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Journal of Petroleum Geology, Vol. 46 (3), July 2023, pp 1 - 34
THE ZECHSTEIN Z2 HAUPTDOLOMIT
PLATFORM IN THE SOUTHERN UK MID NORTH
SEA HIGH AND ITS ASSOCIATED PETROLEUM
PLAYS, POTENTIAL AND PROSPECTIVITY
Peter Browning-Stamp*+, Carlo Caldarelli*, Graham Heard*,
James Ryan* and James Hendry#
The Mid North Sea High (MNSH) region represents one of the least explored areas for the
Late Permian Zechstein Hauptdolomit play in the Southern Permian Basin although some of
the rst offshore wells drilled in the UK were located here. In other parts of the basin such
as onshore Poland, the Hauptdolomit Formation (“Hauptdolomit”) is an active and attractive
exploration target, with oil and gas production from commercial-sized elds. In the UK, the
play has been overshadowed by drilling campaigns in areas to the south of the MNSH
which tested plays in the underlying Rotliegend and Carboniferous successions. However,
with these areas now in decline, there is increased exploration interest in the Hauptdolomit
in the MNSH region, particularly since 2019 when 3D seismic data were acquired and the
rst hydrocarbon discovery was made at Ossian (well 42/04-01/1Z). Geochemical data from
the latter discovery have pointed to the presence of a prolic petroleum system with the
potential for Hauptdolomit reservoirs to be charged both by Zechstein-generated oils and
Carboniferous condensate/gas. With regard to hydrocarbon migration and preservation in the
southern MNSH, a detailed evaluation of the effects of the Mid Miocene Unconformity has
allowed for a greater understanding of the main factors controlling hydrocarbon preservation
and remigration. Reservoir characterization of the Hauptdolomit play has been achieved by
integrating petrographic microfacies analyses, core data and petrophysical interpretations.
The most important factors controlling reservoir quality are the presence and extent of
anhydrite cementation and the presence of high energy shoal facies. Thicker and coarser
grained shoal facies are expected to occur along the yet-to-be explored Orchard platform
margin where numerous prospects have been mapped and rened using recently acquired
3D seismic data.
* Horizon Energy Partners Limited, London, UK
# Iapetus Geoscience Limited, Dublin, Ireland
+ Correspondence:
peter.browning-stamp@horizonenergyglobal.co.uk
Key words: Hauptdolomit, Mid North Sea High, UKCS,
Zechstein Supergroup, Permian, North Sea, Southern
Permian Basin, Z2 cycle, carbonate platform, reservoir
rock.
INTRODUCTION
The UK Mid North Sea High (MNSH) represents a
Variscan palaeohigh spanning quadrants 34 to 39 and
the northern portions of quadrants 40 to 44 (Fig. 1). It is
bounded to the west by the UK mainland and to the east
and NE by the roughly north-south trending Central
Graben system. Together with the Ringkøbing-Fyn
High to the east, the MNSH separates the Northern and
Southern Permian Basins (Fig. 2), the latter extending
from NE England in the west to Lithuania and Poland
© 2023 The Authors. Journal of Petroleum Geology © 2023 Scientic Press Ltd
2Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
in the east / NE (Peryt et al., 2010; Doornenbal et al.,
2010). The MNSH represents one of the least explored
hydrocarbon provinces on the UK Continental Shelf
although pioneering oshore wells were drilled here
in the 1960s. A primary exploration target is the Z2
Hauptdolomit Formation (“Hauptdolomit”) of the
Upper Permian Zechstein Supergroup (Fig. 3) which
historically was considered more of a drilling hazard
than a potential reservoir.
Until recently, investigation of the MNSH area was
hampered by a lack of modern, long oset, regional 2D
and 3D seismic data. However, in 2015, the UK Oil
and Gas Authority (OGA, now the North Sea Transition
Authority) commissioned the rst regional 2D survey
and since then numerous proprietary and multiclient
2D and 3D surveys have been acquired. Interpretation
and mapping of new seismic data, integrated with
knowledge from the re-evaluation of historical
wells and new discoveries, has revealed the extent,
palaeogeography and architecture of Hauptdolomit
carbonate platforms across the region. Key events in
the development of the Hauptdolomit play in the area
occurred in 2019 with the Ossian oil and gas discovery
in quadrant 42, and in 2022 / 2023 with the oil and
gas discovery reported from the Pensacola prospect
in quadrant 41 (locations in Fig. 4). These results
have brought the MNSH into focus as an emerging
hydrocarbon province, and the economic potential of
the Hauptdolomit play in this area is now beginning
to be understood.
The MNSH is located outside the gas-mature
kitchen of Westphalian coaly source rocks which are
prolic in the informally-named Southern Gas Basin
to the south of the MNSH (Fig. 1) (Monaghan et al.,
2017; Doornenbal and Stevenson, 2010). However,
well data have highlighted the presence of eective
hydrocarbon source rocks both within the Zechstein
Supergroup and in the Lower Carboniferous with a
multi-phase charge scenario becoming evident.
Zechstein source rocks are associated with
organic-rich levels within lower slope and basinal
“Stinkdolomit” facies of the Hauptdolomit together
with a contribution from the underlying Zechsteinkalk
Formation and the Kupferschiefer shale. These source
rocks are present on the MNSH and in the immediately
surrounding areas. They have been modelled and typed
to hydrocarbons in the recent discoveries and have been
shown to be suitable for both oil and gas generation
over a wide area. Recent discoveries and shows in
legacy wells in the Hauptdolomit demonstrate the
mobility of hydrocarbons which have migrated from
the mature source kitchens located to the south of the
MNSH, as well as demonstrating eective migration
fairways on a local to sub-regional scale.
Additional plays proposed for the MNSH include
Lower Carboniferous channel and shoreface sandstone
Fig. 1. Location map of NE England and adjacent parts of the Southern North Sea. The study area for this paper
(purple box) covers an area of about 31,000 km2 over the Mid North Sea High and extends from the English
coastline to the UK Continental Shelf boundary in the east. Cygnus and Breagh are the closest producing gas
developments; other gas elds in the Silverpit Basin to the south of the study area are labelled. Licence blocks
(correct as of May 2023) are shown in light grey. Orange shading shows the area of Fig. 10; the prole line of the
roughly north-south oriented seismic section in Fig. 5 is marked by the red line.
Gas
(MSL)
Focus Area
Limit of United Kingdom EEZ
Hull
Middlesbrough
Newcastle
55° North
Sunderland
Berwick-upon-Tweed
Licence
P2331
Licence
P2487
Licence
P2327
Licence
P2559
Licence
P2252
Licence
P2332
Licence
P2556
Licence
P2500
Licence
P1630
Licence
P2558
Licence
P2428
Licence
P2567 Licence
P2429
Dogger
Bank
Cleveland
Basin
Stainmore
Trough
Northumberland
Trough
Mid North Sea High
Southern
Gas Basin
Elbow Spit
High
Netherlands Offshore Area
Silver
Pitt
Basin
Flamborough Head
Fault Zone
Auk-Flora Ridge
Context Map
Vale of Pickering
Fields
Breagh Gas
Field
Cygnus Gas
Field
Licence
P2329
Licence
P2486
Licence
P2427
Licence
P2557
Bathymetry
Focus area
Exploration and
Production Licences
May 2023
Legend
Hydrocarbon Fields
Figure 5 Location Line
Figure 10
Focus Area
3
Browning-Stamp et al.
reservoirs in the Viséan Yoredale and Scremerston
Formations, which are proven to be eective at the
Breagh gas field located in the southern MNSH
(location in Fig. 1; Nwachukwu et al., 2020; Booth
et al., 2020), and the high net:gross Fell Formation
sandstones. Middle Devonian potential in the MNSH is
represented by thick, amalgamated porous sandstones
in the Buchan Formation and in karstied limestones
of the Kyle Group (Brackenridge et al., 2020; Kearsey
et al., 2016). Although less studied, there is potential
for billions of barrels of oil in-place in large structures
which have been revealed by modern seismic data.
The MNSH area was long considered a frontier
province with limited prospectivity but now represents
an exploration “hot spot”. It is a shallow-water area
suitable for relatively inexpensive jack-up drilling, and
the Hauptdolomit and Carboniferous reservoir targets
are in general at depths of less than 3000 m TVDSS.
Future discoveries could be produced as stand-alone
developments but are within tie-back range of existing
gas production hub platforms to the south. Whilst the
MNSH may represent one of the nal regions to be
explored for conventional hydrocarbons in UK waters,
synergies with new-era geoenergy projects position the
MNSH also to play an important and multi-faceted role
in the energy transition.
An iterative, regional, and integrated data-
led approach is required to fully understand the
Hauptdolomit play in the MNSH but has not been
possible in the past due to a lack of high quality 3D
seismic data and modern well data interpretations.
This paper attempts to present a multi-disciplinary
study of the Hauptdolomit play in a study area in
the south of the Mid North Sea High (Fig. 1), and
in particular investigates a Hauptdolomit carbonate
platform referred to here as the Orchard Platform
(Fig. 2). The study area spans the southern half of UK
oshore quadrants 34 to 39 and the northern portion of
quadrants 40 to 44 (Fig. 1), comprising about 31,000
km² or the equivalent of 4.7 quadrants. The main
ndings of the study can be extrapolated to the Dogger
Bank/Dogger High, Elbow Spit High and the southern
limits of the Auk Flora Ridge (locations in Fig.1). The
northern half of the MNSH was also studied but lacks
good well control, and Hauptdolomit platforms there
are therefore more challenging to map.
REGIONAL GEOLOGICAL SETTING OF
THE MID NORTH SEA HIGH
Tectonic Context
The Mid North Sea High was rst named by Rhys et al.
(1974) and Kent (1975). These authors utilised limited
well data and short oset 2D seismic to investigate the
highly complex geological history of the region. In the
Late Permian, the MNSH represented the western part
Fig. 2. Generalized palaeogeographic reconstruction of the Northern and Southern Permian Basins and
adjacent areas showing the distribution of Z2 Hauptdolomit platform carbonate along the basin margins and
over the Mid North Sea – Ringkøbing-Fyn Highs. The study area over the Orchard Platform in the southern
MSNH is marked by the purple box. Onshore hydrocarbon elds in the UK and in Poland/ Germany are shown
(see text for details). Regional mapping is based on the work carried out by Horizon Energy including in the
UK, the Netherlands and Germany, with additional information from Grant et al., 2019.
Vale of Pickering
Fields
West Newton
Blythe
Schoonebeek
Coevorden Guhlen
Netherlands
UK
Germany
Poland
Denmark
Norway
Southern Permian Basin
North Permian Basin
Central Graben Rift
Sulecin
BMB LMG
South Western
Attached
Platform
Orchard Platform
Ringkøbing
High
Texel
High Wielkopolska
Platform
Onshore UK
Attached
Platform
North East
Poland
Attached
Platform
Kamen
Pomorski
Miedzyzdoje
Herringsdorf
Bansin
Krummin
Fennoscandian
Shield
Focus Area
Hydrocarbon Fields
Focus Area
O/G
Slope / Interior shallow basin
Sabkha - Back Platform
Platform Carbonate Facies
Terrestrial - continental
Reduced or absent Z4-Z5
cycles (Orchard Platform)
Legend
Oil Gas
4Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
Fig. 3. Schematic diagram showing the depositional geometry of Zechstein carbonate-evaporite cycles along an approximate SW-NE section across the study area in
the NW margin of the Southern Permian Basin. The diagram illustrates the shelf-slope-basin geometry of the Z1-Z3 cycles. Zechstein stratigraphy and nomenclature
are after Peryt et al. (2010: see text for details). Note that the Z2 Hauptdolomit Formation (“ Hauptdolomit”), the focus of this paper, passes from relatively coarse-
grained platform-top carbonates through slope facies into the lower slope and basinal “Stinkdolomit” which includes laminated organic-rich intervals with source rock
potential.
Carboniferous - Devonian Kupferscheifer
Werra Carbonate
Werra
Salts
Basalanhydrit
Stassfurt Halite
Leine Halite
Roter Salzton
Pegmatiteanhydrit
Upper Werra
Anhydrite
Hauptdolomit
Stinkdolomit/Stinkschiefer
Lower Werra
Anhydrite
Zechsteinkalk
Stassfurt Halite
Toe of Slope
Platform and Isolated Reefs
Werraanhydrit
Plattendolomit
Dissolution
Effect
Hauptanhydrit
Rotliegends
Aller Halite
Major Errosive
Surface
Platform Basin
Triassic - Cretaceous
Overburden
PolyHalite
Wedge
Top Z5
Top Z4
Top Z3
Top Z2
Top Z1
Wuchiapingian
Upper P ermian - Zechstein
Changhs
257 Ma
256 Ma
255 Ma
254 Ma
Z5
Z4
Z3
Z2
Z1
50-150km
Under/Over Burden
Kupferscheifer
BPU - Kupferschieifer
Siliciclastics and Evaporites
Rotliegend Sandstone
Halites
Anhydrites
Carbonates Platformal/Basinal
System Tracts; TST - transgressive; HST - highstand; FSST - falling stage; LSW - lowstand wedge; LST - lowstand
TST
TST
TST
TST
TST
HST
HST
HST
HST
HST
HST
FSST
FSST
FSST
FSST LSW
LSW
LST
LSW
LST
LST
TST
LST/TST
Polyhalites
5
Browning-Stamp et al.
of an east-west trending chain of pre-Permian massifs
that separated the Southern and Northern Permian
Basins and which was punctuated by embayments,
seaways and intrashelf basins (Glennie et al., 2003;
Peryt et al., 2010) (Fig. 2). Basement mapping of
the Mid North Sea High area (Frogtech Geoscience,
2016) identified six basement terranes associated
with the collision of Avalonia, Baltica and Laurentia
during the Caledonian orogeny, and the study area
was part of the Avalonian Leinster–Lakesman terrane.
Several granitic intrusions have been interpreted here
from legacy seismic and potential eld data and are
associated with volcanic activity during closure of the
Iapetus Ocean during the Silurian to Early Devonian.
However, modern deep-imaging seismic data raise the
possibility that some of these “plutons” could in fact
be rotated fault blocks of crystalline metasediments;
indeed the only such feature to have been penetrated
by drilling (Exxon-operated Corbenic well 37/25-01)
encountered quartzites at a depth of 2312 m TVDSS.
A complex of post-orogenic extensional basins
formed in association with opening of the Rheic Ocean
to the south of the study area. Signicant volumes
of sediments were transported southwards from the
eroding Fennoscandian Caledonide hinterland, burying
the Palaeozoic core of the MNSH and covering the
area with thick successions of Devonian and Lower
Carboniferous deposits (Booth et al., 2020, Leeder
and Hardman, 1990). Depositional environments
varied in time and space, largely in response to eustatic
and climatic changes, ranging from oshore marine
to coastal plain, alluvial, lacustrine and arid-zone
continental with dune elds and sabkhas (Kearsey et
al., 2018, Booth et al., 2020).
During the Givetian, transgressive shallow-marine
sandy limestones were locally deposited around the
future MNSH. The Kyle Group consists of one or
two partly to fully dolomitized limestone intervals
with a shallow-water, open-marine fauna of crinoids
and brachiopods (e.g. at well 37/12-1) plus corals and
bryozoans (well 38/03-1) interbedded with shales.
The variable overlying succession includes red-brown
mudrocks with anhydrite, dolomites, siltstones,
carbonaceous shales and shales with marine fossils. In
the Frasnian – Famennian, the MNSH area was part of
a silty-sandy continental sabkha – playa environment
cut by braided and low-sinuosity river channels that
deposited thick bars, dunes and sheetoods of coarse
sand and gravel of the Buchan Formation (Marshall
and Hewett, 2003). These grade up into interbedded
sandstones and shales of the alluvial-uvial Tayport
and (Tournaisian) Cementstone Formations which
were deposited in relatively less arid conditions.
A change in the regional stress regime during the
Carboniferous led to the development of transtensional
and extensional basins on reactivated Caledonian
Iapetus and Tornquist structural trends, with continued
sediment supply via long-lived southward-draining
fluvio-deltaic systems (Glennie and Underhill,
1998; Monaghan et al., 2017). The MNSH area was
Fig. 4. Hydrocarbon shows and discoveries within the Hauptdolomit in the study area across the Orchard
Platform (the extent of the platform is shown by grey shading). Numerous oil and gas shows occur in the
Hauptdolomit and vary from minor shows and dead oil through to major shows and discoveries. Onshore
discoveries and elds producing from the Hauptdolomit are shown for reference together with offshore
discoveries.
6Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
covered by an alluvial to deltaic coastal plain with
sporadic marine transgressions, leading to deposition
of heterolithic successions of channel and mouth bar
/ shoreface sandstones, shales, coals, rare lacustrine
oil shales, and laterally extensive shallow-marine
limestones. A transgressive-regressive trend, as
sediment supply outpaced accommodation space,
led to deposition of the Viséan Fell, Scremerston and
Yoredale and Namurian Millstone Grit Formations
(Monaghan et al., 2017).
During the Westphalian – Stephanian, a broadly
north-south compressional regime became established,
likely associated with the far-eld impact of closure
of the Rheic Ocean and the subsequent Variscan
orogeny (Glennie and Underhill, 1998). Uplift and
inversion produced the MNSH with its core of faulted
and erosionally truncated anticlines and created the
prominent base-Permian unconformity (BPU). The
overstepped and up-warped Lower Carboniferous and
Devonian succession was progressively subaerially
exposed, oxidised and eroded during this event. The
BPU represents up to 80 Ma of time and 100s – 1000s
of metres of missing section; yet when mapped on
regional 3D seismic data, it appears as an irregular
rather than a peneplaned surface (Brackenridge et al.,
2020). Its topography was important for later Zechstein
deposition, including that of the Hauptdolomit. To the
east, similar Variscan upfaulted and folded strata core
the Elbow Spit and Ringkøbing-Fyn Highs which
dene the 800 km long boundary between the Southern
and Northern Permian Basins (Fig. 2), and through
which there were intervening connections such as the
proposed “Jenyon Gap” (Jenyon et al., 1984).
Post-orogenic collapse in the Early Permian
was interspersed with episodic volcanism in the
Southern North Sea and adjacent areas. During this
time the study area was located within Pangea, and
intracontinental deposition in areas anking the MNSH
uplands initially occurred in erg margins, desert wadis
and playa lakes. In the Silver Pit Basin to the south,
equivalent deposits are represented by Rotliegend
Group aeolian-fluvial sandstones and siltstones
(Roscher and Schneider, 2006). These deposits pinch
out on the anks of the MNSH and nowhere directly
underlie the Hauptdolomit.
Zechstein Deposition
The Zechstein Supergroup was deposited within an
interval of 2.8-3.5 Ma in the last 5 to 7 Ma of the
Permian (Wuchiapingian – Changhsingian) (Peryt et
al., 2010, Szurlies, 2013). The Zechstein deposition
occurred following rapid marine inundation of the
Late Permian intracontinental Northern and Southern
Permian Basins, most likely from the Boreal Sea to the
north; ooding was related to post-orogenic collapse
and eustatic rise as a result of polar deglaciation. The
relief of the denuded Early Permian surface of the
MNSH and surrounding wadi basins was preserved
in the palaeobathymetry of the Zechstein Sea, with
fully-marine conditions re-established for the rst time
since the Early Carboniferous (Ziegler, 1990; Taylor,
1998; Glennie et al., 2003). The MNSH massif was
partially and intermittently covered by a shallow sea
with deeper-marine environments in the surrounding
areas (Patruno et al., 2018; Brackenridge et al., 2020).
The geographically restricted area of the Zechstein
Sea, the lack of nearby emergent highlands and the
persistently arid climatic conditions were factors
which resulted in the deposition of both carbonates and
evaporites in the Zechstein Supergroup together with
subordinate clays. In the UK, the Supergroup consists
of four depositional cycles related to progressive
evaporation and subsequent marine ooding (Z1 to Z4:
Fig. 3). Three thinner clastic-evaporite cycles (Z5-Z7)
are limited to the German and Polish Trough areas
(Slowakiewicz et al., 2018; Doornenbal and Stevenson,
2010). Cyclical deposition of shales, anhydrites,
limestones / dolomites, halite and K-salts resulted
from a combination of hydrodynamic restriction and
hypersalinisation against a background of regional
post-rift subsidence, post-glacial eustatic sea-level
uctuations and marine replenishment in a hot, arid
palaeoclimate. Sequence stratigraphic interpretation of
the Zechstein typically recognises a distinct cyclicity
of onlapping transgressive shales, late transgressive
to late highstand carbonate platforms, lowstand
basinward-prograding anhydrite wedges, and lowstand
salts filling residual accommodation space, with
potash salts and polyhalites representing the highest
degree of evaporation (Tucker, 1991) (Fig. 3). Local
variations are in general due to the inuence of subtle
syn-sedimentary tectonics and/or palaeotopography on
the development and lling of accommodation space,
and to spatial variations in shallow-water evaporite
facies and mineralogy during major drawdown events.
Formation names for the Zechstein Supergroup vary
by country and region, and in the UK between oshore
and onshore areas (e.g. Tucker, 1991; see also Fyfe
and Underhill, 2023 this issue). Local names have
been standardised in this paper and the most widely-
accepted formation names are used (c.f. Peryt et al.,
2010) (Fig. 3).
The basal-Zechstein Z1 cycle commences with the
Kupferschiefer, a 1-2m thick, widespread organic-rich
shale that rests directly on the BPU (Fig. 3). Subsequent
deposition evolved from a ramp-style carbonate
platform to a more shelf-like platform in places, where
reefs locally developed. These reefal deposits of the
Zechsteinkalk Formation are well developed and
clearly mappable across the study area. Equivalent
deposits are present onshore in NE England (where
they are known as the Cadeby Formation: e.g. Fyfe
7
Browning-Stamp et al.
and Underhill, 2023, this issue), and also oshore in
the Southern North Sea for example at the Hewett
eld where they are the primary Zechstein reservoir
(Hook, 2020). On the MNSH, the Werraanhydrit
Formation overlies the Zechsteinkalk forming thick
anhydrite deposits (up to ~186 m in thickness – as
seen in well 44/02-01) that prograde basinwards from
the BPU faulted palaeo-highs (and also from the
basin margins). Well data suggest the Werraanhydrit
is lithologically uniform albeit with sporadic minor
shales and dolomites locally recorded as well as halite
pods (salt lakes on the sulphate platform) recorded
from areas south of the MNSH (Grant et al., 2019).
No other halites are recorded in the Z1 cycle.
The Z2 cycle commences with the Hauptdolomit
Formation and its basinal, hemipelagic organic-
rich correlative which is colloquially termed the
“Stinkdolomit” or “Stinkschiefer” facies (Fig. 3),
(Raven et al., 2018). Hauptdolomit carbonate platforms
developed coevally around the margins of the Southern
Permian Basin and in the intra-basinal MNSH,
mantling the Werraanhydrit banks and inheriting
their steep slopes that define the most important
facies belts in the carbonates. Because the anhydrites
overstepped the underlying Zechsteinkalk sequences,
the Hauptdolomit extends further out into the basin
than the earlier Z1 carbonates. The physiography of the
MNSH meant that the Hauptdolomit platform in this
area was quasi-isolated (i.e. with deep water seaways
to both north and south) rather than being land-attached
as it was on the eastern margin of the Southern Permian
Basin and onshore in the UK as well as in Poland and
Germany (Doornenbal et al., 2010). However, the
Hauptdolomit platform carbonates lack the diverse
fossil assemblages and reefs of the Zechsteinkalk,
indicating that the Z2 transgression did not occur with
fully open-marine conditions and that salinities were
possibly higher.
Seasonal winds and tidal currents may have
inuenced Hauptdolomit facies distributions. Based
on compiled observations from Rotliegend Formation
palaeodunes, Kiersnowski (2013) proposed a subsidiary
southeasterly monsoon-type wind direction in the
Southern Permian Basin. The Hauptdolomit platform
was likely located oblique to the prevailing winds,
with its southern margin in a leeward context to the
dominant northeasterly trade winds. By analogy to the
present-day Bahama Banks, tidal currents may have
been focused in embayments and across the intervening
open platform top.
Previous models have postulated the concept
of discrete deep-water “seaways” breaching the
Hauptdolomit platform area and providing direct
connection between the North and South Permian
Basins. Regional mapping on the recent 3D seismic
data negates this, at least across quadrants 36 / 42 to
38 / 44. The lack of seaways would also have focused
wave energy and tidal currents over the platform top
with positive implications for oolite shoal deposition
around the Orchard Platform margins (Fig. 2). This is
dicult to test owing to a paucity of well control on
the eastern and NE platform margins but is supported
by the presence of oolites in wells 44/02-1 and 38/29-1.
It should be noted, however, that there are alternative
interpretations of this data (e.g. Garland et al., 2023
this issue).
The Hauptdolomit is the primary reservoir
interval on the MNSH both in platform margin and
platform interior facies, and potentially also where
these deposits have been reworked into slope apron
wedges (see below). It is onlapped and overlain by
the Basalanhydrit Formation (up to 130 m thick) and,
in places, subsequently by the Z3 Stassfurt Halite (up
to 300 m thick) (Fig. 3), which together constitute
regional seals to the underlying reservoirs.
The base of the Z3 cycle is marked by the
transgressive Grauer Salzton calcareous shales which
are overlain directly by the Plattendolomit Formation
platform carbonates. These carbonates extend further
out into the basin than those in the Z2 Hauptdolomit.
With limited core and cuttings data, the Plattendolomit
has been interpreted to contain low energy, muddy
peritidal (sabkha) to subtidal inner platform facies (e.g.
wells 36/13-1, 36/15-1, 38/29-1) with moderate energy,
ne-grained backshoal facies in well 38/25-1, possibly
deposited close to a windward margin.
The reservoir characteristics of the Plattendolomit
on the MNSH are poor, with the exception of the
northernmost sectors of the Orchard inner platform
(wells 36/13-1 and 36/15-1). Where the formation is
productive in the Southern North Sea (e.g. Wissey and
Hewett elds, oshore UK), reservoir properties are
enhanced by the development of secondary porosity
and fault-associated fracturing (Duguid and Underhill,
2010; Hook, 2020). On the MNSH, hydrocarbon
charging is restricted by the underlying evaporites
which provide a barrier to hydrocarbon migration into
the Plattendolomit from deeper source rocks.
On seismic data, the Plattendolomit in the MNSH
uniquely displays a reticulate morphology of ridges
and depressions up to ~40 m deep and up to 5 km wide
(Patruno et al., 2017; Horizon Energy, unpublished
report). These features have been interpreted as the
result of extensive karstication of the Plattendolomit
during the drop in sea level that led to the initial
deposition of the overlying evaporites of the Leine
Halite Formation (Peryt et al., 2010). An alternative
interpretation considers salt mobilization aecting
the underlying Strassfurt Halite and causing the
collapse of Plattendolomit platform blocks (Peryt
et al., 2010; Patruno et al., 2017). The prominent
rugosity of the Plattendolomit does not aect the
8Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
underlying Hauptdolomit which is characterised by
laterally continuous platform sequences in the Orchard
Platform, as dened by the analysis of seismic data
(Fig. 5, Fig. 6).
Meteoric diagenesis within the Plattendolomit
may locally have inuenced reservoir quality in the
underlying Hauptdolomit; however this cannot be
conclusively established at present, given the limited
core data and petrographic analyses. Across most of
the MNSH and basin areas, the Z3 cycle is completed
by thick Leine Formation halites with local potash and
polyhalite salts in the uppermost interval.
Z4 cycle deposits are thin, ne-grained siliciclastic-
evaporite (sabkha-type) alternations that are poorly
developed in the UK region. These lack carbonates
and have no reservoir potential. Additional halite and
anhydrite deposits of the Aller Group complete the
Zechstein in the Southern Permian Basin.
Post Zechstein Deposition
Renewed extensional tectonics in the Triassic resulted
in the development of the NW-SE trending Dowsing
fault zones in the Southern North Sea, which are in
general associated with the SW margins of the main
“salt” basin and with additional subsidence in the
Sole Pit area (Pharaoh et al., 2010). Based on regional
correlations, preserved Triassic deposits overlying the
MNSH are likely dominated by shales and sandstones
with local evaporites and include sabkha, playa, uvio-
lacustrine and oodplain deposits. A latest Triassic
marine transgression probably resulted in widespread
mud deposition across the eastern UK onshore and the
adjacent oshore region (Doornenbal and Stevenson,
2010).
In the Middle Jurassic, mantle plume driven thermal
doming caused inversion and volcanism in the central
North Sea (Underhill and Partington, 1993) with
subsequent major episodes of Late Jurassic and Early
Cretaceous rifting and subsidence along the Central
Graben – Viking Graben – Moray Firth axes. Central
Graben extension separated the MNSH and Elbow Spit
High from the Rinkøbing–Fyn High and reactivated
existing structures or created new faults in the Mesozoic
cover across the study area. Variable amounts of uplift
and erosion occurred on the MNSH creating the mid-
Jurassic Mid-Cimmerian unconformity (Brackenridge
et al., 2020; Underhill and Partington, 1993). Jurassic
strata appear to be cut by the Mid-Cimmerian
Unconformity in most places, including the Ossian area
(quadrant 42). In the northern sectors of the Orchard
Platform (within quadrants 36 and 38), well data has
shown upper Zechstein evaporites have been eroded
and/or subaerially dissolved. This likely occurred
during the Early Jurassic uplift that also caused the
removal of the Triassic overburden and would have
thus permitted groundwater penetration (Mulholand
et al., 2018).
Triassic and Jurassic extension, whilst responsible
for substantial halokinesis and diapirism in the basinal
areas to the south of the MNSH, had less impact on the
Fig. 5. Regional roughly north-south oriented seismic section (above, main panel) crossing the centre of the
study area (see line location in Fig. 1). The Z2 Hauptdolomit carbonate platform in this part of the southern
Mid North Sea High is referred to in this paper as the Orchard Platform. Below, Panel 1 is a zoomed section
showing changes in the seismic character of the Hauptdolomit from basinal through slope to platform-top
facies (note this is shown on full stack data without de-tuning). Panels 2 and 3 show identical sections on EEI
(Extended Elastic Impedance) and full stack seismic data, with EEI interpretation draped onto the full stack
data. Note the zero-crossing interpretation and the continuity of the pick in the EEI volume.
Yarlington Mill Fault
South North
1s
2s
60kms
Base Cretaceous Unconformity
Muschelkalk
Zechstein
Plattendolomit
Hauptdolomit
Base Permian Unconformity
Basinal Slope Platform Margin Platform Interior Platform Interior
Platform Interior Low / ‘Salt Filled Lows’
Kyle Limestone Formation
TWT in Seconds
Regional Hauptdolomit Facies
Panel 3 - EEI Picks Plotted on Full Stack
Plattendolomit
Plattendolomit
Base Permian
Unconfomity Base Permian Unconformity
Hauptdolomit EEI Pick
Hauptdolomit EEI Pick
Basalanhydrit EEI Pick
Werraanhydrit EEI Pick
Basalanhydrit EEI Pick
Werraanhydrit EEI Pick
Soft
Hard
Panel 2 - EEI Seismic Volume Used to Define Hauptdolomit
Panel 1 - Zoomed Seismic Character Change Example
Base Permian
Unconformity
Plattendolomit
Halite
Wedge
Basinal Haupt
Slope
Platform Hauptdolomit
Main Panel - Regional Full Stack PSDM (Scaled to Time) - MNSH Prime 3D 2021
Scremerston Formation
9
Browning-Stamp et al.
thinner evaporite deposits in the study area. Thickness
variations and salt “pillows” seen in 3D seismic
likely reect minor amounts of salt mobility and local
withdrawal (Brackenridge et al., 2020).
The Early to Middle Cretaceous was characterised
by thermal subsidence with the widespread deposition
of oshore marine marls of the Cromer Knoll Group
followed by pelagic microfossil limestones of the
Chalk Group (Doornenbal and Stevenson, 2010).
Compression, uplift and fault reactivation from the
Late Cretaceous to the middle Cenozoic is commonly
recorded across the MNSH area and was associated
with the Alpine orogeny and North Atlantic opening.
An important east-west tilting event and associated
erosion took place in the middle-late Miocene, with the
westernmost boundary of quadrant 36 approximately
at the fulcrum of the tilting. Within the study area,
this event resulted in signicant uplift to the west
with extensive erosion at the seabed and a progressive
subcropping of Mesozoic strata westwards (Fig. 7),
(Brackenridge et al., 2020).
Dierent geodynamic models have been proposed
to explain the tilting: magmatic underplating of the
crust due the interaction of the Atlantic seafloor
spreading with the Iceland plume (Jones et al., 2002);
asthenosphere diapirism following the eastward
advance of the Icelandic plume towards Europe
(Rohrman and Van Der Beek, 1996); and crustal
compression from plate boundary forces (Hillis et al.,
2008). The tilting is thought to be associated with a
highly diachronous erosional surface: this event is often
referred to as the Mid Tertiary Unconformity (MTU).
with erosion taking place from the early Oligocene
to the late Miocene (Brackenridge et al., 2020).
Biostratigraphic data within the study area suggest that
the main erosional event took place in the middle-late
Miocene and the unconformity is therefore referred
to here as the Mid-Miocene Unconformity (MMU).
Subsidence followed by uplift and erosion resulted
in enhanced fracturing and diagenetic modication of
Zechstein carbonate facies leading to an improvement
in reservoir quality. To the east of quadrants 36-42,
burial continued through the Plio-Pleistocene with
progradation of sediments from the east (Fig. 7).
This regional tilting event had a signicant eect on
the petroleum system of the MNSH, including the
migration and charge histories.
EXPLORATION HISTORY OF THE
HAUPTDPLOMIT PLAY
1. Southern Permian Basin
The Hauptdolomit represents one of the oldest
hydrocarbon exploration targets in Europe. In the
UK onshore, hydrocarbons were encountered in the
formation during early potash working around 1890;
Fig. 6. Generalised palaeogeographic map of the study area showing variations in Hauptdolomit facies. Note the
attached carbonate platform area, the interior shallow basin to the north of the study area, and the main Jazz
and Orchard carbonate platforms. Question marks denote areas of poor understanding/mapping. Gas elds
are shown for context. Key wells of the Hauptdolomit platform are highlighted and named. The areas with
blue cross-hatching to the north and east of the main Orchard Platform denote zones with a thin Zechstein
Supergroup likely due to the absence of the Z4 to Z5 cycles as result of uplift and erosion from the Mid-
Cimmerian unconformity event.
10 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
in 1938, the Eskdale-2 well spudded by D’Arcy
Exploration resulted in the rst Hauptdolomit discovery
in the UK (Haarho et al., 2018). In the Netherlands,
the Slochteren-01 well was drilled in 1959 to test the
Hauptdolomit; it discovered the giant Groningen eld
which produces from the Rotliegend Formation.
Across the Southern Permian Basin, hydrocarbons
have been proven in, and are commercially extracted in
signicant quantities from, the Zechstein Supergroup.
Eective reservoirs are present in reefal carbonates of
the Z1 Zechsteinkalk Formation, rarely in the oolitic
shoal carbonates of the Z3 Plattendolomit Formation,
but most commonly in platform margin shoal, platform
top and occasionally toe-of-slope deposits in the Z2
Hauptdolomit Formation. Fields producing from the
Hauptdolomit are located onshore UK, onshore and
oshore the Netherlands, and onshore Germany and
Poland (Fig. 2); minor production occurs onshore
Denmark and also takes place (at present) in the
oshore UK (see Peryt et al., 2010).
Substantial production occurs in Poland where the
Hauptdolomit (locally known as the Main Dolomite)
has been targeted with 3D seismic since the early
1990s. The Wielkopolska Platform in SW Poland is
composed of numerous peninsulas and isolated micro-
platforms that protrude northwest into the basin from
the Polish margin (Kosakowski and Krajewski, 2014;
Zdanowski and Solarski, 2018). This area represents
a good analogue for the MNSH with many similar
features despite the platform being land-attached.
The area contains multiple producing elds such as
Barnówko – Mostno – Buszewo (BMB) (location in
Fig. 2).
The BMB complex represents the largest oil
accumulation in Poland and the fields cover an
approximate area of 32.5 km². They were initially
thought to be three separate structures: the Mostno gas
pool, the Barnówko gas accumulation at the top of the
Hauptdolomit and an oil accumulation at its base, and
the Buszewo oil pool (Mamczur and Czekański, 2000).
Recoverable reserves were estimated as 95 MM st brl
and 270 Bcf (PGNiG, 2014). The oil ranges from 45°
API directly below the gas-oil contact to 39° API in the
deeper parts of the reservoir. Production started in 2000,
with oil rates peaking at approximately 10,000 stb/d
prior to gas-cap production, and gas rates increasing to
30 MM scf/d by 2014 (PGNiG, 2014). Pre-production
well test rates ranged from 31 to 110 MM scf/d for
gas producers and 329 to 1290 stb/d for oil (Gorski et
al., 1999). The Hauptdolomit was overpressured with
limited aquifer connectivity and the main recovery
mechanism is gas cap expansion. Reported recovery
factors of 15% for oil and 37% for gas are low and
probably reect early gas-cap production and a low raw
gas hydrocarbon content of 43.7%, mostly due to the
signicant presence of nitrogen with reported values
up to 52%. Other elds in the area such as Kościan,
Grotów and Lubiatów also produce signicant volumes
of hydrocarbons, and additional Z2 discoveries have
been made on trend to the west in the Lower Saxony
Basin of NE Germany and in the northern Polish basin
margin in Pomerania (Zdanowski and Solarski, 2018;
Doornenbal and Stevenson, 2010). These Polish elds
have been one of the key inspirations for exploration
in the UK oshore.
Optimal reservoir quality in the Hauptdolomit in
NE Germany and western Poland is in platform-margin
ooid-oncoid packstones to grainstones, platform-top
bioclastic packstones with mouldic porosity, and
locally in stromatolites and thrombolites (Depowski
and Peryt, 1985; Peryt et al., 2010). These carbonates
provide instructive analogues for reservoir prediction
in the Hauptdolomit on the MNSH. The pore system
is mostly interparticle with subordinate mouldic and
vuggy porosity, and fractures aid pore connectivity;
anhydrite cementation is the main process reducing
eective porosity and permeability (Słowakiewicz et
al., 2013). Very good reservoir quality is sometimes
associated with oomouldic and vuggy porosity where
platform edge grainstones were subaerially exposed or
subject to subsurface dissolution from CO2-rich uids
transmitted up faults (Mikołajewski and Słowakiewicz,
2008; Słowakiewicz et al., 2010; Peryt et al., 2010;
Kosakowski and Krajewski, 2014; Biehl et al., 2016).
Syn-depositional emergence seems to be associated
with less reservoir quality enhancement compared
to post-depositional exposure (karstication), and
fracturing is often considered as important. Isolated
carbonate platforms seem to develop better reservoir
properties than land-attached platforms, although the
reasons for this are unclear (Peryt and Dyjaczynski,
1991; Czekański et al., 2010; Mikołajewski et al.,
2019).
Limited public domain data are available for the
Polish Z2 elds. In the BMB elds, porosity ranges
from 4 to 27% and permeabilities up to 28 mD (Peryt et
al., 2010 and references therein). For the Hauptdolomit
in the Kościan eld, typical porosities are 13-15%
and permeabilities 5-25 mD. In the Lubiatow eld,
porosities of 5-31% and permeabilities of 1-136 mD
are reported (Krogulec et al., 2020). In the Grotów
eld, average porosity of 10% and permeability of
only 0.25 mD is reported from an unusual oomouldic
reservoir interpreted to have been deposited in a toe-
of-slope apron setting (Jasionowski and Zsanowski,
in Peryt et al., 2010).
In onshore NE England, the Eskdale Moor eld
was discovered by D’Arcy Exploration (now BP)
in 1938 and produced gas from the Hauptdolomit
Formation (location in Fig. 4). Other discoveries
were made nearby at Lockton in 1966 and in the
Vale of Pickering between 1985 and 1991 (Haarho
11
Fig. 7. West-east oriented regional-scale seismic section across the study area (see line location in the inset at lower left), reproduced with kind permission of TGS. Major stratigraphic boundaries are marked by coloured lines and include the top-Hauptdolomit (purple). Note the prominent Mid Miocene Unconformity (MMU). Regional tilting associated
with the MMU resulted in uplift of the Zechstein Supergroup (including the Hauptdolomit) in areas to the west and increased burial to the east. This event was coeval with hydrocarbon charging across the area. See text for detailed discussion. Wells are plotted (and projected) onto the line.
Browning-Stamp et al.
Focus Area
Hull
Middlesbrough
Newcastle
55° North
Sunderland
Berwick-upon-Tweed
Licence
P2331
Licence
P2487
Licence
P2329
Licence
P2486
Licence
P2427
Licence
P2557
Licence
P2327
Licence
P2559
Licence
P2252
Licence
P2332
Licence
P2556
Licence
P2500
Licence
P1630
Licence
P2558
Licence
P2428
Licence
P2567
Licence
P2429
Dogger
Bank
Cleveland
Basin
Stainmore
Trough
Northumberland
Trough
Mid North Sea High
Southern Gas Basin
Elbow Spit
High
Netherlands Offshore Area
Silver
Pitt
Basin
Flamborough Head
Fault Zone
Auk-Flora Ridge
Limit of United Kingdom EEZ
Focus Area
Line of Section
Part Line - ECD 1986 2D (NDR)
Arbitary Line - ION MNSH Prime 3D 2020/2021 and Legacy 2D Data. Shown with Kind Permission of TGS
West
1 S
2 S
0 S
?
?
?
?
?
Interpretation key
Permian - Base Zechstein Group
Devonian - Top Kyle Limestone
Devonian - Old Red Sandstone?
Permian - Top Z2 Hauptdolomit
Permian - Top Zechstein
Base Cretaceous Unconformity
Cretaceous - Top Chalk
Mid Miocene Unconformity
Sea Floor
Faults
Quad 40 UKCS Quad 41 UKCS Quad 42 UKCS Quad 43 UKCS Quad 44 UKCS Quad 38
41/01-01 41/05-01 41/05-02
Pensacola
42/04-01
Ossian
42/05-a
Dabinett 43/02-01 43/03-01 43/05-01 38/29-01
Mid North Sea High
Field Trip
Locations
Line Section - OGA MNSH 15 2D
Total Section Length
300 km
East
Base Cretaceous
Unconformity
Mid Miocene
Unconformity
Base Permian
Unconformity
Kyle Limestone
Formation
Top Chalk
Hauptdolomit
Zechstein
Old Red Ss?
12 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
13
Browning-Stamp et al.
et al., 2018). All these elds produced gas from the
Hauptdolomit (which is locally termed the Kirkham
Abbey Formation) from traps in discrete fault blocks.
The dominant porosity in these reservoirs is oomouldic
and is primarily related to dissolution during subaerial
exposure (Słowakiewicz et al., 2013).
To the south of Eskdale Moor, the Malton, Marishes,
Kirby Misperton and Pickering elds (locations in Fig.
4) have collectively produced about 35 Bcf of gas with
estimated ultimate reserves of 77 Bcf. The estimated
GIIP value of 148 Bcf implies a recovery factor of
52% (Third Energy Ltd, 2013). Wells produce from
Hauptdolomit grainstones and packstones, attributed
to deposition in a back-platform setting; the attached
platform is interpreted to have developed on the western
margin of the Southern Permian Basin in Z2 time (Fig.
6; Fig. 2). The reservoir section is heavily fractured
due to Paleocene activity on the Flamborough Head
fault zone to the south, providing additional secondary
porosity and permeability to aid production. However,
detrimental eects due to fracturing include the likely
uid communication between the Hauptdolomit and
the underlying Carboniferous aquifer, giving rise to a
signicant water cut on production (Harrison et al.,
2020).
Also in onshore NE England, the West Newton-1
well (location in Fig. 2) was drilled in 2013 by
Rathlin Energy in the PEDL 183 licence encountering
significant volumes of gas and condensate in the
Hauptdolomit (Kirkham Abbey) Formation. The
discovery has subsequently been appraised by two
additional wells (plus a sidetracked well) that have
tested the eld extent and reservoir productivity. A
rst production well is planned for 2023. Contingent
unrisked resources are reported as 198 Bcf from a
GIIP of 304 Bcf. Recoverable volumes are risked at
85% chance of commerciality (RPS Energy Canada
Ltd, 2022).
Oshore to the south of the MNSH, the Blythe
gas discovery well 48/22-01 (location in Fig. 2) in
the Permian Rotliegend sandstones, operated by
Independent Oil and Gas, also produced oil and
gas from the Hauptdolomit. The discovery lies in a
Hauptdolomit mid-platform location some 13 km from
the platform margin. Reservoir primary porosity is low
in this area (< 5%) due to deep burial and associated
diagenesis, secondary porosity, mostly from fault
related fractures, contributes to reservoir deliverability.
2. The Mid North Sea High
Pre-2019
A lack of well penetrations oshore UK along with
poor quality 2D seismic data hampered exploration
of the MNSH area for many decades. Seismic data
from the MNSH region has historically been sparse
with only limited 2D lines shot between 1965 and
1995, in contrast to the extensive regional 2D data that
progressively covered the adjacent Southern Gas Basin
(Fig. 8). Legacy seismic campaigns were dominated
by proprietary and generally small 2D exploration
campaigns that targeted single blocks or prospects,
with occasional long, non-regular grid, tie lines
between wells. All this 2D seismic data was acquired
with short oset congurations. Some of the older
legacy data sets were scanned and vectorised to SEG-Y,
with eld tapes lost or unavailable for reprocessing.
The MNSH area has only limited 20th century multi-
client seismic data. Regular grid 2D seismic data were
acquired by Geco Prakla (now Schlumberger) between
1979 and 1995 in several discrete surveys that together
provide fairly good regional coverage with moderate
imaging quality within the Zechstein section.
Modern 2D multiclient seismic data has more
recently been acquired over the area; for example, PGS
shot the MC2D-MNSG2009 survey in 2009 which was
the rst long-oset 2D acquired in the MNSH area.
It was followed in 2013 by a large scale (~ 4000 km)
tight-grid 2D dataset acquired by Spectrum Geo (now
TGS) (Fig. 8).
One of the most important pre-2019 seismic data
sets is the OGA 2015 MNSH 2D, a ~10,000 km regional
well tie survey funded by the OGA. Reprocessing of
this data with modern workows has signicantly
improved the imaging in the pre-Cretaceous section.
An additional well tie survey was acquired in 2017
by Geo-Partners Limited, funded by Spirit Energy.
The MNSH area has also been covered by a long-
oset, deep imaging 2D programme by GeoEx MCG.
This survey has recorded seismic data to 16 seconds,
imaging the Moho and deep-lying basinal features for
the rst time.
Discrete 3D seismic surveys were initially acquired
in the MNSH region between 1999 and 2012, but
the coverage was sparse compared to the Southern
Gas Basin where abundant 3D campaigns resulted
in extensive regional coverage between the UK
and Netherlands and large-scale merged products.
Multiple older surveys close to the MNSH were
designed to map the feather-edge of the Rotliegend
sandstone play or deeper Carboniferous anticlinal
structures. By extending these plays onto the northern
margins of the Southern Permian Basin, these surveys
encroached the southern margin of the Hauptdolomit
platform. Subsequent 3D data sets in the MNSH area
were dominated by sub-regional multi-client surveys,
with some smaller prospect-specic or part-prospect
proprietary campaigns.
The Silverpit 3D survey (~1200 km²), acquired
in 2009 by PGS in the area of block 44/02, imaged
the southern Hauptdolomit Platform margin. In 2011,
Sterling Resources commissioned the Darach 3D to
image a set of large-scale, plunging Carboniferous
14 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
anticlines truncated at the BPU, and this survey
also provided invaluable data for the overlying
Hauptdolomit platform. This survey was extended in
2013 as the PGS MC3D-SNS2013, covering parts of
quadrants 36, 37 and 42. Marginal eastern areas of
the Hauptdolomit platform (northern part of quadrant
44) were also covered by the CGG 2013 Loadstone
3D survey.
Relative to adjacent hydrocarbon provinces, the
MNSH has a very low well density. Across the ~ 31,000
km2 focus area of this study, a total of only 45 wells
have been drilled, roughly equating to one well per 690
km². The oldest of these wells spudded in 1964, and
many of the wells drilled in the area were amongst the
rst oshore wells to be drilled on the UKCS, reecting
the shallow water depths (<50 m) which allowed early
pioneers to explore with drilling technology of the
time. Older wells frequently lack key data, reports,
and comprehensive or complete log suites; however,
many were cored or have reported well test data that
provide important information for reservoir analysis.
Extracting all the useful data from these legacy wells
has therefore been a key aspect of validating and de-
risking the Hauptdolomit play on the MNSH.
Within the focus area, few wells have been drilled
specically to test a primary Hauptdolomit target.
Many wells penetrate the formation on the way to
deeper targets in the Devonian – Carboniferous
section, and some wells also explore the limits of the
Rotliegend sandstone fairway (Fig. 8). In 1969, Home
Oil drilled the 41/08-01 well, back-to-back with 36/23-
01 (location in Fig. 6) in order to explore the potential
on the MNSH for multiple reservoirs, as proven by the
Auk discovery to the north. The former well had minor
gas shows but subsequent analysis showed it to be o-
structure, whilst the latter was drilled in a Hauptdolomit
basinal location and was dry. Well 41/05-01 was
drilled by Walter UK E&P in 2004 with the objective
of testing the Z3 Plattendolomit and Z2 Hauptdolomit
as primary targets. The well was classed as a dry hole
with minor gas, and subsequent analysis again showed
the Hauptdolomit to be in a basinal setting.
The Crosgan prospect, located at the southern
margin of the MNSH (location in Fig. 4), was drilled
in 1990 by well 42/15a-2 with a Lower Carboniferous
primary objective within a structural high below
the BPU (Base Permian Unconformity). Gas was
encountered in the Viséan Yoredale and Scremerston
Formations, but the well also tested gas from a 17.5 m
net interval of Hauptdolomit with 6.4% porosity (Table
1). 3D seismic data indicate that the Hauptdolomit is in
an isolated micro-platform setting, with wells drilled
in the centre of the platform.
Oil and gas shows in the Hauptdolomit were
recorded in the 43/05-01 well drilled in 1994 by
Hamilton pursuing deeper Carboniferous targets;
subsequent analysis demonstrated no structural closure
at Z2 level.
Fig. 8. Map of the study area showing pre-2019 exploration data and the locations of all the wells drilled (colour-
coded by age); most of the wells in the northern part of the area were drilled in the late 1960s and 1970s. Key
wells are named. Available 2D seismic data (coloured lines) is colour-coded by age; the majority of this data is
pre-1990 with some later, sporadic and localised 2D shoots (yellow/white). 3D data (grey shading) is widespread
in the south of the study area but released data becomes sparser to the north.
Netherlands Offshore Area
1960's 1970's 1980's 1990's 2000's 2010's
Exploration Data Timeline
2D
Seismic
Wells
3D
Seismic
Quad 29 3D - PGS/TGS
SNS 2013 3D - PGS
Breagh 3D Merge - TGS
Lythem
3D - NDR
Breagh 3D - NDR
Crosgan 3D - NDR
Mitchell 3D - NDR
Southern North Sea
Mega-Merge (Mixed Vintage) - NDR
St98
3D - NDR
Cornerstone 3D - CGG
Silverpitt 3D - PGS
Pa09 3D - NDR
AT95 3D - NDR
Gb03 3D - NDR
Limit of United Kingdom EEZ
Focus Area
UK Onshore Seismic
Data Not Shown
Dense 1980's-2010's 2D/3D
Data Targeting
Feather Edge Play
Main Area of SGB
Data Coverage
Area of Poor 2D
Coverage and Quality
TGS SNS 2013 2D Data
38/25-01
38/24-01
38/29-01
38/18-01
38/22-01
38/16-01
37/25-01
43/05-01
44/02-01
43/03-01
43/02-01
37/23-01
36/15-01
36/13-01
36/23-01
36/26-01
41/01-01
41/08-01
41/08-02
42/10b-02
42/15a-02
42/15a-03
55° North
1960's 1970's 1980's 1990's 2000's 2010's
Exploration Data Timeline
2D
Seismic
Wells
3D
Seismic
Focus Area
Legend
Wells with Hauptdolomit
platform to slope facies
15
Browning-Stamp et al.
Post 2019
The year 2019 was important for Hauptdolomit
exploration in the MNSH area, with the rst hydrocarbon
discovery made on the carbonate platform play. One-
Dyas with partners Spirit Energy and Neptune Energy
drilled well 42/04-01 with a primary target of a highly
truncated Lower Carboniferous anticlinal structure
known as Darach (location in Fig. 9), and a secondary
objective to test the overlying Hauptdolomit with the
prospect called Ossian.
The well encountered 19° API oil and gas in the
Hauptdolomit; however, significant overpressures
meant that lost circulation materials, cement and a high
mud weight were required to stem the inow. These
well control mitigations resulted in all logging of the
reservoir being acquired after cementing the 7” liner
and with LCM contamination causing spurious values.
Notwithstanding these limitations, a detailed review
of drilling mud gas analyses carried out by Horizon
Energy has allowed an estimation of the OWC. A thin
hydrocarbon column (~ 9-10 m) is consistent with the
minor structural closure at the well location although
there is additional potential for stratigraphic trapping.
A Hauptdolomit gross thickness of approximately 65
m has been estimated from the log data by Horizon
Energy and corresponds to log characteristics in
oset wells with similarly thick and porous platform
sequences (e.g. Hauptdolomit of 59.8 m in well 43/05-
1). The operator has proposed a thinner interval (44.5
m), but this seems unlikely for the platform setting;
subsequent work by Spirit Energy has also conrmed
a thicker interval (Garland et al., 2023 this issue).
A side track (42/04-1Z) was drilled to test the
Hauptdolomit at a 50 m step-out location, again
encountering overpressure (approximately 2015-
2130 psi above the average computed hydrostatic
gradient at 2218.9m TVDSS). The Hauptdolomit
was tested with an extensive open hole section of the
overlying Zechstein and 3.3 m penetration into the
top of the reservoir. Oil samples recovered showed
moderate biodegradation that may be due to the
recovery in close proximity to the OWC. As such,
biodegradation is expected to be less severe up-dip.
Biodegradation has been identied by saturates GC
analyses which consistently show the presence of
an unresolved complex mixture (UCM) beneath the
heavier hydrocarbons (nC25+) (Bastow et al., 2019;
Core Lab, 2019). The well achieved a stabilised liquids
ow rate of approximately 800 b/d at 80-90% water
cut (ca.100 b/d oil rate) for 48 hours during clean-up
ow period; however the test was terminated after
flow conditions changed considerably with rates
peaking at 3400 b/d with salt and pebbles produced
to surface equipment. Due to gauge location about
300 m above the reservoir interval, there is signicant
uncertainty in estimating reservoir pressure and
whether depletion occurred owing to possible phase
segregation. Post-well analyses report a solution gas
Fig. 9. Map of the study area showing post-2019 exploration data. Three key wells have been drilled in the
Hauptdolomit play since 2019: Ossian, Pensacola and the Crosgan appraisal well. A single long-offset deep
imaging 2D campaign was completed in 2021 (pale green lines). Since 2019, ve 3D surveys (grey shading) have
been acquired (some merged) to give full regional 3D coverage to the bulk of the Orchard Platform, whose
outline is shown in transparent orange.
Netherlands Offshore Area
Post 2019
Pre 2019
Exploration Data Timeline
2D Seismic
Well Data
55° North
Limit of United Kingdom EEZ
Focus Area
UK Onshore Seismic
Data Not Shown
Area of Poor 2D
Coverage and Quality
Pre-2019 Data
TGS NMSH Prime 3D Phase 2 - 2021
TGS - ZC3D19
TGS MNSH
Prime
Phase 3 - 2021
TGS NMSH Prime 3D Phase 1 - 2020
MCG Regional Deep Imaging 2D - 2021
3D Seismic
Shell - Bluewater
3D - 2019
Pensecola Gas
Discovery
Well 41/05-02
Ossian Oil
Discovery
Well 41/04-01/1Z
Crosgan Gas
Appraisal
Well 42/15a-04
Focus Area
Legend
Exploration Data Timeline
Post 2019
Pre 2019
2D Seismic
Well Data
3D Seismic
16 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
ratio of 430 scf/stb and a saturation pressure equal to
the initial reservoir pressure, indicating a saturated
reservoir with oil at bubble point and the potential
for an up-dip gas cap. Thus, whilst the 42/04-1z well
conrmed the presence of hydrocarbons and reasonable
productivity, signicant uncertainties remain regarding
the evaluation of reservoir extent and properties, and
specically the existence and nature of a dual porosity
system.
The first modern well to primarily target the
Hauptdolomit was 41/05a-02 drilled in late 2022 –
early 2023 by Shell partnered with Deltic Energy
and One-Dyas (location in Fig. 9). Gas and oil were
discovered in the Hauptdolomit at a prospect known
as Pensacola, in an isolated micro-platform setting
adjacent to the SW margin of the main carbonate
platform.
Well data are yet to be released, and at the time of
writing (early 2023) most information has come from
press releases. It appears that the well was testing a
Hauptdolomit platform model involving prograding
and fringing reefs, but analysis of seismic data across
the region suggests that the wedging geometries are
linked to halite, anhydrite and polyhalite lithologies
abutting the carbonate platform slope. This was the
case where such features were previously penetrated
in the 43/02-1 well. The prograding margin model
has been supported by well-based interpretations
from Germany and from the Shoonebeek oil eld in
the Netherlands, and it has been adopted by previous
authors for the MNSH (e.g. Patruno et al., 2017)
but with little compelling well or seismic evidence.
Misinterpretations can be made where progradation is
genuinely displayed by the underlying Werraanhydrit
but the Hauptdolomit can be dicult to discriminate
on legacy seismic data.
It seems likely that the Pensacola well encountered
the Hauptdolomit in a slope location. A reported gas/oil
contact at 1755 m TVDSS shows limited conformity
to the structural closure, which is spilling to the north,
highlighting the possibility of a stratigraphic trapping
mechanism (Deltic Energy Corporate Presentation,
March 2023). A slope setting would also question
the validity of the well test in terms of hydrocarbon
volumes discovered, whilst a relatively high average
porosity reported for the Hauptdolomit (16%) may
be linked to the presence of high energy oolitic facies
redeposited on the slope. Despite the uncertainties,
the Pensacola well has proven the presence of oil and
gas reservoired in the Hauptdolomit, possibly in a
slope apron setting with oil being the dominant phase.
Alternatively, if the well is located above a bypass
zone, it could represent a micro-platform interior gas
accumulation with a minor oil leg. Multiple appraisal
wells may be needed in order to fully evaluate the
discovery.
Future drilling activities targeting the Hauptdolomit
include the appraisal of the Crosgan structure, located
in the southern margin of the MNSH. The licence is
currently operated by One-Dyas with partner Shell and
is undergoing appraisal with a 2023 well being drilled
to the south of the structure (location in Fig. 9).
Both Ossian and Pensacola were drilled on
modern 3D seismic data that has allowed improved
mapping, modelling and understanding of the
Hauptdolomit carbonate platform play and unlocking
its prospectivity. However, it should be noted that the
Zechstein in Ossian was a secondary target, and was
therefore not drilled in optimum facies.
Two 3D surveys have been instrumental in building
a regional understanding of the Hauptdolomit platform
on the MNSH (Fig. 9). A 1400 km² multiclient 3D
survey was acquired by Spectrum (now TGS) in 2019,
partly pre-funded by Horizon Energy. This was the rst
3D survey to capture the northern and southern limits
of the Z2 carbonate platform and to focus on imaging
of the Hauptdolomit as primary objective.
ION Geophysical (now TGS) acquired the rst
regional multiclient 3D in the area. The MNSH Prime
Survey was acquired over the summers of 2020 and
2021 and the combined data comprise 11,610 km²
coverage across the southern parts of quadrants 35, 36,
37, 38 and the northern areas of quadrants 41, 42, 43, 44
(Fig 9). Data was acquired with a shooting angle of 90°
using ten 7200 m streamers. Raw data was processed
through a PSDM sequence with FWI (full waveform
inversion) applied, and resultant data quality in the
Zechstein and Carboniferous is excellent.
In addition to the regional surveys, the proprietary
Blue Water 3D survey was acquired in 2020 over the
Pensacola prospect in block 41/05 on behalf of the
licence operator Shell. This survey tied the legacy 3D
giving full prospect coverage and regional context of
the Pensacola isolated carbonate platform.
HAUPTDOLOMIT INTERPRETATION
METHODOLOGY
This paper is based on a detailed regional evaluation
by Horizon Energy Partners Ltd of the Hauptdolomit
platform on the southern margin of the MNSH,
referred to here as the “Orchard Platform” (Fig. 2),
integrating all available data sets and legacy data as part
of the prospect maturation in their licences. Increased
emphasis has been placed on the southern MNSH
margin because data quantity and quality are relatively
good compared to the northern margin and multiple
geological factors coalesce to high-grade the play.
Well Data and Sample Analyses
The presence of Hauptdolomit reservoirs across the
MNSH has been unequivocally demonstrated by a
17
Browning-Stamp et al.
Well Name Operator Facies Comments
Thickness Porosity
Well Information
Year Target
Qd
Haupt Dolomite
Quad 36
Quad 37
36/13-01
36/15-01
36/23-01
36/26-01
37/23-01
37/25-01
Quad 38
38/16-01
38/18-01
38/22-01
38/24-01
38/25-0138/25-01
38/29-01
Quad 41
41/01-01
41/08-01
41/08-02
41/05-02
Quad 42
42/10b-02/2Z
42/04-01/1Z
42/15a-02
42/15a-03
42/15a-04
43/02-01
43/03-01
43/05-01
Quad 43
44/02-01
Quad 44
44/07-01
1967 Arpet Petroleum
TD
1371.6m Carboniferous
1967 1798.3m Arpet Petroleum Carb/Devonian
Home Oil
1969 1819.7m
Placid Oil Co
1970 1502.7m Rotligendes/Carb
Texaco
1970 2537.2m Rotligendes/Haupt
2009 ExxonMobil
2359.0m Devonian
Amoco
1967 2199.7m Zechstein/Carb
Arpet Petroleum
1967 2465.2m Zechstein/Carb
1967 2252.4m American Overseas Zechstein
Amoco
1983 2779.8m
Burmah
1969 2275.6m
American Overseas
1964/65 2835.9m Zech/Rot/Carb
Shell/Esso Carboniferous
2243.0m
1992
2022/23 Shell
1996.0m Haupt
Home Oil
1969 1244.2m Zechstein/Carb
Zechstein
Zechstein
Zechstein
Conoco
1994 2061.4m Haupt/Carb
2019 Carboniferous
One-Dyas
3300.0m
Mobil North Sea
4028.8m Carboniferous
1995
1990 Total Carboniferous
2659.0m
2379.9m RWE Haupt/Carb
2023
3854.8m
Unknown One-Dyas Haupt
1988/89 Amoco Carboniferous
1966 3157.7m Rotligendes/Carb
Whitehall
Hamilton Oil
1994 3614.6m Haupt/Carb
Shell
3496.1m
1965 Zech/Rot/Carb
Amoco
1967/68 3046.2m Zech/Rot/Carb
Core
Gross:67.5m / Net:66.9m
Gross:34m / Net:33.4m
Gross:48.7m / Net:48.3m
Gross:28.1m / Net:4.7m
Gross:28.0m / Net:16.8m
Section Condensed
Gross:48.7m / Net:48.3m
NTG
99%
98%
99%
17%
60%
?
99%
Gross:17.2m / Net:16.9m 98%
Gross:44.5m / Net:40.2m 90%
Gross:46.9m / Net:41.1m 88%
Gross:31.5m / Net:27.4m 87%
Gross:55.2m / Net:52.4m 95%
Gross:18.8m / Net ? ?
Av Por (Net): 22%
Av Por: 16%
Gross:65.0m / Net? Av Por ?
?
Gross:12.8m / Net: 0m
Unknown - Drilling ? Av Por ?
Gross:23.5m / Net ?
0%
<10%
Gross:59.7m / Net:38.1m 64%
Gross:14.7m / Net:9.9m 67%
Gross:25.5m / Net:5.3m 37%
Av Por (Net): 21.5%
Av Por (Net): 10%
Av Por (Net): 8.8%
Av Por (Net): 9.6%
Av Por (Net): 21.4%
Av Por ?
Av Por (Net): 14.6%
Av Por (Net): 15.1%
Av Por (Net): 16.4%
Av Por (Net): 9.2%
Av Por (Net): 9.2%
Av Por (Net): <5%
Av Por (Net): <5%
Av Por (Net): 15%
Av Por (Net): 11%
Av Por (Net): 9%
Gross:58.8m / Net:54.2m 92% Av Por (Net): 17.3%
Gross:57.0m / Net:52.8m 93% Av Por (Net): 20.1%
Gross:62.6m / Net:2.5m 4% Av Por (Net): <5%
Av Por (Net): 6.4%
Av Por (Net): 6.0%
Gross:50.8m / Net:17.5m
Gross:55.0m / Net:21.0m
34.5%
38.3%
Platform Interior ?
Toe of Slope?
Slope - Bypass Zone
Toe of Slope/Micro Plat
Slope - Bypass Zone?
Condensed Zechstein
Platform Interior - Low
Platform Interior - Shoal
Platform Interior - High?
Platform Interior - Shoal
Slope - Distal
Slope - Olistolith?
Platform Interior -Shoal
Micro Platform
Slope - Distal
Platform Interior
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
14.63m
18.28m
12.00m
3.05m
9.14m
Resedimented shoal facies
Oil shows but no structural closure
Distal slope no identified prospectivity
Distal slope no identified prospectivity
Likely bypass zone with no prospectivity
Drilling ongoing
Crosgan gas discovery
Ossian Oil/Gas Discovery
Platform Interior
Late structuration - probably breached
Pensacola Gas/Oil Discovery
High porosity reservoir - No charge
Intra platform high?- No charge
Intra platform low - No charge
High porosity reservoir - No charge
High porosity reservoir - No charge
High porosity reservoir - No charge
Condensed reservoir and no charge
Moderate porosity reservoir - No charge
Likely bypass zone no prospectivity
Likely toe of slope setting - No charge?
High porosity reservoir - Minor Shows
High porosity reservoir - No charge
Micro Platform
Micro Platform
Micro Platform
Micro Platform
Micro Platform
Micro Platform
2009
Gross:68.9m / Net:67.9m
99% Av Por (Net): 18.9%
Slope ?
Margin to slope?
Slope ?
Crosgan gas discovery
Crosgan gas discovery
Late structuration - probably breached
Late structuration - probably breached
Table 1. Table summarising key information and reservoir parameters for the 26 wells in the study area which are relevant to the Hauptdolomit play. These wells
are in Hauptdolomit platform (or micro-platform) top to slope locations, basinal wells are not discussed. Facies interpretations were derived from an integration of
petrographic analyses, petrophysics and seismic interpretations. Net reservoir has been dened using a porosity cut-off of 5%.
18 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
systematic review of data and reports from more than
50 historical exploration wells located both on and
surrounding the MNSH. This was augmented by new
advanced petrophysical interpretations where log
suites permitted, and revised formation tops have been
picked. Well data have been digitally re-mastered and
full synthetic- based seismic ties created. Test data
(where available) have been analysed to determine
reservoir properties. An integrated dry hole well
analysis exercise has been conducted for 46 key wells
in the study area at the Hauptdolomit level.
Core examination and ditch cuttings analysis
played an important role in the interpretation of
the Hauptdolomit. A total of 184 thin sections (125
cuttings samples and 59 core trims) from 11 wells
have been studied for microfacies characteristics and
interpretation of diagenetic fabrics and pore system
evolution, with core samples described in detail
and evaluated with regard to the nearest available
core plug poro-perm data. Despite severe diagenetic
modication, it has been possible to identify recurrent
microfacies, make environmental interpretations and
propose a geological model that is conditioned by well
data and informed by the latest academic research
on analogue platform carbonates from around the
Southern Permian Basin.
This work stream also focused on determining the
hierarchy of controls on eective (interconnected)
porosity and providing a sedimentological process basis
for predicting its spatial distribution. Both existing and
new core samples were used, the latter selected during
a core viewing and description exercise at the National
Geological Repository (British Geological Survey,
Keyworth, UK). Integration of sedimentological and
diagenetic observations with available wireline log
interpretations and routine core analysis results allowed
discrepancies to be assessed (for example, where
cuttings may have failed to adequately sample high-
porosity intervals that were prone to disaggregation).
Samples from lower slope / basinal wells have been
analysed for source rock characteristics, including total
organic carbon (TOC) values and generative potential
using Rock-Eval pyrolysis.
Remote Sensing and Potential Field Data
Integrated gravity and magnetic data have been used
to understand the deep geology of the study area,
the distribution of granites and other intrusives (e.g.
dykes), and the large-scale fault block geometry.
Merged satellite and surface gravity data, including
data-sets acquired with the 2015 MNSH 2D seismic
survey, were used to map the study area and identify
palaeohighs that were possible “seed areas” for
Zechstein carbonate-sulphate platforms.
Multiple proprietary oil slick studies provided
coverage of the MNSH and surrounding region, and
more than 2550 slicks have been identied from nearly
50 years of satellite data. Many of these repeated and
clustered slicks have assisted the petroleum systems
interpretation when fully integrated with seismic data
and reports from shallow seabed core samples. These
results supported hydrocarbon generation and charge
across the region and helped to identify those fault
trends associated with migration and those which are
more likely to be sealing.
Seismic Interpretation
A seismic structural and stratigraphic interpretation has
been undertaken, focusing on the MNSH but including
data from onshore UK, the Netherlands, Germany
and Poland, and oshore UK, the Netherlands and
Denmark. In total, > 130,000 line km of mixed vintage
2D data and over 40,000 km² of 3D data have been
reviewed, the interpretation progressing from legacy
2D lines through to new 3D data. Reprocessing was
performed on various key oshore 2D and 3D surveys
on and around the MNSH, increasing resolution and
imaging quality of the Zechstein and specically the
Hauptdolomit. Pre-stack depth migration (PSDM)
and FWI have proved invaluable, along with gather
conditioning and the application of various seismic
attributes including frequency decomposition blending
and various forms of seismic inversion.
Horizon Energy Partners Limited current
interpretation, completed in 2023, is predominantly
picked on modern, reprocessed and newly acquired
3D seismic data (Fig. 9). Numerous horizons have
been mapped across the MNSH and beyond, tied by
synthetic seismographs created for the wells across the
region. Horizons were picked on the nal depth-to-time
scaled full stack data, and also on regional relative
impedance and elastic impedance volumes, with a
subsequent depth conversion performed. Seismic
polarity convention is ‘North Sea Normal’ such that an
increase in acoustic impedance or “hard” reector is a
trough (red on the colour bar used) and an acoustically
“soft” reector is a peak (black). The Hauptdolomit and
Plattendolomit are generally picked on red troughs,
but this is sensitive to the local stratigraphy and tuning
eects (see below). Example picks can be seen in the
main panel of Fig. 5.
Seismic Character of the Hauptdolomit
The seismic character of the Hauptdolomit is
predominantly governed by its depth, thickness (which
impacts the amplitude spectrum), depositional setting,
and contrasts in acoustic impedance between the
overlying and underlying anhydrites (and sometimes
the Z3 and Z1 carbonates). Across the MNSH, the
Hauptdolomit occurs between 1500-2600 m TVDSS,
well within the target range of modern long oset
seismic data. However, unique imaging challenges
19
Browning-Stamp et al.
arise owing to very high velocities in halite, anhydrite
and dolomite that give a much lower resolution
than softer / slower formations would at this depth.
Additionally, the Hauptdolomit changes laterally in
thickness, gross lithology (e.g. carbonate mineralogy,
anhydrite content), porosity and potentially the
uid type contained within it. The interaction of all
these factors is compounded by the underlying and
overlying anhydrites which are of variable thickness,
giving very dierent seismic responses. If present,
complex and irregularly bedded polyhalite sections
onlapping the carbonates will add complexity owing
to their high velocities. All these factors lead to the
Hauptdolomit being a generally low amplitude,
laterally discontinuous seismic event that is typically
variable in character and challenging to map (Fig. 5,
panel 1). Interval velocities range from approximately
5000 m/sec to over 6000 m/sec.
Understanding the vertical resolution of the seismic
data is critical. For the modern 3D seismic data used
in the current interpretation, this ranges between 41 m
and 50 m at typical Hauptdolomit depth (λ/4, dependent
on velocities used and average frequency). This can
then be compared to gross vertical thicknesses of the
Hauptdolomit in wells drilled in contrasting platform
locations / facies across the region. In basinal, toe-
of-slope and slope settings, the Hauptdolomit thins
well below resolution of the seismic data giving a
constructive tuning response and a very high amplitude
trough, likely with contribution from both the overlying
and underlying evaporites. This tuning feature is
easy to map but does not pinpoint the Hauptdolomit.
In platform areas the Hauptdolomit Formation is
generally 44.5 to 67.5 m thick (derived from wells),
straddling the envelope of true resolution. Where
overlain by halite with thin intervening anhydrite, there
can be destructive tuning and amplitude reduction.
Interpretation of the top and base Hauptdolomit on full
stack data therefore has a margin of error with possible
phase rotation and the top Hauptdolomit sometimes
lying on zero crossings. This can impact well ties, depth
conversion methods and prospect denition.
To address these imaging challenges in the
Hauptdolomit, advanced geophysical quantitative
interpretation (QI) workows have been tested and
utilized. This has included systematic review of
available well VSP (Vertical Seismic Proles), sonic
and shear sonic well logs along with density log data.
Zechstein lithologies including halite and anhydrite
have displayed well dened and predictable consistent
P/S-wave, and density properties. Pre- and post-stack
seismic conditioning using angle stacks and gathers
have been a valuable tool to better map and understand
the distribution, thickness and reservoir development
of the Hauptdolomit across the study area. Application
of detuning algorithms (removal of tuning-induced
variations) to the seismic data is a key processing step
that signicantly improves results.
Extended Elastic Impedance (EEI) methodology
has been successfully applied utilising a standardised
Chi-angle rotation set and angle stack content at 40°
degree rotation. The data has been manipulated in
such a way as to alter the apparent contrast between
dierent lithologies based on their intercept/gradient
analysis, resulting in an overall positive reection
coecient on a relative impedance extraction. This
workow has provided seismic sections showing clear
contrast between the Hauptdolomit, Basalanhydrit and
Werraanhydrit Formations for the rst time (Fig. 5,
panels 2 and 3). Following the application of the EEI
methodology, a regional relative impedance volume has
been used for the seismic interpretation, substantially
improving the imaging in the Hauptdolomit section
and producing a reliable top Hauptdolomit seismic pick
across the platform area. It has also allowed mapping of
predicted (albeit uncalibrated) porosity across a section
of the Orchard Platform area (Fig. 10).
Within the 3D seismic volume used for the
evaluation, there are no wells with platform facies
available for a direct calibration between seismic data
and measured petrophysical properties. The results
obtained should be interpreted in relative terms, with
porosity values displayed in Fig. 10 being within
the range of porosity derived from oset wells with
equivalent platform facies outside of the 3D seismic
volume. Despite these limitations, the results of the
seismic quantitative interpretation work show a general
improvement in porosity towards the southern margin
of the Orchard Platform, with lower porosity values
occurring within shallow intra-platform depressions;
these areas are overlain by the Strassfurt Halite
Formation which is otherwise absent in the Platform.
HAUPTDOLOMIT PLAY ELEMENTS ON
THE MID NORTH SEA HIGH
Source Rocks
Given previous exploration efforts on the MNSH
were focused on the evaluation of the Carboniferous
petroleum system, limited geochemical data have been
acquired to assess the Zechstein source rock potential.
The available results from the MNSH area (British
Geological Survey, 2017; Horizon Energy, unpublished
geochemical data), and analogue source rock
sequences in Poland (Zdanowski and Solarski, 2018),
suggest low to moderate organic carbon enrichment
within o-platform facies of the Hauptdolomit and
Zechsteinkalk Formations. Original TOC values are
believed to be between 1 to 3% wt% with only sporadic
values up to 4-5 TOC wt%.
Despite the limited TOC values and source rock
thicknesses (likely not exceeding 3-4 m), areally
20 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
extensive source rock kitchens could potentially have
generated substantial hydrocarbon volumes. Thicker
organic-rich Hauptdolomit source rocks may be present
in undrilled base-of-slope facies of the Hauptdolomit
in the MNSH, a possibility suggested by geochemical
data from analogue Z2 carbonates in Germany and
Poland (Slowakiewicz et al., 2013; Zdanowski and
Solarski, 2018). Based on geochemical analyses of
39 oil occurrences across the eastern and southern
sector of the Southern Permian basin, Slowakiewicz
et al. (2018) identified predominantly algal-rich,
marly carbonate Hauptdolomit source rocks located
in lower slope facies and also in shallower-water
restricted lagoonal facies. Zdanowski and Solarski
(2018) highlighted the presence of oil-prone Type II
kerogen with calculated original TOC values ranging
from 1 wt% to about 5 wt% in slope sequences of
the Zechstein Z2 carbonate platform in the Gorzów
Wielkopolski–Międzychód–Lubiatów area.
The eectiveness of Zechstein source rocks in the
MNSH area has been conrmed by recent geochemical
analyses of oil samples (19o API) from the Ossian
discovery (Bastow et al., 2019; Core Lab, 2019). The
presence of gammacene, Pr/Ph ratios below 1, elevated
relative abundance of C34 and C35 hopanes and other
characteristic biomarker parameters such as Ts/Tm<1
indicate a carbonate-evaporitic source rock setting.
Within the Southern Permian Basin, such a depositional
environment is typically associated with source rocks
of Late Permian age in the Zechstein Supergroup
(Slowakiewicz et al., 2020). By contrast, hydrocarbons
generated by the underlying Lower Carboniferous
shales and coals are always related to marine
siliciclastic, terrestrial or lacustrine organofacies (Vane
et al., 2015; Slowakiewicz et al., 2020).
Additional geochemical analyses funded by
Horizon Energy Partners Limited have pointed to a
characteristic carbonate source rock signature which
can be assigned to lower slope sequences of the
Hauptdolomit by comparison with biomarker data
from Polish and German Zechstein oils (Slowakiewicz
et al., 2018).
The basal Z1 Kupferschiefer Formation shale is
characterized by excellent source rock properties with
a distinctive oil-prone marine/lacustrine organofacies
with high TOC values (generally from 8 to 20 wt%) and
high present-day hydrogen index values (between 400
and 700 mg HC/gTOC) (British Geological Survey,
2017). The Kupferschiefer is laterally extensive across
the MNSH area but is typically only about 2 m thick
and is therefore expected to provide only a limited
contribution to the petroleum system.
Despite limited primary geochemical data from
Zechstein source rocks in the MNSH, Zechstein-
Haupt Below
Seismic Resolution
Yarlington Mill Fault
Dabinett
Promintory
Toe
of
Slope Lead?
Intra Platform
‘Salt Hole’
Lows
Platform
Margin
Platform
Margin
Seismic Defined Porosity
20%? 5%
High Porosity Low Porosity
High Porosity
Southern Margin Area
43/02-01
Quad 43
Quad 37
Quad 36
Quad 42
Pomeroy Fault
Fig. 10. Seismic dened porosity distribution across the main area of the Hauptdolomit Orchard Platform
(location shown in Fig. 1) covering an area of roughly 3000 km2. Note the highly irregular southern platform
margin with promontories and embayments. The porosity distribution is based on seismic quantitative
interpretation work and subsequent EEI (Extended Elastic Impedance) volumes. The 42/02-1 well displayed was
not used for the determination of porosity, because the Hauptdolomit is anomalously thin here and likely below
seismic resolution (see text). Extract from TGS MNSH Prime 3D Survey.
21
Browning-Stamp et al.
generated oils in analogue basins in Poland and
Germany, which have a very similar tectonic and
stratigraphic evolution to that of the MNSH, are often
associated with large hydrocarbon accumulations
(Zdanowski and Solarski, 2018). The BMB field
onshore Poland, regarded as a key analogue for the
Hauptdolomit play in the MNSH, contains oil sourced
from Hauptdolomit basinal/slope facies deposited
adjacent to the platform margin together with gas
derived from underlying Upper Carboniferous coaly
source rocks; in-place volumes are estimated to be
about 498.2 million brl of oil and condensate and in
excess of 1 Tcf of gas (Gorski et al., 1999).
The main Carboniferous source rocks in the MNSH
and adjacent areas occur within the Viséan Scremerston
and Yoredale Formations which contain good to
excellent quality source rocks and coals with Type II-III
kerogen providing signicant gas generation potential
as well as oil potential (Monaghan et al., 2017; Vane et
al., 2015). Due to erosion at the BPU, these formations
progressively truncate towards the central sections of
the MNSH and Hauptdolomit platform. Deposition
took place in alluvial and delta plain environments
with sporadic marine inuence.
Local Carboniferous sequences have conventionally
been associated with gas potential due to the
commercial gas success south of the MNSH, e.g. the
Breagh and Cygnus elds and the Crosgan discovery
(locations in Fig. 1). However, potential for oil-prone
source rocks is suggested by the sporadic presence of
oil shows showing stable isotope values consistent with
Carboniferous sources (e.g. wells 44/21-1, 44/21-4), as
well as mature oil shales and oil-prone coals. Examples
of the latter include coals and shales from cored
Scremerston and Yoredale Formations in the 43/02-1
well (location in Fig. 6), with hydrogen indexes in the
range 200 to 478 mgHC/gTOC and vitrinite reectance
(VR) values of 0.6-0.7 Ro% consistent with the early
to mid-maturity window (Geochem Group, 1989).
Notwithstanding the signicant local oil potential, the
predominance of gas-prone coals suggests that overall
Carboniferous source rocks are characterised by gas-
generating potential.
Hydrocarbon Generation and Migration
The available geochemical data (British Geological
Survey, 2017) and the results of basin modelling carried
out by Horizon Energy indicate that Kupferschiefer
and “Stinkdolomit” source rocks in the MNSH area
are early mature for oil generation, with vitrinite
reectance (VR) values of 0.5-0.6 Ro%. Hydrocarbon
charging relies on eective migration up-dip into
the Hauptdolomit platform from the adjacent basinal
area to the south (Fig. 11), where signicantly higher
thermal maturities are modelled (mid oil window,
VR of 0.7-0.9 Ro% for the base of the Zechstein
Supergroup). Both Carboniferous and Zechstein source
rocks are suciently thermally mature in this area to
have generated hydrocarbons.
The similarity of the burial history and tectono-
stratigraphic evolution of the area around the 42/04-1
Ossian discovery to that of the main Hauptdolomit
platform margin has allowed data on the Ossian oil
to be used for a wider petroleum systems evaluation.
The Ossian oil is a 19° API crude with a low to
moderate degree of biodegradation. The presence of
biodegradation has been identied by GC analyses of
saturates (Bastow et al., 2019; Core Lab, 2019) which
consistently show the presence on chromatograms of a
prominent unresolved complex mixture (UCM) beneath
the heavier hydrocarbons envelope (nC25+); and also
by the higher proportion of resins and asphaltenes in
Fig. 11. (left) Schematic SE-NW oriented geoseismic interpretation across the southern margin of the Orchard
Platform showing the migration of hydrocarbons from Carboniferous source rocks up into Hauptdolomit
carbonates in off-platform (basinal) areas and subsequent migration into platform-top Hauptdolomit reservoirs,
as believed to occur in the vicinity of the Ossian discovery (see text for details). The zoomed palaeogeographic
map (right) shows the oil- and gas-mature source rock kitchen to the south of the Orchard Platform and the
location of the geoseismic section. There is also potential for intra platform lagoonal microbial source rocks in
the area although they may not be sufciently mature for hydrocarbon generation.
Basalanhydrit
Stassfurt Halite
Leine Halite
Hauptdolomit
Hauptanhydrit
Plattendolomit
Werraanhydrit
Carboniferous
Source Rocks
Polyhalite
Wedge
Base
Permian
Unconformity Panel 1
Location
Main Local Source
Rock Kitchen
Hauptdolomit
Possible
Intra-Platform
Lagoons?
Secondary
Source?
Panel 1 Panel 2
Ossian
discovery
Hauptdolomit
22 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
these samples relative to unaltered oils. Based on the
analysis of alkane and aromatic maturity parameters
(Bastow et al., 2019; Core Lab, 2019), a low to mid
maturity source rock has been determined (0.7- 0.85
%Ro) (unpublished report for Horizon Energy). This
level of maturity is similar to that of the source rock
which generated the 29.3° API oil in the Blythe eld
on the western margin of the Southern Permian Basin
(location in Fig. 2) and which was also typed to a
Zechstein source rock (unpublished report for Horizon
Energy). This suggests an original API gravity for the
Ossian oil in the range 25°-30°. However, the thermal
maturity of the source rock which generated the Ossian
oil samples is likely to be signicantly higher than that
of potential Zechstein source rocks in the well, which
are early mature: calculated VR of 0.5-0.6 Ro% in
the Hauptdolomit, measured VR of 0.6-0.75 Ro% in
the Carboniferous (Panterra, 2019). Therefore, such
results lend support to a model of active migration
from a kitchen in the deeper basin to the south of the
MNSH (Fig. 11).
Despite the mid-mature Zechstein source ngerprint
in the Ossian oil, geochemical data point to a dierent
source for the gas phase which is characterised by a
signicantly higher degree of thermal maturity. This
has been evidenced by stable isotope analyses from
hydrogen:deuterium and light (C1) hydrocarbons
indicating a condensate associated to post-mature
gas (ẟ13C1: -35.5 to -35.3, ẟ D: -168 to -93) (Bastow
et al., 2019). These levels of thermal maturity (likely
Ro%>1.2) are much higher than those in the deepest
Zechstein sequences of the adjacent basin (VR of
0.7-0.9 Ro%), implying a contribution from a deeper
Carboniferous source. The presence of up to 17.4% of
nitrogen in the Ossian PVT gas samples (Core Lab,
2019) lends further support for gas generation from a
deep Carboniferous source. In the Southern Permian
Basin, the occurrence of nitrogen has been linked to the
preferential trapping of late generation products from
Carboniferous coals, possibly supplemented locally by
inorganic sources (Gras and Clayton, 1998).
Hydrocarbon migration from Carboniferous source
rocks into Hauptdolomit platform reservoirs relies on
the presence of faults allowing communication between
Carboniferous and Permian intervals. Multiple faults
have been identied in proximity to the Z2 platform
margin and in the adjacent basin which are candidate
migration pathways (Fig.11). Furthermore, the
presence of a thin (< 15 m) Werraanhydrit Formation
in the basin with discontinuous anhydrites enhances
the potential for uid communication between the
Zechstein and Carboniferous intervals.
The hydrocarbon charge interpretation by Horizon
Energy for Ossian includes an initial phase of oil
emplacement that has been slightly biodegraded close
to the oil-water contact, and then a secondary gas
charge. Maturity modelling carried out by Horizon
Energy has shown that the Ossian oil was likely
generated in the adjacent basin and was emplaced
in the reservoir in the Late Jurassic prior to the Mid-
Cimmerian unconformity event, when low reservoir
temperatures (< 70 °C) allowed biodegradation to take
place. Gas generation is modelled by Horizon Energy
to have occurred during the early to middle Miocene
peak burial stage prior to subsequent Miocene uplift,
tilting and erosion.
Unpublished modelling results by Horizon Energy
suggest that hydrocarbon generation from all source
rock intervals was insignicant prior to the Variscan
orogeny in the main kitchen area south of the MNSH.
This contrasts with some previous studies which have
pointed to pre-Variscan (Early Permian) peak burial
including in areas on the southern ank of the MNSH;
for example 1D basin modelling results of well 42/13a-
6 indicated pre-Variscan peak maturity in the Breagh
eld (Grant et al., 2020). The possibility remains of
pre-Variscan peak burial in the innermost area of the
MNSH, but this is not relevant to the prospects on its
southern margin. Peak burial during the early-mid
Miocene provides a good match between calculated
and measured thermal maturity in the wells of the
focus area, and correctly predicts the hydrocarbon
distribution in the MNSH and surrounding area.
Hydrocarbon preservation
and the eects of Miocene tilting
Hydrocarbon preservation in the MNSH area is largely
related to the eects of middle-late Miocene uplift and
regional tilting. The associated erosion produced the
Mid Miocene Unconformity (MMU) which is readily
identiable on seismic data by the onlap of Plio-
Pleistocene strata (Fig. 7) (Brackenridge et al., 2020).
The impact of the deformation becomes progressively
more significant to the west: Zechstein rocks are
exposed at the surface onshore in NE England, with
uplift and erosion in excess of 1.5 km; oshore, the
Mesozoic sequence crops out at the sea oor (e.g.
wells 41/14-1, 41/10-1). Breaching of hydrocarbon
seals resulting in reduced Zechstein trap integrity is
thought to have aected these areas.
The amount of mid-late Miocene uplift and erosion
decreases progressively to the east. Approximately 400
m of Cenozoic erosion is estimated to have occurred
in the area of the Ossian discovery and <100 m in
the easternmost part of the MNSH close to the UK-
Netherlands maritime boundary.
The uplift and east-west tilting post-date all
hydrocarbon generation and migration episodes on and
around the MNSH. Therefore in order to characterise
charge, ll and retention of Hauptdolomit traps, it is
critical to evaluate structures at middle Miocene time
and to assess the impact of progressive east-west tilting
23
Browning-Stamp et al.
on the structures through to present day. This has been
achieved by restoring seismic sections to the time of
middle Miocene peak oil and gas generation, and by
building regional isochore maps which incorporate the
interval from the BPU to the MMU seismic picks. The
latter includes a “pseudo” seismic pick coming above
sea oor in the areas most aected by uplift towards
the UK coastline.
Integration of restored seismic sections, isochore
maps and 1D basin modelling results has shown the
current regional dip between the Hauptdolomit platform
and the adjacent deeper basin to the south was even
more pronounced prior to the middle Miocene uplift
and erosion, providing a very favourable structural
conguration for hydrocarbon charging. Additionally,
the east-west regional tilting may have promoted
remigration of hydrocarbons spilled from deep traps to
the east-southeast of the MNSH area towards the SW
sectors of the Hauptdolomit platform margin including
the area surrounding the Ossian discovery. Areas with
a moderate degree of Cenozoic uplift and erosion (in
the range 100 to 600 m) are considered optimal for
hydrocarbon migration and preservation. Their relative
position with respect to the regional dip would allow
for additional remigrated hydrocarbons to be trapped,
whilst allowing for seal integrity to be maintained.
Backstripping the mapped top Hauptdolomit
surface prior to the MMU has shown even larger
structural closures within the Hauptdolomit platform
margin during the main phase of hydrocarbon charge.
Whilst the regional tilting may have resulted in the
partial loss of hydrocarbon volumes, the integration
of structural restoration and hydrocarbon generation
/ migration modelling carried out by Horizon Energy
indicate that present-day structures are likely to be
full to spill.
A significant feature of Zechstein reservoirs
that is likely linked to the effects of the Mid-
Miocene Unconformity is the presence of signicant
overpressure which has been recorded in several wells
with hydrocarbon occurrences including the Ossian
oil and gas discovery (see above). A regional pore
pressure study for Horizon Energy has concluded that
disequilibrium compaction of reservoirs with self-
sourced hydrocarbons would be sucient to generate
some overpressure, but would not be sucient to
explain the signicant level in the 42/04-1 Ossian
well. A majority of wells with associated overpressure
in the MNSH and surrounding area include gas from
late mature to overmature Carboniferous source rocks,
requiring communication between Zechstein reservoirs
and the deeper stratigraphy. Pressure communication
negates the compaction disequilibrium model and gas
expansion / pressure transfer processes are therefore
considered to be the primary overpressure generating
mechanism. This further implies that Ossian contains
associated free gas because oil does not have sucient
compressibility to generate the overpressure.
Zechstein reservoirs likely behaved as large-scale
sealed compartments during the mid-late Miocene
uplift event, allowing previously migrated gas to
expand and cause overpressure whilst the underlying
Carboniferous reservoirs maintained near-hydrostatic
pressure regimes. The degree of uplift can therefore
be related to the amount of overpressure and the seal
integrity, with prospects east of Ossian likely to be less
severely (if at all) impacted.
Hydrocarbon accumulations in the Hauptdolomit
with multiple effective source rocks (Zechstein-
generated oils and Carboniferous condensate/gas) are
known in other parts of the Southern Permian Basin
such as the BMB eld in onshore Poland (see above).
However, this represents a new play concept for the
MNSH area. A greater understanding of the impact
of the Mid Miocene Unconformity on hydrocarbon
migration, overpressure and trap preservation will help
to focus future exploration eorts in this underexplored
oil and gas play.
Hauptdolomit Reservoir Architecture and Facies
Unlike the Hauptdolomit platform bordering the UK
coastline (and which extends onshore), the Orchard
Platform on the MNSH was quasi-isolated; it extended
laterally for approximately 250 km in an east-west
direction and approximately 100 km north-south in
the study area (Fig. 2). The northern margin of the
platform includes a broad embayment facing NE,
whereas the steep southern margin is highly indented
with promontories and isolated micro-platforms,
suggesting a greater degree of structural control and
instability (Fig. 12). A similar indented and collapsed
Z2 platform edge has been mapped in the Amethyst
area on the western margin of the Southern Permian
Basin (Grant et al., 2019).
Depositional components of the Hauptdolomit
platform identified from petrographic analysis
include abundant green algae, reworked microbialite,
calcispheres, subordinate molluscs, rare foraminifera
and ostracods. These are locally accompanied by
peloids, oncoids and ooids. In ne-grained or muddy
facies, microbialites form laminated mats whereas in
grainier facies this material is reworked from small
build-ups into part-micritized amorphous grains
and/or binds other grains together as aggregates. No
stenohaline marine taxa have been recorded anywhere
in the Hauptdolomit platform and are rarely observed
in basinal deposits. The lack of framework-building
organisms meant that platform margins are shoal-
rather than reef-dominated, and only small microbialite
build-ups can be inferred from reworked material
in platform interior locations. In cored platform
interior locations, the dominant facies are packstones,
24 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
grainstones and wackestones. Thin sections analysis
from cuttings suggest muddy inner platform deposits
are more abundant to the northeast where the platform
interior is more extensive (e.g. wells 36/13-1 and
36/15-1).
Based on core and thin sections, and analogue Z2
carbonates in western Poland and elsewhere, multiple
sedimentary facies have been dened and used to
propose a conceptual facies model reflecting the
depositional environments and processes relative to the
physiography of the MNSH platform. This predicts a
peripheral platform margin belt of ooid and bioclastic
shoals, finer-grained but still mud-poor, subtidal,
backshoal and open platform environments, and a
facies mosaic of muddy peritidal to grainy shallow
subtidal deposits in the inner parts of the platform
where attenuated tide and wave energy allowed
mud to accumulate and emergent islands to develop.
Localised intrashelf basins accumulated deeper water
muddy carbonates. Based on core examination,
carbonate muds, calciturbidites and possible debrites
accumulated in lower slope to basinal settings. The
turbidites consist of laminated to graded thin beds
with sharp, locally scoured bases, whereas debrites
are thicker and more massive.
The presence of fault controlled shallow intra-
platform basins is suggested by re-interpretation
of sedimentary facies from the cored interval of
well 38/24-1. Despite being located in the platform
interior, the facies and petrographic microfacies
imply signicantly lower energy and deeper water
than those in surrounding wells such as 38/22-1 and
38/29-1. There are also resedimented carbonates which
appear to have been eroded from surrounding shallow
peritidal environments. These conclusions dier from
previous published interpretations which described the
presence of in situ evaporite collapse brecciation and
a platform slope setting from the 38/24-1 well cores
(Amiri-Garroussi and Taylor, 1992).
Seismic interpretation shows that the Hauptdolomit
in the Orchard Platform area of the MNSH has
an aggradational motif and lacks progradational
clinoforms proposed by Patruno et al. (2017). Also,
in contrast to that paper and Garland et al. (2023,
this issue), we present an alternate interpretation that
the Hauptdolomit platform extends across the entire
breadth of the MNSH, rather than being conned to
its margins. The Orchard Platform therefore comprises
a variably restricted shallow subtidal to peritidal
aggrading platform with abundant grainy facies and
shoal development close to its margins, a steep slope
(in the south) and local intraplatform depressions (Figs
6, 10 and 13).
Hauptdolomit Reservoir Diagenesis
A consistent paragenetic scheme has been determined
for the Hauptdolomit (Fig. 14) based on petrographic
analysis of core and cuttings from 11 wells on the
Fig. 12. Generalised palaeogeographic map of the study area showing Hauptdolomit platforms and facies
variations (c.f. Fig. 6), and showing key identied prospectivity including gaselds and prospects/ leads. Only
the most prominent prospects are marked, i.e. those with the largest potential reserves. Note the absence of
prospects to the west of the Jazz platform due to the shallow depth of the Hauptdolomit and the lack of data
(as indicated by the question marks). The dashed blue line shows the area of Fig.13.
25
Browning-Stamp et al.
Fig. 13. Static carbonate model for the study area (see location in Fig. 12) showing the morphology and extent of Z2 Hauptdolomit carbonate platforms (orange-red
colours) and intervening seaways. Dark grey shows lower slope and basinal “Stinkdolomit” mudstones which may have source rock potential. Ooid shoals are present in
platform margin locations. Key wells are labelled. Model based on an integration of regional seismic data, petrographical analyses and well based datasets. The precise
distribution of shoals and high energy facies is not fully understood at present and will only be de-risked by additional drilling.
Carboniferous
Anticline
Basin Floor
mudstones –
Stink Dolomite
Oil Source
Quad 38
Quad 42 Intra-basinal
highs
With Z2 Reefal
and Ooid
Facies
(Crosgan)
42/04-01Z
Ossian
37/23-01 43/05-01
44/02-01
38/24-01 44/07-01
Ooid shoals
and banks,
minor reefal
growth
Tidal
Channels
Re-activated
Caledonian
Aged Faulting
~1.5km
~100 kms
Quad 44
Quad 36
~100 kms
42/15a-02
Crosgan
Intrashelf
Basin/Shallow
Subtidal
Lagoon
43/02-01
43/03-01
42/05-a
Dabinett
(undrilled)
Ossian
Platform
36/15-01
Legend
Slope/Slope Apron – Occasional Talus
Basinal Areas – Stink Dolomite Facies
Werra Anhydrite Platform – Anhydrite
Z1 Carbonate - Kalk
Pre-Permian – Carboniferous To Devonian Sandstones, Coals and Shales
Base Permian Unconformity
Peritidal Flats - Microbial Mats
Z2 Cycle
Z1 Cycle
Top Devonian Limestone– Tayport/Old Red Ss.
Pre
Zechstein
Carboniferous – Fell Ss.
Carboniferous – Scremerston Ss.
Lower Permian – Leman Ss.
Shallow Subtidal Lagoon
Open Subtidal Lagoon – Tidal Channels – Intrashelf Basins
Platform Margin/Inner Platform Shoal
Interior Shallow Basin
Oil Source
Kitchen
38/22-01
38/29-01
26 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
MNSH, and on the interpretation of diagenetic
processes aecting analogue platform carbonates from
elds in Poland such as the BMB and LMG elds
(Krogulec et al., 2020; Kosakowski and Krajewski,
2014; Kwolek and Mikolajewski, 2010). The most
important diagenetic processes aecting reservoir
quality were dolomitization, partial to complete
dissolution of the abundant aragonitic grains (green
algae, molluscs, some microbialites), anhydrite
cementation, dedolomitization, and localised late-stage
dissolution of dolomite and anhydrite.
Widespread, pervasive dolomitization of the
Hauptdolomit most likely resulted from a seepage-
reux mechanism that commenced during deposition
from mesohaline brines and continued and intensied
during the subsequent deposition of the onlapping
Basalanhydrit Formation with hypersaline brines
encroaching the platform top. In reux systems, the
thickest fully-dolomitised intervals occur below the
highest salinity part of the carbonate platform top and
are predicted to taper and become less pervasively
dolomitised towards the platform margin (Machel,
2004). This model is consistent with observations from
the Hauptdolomit of the MNSH, where platform-top
sequences are completely dolomitized and decreasing
dolomitization occurs towards the lower slope and the
basin (although dedolomitisation may also play a part
in this). The pervasive nature of dolomitization in the
Hauptdolomit may reect the inherent reactivity of an
aragonite-rich and largely permeable (grainy) original
carbonate composition, as well as the density drive
being augmented by the elevation of the platform with
respect to the surrounding basin, in part reecting the
underlying Werraanhydrit bank.
Depositional micrite and ne-textured or micritised
grains in the Hauptdolomit are mostly replaced by tight
planar-S dolomite with very limited inter-crystalline
porosity. The same is recorded from Polish Z2
platforms and shows that dolomite replacement was
accompanied by localised cementation. Consequently,
the post-dolomitization interconnected pore network
was essentially the same as that defined by the
primary facies, dominated by modied but essentially
intact interparticle pores, implying that the reservoir
potential was strongly related to the original facies
(and especially to the distribution of grainy, high-
energy shoal facies). Almost all the samples analysed
came from wells located a few km inboard of the
platform margin, and it is signicant that conceptual
and numerical models of reflux dolomitization,
whilst revealing complex physio-chemical controls
and interdependencies, concur in predicting that the
highest porosities and the least cementation occur in
the platform margin to upper slope settings.
Hauptdolomit peritidal facies contain some syn-
depositional anhydrite (replacing or pseudomorphing
after gypsum), and some anhydrite cementation may
have accompanied dolomitization as a mineral sink
for excess calcium released in the process. However,
most of the blocky pore-filling anhydrite cement
likely precipitated during early burial in response to
uids derived from the dehydration of gypsum and/
or pressure dissolution of anhydrite in the basin and
directly underlying the Hauptdolomit. These warm,
hypersaline uids moved laterally and upwards in
response to pressure gradients and exploited the most
permeable pathways through the Hauptdolomit.
Predicting the distribution and extent of anhydrite
cementation in the Hauptdolomit remains a key
uncertainty in the study area. Available well data
indicate cm-dm scale and km-scale (inter-well)
heterogeneities in the severity of anhydrite cementation
with no obvious control. Anhydrite cementation is
likely to be most signicant where thick anhydrite
deposits juxtapose or overlie the platform, or in
the presence of faults providing uid access from
underlying sulphate banks. There is a crude qualitative
relationship between Werraanhydrit thickness and
anhydrite abundance in the Hauptdolomit. Although
anhydrite cements have frequently been identied in
core and thin sections, petrophysical evaluations of
the available wells within the Hauptdolomit platform
suggest low anhydrite contents (<5% in all cases). As
such, the cored intervals may give an unduly pessimistic
impression. However, anhydrite cement remains the
most signicant factor in determining the relationship
between facies-controlled reservoir potential and actual
reservoir quality. By plugging the interconnected pore
systems, permeability is signicantly reduced, even
if signicant vuggy porosity (in dissolved or hollow
grains) remains.
Deep burial diagenesis was not important in the
Hauptdolomit of the MNSH, and there is a lack of
correlation between porosity or permeability and
maximum burial depth. Stylolites are rarely developed
and dolomitisation likely reduced the efficacy of
mechanical and chemical compaction at the <
3000 m burial depths. Very localised dissolution of
dolomite and/or anhydrite is associated with fractures
and “exotic” minerals such as uorite, pyrite and
sphalerite. The dissolution is particularly notable in
well 38/24-1 and the solution-enhanced fractures (plus
local brecciation) are probably linked to the faulting
that is interpreted to control the local development
of an intra-platform depression (Fig. 6). This late
dissolution explains why deeper water facies in well
38/24-1 yield similar porosity-permeability ranges to
shallower facies in surrounding wells. It shows that
faulting and fracturing can enhance reservoir quality
signicantly in the Hauptdolomit by focusing reactive
uids. Similar features are more extensively developed
in the Z2 Main Dolomite in NE Germany, associated
27
Browning-Stamp et al.
with Carboniferous-sourced CO2-rich hydrothermal
uids gaining access via fault-fracture systems. Similar
fault conduits with associated corrosive uids could
be an additional factor leading to an improvement
of reservoir properties in platform margin sequences
where the steep, scalloped southern Hauptdolomit
platform margin suggests that there was some tectonic
control.
Dedolomitization was not recorded in any of the
Orchard Platform wells that were the focus of the
current study but is displayed in cores of middle
to lower slope facies from well 41/18-1 and to a
lesser extent in the 41/15-1 well. Dedolomitization
processes replace fine crystalline dolomite with
nodular to bed-parallel arrays of coarse sparry calcite,
signicantly reducing any intercrystalline porosity.
Dedolomitisation probably results from the migration
of Ca-rich uids during burial, and conceptual models
for dedolomitization predict its impact to be greatest
in slope settings where thick anhydrites are juxtaposed
with dolomites, whereas it is rare in platform margin
and interior settings. In the Hauptdolomit of NE
Germany, extensive dedolomitization of the lower
slope has been documented, locally extending to
the upper slope and platform margin, and resulting
in average 5-10% reduction in matrix porosity
(Schoenherr et al., 2018).
Hauptdolomit Reservoir Seals and Preservation
The Hauptdolomit reservoir interval is overlain by the
Basalanhydrit and Stassfurt Halite Formations. These
provide an excellent top seal for any hydrocarbon
accumulation, with caprock integrities capable of
containing large overpressured gas columns. These
sequences also constitute lateral seals against the
dipping anks of the platform. Core and cuttings
samples show the Basalanhydrit Formation is
dominated by tight, nely crystalline anhydrites with
some clays and thin, tight dolomites.
Based on available well data, the Basalanhydrit
Formation is a uniform anhydrite-dominated interval
across the Orchard Platform area with the thickest
sequences being close to the platform margin (e.g. 57
m in the Ossian discovery, 107.9 m in well 43/05-1),
and thinner but adequate thicknesses reported in the
platform interior (e.g. 32 m and 14 m in the 36/13-1 and
38/24-1 wells, respectively). The Basalanhydrit is the
main sealing unit for Hauptdolomit platform interior
areas located at the greatest distance from the basin
and platform margin, due to the reduced thickness or
absence of the overlying Stassfurt Halite (< 20 m). In
the case of breached Basalanhydrit sequences, this could
have allowed charge into the overlying Plattendolomit
Formation. The main risk to Basalanhydrit seals is
fracturing, which is only expected close to major faults.
Late stage minor replacement of anhydrite by gypsum not shown
Paragenetic time/Burial Depth
Mesogenetic
Eogenetic
Micritisation
Syn-sedimentary Gypsum
Desiccation Cracks
Aragonite or Mg-Calcite Marine Cement
Mechanical Compaction
Emergence-related dissolution of shoals
Aragonite dissolution
Aragonite and Mg-calcite dolomitisation
Gypsum to Anhydrite
Dolomite cement
Anhydrite cement
Fracturing, incipient brecciation
Dolomite dissolution
Anhydrite dissolution
Pyrite
Fluorite
Sparry calcite (dedolomite)
Chemical compaction
Hydrocarbon
Not commonly seen in the area
38/24-01
Slope Wells
Fig. 14. Diagenetic history for the Hauptdolomit derived from petrographic studies. Thickness of bars signies
the relative importance of the diagenetic processes: red indicates porosity destruction, green indicates porosity
creation, orange indicates neutral or minor impact on porosity. Subaerial exposure of the Hauptdolomit is
unproven from core studies but likely occurred based on analogue studies from other areas. Dedolomitization
has not been identied in the samples analysed but occurs in the Hauptdolomit in surrounding areas.
28 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
The thickest halite sequences occur in basinal
areas and progressively pinch out towards the inner
platform domain. Generally, across the MNSH the
Basalanhydrit thickens where the Stassfurt Halite
is attenuated and seals are considered eective and
low risk. The seal effectiveness is proven by the
Ossian discovery where the thin but significantly
overpressured hydrocarbon column underlies a 57 m
Basalanhydrit but a poor quality Stassfurt Halite (11
m thick, with only minor traces of halite). No seeps
have been observed in the vicinity of the Ossian well,
whereas they have been recorded above other parts of
the Hauptdolomit platform (especially near mapped
faults). It is important to note that seeps can occur
above proven oil elds, and so this is not necessarily
an indication of seal failure for prospects.
HAUPTDOLOMIT
RESERVOIR QUALITY EVALUATION
The Orchard Platform Play
The best reservoir quality in Hauptdolomit platforms
across the Southern Permian Basin is nearly always
associated with the presence of coarse-grained
shoal facies (platform margin shoal, back-shoal),
and sometimes in current-swept platform top facies,
where anhydrite cementation has been limited.
High energy facies include oolitic, oncoidal and/or
skeletal grainstones and in situ or reworked microbial
boundstones. These facies have been observed in
samples from wells which appear to have intercepted
inner platform shoals and mounds in the Orchard
Platform (e.g. wells 38/16-1 and 38/29-1), particularly
in the upper section of the Hauptdolomit. Core data
from well 44/02-1 provides an additional example of
moderate thickness oolitic grainstone intervals but this
well was drilled in an upper slope setting (Fig. 13),
hence these facies are likely resedimented from the
overlying platform margin or from a collapsed platform
margin block. The slope context may also explain a
pervasive anhydrite cement in these samples. Shoal
deposits are expected to be thickest and to have the
best petrophysical properties around the Hauptdolomit
carbonate platform margin, even though the current
lack of well penetrations in these areas hampers
assessment of their full reservoir potential.
In general, steep carbonate platform margins are
commonly associated with optimal reservoir quality
because of a combination of clean-washed, high-energy
facies with the local potential for subaerial exposure
and karstification, together with dolomitization
conditions that retain porosity, and frequent fracturing
related to dierential compaction and gravitational
instability. Where margins are controlled by basement
faulting, the same faults may focus reactive uids from
deeper maturing source rocks that enhance porosity,
and platform margin collapse episodes can deposit
slope apron breccias with additional reservoir potential.
Within the platform facies of the Hauptdolomit,
the uppermost section of the unit likely represents
the best reservoir section. This may reect evolving
hydrodynamic conditions as the platform aggraded.
There could be an inuence from increased subaerial
emergence towards the top of the Hauptdolomit as the
platform shallowed, but direct evidence is currently
lacking because all the cored intervals from wells on
the MNSH are from the middle section of the formation
and cuttings are poorly able to capture evidence of
exposure such as vuggy porosity. Nevertheless, it can
be surmised that the Hauptdolomit platform margins,
and potentially the younger platform top, are where
reservoir quality is likely to be optimal.
Available well data indicate typical gross
thicknesses between 44.5 m and 67.5 m for platform
interior Hauptdolomit sequences (Table 1). Net
thickness in well sections has been dened using a
porosity cut-o of 5% which generally corresponds to
permeability values above 1 mD based on core analysis
data. Overall net-to-gross (NTG) values dened by
the petrophysical evaluation are mostly between
65 and 90%, with wells in the northern part of the
Hauptdolomit platform displaying the highest values.
Average porosity ranges from 13 to 23% within the net
section, with the 43/05-1 well showing 15% average
porosity in a platform interior setting (Table 1, location
in Figs 6 and 13). This well can be considered as an
example of a well drilled in relatively close proximity
to the southern Hauptdolomit platform margin (albeit
still 7 km inboard), hence a reasonably good reservoir
analogue for prospects in the area.
A discrepancy has been noted between the high
average log-derived porosities of up to 22% in some
wells in the NE of the Orchard Platform (notably
36/13-1 and 36/15-1, Table 1) and the scarce visible
porosity in corresponding thin sections from ditch
cuttings. This suggests that microporosity may be
signicant in these dolomitised muddy inner platform
facies, and/or poorly interconnected vuggy porosity
that is not sampled in the small cuttings. Petrophysical
analyses corroborate these possibilities. Combining
and analysing variations between porosity, shallow
and deep resistivity and evaluating sonic velocity
deviations from the Wyllie equation has shown only
a modest portion of these inner platform reservoirs
can be expected to be productive (e.g. effective
intergranular and connected vuggy porosity estimated
in approximately 20-25% of the reservoir interval in the
36/13-1 well). Interconnected macroporosity is more
likely to occur in outer platform and platform margin
settings, even if porosity totals are similar.
Platform interior shoal reservoir facies have been
detected, not just in the main area of the Orchard
29
Browning-Stamp et al.
platform but also are indicated by well and seismic
data to occur in a number of isolated micro-platforms
that formed around the area, especially situated above
Carboniferous highs to the south of the main platform.
Various named features exist and are currently at
dierent stages of the exploration cycle. The Crosgan
field represents the most developed of the micro
platforms in the UK sector, with three existing wells
and a fourth appraisal well drilled in mid-2023.
Slope and Basinal Hauptdolomit Plays
The Hauptdolomit platform margin to upper slope
transition is represented by a characteristic clinoform
shape on seismic data across the region, albeit without
signicant thickening. Slope length and angle are
locally variable but on average the slope angle is
8-10°. The upper slope is not sampled except in well
44/02-1 which, as explained above, likely represents
redeposition of shoal facies or a foundered block of
the platform margin above a detachment. Well 43/02-
1 appears to be located in an upper slope setting
according to the seismic interpretation; however; the
anomalous Hauptdolomit thickness of only 12.8 m
and the absence of high energy facies may suggest the
well has been drilled in a by-pass zone. Other wells to
have tested slope facies include 36/23-01 and 41/05-
02 (Pensacola), both of which lack public domain
data. Onshore, West Newton B1 also penetrated
Hauptdolomit slope facies (RPS Energy Canada Ltd,
2022).
The slope areas of the Orchard Platform have
been mapped using seismic attributes and spectral
decomposition techniques, highlighting highly
indented and scalloped morphologies with possible
slope apron deposits. With the possible exception of
Pensacola, the toe-of-slope hydrocarbon play is still
unproven in the MNSH area. Based on the prevailing
wind direction and platform morphology, the episodic
shedding of shoal facies down slope to form slope-
apron wedges abutting the southern platform margin
is considered very likely. The scalloped and gullied
slopes would in general be associated with platform
margin collapse and deposition of talus breccias,
debrites and calciturbidites, especially where the
margin was weakened by subaerial exposure and syn-
sedimentary or reactivated tectonic faults. By analogy,
Clark (1986) noted that Hauptdolomit platform
sequences on the southern ank of the Ringkøbing-Fyn
High in the Netherlands sector are also characterised
by a narrow, fault-controlled platform margin. In
Poland and Germany, hydrocarbons traps are present in
Hauptdolomit (Main Dolomite) toe-of-slope deposits
(Gorska et al., 2003; Słowakiewicz and Mikołajewski,
2009; Kwolek and Mikołajewski, 2010). Thus, whilst
this play concept is yet to be proven for the MNSH, it
represents signicant upside potential especially if the
toe-of-slope reservoirs are in pressure communication
with platform accumulations as in Poland. Additional
drilling, testing and seismic attribute analysis may de-
risk this hydrocarbon play in the future.
In basinal areas, the deposition of the Hauptdolomit
occurred in a lower energy marine setting with
laminated deposits of “Stinkdolomit” source rock
facies (Fig. 13 and Fig. 3), thin laminated to graded
calciturbidites and interbedded hemipelagites. The
Hauptdolomit is thinner than on the platform (generally
< 20 m) and lithologically is dominated by tight
limestones, ne-grained dolomite or coarse replacive
calcite where dedolomitisation has occurred.
Reservoir Deliverability
The Hauptdolomit is proven to produce at economic
rates in onshore Poland where the depositional
environment can be considered to be analogous to that
expected in equivalent reservoirs of the MNSH. Fields
such as BMB are typically developed by vertical wells
with no pressure support, and production rates and
hydrocarbon sweep are maintained by high well counts
(the BMB development includes in excess of 30 wells).
Oshore, where drilling costs are considerably
higher, horizontal wells would be required to produce at
economic rates. For a typical BMB reservoir thickness
and quality, scaling up a vertical well productivity to
a 500-1000 m horizontal well length could result in a
productivity ratio between 1:6 and 1:10 (Dankwa et
al. 2018). This would imply increasing a BMB oil test
rate of 1200 stb/d (Gorski et al.,1999) to between 7500
and 12,500 stb/d for an equivalent horizontal well with
500 m to 1000 m reservoir section.
The Hauptdolomite is expected to be overpressured
and therefore not in pressure communication with a
large regional aquifer. In the absence of signicant
natural water drive, water injection/removal would
likely be required in an oil scenario to maintain
pressure support and aid sweep, and achieve a recovery
factor greater than 30% in Orchard Platform elds.
Recovery factors greater than 60% are expected in a gas
scenario under natural depletion, where lack of aquifer
inux reduces bypassed gas, and the requirement for
workovers to shut-o water production.
HAUPTDOLOMIT PROSPECTIVITY
The Hauptdolomit play is relatively underexplored
and any future discoveries will have to be developed
within the context of energy transition objectives.
The MNSH is the nal frontier area to be explored
in the UKCS, with uncertainty remaining around
the role of future licensing rounds and a trend of
only infrastructure-led and hub-focused exploration.
However, the Hauptdolomit has potential to deliver
much-needed domestic reserves in easily developed
30 Hauptdolomit platform in the southern UK Mid North Sea High: petroleum plays
shallow waters close to regional hubs with available
pipelines and production capacity.
A portfolio of prospects and leads has been dened
from legacy 2D data, rened using modern 3D data
and brought to the current drill-ready status across the
region in the last two years. In total, > 75 prospects
have been mapped across the Orchard Platform area
by Horizon Energy Partners Limited, so the selected
examples described below and shown in Fig. 12 are by
no means exhaustive of the play opportunities. These
prospects are volumetrically substantive for oil and/or
gas. Prospectivity is identied in multiple closure types
and depositional facies, with varying risk elements.
Example mean unrisked prospective resources
for Hauptdolomit prospects on the Orchard Platform
margin illustrate this potential, such as Ossian
Northwest (open acreage – 282 MM brl in the oil
case, 1.3 Tcf in the gas case) and Dabinett (Horizon
Energy – 139 MM brl in the oil case, 663 Bcf in the
gas case), and are likely to have the highest quality
reservoirs. These are supplemented by platform interior
carbonate shoal prospects such as Yarlington Mill
(Horizon Energy – 807 MM brl for the oil case, 3.5
Tcf for the gas case), Cox’s Orange Pippin (Horizon
Energy – 52 MM brl in the oil case, 236 Bcf in the
gas case), and Balvenie (Draupner Energy – 1.16
Tcf). Due to the uncertainty with regard to assessment
of the relative proportion of reservoired Zechstein
oil versus Carboniferous gas, volumetric estimates
in each of the above prospects have been presented
with two end-member scenarios: an oil case (which
assumes no Carboniferous gas) and a gas case (no
Zechstein-generated oil). Both oil and gas are likely
to be present, however overall the gas potential is
regarded as most signicant, based on a volumetric
assessment of source rocks generative capacity and the
established Carboniferous gas presence in the area to
the south of the MNSH (e.g. Breagh eld and Crosgan
gas discovery) .There are also combined isolated/main
platform prospects such as Jazz (Spirit Energy/Shell –
volumes unknown).
The Bonnie Bray micro-platform to the southwest
of the Ossian discovery (location in Fig. 12) remains
as the only unlicenced and undrilled isolated platform
in the area and is an attractive exploration target due
to the robust closure oered by these structures. Mean
unrisked prospective resource volumes are estimated
as 390 Bcf (Spirit Energy, 2021).
CONCLUSIONS
The Hauptdolomit play of the UK MNSH has been
signicantly de-risked by recent drilling activities and
the availability of new, high-quality regional 3D seismic
data. A key development for the play occurred with the
Ossian discovery in 2019, which proved the presence
of an overpressured hydrocarbon accumulation
in Hauptdolomit platform interior facies. Despite
signicant limitations in data quality and availability,
the Ossian discovery has highlighted the potential for
good productivity in thick Hauptdolomit platform
sequences, together with multiple hydrocarbon phases
and a potential for gas up-dip in the structure. The
results from the Pensacola well, spudded in late 2022,
have also validated the presence of both oil and gas
in the Hauptdolomit, despite the well test proving
inconclusive as to the hydrocarbon volumes given the
slope location of the well.
Seismic attribute analysis and the implementation
of advanced QI workows has signicantly improved
the imaging in the Hauptdolomit section, allowing
for the rst time a reliable top Hauptdolomit seismic
pick to be produced across the Orchard Platform
area. The results have provided a new outline for the
Hauptdolomit platform margin, presented in this paper.
The presence of thick and eective source rocks in
the MNSH required to generate economic quantities
of hydrocarbons has often been perceived as a key
uncertainty, especially considering the historical
paucity of geochemical data from the Zechstein
Supergroup. However, the latest geochemical analyses
from the Ossian discovery have shown the potential
for the Hauptdolomit play to benet from multiple
eective source rocks, with both Zechstein generated
oils and Carboniferous condensate/gas. The co-
existence of hydrocarbons from dierent sources in
the Hauptdolomit is well known in other prolic areas
of the Southern Permian Basin, such as in the BMB
elds onshore Poland, yet it represents a new concept
for the MNSH.
Burial history modelling results integrated with the
available source rock and thermal maturity data have
delimited extensive hydrocarbon kitchen areas located
in basinal sequences adjacent to the southern margin
of the Hauptdolomit platform. Whilst Pre-Variscan
peak burial (Early Permian) may have occurred in the
innermost sectors of the MNSH, the latest modelling
results have ruled this out in the main kitchen area. As
a result, source rocks generative capacity was likely
preserved during the main hydrocarbon charge and
migration events in the Late Jurassic and during peak
burial in the early to middle Miocene.
Hydrocarbon preservation on the MNSH has to
be understood in the context of the Mid Miocene
Unconformity (MMU). This key tectonic event is
expressed by a prominent regional tilted surface
easily identiable on seismic data by onlapping Plio-
Pleistocene strata. In order to characterize hydrocarbon
migration and hydrocarbon ll within Hauptdolomit
prospects, it is critical to evaluate structures at mid
Miocene time (peak burial) and then to assess the
impact of the progressive tilting and uplift on the
31
Browning-Stamp et al.
structural traps through to present day. The integration
of restored seismic sections, isochore maps and 1D
basin modelling results has shown that areas with a
moderate degree of Cenozoic uplift and erosion (ca.
100-600 m) are optimal with respect to hydrocarbon
migration and preservation. Such a degree of uplift is
considered favourable for the play with the possibility
for additional remigrated hydrocarbons to be trapped
up-dip due to the eects of the regional tilting, whilst
allowing for seal integrity to be maintained.
An extensive review and integration of petrographic
microfacies and diagenetic analyses, core data, and
petrophysical interpretations has shown that the
best reservoir quality in Hauptdolomit platforms is
associated with oolitic-bioclastic outer platform shoal
facies with limited cementation, and sometimes in
current swept platform interior shoal facies. Overall, the
most important factors in controlling reservoir quality
are the presence of original high energy facies and the
extent of anhydrite cementation, whilst dolomitisation
only had a marginal role. To date, shoal facies have
been identied from wells inboard of the platform
margin, and are predicted to be thicker, coarser grained
and have enhanced poro-perm properties along the
yet-to-be explored platform margin and upper slope.
Public domain data from onshore reservoir
analogues, such as the BMB elds onshore Poland,
are noteworthy in showing significant levels of
productivity in reservoirs with a similar depositional
environment and diagenetic history to the MNSH.
This provides encouraging indications, based on
modelling results, that developing such reservoirs will
be economically viable in the shallow water oshore
MNSH environment.
Additional exploration drilling is required to further
de-risk the petroleum system and the Hauptdolomit
play in the MNSH. More wells are needed to fully
calibrate seismic, better dene carbonate depositional
environments, rene the petroleum system model and
most importantly to nd viable, producible resources
of oil or gas and so prove the commerciality of the
Hauptdolomit play. A multitude of highly attractive
exploration opportunities exist in the Orchard Platform
area in a variety of depositional settings within the
carbonate platform, and with additional potential for
resedimented carbonates in base of slope plays.
ACKNOWLEDGEMENTS
The authors would like to acknowledge Horizon
Energy Global Corporation’s management for allowing
us to put together the paper during a challenging time
in the UK Oil and Gas Industry. We would also like to
thank TGS for their kind permission to publish seismic
gures and for their continued support. Technical
review comments from Giancarlo Rizzi (Task Fronterra
Geoscience) and Jo Garland (Cambridge Carbonates)
have been particularly instructive and helped to
improve the submitted version of the paper, along with
helpful suggestions from JPG editorial sta.
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