19th November 2019
Abstract
Joint lecture with
Food and drink served from 6:00pm, lecture starts at 6:30pm. PESGB or Geological Society of London Members do not need to register to attend.
‘The Neglected Reefs of The Zechstein Haupt Dolomite – Future Exploration Target’
With Peter Browning-Stamp – Geoscientist, Ardent Oil
The Zechstein (Z2 Cycle) Haupt Dolomite has for many years been a forgotten and neglected exploration target In the UK Offshore, despite successful production from other areas of the Southern Permian Basin, including Onshore in the UK. There are a number of reasons for this. Since the earliest days of Southern North Sea exploration, the play has been overshadowed by drilling campaigns focussed on Rotliegend, Bunter and deeper Carboniferous targets. This is not hard to understand, given the early impetus of UK explorers hunting the next Groningen Field, mapping sizeable prospects, even in those days, on early 2D seismic data. Vastly improved seismic over the years then led to a ceaseless stream of prospects mapped at these target levels, and the reservoirs, by comparison with the fickleness of Zechstein carbonates, were very user friendly. From the earliest wells however, encouragement was there to find within the Zechstein carbonates if you looked for it, with tantalising shows, porosity and several discoveries.
Detailed palaeogeographies were however little understood, a situation not helped by the diagenetic overprint on the original depositional environment, and a general lack of core data to study. Notwithstanding this, Ardent Oil have constructed palaeographical maps for the Zechstein, and it immediately becomes clear that the ‘sweet spot’ areas for optimal depositional reservoir facies lie mainly in those parts of the basin that have not benefited historically from the luxury of extensive 3D data coverage. The focus area for the Zechstein lies near the southern margin of the Mid North Sea High, in the southern portions of quads 36 and 37. This area is close enough to infrastructure to be of interest and lies away from any environmental sensitivities.
Another important positive step for this part of the basin is that the historical lack of 3D data, and hence our lack of detailed understanding of the seismic facies, is currently being addressed by a new seismic survey sponsored by Ardent. But this is not all that is happening. 2019 has heralded the long overdue renaissance of the Zechstein Play, with at least one well offshore well drilled targeting the Zechstein Carbonates (currently of tight hole status), along with testing at West Newton onshore UK in PEDL183. Serious interest is now being shown by several companies, both large and small. Additional 3D as well as 2D data are being acquired over key areas, as the play gears up to become a future exploration hotspot.
Based only on the historical 2D data available to us, across the southern boundary of quads 36/37, we have extensively mapped a large reefal promontory (affectionately named the Elephants Foot). The principal reservoir comprises of reefal build-ups of the Z2 cycle Hauptdolomite, deposited as non-skeletal limestones in shallow water settings (well within the photic zone), above a legacy topography composed of Carboniferous anticlines that were subsequently draped by thickened banks of Z1 Werra Anhydrite. These were subsequently covered by the overlying Z2 Strassfurt Halite and Anhydrides. The Formation is believed to have been extensively altered post deposition by dolomitization and heavy fracturing, which can (even pre-3D) be seen on the legacy 2D seismic. It is expected that very little of the original fabric remains. The source rocks for the play are believed to be lowermost Namurian and Visean shales, which source the nearby Breagh Field (500bcf), and the cluster of Zechstein gas fields in the Cleveland Basin. The Zechstein may also be self-sourcing and potentially oil prone in the area!
Being armed with new state of the art 3D seismic gives us the opportunity to pursue a variety of exploration plays, seen and illuminated clearly for the first time. These range from isolated patch reefs (as seen at Crosgan), shoal features (as seen at West Newton), reefal front build-ups, talus slopes and back reef ooid banks and lagoonal areas- to name but a few. Ardent Oil have been working on this play for a number of years now, but certainly do not claim to have all the answers! By its very nature the Zechstein is complex, requiring detailed facies mapping and a clear understanding of the prospect geometries, as traps are likely to involve components of stratigraphic as well as structural seal, both lateral and vertical, along with diagenetic and / or fracture related components of either porosity destruction or enhancement. The ability to map in this detail is what has been historically lacking in the play. Explorers of 2019 and beyond should be able to carefully map Zechstein prospect geometries, using 3D data to best effect, while simultaneously hunting porosity using inversion or other techniques, depending on the available calibration data.
Figure 1. Paleo-Geographical map showing the area referred to as the Elephants foot.
