As the United Kingdom reduces its CO2 emissions in order to meet its 2050 net zero greenhouse gas targets, there will be a significant evolution of the UK's energy mix. The reliance on hydrocarbons will decrease while there is predicted to be an increase in low carbon energy sources such as renewables and nuclear. In order to decarbonise and achieve the net zero emissions targets while concurrently producing enough energy to provide for national energy needs, large-scale, low carbon energy generation projects need to be developed alongside energy storage facilities to provide flexibility within a low carbon energy supply. Robust CCUS programmes will need be developed in order to capture and store unavoidable carbon dioxide emissions. The subsurface geology of the UK provides opportunities for the development of low carbon energy generation, energy storage and CCS, and the Upper Permian Zechstein Supergroup deposited in eastern England and offshore in the Southern North Sea is a potential host for these new developments. In NE England, salt cavern gas storage sites have been developed in thick Zechstein evaporites since the mid 20th centrury. In this paper we present new isopach maps and well correlation panels which will help to outline optimal locations for the development of additional salt caverns for gas storage. A review of the Zechstein Supergroup indicates that it does not exhibit great potential for the development of CCS, due both to its complex reservoir characteristics and to difficulties with both subsurface imaging and monitoring. However thick Zechstein evaporites could provide an excellent seal for CO2 storage in the underlying Lower Permian Rotliegend Group.
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 first 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 fields. 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 first hydrocarbon discovery was made at Ossian (well 42/04-01/1Z). Geochemical data from the latter discovery have pointed to the presence of a prolific 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 refined using recently acquired 3D seismic data.
The preserved Zechstein succession on the Utsira High in the NE part of the Norwegian North Sea is 25-100 m thick and is dominated by shelf carbonates. Internal subdivision of the succession is based on the recognition of key surfaces in petrophysical logs and cores, and suggests that the carbonates mainly consist of ZS2 and ZS3 deposits and that younger ZS4 and ZS5 deposits are only locally preserved. The carbonates have undergone early, syn-depositional dolomitization followed by later dolomite recrystallization and calcitization. Calcitization, interpreted as dedolomitization, is restricted to the upper part of the ZS3 carbonate unit and based on U/Pb dating took place during the Triassic, with a later phase of recrystallization linked to mid-Jurassic uplift. Both dedolomitization and dolomite recrystallization relate to fresh-water infiltration with the resetting of δO18 values prior to the Late Jurassic drowning of the Utsira High. The reservoir quality of the carbonates is directly linked to post-depositional meteoric diagenesis, and the best reservoir properties are recorded in intervals dominated by recrystallized dolomites in ZS2 and lower ZS3 carbonates. Dedolomitization significantly reduced porosity in the upper ZS3 carbonates.
The Upper Permian Zechstein Supergroup has the potential to play an important role in the UK's future energy production and energy transition. However the Supergroup is comparatively poorly understood in the UK, particularly the link between the onshore and offshore geology. In this paper we re-evaluate available data in order to present a consistent regional interpretation of the Z1 to Z3 Zechstein Supergroup cycles. This review is based on an interpretation and re-evaluation of 620 offshore wells located in the UK portion of the SW Southern North Sea and 83 onshore wells located in Yorkshire and Lincolnshire (eastern England). The Zechstein Supergroup was interpreted in each well, and the data was used to compile seven SW-NE oriented correlation panels which show the development of the Supergroup in the study region. Five isopach maps for key formations in the Zechstein Supergroup were created, together with depositional environment maps for each of the main Zechstein carbonate formations. In combination, these regional-scale maps and diagrams have resulted in a consistent interpretation of the Zechstein Supergroup over an area which extends from the onshore outcrop in the west to the UKCS boundary in the Southern North Sea in the east.
A multidisciplinary approach combining geological mapping based on seismic and well data with petrographic analyses of core and cuttings samples was used to gain a better understanding of the distribution of Upper Permian (Zechstein, Z2) Hauptdolomit platforms and their depositional facies around the Elbow Spit High in the northern Dutch offshore. A detailed understanding of the Hauptdolomit's lateral facies variability is of great importance for assessing its reservoir potential, since both the thickness and reservoir properties of these carbonate platforms greatly depend on local accommodation within different palaeo-depositional environments. The platforms generally contain the thickest Hauptdolomit sequences and are largely characterised by a mix of oolitic and coated grainstones, as well as by some dolomicrites. Porosities of around 15% are reached at well E02-02 within the grainstone intervals, and interconnectivity between the pores is generally present. Seismic mapping has indicated a rim of isolated Hauptdolomit platforms, which are up to 10 km wide, around the southern and NW margins of the Elbow Spit High. No Hauptdolomit platforms are present on the NE margin of the High, likely because the palaeo- basin margin was too steep and hence lacked accommodation for carbonate growth. Discoveries made in recent years in the UK sector of the southern North Sea have highlighted the importance of the Hauptdolomit hydrocarbon play, and the results of the current study provide a solid base for assessing the reservoir potential of this play in the relatively underexplored northern part of the Dutch offshore.
Oil in the Jarvis structure underlying the main Upper Jurassic reservoir at the Ettrick oilfield (Outer Moray Firth, UK northern North Sea) is present in Upper Permian (Zechstein) carbonates. The origin of this “Jarvis oil” is investigated in this paper using a multidisciplinary approach based on data from well-logs and cores from wells 20/02-2 and 20/02-3. Reservoirs at the Jarvis structure consist of carbonates in the upper part of the Halibut Carbonate Formation (Ca2) and in the Carbonate Member of the Turbot Anhydrite Formation (Ca3). These carbonates are typical Zechstein dolomites composed of a range of facies from mudpackstones with storm beds deposited at moderate water depths to shoreface bioclastic-oolitic packstones to shallow-subtidal and intertidal microbial laminites. Interbedded anhydrites replace sabkha and lagoonal selenitic gypsum. Several shallowing-upward units are recognised. Molecular analysis of the Jarvis oil, and comparisons with biomarker and other geochemical data from extracts of Zechstein cores and published data from different source rocks from the North Sea area, suggest that the oil was generated by marine, OM-rich shales in the Upper Jurassic Kimmeridge Clay Formation. The oil was generated at peak oil window maturity and is characterised by high Pr/Ph, BNH/H and DBT/P ratios, and abundant C28 steranes and C28+29 monoaromatic and C26R + C27S triaromatic steroids. The molecular composition of organic material in extracts of core samples of Zechstein carbonates from wells in the Jarvis structure differs significantly from that of the Jarvis oil. Biomarkers such as BNH are absent in the core extracts, and there are different distributions and abundances of saturated and aromatic hydrocarbons, likely controlled by thermal maturity.
This paper provides an updated understanding of the reservoir stratigraphy, sedimentology, palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the Mid North Sea High. The paper is based on the examination and description of core and cuttings data from 25 wells which were integrated with observations based on existing and new 3D seismic.
Based on thin-section petrography of cuttings and core from the wells studied, it is evident that Hauptdolomit microfacies are distributed in a relatively predictable way, and well-defined platform interior, platform margin, slope and basin settings can be distinguished. Platform margins are typically characterised by the development of ooid shoals and, to a lesser-extent, by microbial build-ups. High-energy back-shoal settings are characterised by a more complex combination of peloid grainstones, thrombolitic and microbial build-ups, and fine crystalline dolomites. Lower energy lagoons which developed further behind the platform margin are characterised by a variety of microfacies types; fine crystalline dolomites are common in this setting as well as peloidal facies and local microbial build-ups. Intertidal and supratidal settings are typified by increased proportions of anhydrite and the development of laminated microbial bindstones (stromatolites). Platform margins are in general relatively steep and pass into slope and basinal settings. Only a few wells have penetrated Hauptdolomit successions deposited in a slope setting, and these successions are characterised by a range of resedimented shallow-water facies together with low-energy laminated dolomicrites and fine crystalline dolomites. Slope zones in the study area are interpreted from seismic data to be typically 1-1.5 km in width. Basinal Hauptdolomit deposits have been strongly affected by post-depositional diagenesis and are dedolomitised to variable degrees. The original depositional facies are rarely preserved.
Diagenetic studies show that dolomitisation has affected almost the entire Hauptdolomit Formation throughout the study area in both basinal and platform settings. The dolomite is considered to result from seepage-reflux processes and is an early diagenetic phase. Mouldic porosity is present in many facies types as a result of dissolution, especially in ooid grainstones, thrombolitic build-ups and peloidal facies. The dissolution cannot be associated with any one diagenetic phase but was most likely a result of the dolomitisation process itself. Stable isotope analyses indicate that all dolomites were precipitated from Permian marine-derived pore fluids. Fluid inclusion analyses of dolomite cements indicate that cementation continued into the burial realm. Anhydrite cementation occurs in two phases: early anhydrite precipitation was associated with dolomitisation, and can be distinguished from a later, pore-filling cement
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