The Zechstein play of the Mid North Sea High - an analogue study

Updated: Sep 23, 2021

Over the summer of 2021, 1st Subsurface provided three University of Aberdeen students with MSc projects. Focussing on the Southern Permian Basin, and using TROVE KnowledgeBases to inform their research, these projects were a great success.


Over 3 weeks our students (Alexzandra, Queen & Angeliki) will be sharing their findings. We'd like to thank all 3 of the students for their hard work throughout and wish them all the best for their careers.


This week, Queen describes her project focussing on the Permian of the Mid North Sea High, and potential Polish analogues.


If you would like to reach out to Queen, you can do so here: nokuzothabhebe@gmail.com


 

For my MSc Project this summer I compiled publicly derived data (well reports, seismic, 3D outcrops etc.) from literature and other public databases. I used this to discuss my thesis title “Lessons from a Permian Play: Zechstein Onshore Poland - An analogue for the Central North Sea (Supported by North East England)".


The main aim was to present the potential of the Zechstein hydrocarbon play on the MNSH, by gaining insight from reservoir analogues in onshore Poland and Northeast England. The objective was aimed at understanding whether this comparison can help improve qualitative understanding of the Zechstein supergroup and its application to the MNSH.

Fig 1: Palaeography of Zechstein in the Permian Basin NW Europe. Boundaries are North Permian Basin (NPB),Mid North Sea High ( MNSH)/ Central North Sea (CNS) and South Permian Basin (SBP) showing study area.

Perceived Challenges


Exploration in the CNS has predominantly focused on the Lower Permian Rotliegend group and Upper Carboniferous reservoirs, whilst the Upper Permian play has remained relatively under-explored. Historically, challenges in exploration and production of the Zechstein have been due to issues such as poor seismic imaging quality and in interpreting reservoir evaporative cycles in NE England and the MNSH, respectively. This, in addition to the Variscan erosion in the area, has resulted in the potential of a working Zechstein Play in the Upper Permian being largely overlooked.


Success of Poland


1961 marked the discovery of the first Zechstein Main Dolomite field “Rybaki”; an onshore oil field within the Polish Lowlands. Since then, the Polish lowlands have provided plays which form the bulk of the country’s oil and gas production. Major oil and gas fields include Barnówko–Mostno–Buszewo (BMB), with up to 444 mmbbls and 28.43 Bcm, and Lubiatów-Międzychód-Grotów (LMG) with 296 mmbbls and 21.9 Bcm of in-place oil and gas. The success of exploration and production in Poland formed a good framework analogue for the success the UK may eventually have.


Zechstein Play


The Zechstein Limestone and Main Dolomite form the main oil and gas reservoirs within the marine carbonate formation. Good reservoirs tend to be within carbonate platforms and reefs. Organic matter accumulates within basinal facies of the carbonate platforms forming source rocks. Finally, evaporative units form effective seals, with a combination of structural and stratigraphic traps.


Outcrop Analogue Study, NE England


The results of studying virtual outcrops in South Shields, UK, showed that degree of deformation is not always the same due to the fact that it may be gradual. Diagenetic effects appear to be complex within potential reservoir units making accurate prediction of porosity and permeability difficult.


Mechanical properties within a unit control the intensity and distribution of deformation within a section [1] explaining the variations of collapsed carbonates. Large areas of collapse breccia may increase reservoir quality e.g the success of Auk and Argyll fields in the UK CNS. As reservoir units in England appear to be more affected by brecciation in comparison to Poland, where outcrops are scarce, less information is known on this. Microphotographs from well samples confirm dissolution largely impacts pore systems and permeability in reservoirs. Studying the outcrops helps predict fluid flow and connectivity [1], useful in understanding the subsurface.

Fig 2: Dissolution of Hartlepool Anhydrite (Z1) causing collapse of Roker Formation (Z2) Trow Rocks , South Shields, Taken from (SAFARI Database).

Source Rock Analysis, Poland


Source rocks fall within Type 2 to Type 3 kerogen indicating oil or gas to gas prone rocks. Pristane and Phytane values indicate a restricted reducing environment within a transitional to algal marine depositional zone, such as a lagoon for both Polish and English source rocks. TOC values for Kupferschiefer shales are within 0.2-10%, making an excellent source rock despite concerns of thickness and Main dolomite TOC within 0.2-0.8%. Data confirms that Upper Carboniferous coals and shales, and basinal Zechstein units, are working source rocks for the Zechstein reservoir.

Fig 3: Plot of Hydrogen Index (HI) versus Oxygen Index (OI) obtained from Rock Eval Pyrolysis data for Upper Carboniferous bituminous coals, Zechstein Main Dolomite , Westphalian A&B and Kupferschiefer shales.

De risking the Zechstein Play


Oil seeps were found within the Boulby Halite Fm (Z3) in Yorkshire, UK, 30km away from the MNSH. Sterane data shows oil samples derived from the Boulby Halite Fm (Z3) are thought to be derived from algae/plankton reducing environment, similar to samples within Main Dolomite (Z2).


Migration occurred through the fractured dolomite rock and evaporite units meaning there must have been conduits to aid this. This example de-risks the CNS in terms of providing more evidence to a working source rock within the Main dolomite. This means there is in fact a charge that can be reservoired within the Z2, although we may question the volume of the potential source rock. Additionally, there is evidence in Poland confirming the likelihood of a working source rock / ‘in situ’ oil generation within the Z2 as well as long-distance migration.

Fig 4 : Oil seep, Boulby mine. Taken from ([4] M. Słowakiewicz et al., 2020)

Conclusions and Recommendations


Analogues provide evidence that the Zechstein unit has both conventional and unconventional reservoirs. The supergroup has proven to contain all successful components for a working petroleum system onshore and offshore UK and onshore Poland. It is valuable to consider grouping the Zechstein basin as one geological system, which may help improve understanding of the play.


The scope of Zechstein sourced hydrocarbons have been largely ignored by industry and to

date, there are no known Zechstein sourced fields in the UK MNSH. Completing this project

provided insights on the great potential this Permian play has.


 

Thank you to Jeremy Lockett and Matthew Belshaw for all their help during the duration of my project. Thank you for the support and expertise the team at 1st Subsurface provided and the newfound knowledge I now have.




References

Publicly available industry reports available in the UK Onshore Geophysical Library at: https://ukogl.org.uk/


[1] Daniels, S., Tucker, M., Mawson, M., Holdsworth, R., Long, J., Gluyas, J. and Jones, R., 2020.Nature and origin of collapse breccias in the Zechstein of NE England: local observations with cross-border petroleum exploration and production significance, across the North Sea. Geological Society, London, Special Publications, 494.


[2] Peryt, T.M, Geluk, M.C., Mathiesen, A., Paul J. & Smith, K. 2010. Zechstein. In: Doornenbal, J.C. and Stevenson, A.G (Eds): Petroleum Geological Atlas of the Southern Permian Basin Area. EAGE Publications by (Houten) 123-147


[3] PGI-NRI's Brochure "Oil&Gas in Poland. New Opportunities" [online] Polish Geological Institute. Available at: <https://infolupki.pgi.gov.pl/en/international-news/ > [Accessed 11 July 2021].


[4] Słowakiewicz, M., Gluyas, J., Kowalski, A., Edwards, T., Słama, S., Mawson, M., Tucker,

M., Scovell, P. and Polonio, I., 2020. A new and working petroleum source rock on the UK

Continental Shelf (Upper Permian, offshore Yorkshire). Marine and Petroleum Geology, 121.