New insights into the unexploited reservoir potential of the Mid North Sea High (UKCS quadrants 35–38 and 41–43): a newly described intra-Zechstein sulphate–carbonate platform complex

The Mid North Sea High (MNSH) is located on the UKCS in quadrants 35–38 and 41–43. It is a large structural high that is flanked by the mature hydrocarbon provinces of the Central North Sea (CNS) to the NE and the Southern North Sea (SNS) to the SE. In the MNSH region, the source and reservoir intervals that characterize the SNS (Westphalian, Lower Permian) are absent and therefore the area is relatively underexplored compared to the SNS Basin (c. one well per 1000 km2). Nevertheless, two discoveries in Dinantian reservoirs (Breagh and Crosgan) prove that a working petroleum system is present, potentially charged either via lateral migration from the SNS or from within the lower Carboniferous itself. Additionally, gas was found in the Z2 carbonate (lower Zechstein Group) in Crosgan, with numerous other wells in the area reporting hydrocarbon shows in this unit. The results of the interpretation of recently acquired 2D and 3D seismic reflection datasets over parts of UKCS quadrants 36, 37 and 42 are presented and provide insight into both the geology and prospectivity of this frontier area.This study suggests that intra-Zechstein clinoform foresets represent an attractive, hitherto overlooked, exploration target. The Zechstein Group sits on a major unconformity, probably reflecting Variscan-related inversion and structural uplift. Below it, fault blocks and faulted folds occur, containing pre-Westphalian Carboniferous and Devonian sediments, both of which contain potential reservoirs. In the lower Zechstein, a large build-up is observed, covering a total area of 2284 km2. This is bounded on its margins by seismically defined clinoforms, with maximum thicknesses of 0.12 s two-way time (c. 240–330 m). This rigid, near-tabular unit is clearly distinguished from the overlying deformed upper Zechstein evaporites. In map-view, a series of embayments and promontories are observed at the build-up margins. Borehole data and comparisons with nearby discoveries (e.g. Crosgan) suggest this build-up to represent a Z1–Z2 sulphate–carbonate platform, capped by a minor Z3 carbonate platform. Interpreted smaller pinnacle build-ups are observed away from the main bank. The seismic character, geometry, size and inferred composition of this newly described Zechstein platform are similar to those of platforms hosting notable hydrocarbon discoveries in other parts of the Southern Permian Basin. The closest of these discoveries to the study area is Crosgan, which is characterized by the Z2 carbonate clinothem (Hauptdolomit Formation) as a proven reservoir.