Revealing the Intra-Zechstein hydrocarbon prospectivity on the UK Mid North Sea high

By Paul Bellingham, Will Reid, Emily Kay and Neil Hurst; ION Geophysical

Published at: May 18, 2021

The Intra-Zechstein plays on the flanks of the European Permian basin are complex and highly variable. The sequence includes repeating anhydrite, dolomite, and halite sequences with significant lateral facies variation. While the center of the Permian Basin is dominated by more massive halite units, the basin margins contain higher proportions of platformal carbonate and anhydrite units.

These marginal areas have proven successful in Poland, Netherlands and both onshore and offshore UK, most recently with the Ossian discovery in 2019 on the southern flank of the UK’s Mid North Sea High (MNSH). Interest in the area was further demonstrated by the success of recent licensing in the UK’s 32nd round when Shell, Deltic and Horizon all took new acreage within the play fairway, joining existing players Spirit, Chrysaor, Ardent, One-Dyas and Draupner.

In 2020, ION completed the first phase of the MNSH PRIME 3D survey, covering the entire Zechstein play fairway in the UK MNSH area. The second phase, which is industry-supported, is currently in acquisition and will be complete later this year with products available in early 2022 (Figure 1).

Figure 2: Interim depth section across the first phase of ION MNSH PRIME 3D. Lateral variation in intra-Zechstein seismic facies is clearly seen with important changes between platformal and basinal facies in the lower Zechstein Z1 and Z2 units. Note the unit thins to the east in this image due to decrease in accommodation space. All seismic images North Sea Normal, white is hard. INSET: detailed stratigraphic breakdown of the Zechstein showing the repeating carbonate, anhydrite and halite units and the lateral variation between more platformal areas on the left (carbonate/anhydrite dominated) and more basinal areas on the right (halite dominated), from Patruno et al., 2017.

A newly acquired 3D seismic survey, covering the entire Zechstein play fairway in the UK Mid North Sea High, reveals the hydrocarbon potential of the complex and highly variable Intra-Zechstein plays on the flanks of the European Permian basin.

Figure 1: IONs MNSH PRIME 3D survey. The blue outline shows the first phase acquired in 2020 and the red outline shows the second phase being acquired in 2021. Held acreage is in green and previously identified prospects and leads are shown in pale pink. The black outline marks the interpreted edge of the anhydrite Z2 platform, mapped from previous seismic data. Interpretation of the ION PRIME 3D will further refine this map.

The Zechstein: an ignored and bypassed play

Exploration in the Southern North Sea (SNS) and on the Mid North Sea High (MNSH) area has focused on the clastic plays of the Lower Permian Rotliegend Group and earlier Carboniferous units in dominantly structural traps. The Zechstein, in contrast, is typified by multiple thin dolomite units within a dominantly anhydrite and halite sequence with variable reservoir facies and with complex geometries (Figure 2). With seismic quality unable to easily resolve reservoir variability, it was not the primary target for exploration, also meaning that wells through the Zechstein unit were neither aimed at the most prospective parts, nor significantly investigated.

However, there are several hints toward the potential of this unit. But exploration efforts are hampered by a lack of large, modern, high-quality 3D seismic in the most prospective areas of the play. The ION MNSH PRIME 3D is therefore critical to imaging these complex repeating units, their lateral variability and building an understanding of reservoir distribution and prospectivity.

Time to reconsider

In the UK 32nd round, 40% of the acreage was awarded in the SNS. The UK remains an attractive area for investment with one of the highest global returns per barrel. The presence of existing infrastructure, the location of future CCS (e.g., Endurance) and the location of windfarm development (e.g., Sofia and Dogger Bank), all help to lower the carbon footprint for oil and gas while maintaining profitability, making the SNS an area of industry focus.

The geology also favors the SNS/MNSH area to demonstrate the potential in the Zechstein, especially the Z2 Hauptdolomit unit (inset Figure 2). One of the earliest discoveries offshore in the SNS was Resolution (1966) which found oil in the Z2 Hauptdolomit just offshore Yorkshire. While reservoir quality was poor and the discovery remains undeveloped, Shell have recently farmed-in with a view to further appraisal and potential development.

In 1990, the Crosgan discovery, whilst mainly targeting Carboniferous, discovered gas in the Hauptdolomit. In 2019, the Ossian discovery (Figure 1) tested oil in the Hauptdolomit and will be ready for appraisal in early 2022, and onshore UK, the West Newton oil discovery was made in the same unit, targeting the platformal facies to optimize reservoir quality.

All these results and those on the margin of the Permian basin demonstrate that the Hauptdolomit is an attractive target with excellent reservoir properties in areas of platformal facies. The identification of these platform facies from the less prospective basinal facies, requires regional high quality 3D seismic to fully constrain the distribution. IONs MNSH PRIME 3D survey enables the explorer to do just this.

Spectacular initial results

Imaging of the first phase of the MNSH PRIME 3D is in progress, using a full range of modern processing and imaging techniques. Initial results show a significant improvement in imaging quality over previous datasets. Figures 3–5 show sections and horizon maps of the Top Zechstein, Plattendolomit (Z3) and Hauptdolomit (Z2) respectively.

Figure 3: The top of the Zechstein unit. The west (a and b) shows minor deformation of overlying Triassic and upper Zechstein units forming pronounced salt ridges. In the center and east of the survey (c and d) the upper Zechstein, which is salt dominated, has been evacuated down to the Z3 Plattendolomit, which is also faulted, and infilled with Triassic collapse structures. Significant accommodation space is created but with limited tectonic deformation; note largely undeformed base Zechstein.

The Top Zechstein (Figure 3) shows varying amounts of halokinesis causing collapse and thickening of the overlying Bacton group Triassic strata, itself a potential target. The initiation of these collapse features is inherently related to the edges of the more platformal seismic facies units in the lower Zechstein, highlighting that movement is initiated where there are rapid changes in the amount of halite in the section.

The Z3 Plattendolomit unit is a strong seismic marker but is variable across the area (Figure 4). Where the platformal zones dominate, and the overall Zechstein is thinner in the east, the Plattendolomit is characterized by karstification and the presence of sinkhole-type features. These are not seen further west where there is a thicker sequence and the Plattendolomit is surrounded by more halite-rich units. Here we see more gentle, salt-cored ridges and no evidence for exposure or karstification. Note that the ridges form early and are layer bound.

Figure 4: The Z3 Plattendolomit varies between a set of salt cored ridges on an otherwise smooth horizon in the west (a and b), and a karstified horizon with and sinkhole features in the east (c and d).

The Z2 Hauptdolomit (Figure 5), is more enigmatic seismically but is a critical horizon to pick accurately; it is the most prospective reservoir unit where it is a more elevated, thicker platformal facies, proven by previous successes. Additionally, where it is a more basinal facies it has also been demonstrated to be an effective source rock, the ‘Stink-dolomite’. In the west of the survey, the platform edge is well picked out seismically defining potential targets; in the east we see more erosion and collapse in this unit, giving the potential for karstified secondary porosity development.

Hauptdolomit unit shows progression from more platformal to more basinal seismic facies in the west of the survey (a, b, c) while in the east of the survey the platformal seismic facies is more prominent with frequent collapse features.

Overall, our initial results from this exciting new dataset show how high quality data can illuminate and unlock the potential of this previously largely overlooked play in the UKCS.

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