfurt Halite occur slightly higher
in the section, and are described as shelfal rather than
basinal (Smith et al., 2014; Kemp, 2018). The strong
trough seismic response of the shelf polyhalites in
slope locations connects with the strong trough seismic
response of the basinal polyhalites in the basin, creating
a characteristic wing-like geometry which is observed
throughout the platform margin where polyhalites
are recognized in both basinal wells and slope wells
(Fig. 12). Further to this, the slope wells 43/02-1
and 36/26-1 have a halite section below the shelfal
polyhalites which forms a strong peak within the
Stassfurt Halite. This seismic peak event may appear
to be a continuation of the Hauptdolomit peak within
the platform, but the two slope wells demonstrate that
the peak is related to the Stassfurt Halite rather than
the Hauptdolomit (Fig. 6).
Slope areas in other parts of the Southern Permian
Basin are characterised by a thickened and prograded
Hauptdolomit – such as the Shoonebeek and Emmen-
Nieuw Amsterdam fields in the Netherlands and
South Oldenburg in Germany. However, the slopes
are relatively wide in these areas – 4-5 km in the
Netherlands (Reijers, 2012; Van de Sande et al., 1996),
16 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
Fig. 11. Slope setting:
(a) Well 36/26-1: (1) laminated dolomicrite microfacies, with laminations that are slightly argillaceous/organic-
rich; (2) dolomitised ooid packstone/grainstone microfacies with subtly developed concentric laminae, and
pervasive anhydrite interparticle cement (3). (4) In some samples it is not always clear what the original grains
were, and these were categorised into the dolomitised grainstone microfacies. (5) Fine crystalline dolomite
microfacies. (6) Laminated (microbial?) dolomicrite microfacies; in this microfacies, the laminae have an
irregular, crinkly nature and are locally lamentous, most likely indicating a microbial origin.
(b) Well 43/02-1: (1, 3) ne crystalline dolomite microfacies; (2) ne crystalline dolomite, possibly replacing a
grainy texture; note the common anhydrite cementation; (4) anhydrite microfacies. For both wells the key to
the cuttings microfacies track is in Table 2. All photomicrographs are from cuttings.
(c) Wells 43/2-1 and 36/26-1 penetrate thin Hauptdolomit sections (42.6 ft/13 m, 33 ft/10 m respectively), with a
bright trough at the slope created by an amalgamation of the Werraanhydrit, Hauptdolomit and Basalanhydrit.
Overlying reectivity and geometries are attributed to the strong acoustic impedance contrasts between
polyhalites and anhydrites (strong trough) and halites (strong peak) within the Stassfurt Halite. At well
36/26-1, the bright trough at the slope is interpreted to merge up-slope with the overlying Basalanhydrit.
Seismic courtesy of TGS (36/26-1) and GeoPartners Ltd and Seabird (43/02-1).
36/26-1 43/02-1
900
650
700
750
800
850
1000
950
NS
ms
0.5km
Top Zechstein
Hauptdolomit
Werraanhydrit
BPU
Stassfurt Halite
Polyhalite
WE
1.25km
1700
1400
1500
1600
1900
1800
ms
2000
Hauptdolomit
Werraanhydrit
Hauptdolomit
Werraanhydrit
(a)
(b)
(c)
Wing geometry at slopeWing geometry at slope
FIGURE 11
17
J. Garland et al.
and 15-20 km in Germany (Strohmenger et al., 1996)
– and have a gentle dip. They therefore contrast with
the narrow (1.5 km), steeply-dipping slopes recorded
in the Mid North Sea High. Seismic geometries have
been interpreted to indicate prograded and thickened
Hauptdolomit (for example by Patruno et al., 2018),
but well analysis and more recent seismic evidence,
as presented here, has resulted in a revision of this
interpretation to favour an aggradational seismic
geometry.
The facies variation of the Hauptdolomit in wells
36/26-1 and 43/02-1 is presented in Fig. 11. In 36/26-
1, there is a relatively high proportion of low-energy
depositional facies such as dolomicrites, laminated
dolomicrites and ne crystalline dolomites (Fig. 11a,
photomicrographs 1, 5 and 6), all of which indicate
low-energy deposition well below wave base. Indeed,
the base of the Hauptdolomit appears to contain dark,
silty mudstones (claystones) which may represent the
basinal, organic-rich Stinkschiefer facies. However,
there is also a signicant proportion of microfacies
which would normally be associated with a shallow-
platform setting – in particular dolomitised grainstones,
dolomitised ooid grainstones and microbial bindstone
(stromatolitic) textures (Fig. 11a, photomicrographs 2,
3, 4). The presence of these shallow-water microfacies
can be interpreted in two ways: rstly, it is possible
that they represent shallow-platform material which
has been reworked into a slope setting in the form
of gravity ows, storm deposits or turbidites; this
appears to be likely given the admixture of both grainy
and mud-supported cuttings samples. Alternatively,
the shallow-water platform facies may be in situ, in
which case they record a phase of progradation during
Hauptdolomit deposition. As seismic data clearly show
that the wells are located in a Hauptdolomit slope
setting, it is most likely that the grainy facies represent
redeposited material which has been reworked from
the platform top into a lower slope location.
Well 43/02-1 is important in terms of the
Hauptdolomit’s seismic response, which clearly
demonstrate the characteristic slope morphologies
discussed above (Fig. 11c). However, only two cuttings
samples were recovered from the Hauptdolomit
interval for sedimentological analysis (Fig. 11b).
Both samples were characterised by a mixture of ne
crystalline dolomites and anhydrite, although there
are ghosts of microbial grains within the anhydrite
(?microbial lumps: Fig. 11b).
Basin facies
In most of the basinal wells that were studied, the
Hauptdolomit has been subjected to a degree of
dedolomitisation and calcitisation (as shown in the
map in Fig. 13). Dedolomitisation (discussed in
more detail below) has important implications for the
seismic response which is variable. In the western part
of the Mid North Sea High, the Hauptdolomit can be
mapped on seismic as a peak above a Werraanhydrit/
Zechsteinkalk section that is dominated by carbonates,
rather than anhydrites, and below a thin Basalanhydrit
and a thick Stassfurt Halite (with thick polyhalites at
the base; Fig. 6). This is demonstrated in wells 42/09-
1 and 36/23-1, and average porosities of 8.7% and
10% were calculated for the Hauptdolomit at these
wells. In the same area, wells 41/05-1 and 41/10-
1 have microfacies interpreted as Hauptdolomit
limestones rather than dolomites; the limestones
have AI contrasts that can generate a Hauptdolomit
peak (average porosity of 7% and 5%) above a
Werraanhydrit section that is dominated by anhydrites,
and below a thin Basalanhydrit and a thick Stassfurt
Halite (with thick polyhalites at the base). However,
in the south (well 41/15-1) and east (wells 44/07-1
and 38/18-1) of the Mid North Sea High study area
(Fig. 1), the Hauptdolomit peak becomes less easy
to distinguish from the underlying Werraanhydrit
trough, and becomes an intermittent peak or a trough
Fig. 12. Roughly NNW-SSE oriented seismic prole in Quadrant 38 to the south of well 38/24-1 (prole F in
Fig. 1). In the south of the prole, wing-like geometries above the slope can be observed where polyhalites are
present at the base of the Stassfurt Halite in the adjacent basinal wells (e.g. 44/7-1 offset). However, in the north
of the prole, no wing-like geometries are present and the basinal wells do not have polyhalites and anhydrites
within the Stassfurt Halite (e.g. well 38/24-1). This is supported by the shelf polyhalites seen in the Stassfurt
Halite of wells 43/02-1 and 36/26-1. Seismic courtesy of Western Geco (left) and CGG (right).
Polyhalite
No polyhalite
2100
2000
1600
1700
1800
1900
ms
38/29-1
38/24-1
SENNW
2.5km
2200
Top Zechstein
Hauptdolomit
Werraanhydrit
BPU
Polyhalite
Wing geometry at slope
SSE
18 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
event combined with the Werraanhydrit trough. This
is shown in Fig. 14b.
In the majority of the basinal wells in the Mid
North Sea High, polyhalites are present at the base
of the Stassfurt Halite and have been described in
previous studies as basinal (Smith et al. 2014; Kemp,
2018). This follows descriptions from the onshore
Boulby Mine (located near Middlesborough in NE
England) where polyhalites are mined from the Fordon
Evaporite Formation, the onshore equivalent of the
Stassfurt Halite. The basinal polyhalites inuence the
seismic signature as illustrated by wells 44/07-1 and
38/18-1 (Fig. 14b), and produce a trough above the
Hauptdolomit/Basalanhydrit (e.g. at wells 36/23-1,
41/10-1, 42/09-1, 43/06-1, 44/06-1 and 44/07-1). In
the UK onshore, basinal polyhalites are recorded in
the Great Hateld 1 well (location shown in Knott
and Cross, 1992), 4 km north of West Newton oil
eld (Quirk and Archer, 2022). The seismic response
of these basinal polyhalites in the Stassfurt Halite
connects with the shelf polyhalites which are slightly
higher in the section, and form a distinctive wing-like
geometry at the slope adjacent to the margins of the
platform. They are described in more detail below.
Wells without basinal polyhalites are present in a
few small areas of the Mid North Sea High, for example
at wells 37/23-1, 38/18-1 (Fig. 14b) and 38/24-1
(Fig. 12). In these areas, no wing-like geometries are
developed in seismic proles at the slope adjacent to
the platform margins, supporting the interpretation
that the geometries are created by the presence of
polyhalites with the Stassfurt Halite and are not due
to prograding Hauptdolomit. Browning-Stamp et al.
(2023, this issue) interpret these as shallow basinal
areas.
One well, 44/07-1, has been cored in the
Hauptdolomit basinal facies and is considered to be
representative of this setting. A 10 m core was taken
from the upper part of the Hauptdolomit and reveals
a succession of fine, often laminated, carbonates
and evaporites which have been intensely calcitised,
brecciated and fractured (Fig. 14a). The lower parts of
the core are characterized by repeated, thinly-bedded,
ning-upwards intervals that are interpreted as distal
turbidites (Fig. 14a). Minor scour surfaces are present,
and the background sedimentation is characterised by
bedded, ne crystalline dolomites or microspar which
contains dark, organic-rich partings, sometimes as
thick as 1 cm (Fig. 14a, core photo 6). There is evidence
of small syn-sedimentary faults and minor slumping
(Fig. 14a, core photos 5 and 4). The facies contain a
very restricted faunal assemblage consisting only of
rare ostracods and small benthic foraminifera.
The upper sections of the cored interval are
characterised by an increased proportion of evaporites.
Core observations suggest that the initial sediment
was finely laminated (Fig. 14a, core photos 1, 2,
3, 4). The minerology of the original sediments
is difficult to determine as they have undergone
extreme dedolomtisation, but it is probable they
consisted of carbonate mudstones with ne anhydrite
interlaminations. Frequent layers of crystalline calcite
43/06-1
42/10a-1
43/16-2
42/15a-2
42/10b-2
42/15a-3
42/09-1
42/04-1
41/24a-2
41/20-2
41/20-1
41/24a-1
41/18-1
41/15-1
41/10-1
41/08-1
41/08-2
41/05-1
41/01-1
36/26-1
36/23-1
36/13-1 36/15-1
37/12-1
37/23-1
37/25-1
43/03-1
43/02-1
43/05-1
44/06-1
44/07-1
44/02-1
38/29-1
38/24-1
38/18-1
38/16-1
38/25-1
E02-02
Seismically defined Hauptdolomit platform
Coarse crystalline limestones
Limestones &
dolomite
Limestones &
dolomite
Coarse calcite ONLY
at very base
FIGURE 14
Fig. 13. Map of the study area showing wells in which the Hauptdolomit includes limestone lithologies. The
limestones are interpreted to result from intensive dedolomitization of precursor dolomites in basinal and
slope areas.
19
J. Garland et al.
Fig. 14. (a) Basinal facies in well 44/07-1: (1) thinly bedded ne dolomites and darker organic-rich thin beds; note
also the large open fracture cross-cutting the core; (2) small-scale ?syn-sedimentary faulting within beds;
(3) nely laminated sediments which have been folded; (4) Finely laminated ?carbonate mudstones and
anhydrites which have been intensely neomorphosed; (5) sub-horizontal “layers” of coarse calcite crystals
which are probable calcitised gypsum; note that they transition in the lower layer to very dark sediments
with common ?sphalerite/sulphur. (6) Laminated carbonate with contorted and brecciated laminae, with vugs
cemented by coarse calcite and a bitumen ll.
(b) Left: trough seismic events at the base of the Stassfurt Halite caused by acoustic impedance contrasts
between polyhalites/anhydrites and halites, e.g. well 44/07-1 (where basinal polyhalies attach to the shelfal
polyhalites on the slope to create “wing-like” geometries). Right: basinal well 38/18-1 with halite throughout the
Stassfurt Halite (and no polyhalite or anhydrite); “wing-like” geometries are not seen at the slope adjacent to
these basinal areas (also shown in Fig. 12). Seismic courtesy of CGG (44/07-1), PGS (44/02-1) and GeoPartners
Ltd and Seabird (38/18-1).
(a)
2
3
4
12
345
6
44/07-1
MD
(m)
GR
(API)
0 100
Lithology
Porosity
(%)
30 0
Cuttings
microfacies
Depositional
setting
CORE
5
1
6
(b)
38/18-1
Top Zechstein
Hauptdolomit
Werraanhydrit
BPU
Stassfurt Halite
Polyhalite
2.5km 1.25km
1500
1600
1700
1800
1900
2000
SW NE
ms
1500
1600
1700
1800
1900
2000
ms
2100
44/07-1 44/02-1 NE
SW
Hauptdolomit
Werraanhydrit
Wing geometry at slope
Low-energy open marine
20 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
occur in the upper core intervals and are considered
to most likely represent former coarse bottom-growth
selenitic gypsum crystals (with preserved twins) which
have now been calcitised (Fig. 14a, core photos 2, 3, 4).
Anhydrite nodules are present throughout the section.
Despite the intense dedolomitisation, these
sedimentary features are characteristic of basinal
sedimentation. The thin, ning upwards intervals in
the lower half of the core are interpreted as distal
turbidites, while the laminated upper interval most
likely represents relatively deep-water sedimentation
below wave base. The alternation between carbonate
and evaporite laminae probably indicates deposition
in a stratified water body: thinly bedded pelagic
carbonates and evaporite cumulates are known to
be deposited during periods of basin stratication
(Warren, 2016). The intervals of upright-growth
evaporites (now calcitised) represent phases where
the water body was mixed (as a result for example of
storms or high-frequency sea-level changes), allowing
brine-rich waters to be saturated with sulphate down
to the sediment-water interface. Surface waters
within free-standing perennial brine lakes or seaways
routinely uctuate between stratied and unstratied
conditions (Warren, 2016).
HAUPTDOLOMIT DIAGENESIS
Advanced diagenetic studies were undertaken on
samples of the Hauptdolomit from four cored wells
in the study area in order to determine the diagenetic
history (paragenesis) and to investigate conditions of
dolomitisation, cement precipitation, porosity creation,
fracturing and hydrocarbon migration. Three of the
cored wells are located on the platform (38/22-1, 44/02-
1 and 38/29-1), and the fourth well is in a basinal setting
(44/07-1) (locations in Fig. 1). Whilst the Hauptdolomit
in the platform wells has a dolomitic mineralogy, in
the basinal well it has been signicantly modied by
later calcitisation/dedolomitisation (Fig. 13).
Paragenesis
A total of 19 diagenetic events have been recognised
in the Hauptdolomit sediments analysed, the most
important of which are discussed below (Fig. 15).
Circumgranular cement
A common feature of ooid and coated-grain grainstones
is the presence of a very early circumgranular
cement (i.e. Fig. 16a; Fig. 8b photomicrograph 3;
Fig. 9a photomicrograph 3). Cement crystals are
approximately 60 µm in length and most likely had
an original aragonitic mineralogy which was later
replaced by dolomite. The cements are interpreted to
have been precipitated under marine conditions and
provided some limited mechanical stability against
later compaction.
Gypsum
In the intertidal and supratidal facies (i.e. laminated
stromatolites), very early formed gypsum is common
and takes the form of small, felted nodules or small,
lath-like or acicular crystals dispersed within the
sediment (Fig. 16b). Early formed gypsum also occurs
Fig. 15. Paragenetic sequence for the Hauptdolomit sediments in the study area.
EOGENESIS SHALLOW BURIAL DEEP BURIAL
Circumgranular cements (ooids)
Synsedimentary evaporites
Replacement dolomitisation
Dolomite cement
Dissolution
Polyhalitisation
Dedolomitiasation/calcitisation
Fracturing/dissolution
Ferroan and non-ferroan coarse calcite cements
Coarse dolomite cements
Calcite crystal silt/bituminous vug ll
Sulphur/sphalerite
Anhydrite cement
Compaction
Porosity descrease
Porosity increase
Porosity neutral
Aecting all depositional settings
Mostly occurring in platform sediments
Mostly occurring in slope/basinal sediments
21
J. Garland et al.
A
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
(j) (k) (l)
FIGURE 16
Fig. 16. Photomicrographs showing diagenetic characteristics of the Hauptdolomit in the study area.
(a) Ooid grainstone with dolomitised circumgranular cements around grains. Note also the partial dissolution
of the grains and the anhydrite cementation within interparticle pore space. Cutting sample, well 38/29-1.
(b) Early sulphate nodules (originally gypsum) within peloidal packstones texture. Core sample, well 38/29-1.
(c) and (d) PPL and CL pair. Replaced peloids have a yellow-brown CL colour; peloid cement rims have red-
orange CL. Red CL rims are subhedral. Core sample, well 38/29-1.
(e) Acicular dolomite. Core sample, well 38/29-1.
(f) BSEM image of mouldic porosity after peloids. Note the euhedral crystal margins of the dolomite cements
which have grown into open pore space (arrow). Core sample, well 38/22-1.
(g) and (h) PPL-CL pair. Fine mouldic pores in dolomite. No anhydrite in moulds. Dolomite cements are orange
and dull yellow CL. Intergranular anhydrite is non luminescent. Core sample, well 38/29-1.
(i) Thrombolites in which the micritic/organic rims of the clotted grains have been dissolved. Core sample, well
38/22-1.
(j) BSEM image of thrombolitic grains, where the rims have been partially dissolved (red arrow). The grains
have a well-developed cement that precipitated into open pore space. Coarse blocky anhydrite cemented the
interparticle pores (white). Core sample, well 38/29-1.
(k) Ooid grainstone with dolomitised circumgranular cement occurring around most grains and lling ne
interparticle porosity (yellow arrow). Note that dissolution of ooid laminae is prevalent, and locally there
has been gravitational collapse of the core of the ooids (red arrows). Late anhydrite cement occurs in the
interparticle porosity (A). Core sample, well 44/02-1.
(l) BSEM photo of large ooid in image (k). Note that: greys = dolomite, black = pore space, white = anhydrite.
The ne laminae of the ooids are replaced by a very ne crystalline dolomite, whilst the core of the ooid is
replaced by a ne-medium crystalline dolomite. Core sample, well 44/02-1.
22 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
locally as interlaminations in basinal sediments (Fig.
14).
Replacement dolomitisation
Replacement dolomitisation is prevalent and is
generally mimetic in character. At least three forms
of dolomite are present:
• Very fine crystalline (aphanocrystalline)
replacive dolomite (Fig. 9a, photomicrographs
1, 2 & 4; Fig. 8b photomicrograph 1) which
replaces ooid laminae, the cortices of ooids
and grains such as peloids, and locally replaces
interparticle sediment. This dolomite type is in
general non planar-a to planar-s with poorly
developed crystal shapes.
• Fine to medium crystalline replacive dolomite,
whose crystals have a non planar-a to planar-s
morphology (Fig. 9a photomicrograph 6;
Fig. 8 a photomicrograph 2, 5; Fig. 11b
photomicrographs 1 & 3). The crystals
are approximately 20-80 µm in size, and
commonly replace grains such as peloids and
clotted textures in the thrombolites, carbonate
micrite, and the coarse circumgranular cement
around grains (Fig. 16a). Where peloids have
been replaced, the dolomite crystals have a
characteristic “equant” shape (Fig. 16c, d).
• Replacement dolomite with an acicular/needle-
like character (Fig. 16e) with crystals up to
200 µm in length. This form of dolomite is
especially common in the laminated microbial
bindstone facies and locally within ooids, and
is considered to represent replacement of very
early formed gypsum.
Under cathodoluminescence illumination (CL), the
replacement dolomites have a dull red/orange colour
(Fig. 16d) or locally a cloudy, mottled dull yellow-
green-brown colour. Whilst the dull red/orange colour
is typical of replacement dolomites, the less usual
yellow-green-brown colour may indicate a dierent
chemistry or crystallography. It is likely that these
early dolomites are calcium-rich and are far from
stoichiometric.
Dolomite overgrowth cements
Dolomite overgrowth cements are present in some
microbial facies and represent a continuum of the
dolomitisation process. Cement crystals have subhedral
to euhedral faces and form around clotted grains and
peloids (Fig. 16f, g, h); the cement is generally 15-
20 µm thick but can be up to 60 µm. Where peloids
have been replaced by dolomite, there is often a
dolomite cement overgrowth around the whole grain
approximately 10-20 µm thick (Fig. 16f, g, h). Under
CL, the cements is characterised by a ne red-orange
zonation (Fig. 16h).
Dissolution
Dissolution is common in the samples analysed and
aects:
• the micritic/organic-rich rims of thrombolitic
clotted grains, resulting in a microporous or
dissolved/ mouldic rim (Fig. 16i, j);
• peloids, which are commonly surrounded
by dolomite cement but have frequently
been dissolved, resulting in a mouldic pore
surrounded by a “sheath” of residual remnants
of dolomitised grain and calcite cement ~10-20
µm in thickness (Fig. 16d, f, g, h);
• the cores of ooid grains, which are commonly
dissolved, as are selected ooid laminae
(Fig. 9a, photomicrographs 3 & 4; Fig.
8a, photomicrographs 3 & 4; Fig. 8b,
photomicrograph 3; Fig. 16k, l) and bioclasts
(e.g. bivalves); dissolution has probably
preferentially aected parts of an ooid that
had an original aragonitic mineralogy or which
were microbial and therefore less stable.
Some dolomites have a “chalkied” or “leached”
appearance which may be the result of dissolution.
Finally, dissolution may have resulted in the formation
of small vugs after anhydrite nodules. The vugs are
circular, and many are open although others have been
cemented by a later phase of anhydrite.
Petrographic observations indicate that the
circumgranular dolomite cements pre-dated dissolution
(Fig. 16d, f, g, h). The processes responsible for
dissolution are discussed in more detail below.
Dedolomitisation/ calcitisation
The Hauptdolomit in many of the basinal wells
(and in some slope wells) in the study area has been
aected by dedolomitisation/ calcitisation (Fig. 13),
and the resulting textures range from microspar
to coarsely crystalline and spherulitic fabrics. The
widespread dolomitisation of the Hauptdolomit
precedes dedolomitisation, even in basinal settings;
thus, in well 44/07-1, it is clear that dolomites are
engulfed by, and subsumed within, the replacement
calcite (Fig. 17a).
Dedolomitisation is more intense in the upper parts
of the Hauptdolomit compared to the base. Whilst
macrotextures are generally preserved, microscopic
textures are often obliterated. Replacement calcite
crystals are 50-12000 µm in size (Fig. 17b, c), and
larger crystals commonly have a pronounced fan-
shaped sweeping extinction under crossed-polarised
light. The replacement calcite cross-cuts all former
sedimentological and diagenetic boundaries and locally
has an almost spherulitic or concretionary appearance.
Early sulphate precipitates are also calcitised (Fig.
14). Under CL, neomorphic calcite has a non- to dull-
brown colour.
23
J. Garland et al.
(a) (b) (c)
(d)
(e)
BCB
(g)(f)
(h) (i)
?B
C
D
A
(j) (k)
FIGURE 17
Fig. 17. Diagenetic characteristics of the Hauptdolomit in the study area:
(a) Coarse replacement calcite with small, etched dolomite rhombs engulfed within the calcite (arrows). Core
sample, well 44/07-1.
(b) and (c) PPL and XPL thin section scans of a carbonate mudstone, which has been intensely calcitised to
coarse calcite crystals (as seen in XPL image). Thin section = 21mm wide. Core sample, well 44/07-1.
(d) Microspar replacement. Note the uniform crystal size. Core sample, well 44/07-1.
(e) Thin section scan of fractured and vuggy dedolomitised calcite. The vugs are cemented by both ferroan
and non-ferroan cements (yellow arrows), followed by a nal bituminous crystal silt ll (red arrow). Sulphur/
sphalerite is locally present (black arrow). Core sample; thin section width = 18 mm; well 44/07-1.
(f) and (g) PPL and CL photomicrograph pair showing the coarse calcite cements lining vuggy porosity. Very
coarse euhedral cements both dark brown dull CL (?B) post-dated by bright orange brown zoned CL cements
(C). The bright CL is possibly fracture fed. Very ne matrix ll in duller ne argillaceous grains are calcite
orange CL. Core sample, well 44/07-1. Note that the thin sections are not stained.
(h) and (i) PPL and CL photomicrograph pair showing calcite cements. The cement next to the pore is a coarse
uniform bright orange brown CL (D) and this post-dates ne CL zones (C) that overgrow the cloudy and dull
CL inclusion-rich calcite (A). Well 44/07-1. Note that the thin sections are not stained.
(j) Possible sulphur or sphalerite associated with the bituminous vug ll. Core sample, well 44/07-1.
(k) Anhydrite (white mineral) with bituminous pore ll. Core sample, well 44/07-1.
24 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
Calcite microspar is a dedolomitisation texture in
which calcite crystals are typically only 10-25 µm in
size (Fig. 17d). Microspar is often associated with ne
crystalline dolomites.
Fracturing/brecciation
A significant phase of fracturing/brecciation and
dissolution post-dated dedolomitization in the basinal
Hauptdolomit sediments. This resulted in a vuggy and
fractured texture, commonly with relict porosity within
larger fractures/vugs (Fig. 14a, core photographs 1 and
6; Fig. 17e). The vugs may result from the dissolution
of former anhydrite nodules.
Ferroan and non-ferroan calcite cements
Vugs and fractures are commonly cemented by a
complex zoned suite of ferroan and non-ferroan calcite
cements (Fig. 17e). Crystals are up 3000 µm in size,
and have an initial ferroan followed by a non-ferroan
zonation. The stratigraphy of the calcite cements can be
reconstructed under CL. The initial pore-lining cement
(calcite A) has an orange CL colour is inclusion-rich
and massive (Fig. 17h, i). This is followed by a coarse
euhedral cement which has a dark brown, dull CL
colour (calcite B; Fig. 17f, g, h, i), and which is post-
dated by a zoned cement with a bright orange-brown
CL colour (calcite C; Fig. 17f, g, h, i). A cement with
a uniform brown CL colour is the nal phase and is
present next to open pore space (calcite D; Fig. 1 7h, i).
Saddle dolomite
Minor and localised saddle dolomite is present
associated with some fractures/vugs. Crystals are up
to 4000 µm in size. However it is dicult to establish
the timing of this dolomite with respect to the calcite
cements.
Bituminous calcite crystal silt
A bituminous calcite crystal silt is present is the
remaining pore space. This sediment contains broken-
up calcite crystals which sometimes have a ferroan
core and non-ferroan rim as well as dolomite rhombs
(Fig. 17e and j). The surrounding ll locally appears
to be clay-rich and appears to have been created
through mechanical grinding/breakage of the calcite
cement phases, associated with a phase of fracturing/
brecciation/bitumen emplacement. Sulphur (or
possibly sphalerite) is also associated with the vuggy
bituminous ll (Fig. 17e and j). In addition, pyrite is
common.
Anhydrite cement
The nal cement phase which aects all Hauptdolomit
sediments to variable degrees is a late anhydrite
which occludes pores in both the microbial and the
oolitic facies (Fig. 9a photomicrograph 2; Fig. 9b
photomicrograph 2; Fig. 8a photomicrograph 4; Fig.
11a, photomicrograph 3; Fig. 11b, photomicrograph 2;
Fig. 16a and i), and is also the nal cement phase in
vugs in the basinal dedolomites (Fig. 17k). Anhydrite
cementation has therefore reduced overall reservoir
quality signicantly. The cement is coarse and blocky
or bladed, with crystals generally 100-500 µm in size.
The distribution is patchy and not all interparticle pores
are occluded.
Compaction
There is only limited evidence for compaction in
the Hauptdolomit in the studied samples. Locally,
the oolitic grainstones demonstrate concavo-convex
contacts and spalled ooid laminae prior to anhydrite
cementation. The framework and interparticle porosity
of the microbial facies is generally well-preserved.
Fig. 18. Plot of oxygen and carbon stable isotope data for the Hauptdolomit from the studied wells showing
that most dolomite data plots within the known parameters for Permian marine carbonates. Marine Permian
carbonate is after Leary and Vogt (1986). Dedolomite samples show slightly elevated d18O signatures.
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
-10.00 -9.00 -8.00 -7.00 -6.00 -5.00 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00
Dolomite pla�orm wells
Dolomite slope well
Dedolomite slope well
Dolomite basinal well
Dedolomite basinal well
103
δ
18OVPDB
103
δ
13CVPDB
Marine Permian
carbonates
25
J. Garland et al.
Diagenesis discussion
Although the paragenesis for the samples of the
Hauptdolomit dolomites and limestones analysed can
be established based on petrographic observations
(Fig. 15), further analysis is needed to investigate
the conditions (temperature/ salinity) under which
diagenetic changes occurred. Oxygen and carbon
stable isotope data is plotted in Fig. 18, and shows
that Hauptdolomit dolomites from the wells studied
plot within the known parameters of Permian marine
carbonates (Leary and Vogt, 1986). The stable
isotope data is comparable to that of other Zechstein
dolomites from Germany (Schoenherr et al., 2018)
and the Netherlands (Reijers, 2012), and suggests
that the dolomites were precipitated relatively early
from marine-derived pore uids. Due to the strong
association with evaporites in the samples analysed,
and the observation that the Hauptdolomit sediments
from both basinal and platform wells is dolomitised,
it is probable that the dolomitisation process involved
seepage-reux in near-surface/early burial conditions.
Fluid inclusion analysis was performed on
one Hauptdolomit sample from well 38/22-1.
Homogenisation temperatures for uid inclusions in
the dolomite cement surrounding peloid grains suggest
pore uid temperatures ranging from 65 to 95°C, and
that the uids were highly saline with 22-24 wt.%
NaCl (Fig. 19). Using a geothermal gradient of 32°C/
km (based on bottom-hole temperatures from well
42/04-1 and a surface temperature of ~25-30°C during
the Permian: Wygrala, 1989), this suggests that the
dolomite cements were precipitated between depths
of ~1 and 2 km.
Dissolution is clearly an important event and
provides essential porosity in reservoir facies. Much
of the material which was dissolved was likely either
aragonitic or rich in organic content. These materials
are prone to dissolution during burial, particularly
aragonite which becomes unstable at depth. When pore
uids are oversaturated with respect to dolomite and
thus precipitate dolomite cements, the pore uid can at
the same time be undersaturated with respect to calcite,
and contemporaneously dissolve the residual calcitic
grains or laminae together with the more calcian and
non-stoichiometric early replacive dolomites. This
may result in well-developed mouldic porosity. The
distinctive moulds associated with the microbial and
peloidal facies must have post-dated, or been coeval
Fig. 19. Fluid inclusion data for samples of dolomite cements in the Hauptdolomit from well 38/22-1 and calcite
cements from well 44/07-1. Dolomite cements were precipitated from saline pore waters at temperatures
ranging from 65 to 95oC, whilst calcite cements show an increasing temperature prole from early cements
precipitating from pore uids at temperatures ranging from 77 to 103°C to later cements precipitating from
slightly hotter pore uids (~107-112°C).
00
02
04
06
08
10
12
14
16
18
20
0-10
10-20
20-30
30-40
40-50
50-60
60-70
70-80
80-90
90-100
100-110
110-120
120-130
130-140
140-150
Frequency
Homogenisa�on temperature (C)
Calcite D
Calcite C
Calcite B
Dolomite cement
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10
10-11
11-12
12-13
13-14
14-15
15-16
16-17
17-18
18-19
19-20
20-21
21-22
22-23
23-24
24-25
25-26
26-27
Frequency
Salinity (Wt. % NaCl)
Calcite D
Calcite C
Calcite B
Dolomite cement
27-28
28-29
29-30
26 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
with, cement formation (Fig 16f, g, h). Since cement
formation occurs at temperatures of 65-95°C, early
meteoric diagenesis can be ruled out.
Four dedolomite samples were collected for O and
C stable isotope analyses (Fig. 18). Dedolomitisation
of Zechstein carbonates is well-documented in the
Netherlands (Reijers, 2012; Clark, 1986; Clark, 1980,
Van der Baan, 1990), Germany (Schoenherr et al.,
2018), and Poland (Peryt and Scholle, 1996), and
the petrographic and stable isotope characteristics
of dedolomitised limestones in these studies are
remarkably similar to those from well 44/07-1
(Schoenherr et al., 2018; Fig. 20). In both Germany and
the Netherlands, dedolomitisation is interpreted to have
occurred in shallow-burial conditions, and petrographic
and geochemical observations in the present study
suggest that the process occurred in similar conditions
in the Mid North Sea High. Dedolomitisation was
a shallow burial process which aected precursor
dolomites, whereby excess calcium from the transition
of gypsum to anhydrite during burial combined with
CO2 and organic acids derived from basinal sediments.
The process was triggered by excess calcium reacting
with excess carbonate ions from dissolution.
Post-dating dedolomitsation in the samples
analysed was a phase of fracturing and vug formation,
although vugs may also have developed as a result of
calcitisation. The presence of calcitised evaporites,
solid bitumen and elemental sulphur may indicate
that thermochemical sulphate reduction took place.
Vugs and fractures are locally cemented by a suite of
calcite cements, and uid inclusion analyses allowed
the conditions of precipitation to be constrained (Fig.
19). The evolution of the calcite cements (B to C to
D) in samples from well 44/07-1 shows an increasing
temperature prole: thus early cements precipitated
at temperatures ranging from 77 to 103°C, and later
cements precipitated at ~107 to 112°C. Fluid inclusion
temperatures for the calcite cements (77-112°C,
showing an increasing temperature trend with time),
would indicate burial depths in the range of 1.5 km to
2.6 km. It should be noted that the lower temperature
inclusions were more reliable, precipitation at the
shallower end of that range may be more appropriate.
Hauptdolomit reservoir quality
Reservoir quality in the Hauptdolomit is a function
of both depositional facies and later diagenesis and
burial trends. By comparing porosity logs for the key
wells in the dierent gross depositional environments
(Fig. 8; Fig. 9; Fig. 11; Fig. 14), it is clear that the best
reservoir porosities are associated with high energy,
grain-supported platform margin settings. Mouldic
and interparticle porosities are most common in oolitic
grainstones (Fig. 9a, photomicrographs 3, 4, 5; Fig. 9b,
photomicrographs 2, 3), whilst framework and mouldic
porosities are locally preserved in microbial build-ups
(i.e. cored interval in well 38/22-1; Fig. 21). Anhydrite
cementation is the diagenetic process most detrimental
to reservoir quality (Fig. 9a, photomicrograph 2; Fig.
Fig. 20. Plot of d18O (%o) versus d13C(%o) for Hauptdolomit dolomites and dedolomites (data from Schoenherr
et al., 2018, with data from well 44/07-1 overlain). Note that stable isotope values for the Hauptdolomit in the
study area are comparable with those from Germany.
range for dedolomites in 44/07-1
range for dolomites in 44/07-1
27
J. Garland et al.
9b photomicrograph 2; Fig. 8a photomicrograph 4;
Fig. 11a photomicrograph 1; Fig. 21), but despite
detailed evaluation the distribution and intensity of
late anhydrite cement is dicult to predict.
Only limited core data from wells located on
interpreted Hauptdolomit platforms in the study area
is available for a thorough examination of porosity-
permeability relationships (Fig. 21). In a short core
from well 44/02-1, ooid and coated grain grainstones
are intensely cemented by anhydrite, resulting in
generally poor porosities and permeabilities; the
porosity, where present, is typically mouldic and
partly interparticle. In well 38/22-1, it is clear that
there is a facies control on reservoir quality (Fig.
21). Thrombolitic boundstones oer some of the best
reservoir qualities; framework porosity is only patchily
cemented by anhydrite, and mouldic porosity is also
developed (Fig. 21). By contrast, laminated microbial
bindstones (stromatolites) are strongly cemented by
anhydrite and have poorer reservoir properties.
DISCUSSION
Geometry of the Hauptdolomit
platform margin and slope
The geometry of the Hauptdolomit platform margin
and slope is variable on a regional and basin scale.
In the study area in the Mid North Sea High, the
Hauptdolomit carbonates were developed on pre-
existing sulphate platforms which were not attached
to an exposed landmass, so there was little or no input
of continental clastic material. Seismic observations
from the Mid North Sea High area show that the slope
margins of the Werraanhydrit platforms here appear
Fig. 21. Porosity-permeability cross-plot for core-plug data from the Hauptdolomit for wells in platform settings,
with (below) examples of microfacies. Rock typing class boundaries from Lucia (1999).
0.0001
0.001
0.01
0.1
1
10
100
1000
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0
Permeability (md)
Porosity (%)
Oolic grainstone (44/02-1)
Thrombolic boundstones (38/22-1)
Laminated microbial bindstones (38/22-1)
Cloed peloidal packstones (38/22-1)
Upper limit Class I
Class II-Class I boundary
Class III-Class II boundary
Lower limit Class III
LUCIA Class III
LUCIA Class II
LUCIA Class I
Oolitic grainstones (44/02-1) Thrombolitic boundstones
(38/22-1)
Laminated microbial
bindstones (38/22-1)
28 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
to have been relatively steep, and consequently the
Hauptdolomit is relatively thin in these locations (as
suggested by data from wells 36/26-1 and 43/02-1). At
these well locations, observations from seismic suggest
that Hauptdolomit platform facies appear to have an
aggradational rather than a progradational character.
Similar platform geometries for the have been
reported in Poland. In the Gorzów Platform and Grotów
Peninsula area of western Poland, hydrocarbons are
produced from Hauptdolomit platform carbonates
at numerous fields including BMB and LMG
(Slowakiewicz and Mikolajewski, 2009; Jaworoski
and Mikolajewski, 2007; Czekañski et al., 2010). At
the LMG eld complex, a steeply-dipping “bypass”
margin has developed resulting in the deposition of
a resedimented base-of-slope package which forms
an additional reservoir (Lubiatów eld: Kwolek and
Mikołajewski, 2010; Słowakiewicz and Mikołajewski,
2009).
In contrast, Van de Sande et al. (1996) demonstrated
a dierent slope margin style in studies from onshore
Netherlands, where the position of the platform
with respect to the prevailing wind direction had
a significant control on the aggradational versus
progradational nature of the shelf margin. These
authors proposed that wind-generated, high energy
ooid shoals developed along the windward faces of
the platform margins as a result of persistent wave
action. The shoals formed resistant bars, and relatively
minor amounts of sediment were reworked onto the
ZEZ1
BASIN SLOPE PLATFORM
Progradational system
Wide (4-5km) gently dipping slopes with thick dolomites (150m-200m)
e.g. Schoonebeek
HAUPTDOLOMIT
HAUPTDOLOMIT
WERRAANHYDRIT
WERRAANHYDRIT
Aggradational system 1
Narrow slopes (1-1.5km) with thin dolomite
e.g. COV-58, HBG03, HBG-05,
CLDV & HGW wells in The Netherlands;
MNSH wells
BASIN SLOPE PLATFORM
Thin (~10m) anhydrites & polyhalites
in Stassfurt Halite
HAUPTDOLOMIT
WERRAANHYDRIT
Aggradational system 2
Steep narrow slopes (1-1.5km)
e.g. MNSH wells 43/2-1, 36/26-1, West Newton B1
BASIN SLOPE PLATFORM
Thicker (~30-100m)
Anhydrites &
Polyhalites in
Stassfurt Halite
POLYHALITE
WEDGE
FIGURE 22
Fig. 22. Models of progradational and aggradational Hauptdolomit platform margin styles. Modied from
Van de Sande et al. (1996).
29
J. Garland et al.
Fig. 23. Cartoon block diagram summarising factors which may inuence the distribution of shelf-margin ooid
shoal facies in the Hauptdolomit in the study area.
slope but instead formed thick, aggradational packages
at the shelf margin (Van de Sande et al., 1996; Fig.
22). On leeward margins by contrast, sediment was
comparatively easily shed from the platform to the
slope, creating a gently dipping slope with thicker slope
facies (Van de Sande et al., 1996; Fig. 22).
At the wells evaluated in this study, the Hauptdolomit
margin resembles the aggradational margin of Van de
Sande et al. (1996), and slope facies are relatively
thin. However, there may be local variations in the
slope conguration and careful mapping may identify
toe-of-slope deposits similar to those recorded at the
LMG eld complex.
In addition to wind direction, several other factors
may inuence the distribution and thickness of shelf
margin facies. For example, tidal activity inuences
the shape and size of facies belts, and tidal bars are
typically oriented perpendicular to shelf margins.
In present-day systems such as the Bahamas, areas
of vigorous tidal exchange correspond to localities
with high-energy ooidal bars at the platform margin
(Harris, 2018). Tidal shoals are focussed where there
is a change in geometry of the platform or where there
is an embayment resulting in the funnelling of tidal
currents, and similar inuences may have controlled
the distribution of Hauptdolomit shelf margin facies
(Fig. 23).
Antecedent topography in modern carbonate
settings is often a focal point for the development of
shallow-water, high energy shoals. In the study area
in the Mid North Sea High, the antecedent topography
at the level of the Base Permian Unconformity had a
signicant inuence on the position and development
of the Werraanhydrit sulphate platforms, and maybe
also on Hauptdolomit deposition (Fig. 23).
Analogue porosity-permeability data
The very limited availability of core plug or test
data from the study area means that permeability
measurements for the Hauptdolomit are in general
lacking. However analogue data has been published
in studies from Germany, the Netherlands and Poland
(Fig. 24), although the Hauptdolomit in these areas
may not have undergone the same diagenetic or burial
pathways as it has in the Mid North Sea High. In the
analogue examples, ooid grainstones are the main
reservoir facies in the Hauptdolomit, and mouldic and
vuggy porosity are the main pore types. Leaching is
most often proposed to explain porosity development,
and may be either an early process or occur during
later burial, or may take place as a result of meteoric
dissolution during late-stage uplift. The reservoir
characteristics of the Hauptdolomit is reduced in
all areas as a result of anhydrite cementation whose
distribution and intensity is dicult to predict.
On the whole, the relatively limited poroperm
dataset for the MNSH compares favourably with its
Southern Permian Basin counterparts (Fig. 24). For
example, in NW Germany and Poland, the porosity-
permeability of the Hauptdolomit may be lower than
predicted for the Mid North Sea High because of
signicantly deeper burial (to almost 5 km), which
PALAEOWIND DIRECTION
FROM NE
LEEWARD
LEEWARD
WINDWARD
EMBAYMENTS FOCUSSING
TIDAL CURRENTS
POSSIBLE INFLUENCE OF BPU HIGH
(ANTECEDENT TOPOGRAPHY)
DURING HAUPTDOLOMIT INTERVAL
KUPFERSCHIEFER
30 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
will have resulted in increased compaction and
cementation. In the Netherlands, faults and fractures
in the Hauptdolomit are more signicant than they are
in the Mid North Sea High, and are interpreted to have
served as conduits for uids from which anhydrite
cements were precipitated (Clark, 1986). Average
permeabilities of the Hauptdolomit in this area may
be lower than that in the Mid North Sea High.
CONCLUSIONS
The reservoir potential of the Z2 Hauptdolomit
Formation of the Upper Permian Zechstein Supergroup
(“Hauptdolomit”) in the Mid North Sea High area was
investigated using a regional but detailed approach. The
workow involved evaluating several large 3D seismic
areas supplemented by numerous 2D lines; this allowed
an interpretation to be made of gross depositional
settings, and Hauptdolomit platform margins and
base-of-slope polygons to be mapped. The recently
acquired 2022 TGS MNSH 3D data enabled a robust
seismic mapping approach to be developed, resulting
in signicant advances on previously published data
(i.e. Patruno et al., 2018). Detailed cuttings microfacies
studies allowed depositional models based on
microfacies observations to be developed, resulted in
an improved understanding of how the Hauptdolomit
evolved in dierent depositional settings and how
diagenesis inuenced reservoir quality.
Hauptdolomit basin, slope and platform settings
were distinguished using seismic data integrated with
the results of microfacies analysis and incorporating
seismic-to-well ties. The data shows that large parts
of the study area are characterised by the presence
of polyhalites within the overlying (Z2) Stassfurt
Halite Formation, which may create particular seismic
geometries at the Hauptdolomit slope. These “wing-
like” geometries at the slope are interpreted to be
intra-Stassfurt Halite features, rather than thickened,
prograded Hauptdolomit, as has been suggested in
previous publications (e.g. Patruno et al., 2018). This
interpretation has come from integrating well data and
seismic data, and comparisons with analogues.
Hauptdolomit microfacies appear to be distributed
across platforms in a relatively predictable way.
Platform margins show the development of ooid shoals
and locally microbial build-ups. High-energy back-
shoal settings are characterised by a more complex
combination of peloid grainstones, thrombolitic and
microbial build-ups, and ne crystalline dolomites.
Lower energy lagoons are characterised by a variety
of microfacies types; ne crystalline dolomites are
common in this setting as well as peloidal facies and
local microbial build-ups. Intertidal and supratidal
settings are typified by increased proportions of
anhydrite and the development of laminated microbial
bindstones (stromatolites). Slope environments are
characterised by a range of resedimented shallow-
0.0001
0.001
0.01
0.1
1
10
100
1000
0 5 10 15 20 25 30 35 40 45
Permeability (md)
Porosity (%)
NW Germany Pla�orm Interior Tidal Flat (Schoenherr 2014 & 2018) NW Germany Pla�orm Interior Algal Tidal Flat (Schoenherr 2018)
NW Germany Pla�orm Margin Algal laminated shoal (Schoenherr 2014) NW Germany Pla�orm Margin Grainy Shoal (Schoenherr 2014 & 2018)
NW Germany Pla�orm Margin Ooid Shoal (Schoenherr 2014 & 2018) NW Germany Pla�orm facies (Schoenherr 2014)
NW Germany Pla�orm Interior outcrop (Becker 2018) NW Germany Pla�orm Margin outcrop (Becker 2018)
NW Germany Slope subsurface (Becker 2018) NW Germany Slope outcrop (Becker 2018)
NW Poland Pla�orm Margin ooid shoal Benice-3 NW Poland Pla�orm Margin ooid shoal Ciechnowo-5
NW Poland Pla�orm Margin ooid shoal/slope Czarne-1 NW Poland ooid shoal/slope Gardomino-1
SW Poland Pla�orm Margin Miedzychod-4 SW Poland Pla�orm Margin Miedzychod-5
SW Poland Pla�orm Margin Miedzychod-6 Netherlands Pla�orm Interior
Netherlands Pla�orm Margin Netherlands Slope
Uk well 38/22-1 UK well 44/2-1
Upper limit Class I Class II-Class I boundary
Class III-Class II boundary Lower limit Class III
LUCIA Class III
LUCIA Class II
LUCIA Class I
Fig. 24. Porosity-permeability plot for Hauptdolomit facies derived from published data (data from NW
Germany from Schoenherr et al., 2014, Schoenherr et al., 2018; Becker, 2018; data from the Netherlands
onshore data from NLOG.nl). Rock typing class boundaries from Lucia (1999).
31
J. Garland et al.
water facies together with low-energy laminated
dolomicrites and ne crystalline dolomites. Basinal
Hauptdolomit deposits have been strongly aected by
post-depositional diagenesis and are dedolomitised to
variable degrees.
Platform slopes are in general relatively steep, and
are narrow, typically 1-1.5 km wide, aggradational, and
pass into slope and basinal settings.
Diagenesis is pervasive, aecting all facies types
and modies porosity/permeability characteristics.
Dolomitisation is considered to be an early seepage-
reux process, and aects all depositional facies.
Mouldic porosity is present in many facies types as
a result of dissolution, especially in ooid grainstones,
thrombolitic build-ups and peloidal facies. Minor
dolomite cements are present around grains, and
fluid inclusion analysis helped to conclude that
dolomitisation continued into the burial realm.
Anhydrite is the main burial cement phase, and is
detrimental to reservoir quality.
Dedolomitisation is common in samples from
basinal wells, and is considered to be a shallow burial
process.
These interpretations and concepts will contribute
to further exploration in the Mid North Sea High.
ACKNOWLEDGEMENTS
We would like to thank Oliver Davis and Anne-Sophie
Cyteval from Spirit Energy, and Elena Manzo from
Shell for their excellent technical discussions whilst
working on the Zechstein. Fabrizio Conti, Benoit
Vincent, Peter Gutteridge, Julia Morgan, Francis
Witkowski, Steve Crowley and Jon Bouch are also
acknowledged for their contributions to the work
programme. We would like to thank Shell, ONE-
Dyas, Neptune, TGS, GeoPartners Ltd, Seabird, PGS,
Western Geco and CGGV and for their support and
for their permission to publish this paper. Finally, the
British Geological Survey @ UKRI are thanked for
enabling us to evaluate the cuttings and cores. The
samples were evaluated through loan 277438. Reviews
of a previous version of the paper by Elena Manzo
and C.R. (Kees) Geel are acknowledged with thanks.
Data availability statement
The data used within this study are from open access
sources unless otherwise stated. Open access source
material included in this project has been obtained from
the British Geological Society (Loan number 277438),
UK National Data Repository and the North Sea
Transition Authority. Seismic data is multiclient and
not open-access apart from a 2D survey WG15 (OGA)
which is publicly available. All seismic providers have
been acknowledged in gure captions.
REFERENCES
ARCHER, S. G., KOMBRINK, H., PATRUNO, S., CHIARELLA,
D., JACKSON, C. A-L. and HOWELL, J., 2022. Cross-border
Petroleum Geology, I The North Sea: An Introduction. In:
Patruno, S., Archer, S. G., Chiarella, D., Howell, J. A., Jackson,
C. A.-L. and Kombrink, H. (Eds), 2022. Cross Border Themes
in Petroleum Geology I: The North Sea. Geological Society of
London, Special Publication, 494, 1-11.
BECKER, I., 2018. Structural and diagenetic controls on
reservoir quality in tight siliciclastic and carbonate rocks.
Ph.D Dissertation. Hohenlimburg, Stadt Hagen. Institut für
Angewandte Geowissenschaften (AGW), published by KIT
Scientic Publishing, pp 186.
BROWNING-STAMP, P., CALDARELLI, C., HEARD, G., RYAN,
J. and HENDRY, J., 2023 this issue. The Zechstein Z2
Hauptdolomit platform in the southern margin of the UK
Mid North Sea High and its associated petroleum plays,
potential and prospectivity. Journal of Petroleum Geology, 46
(3), 295-328.
CLARK, D.N., 1980. The diagenesis of Zechstein carbonate
sediments. Contr. Sedimentology, 9, 167-203.
CLARK, D.N.,1986. The distribution of porosity in Zechstein
carbonates. In: Brooks, J., Goff, J.C. and Van Hoorn, B (Eds),
Habitat of Palaeozoic gas in N.W. Europe. Geological Society
of London, Special Publication, 23, 121-149.
CZEKAŃSKI, E., KWOEK, K. and MIKOŁAJEWSKI, Z., 2010.
Hydrocarbon elds in the Zechstein Main Dolomite (Ca2)
on the Gorzów Block (NW Poland). Przegląd Geologiczny,
58, 8, 695-703
DOORNENBAL, J. C., KOMBRINK, H., BOUROULLEC, R.,
DALMAN, R. A. F., DE BRUIN, G., GEEL, C. R., HOUBEN,
A. J. P., JAARSMA, B., JUEZ-LARRÉ, J., KORTEKAAS, M.,
MIJNLIEFF, H. F., NELSKAMP, S., PHARAOH, T. C., TEN
VEEN, J. H., TER BORGH, M., VAN OJIK, K., VERREUSSEL,
R. M. C. H., VERWEIJ, J. M. and VIS, G.-J., 2019 New Insights
on Subsurface Energy Resources in the Southern North Sea
Basin Area. In: Patruno, S., Archer, S. G., Chiarella, D., Howell,
J. A., Jackson, C. A.-L. & Kombrink, H. (Eds). Cross-Border
Themes in Petroleum Geology I: The North Sea. Geological
Society of London, Special Publication 494, 233-269
DUGUID, C. and UNDERHILL, J. R., 2010. Geological controls
on Upper Permian Plattendolomit Formation reservoir
prospectivity, Wissey Field, UK Southern North Sea.
Petroleum Geoscience, 16, 331-348.
FYFE, L.-J. C. and UNDERHILL, J.R., 2023, this issue. A regional
overview of the Upper Permian Zechstein Supergroup
(Z1 to Z3) in the SW margin of the southern North Sea
and onshore Eastern England. Journal of Petroleum Geology,
46 (3), 223-256..
GRANT, R. J., UNDERHILL, J.R., HERNÁNDEZ-CASADO, J.,
JAMIESON, R.J. and WILLIAMS, R.M., 2019. The evolution
of the Dowsing Graben System: implications for petroleum
prospectivity in the UK Southern North Sea. Petroleum
Geoscience, 27 (1),18-64.
HARRIS, P.M., 1979. Facies Anatomy and Diagenesis of a
Bahamian Ooid Shoal. Sedimenta VII. Miami, FL: Univ. Miami.
JAWOROWSKI, K. and MIKOŁAJEWSKI, Z., 2007. Oil- and
gas-bearing sediments of the Main Dolomite (Ca2) in the
Międzychód region: a depositional model and the problem
of the boundary between the second and third depositional
sequences in the Polish Zechstein Basin. Przegląd Geologiczny,
55, 12/1, 1017-1024
KEMP, S.J., SMITH, F.W., WAGNER, D., MOUNTENEY, I., BELL,
C.P., MILNE, C.J., GOWING, C.J.B. and POTTAS, T.L., 2016.
An Improved Approach to Characterize Potash-Bearing
Evaporite Deposits, Evidenced in North Yorkshire, United
Kingdom. Economic Geology, 111, 719-742. https://doi.
org/10.2113/econgeo.111.3.719
KNOTT, I. and CROSS, K.G., 1992. Gas Storage Caverns in East
32 Hauptdolomit Formation, southern Mid North Sea High: reservoir sedimentology and diagenesis
Yorkshire Zechstein Salt: Some Geological and Engineering
Aspects of Site Selection. SPE 24923, 689-698.
KWOLEK, K. and MIKOLAJEWSKI, Z., 2010. Criteria of
identication of lithofacies objects as potential hydrocarbon
traps in the Main Dolomite (Ca2) strata at the toe-of-slope
of carbonate platforms and microplatforms in central-
western Poland. Przegl¹d Geologiczny, 58, 5, 426-435.
LEARY, D.A. and VOGT, J.N., 1986, Diagenesis of Permian
(Guadalupian) San Andres Formation, Central Basin
Platform,. In: Bebout, D. G. and Harris, P. M. (Eds),
Hydrocarbon Reservoir Studies, San Andres/Grayburg
Formations, Permian Basin. Permian Basin Section SEPM
Publ. 86-26, 67-68.
LUCIA, F. J., 1999. Carbonate Reservoir Characterization.
Springer, pp. 226.
MULHOLLAND, P., ESESTIME, P., RODRIGUEZ, K. and
HARGREAVES, P. J., 2018. The role of palaeorelief in
the control of Permian facies distribution over the Mid
North Sea High, UK Continental Shelf. In: Monaghan, A.
A., Underhill, J. R., Hewett, A. J. and Marshall, J. E. A. (Eds),
Paleozoic Plays of NW Europe. Geological Society of London,
Special Publication 471, 155-175.
PATRUNO, S., RIED, W., JACKSON, C.A.-L., and DAVIES,
C., 2018. New insights into the unexploited reservoir
potential of the Mid North Sea High (UKCS quadrants
35–38 and 41–43): a newly described intra-Zechstein
sulphate–carbonate platform complex. In: Bowman, M. and
Levell, B. (Eds), Petroleum Geology of NW Europe: 50 Years
of Learning – Proceedings of the 8th Petroleum Geology
Conference, Geological Society of London, 87-124.
PEETERS, S. H. J., GEEL, K., GARLAND, J. and BOUROULLEC,
R., 2023 this issue. Seismic and petrographic characterisation
of the Zechstein Hauptdolomit platforms around the Elbow
Spit High, Dutch Offshore. Journal of Petroleum Geology, 46
(3), 361-382.
PERYT, T. M. and SCHOLLE, P. A., 1996. Regional setting and
role of meteoric water in dolomite formation and diagenesis
in an evaporite basin: studies in the Zechstein [Permian)
deposits of Poland. Sedimentology, 43, 1005-1023.
PERYT, T. M. , GELUK, M., MATHIESEN, A., PAUL, J. and SMITH,
K., 2010. Chapter 8 – Zechstein. In: H. Doornenbal and
A. Stevenson (Eds), Petroleum Geological Atlas of the
Southern Permian Basin Area. EAGE Publications, Houten.
pp 123-147.
QUIRK, D.G and ARCHER, S.G., 2022. Exploration play statistics
in the Southern North Sea region of the Netherlands and
UK. In: Patruno, S., Archer, S. G., Chiarella, D., Howell, J. A.,
Jackson, C. A.-L. and Kombrink, H. (Eds), Cross Border
Themes in Petroleum Geology I: The North Sea. Geological
Society of London, Special Publication 494, 117-136, https://
doi.org/10.1144/SP494-2020-200
REIJERS, T.J.A., 2012. Sedimentology and diagenesis as
‘hydrocarbon exploration tools’ in the Late Permian
Zechstein-2 Carbonate Member (NE Netherlands).
Geologos 18, 3, 163-195.
SCHOENHERR, J., REUNING, L., HALLENBERGER, M.,
LUDERS, V., LEMMENS, L., BIEHL, B.C., LEWIN, A.,
LEUPOLD, M., WIMMERS, K. and STROHMENGER,
C.J., 2018. Dedolomitization: review and case study of
uncommon mesogenetic formation conditions. Earth-Science
Reviews, 185, 780-805.
SCHOENHERR, J., REUNING, L., WIMMERS, K., FELLMIN, S.,
SMODEJ, J., KUSKU, S., BRAUCKMANN, F.J., CORONA,
F.V., STROHMENGER, C.J. and GUIDRY, S., 2014. The Impact
of Dedolomitization on Reservoir Quality of the Upper
Permian Zechstein-2-Carbonate, NW Germany. AAPG
Search and Discovery Article #51010.
SŁOWAKIEWICZ, M. and MIKOŁAJEWSKI, Z., 2009. Sequence
stratigraphy of the Upper Permian Zechstein Main dolomite
carbonates in western Poland: A new approach. Journal of
Petroleum Geology, 32(3), 215-234.
SMITH, F.W., DEARLOVE, J.P.J., KEMP, S.J., BELL, C.P., MILNE,
C.J., and POTTAS, T.L. 2014. Potash – Recent exploration
developments in North Yorkshire. pp 45-50, In: Hunger,
E., Brown, T. J. and Lucas, G. (Eds), Proceedings of the17th
Extractive Industry Geology Conference, EIG Conferences
Ltd. 202 pp
STEINHOFF, I. and STROHMENGER, C., 1996. Zechstein 2
Carbonate Platform Subfacies and Grain-Type Distribution
(Upper Permian, Northwest Germany). Facies, 35, 105-132.
STEWART, S. A. and COWARD, M. P., 1995. Synthesis of Salt
Tectonics in the Southern North Sea, UK. Marine and
Petroleum Geology, 12, 5, 457-475.
STROHMENGER, C., VOIGT, E. and ZIMDARS, J., 1996.
Sequence stratigraphy and cyclic development of Basal
Zechstein carbonate-evaporite deposits with emphasis
on Zechstein 2 off platform carbonates (Upper Permian,
Northeast Germany). Sedimentary Geology, 102, 33-54.
TUCKER, M.E., 1991. Sequence stratigraphy of carbonate-
evaporite basins: models and application to the Upper
Permian (Zechstein) of northeast England and adjoining
North Sea. Journal of the Geological Society, 148, 1019-1036,
van de SANDE, J. M. M, REIJERS, T. J. A and CASSON, N., 1996.
Multidisciplinary exploration strategy in the northeast
Netherlands Zechstein 2 Carbonate play, guided by 3D
seismic. In: Rondeel, H.E., Batjes, D.A.J. and Nieuwnhuijs,
W.H. (Eds), Geology of gas and oil under the Netherlands,
125-142. Kluwer Academic Publishers.
Van der BAAN, D., 1990. Zechstein reservoirs in The
Netherlands. In: Books, J. (Ed.), Classic Petroleum Provinces.
Geological Society of London, Special Publication 50, 379-398
WARREN, J. K., 2016. Evaporites – A Geological Compendium.
Springer, pp 1813.
WYGRALA, B.P., 1989. Integrated Study of an Oil Field in the
Southern Po Basin, Northern Italy. Ph.D Dissertation, Köln
University, Jülich Research Centre, Jülich, Germany, ISSN
0366-0885, 217 p.