The role of palaeorelief in the control of Permian facies distribution over the Mid North Sea High, UK Continental Shelf

The Mid North Sea High (MNSH) consists of a ridge of Paleozoic strata located in the centre of the North Sea between 55° N and 56° N. In 2010, interest in the Permian of the MNSH was revived by the discovery in Quadrant 44 of the Cygnus gas field. This study focuses on the Zechstein carbonates of the MNSH and uses play concepts that draw an analogy with the Zechstein oil and gas fields discovered in Denmark and Poland, and also with the Wissey gas field in Quadrant 53. New 2D seismic establishes the presence of a significant Zechstein reef that blocks the southern entrance of the Jenyon Gap onto the MNSH. Seismic data show that the reef developed in stages and its presence can be inferred from the occurrence of isolated lagoons within the centre of the build-up. The barrier reef's existence explains both the presence of the rare hygroscopic mineral tachyhydrite in the centre of the MNSH and also the observed isopach difference in the Zechstein cycles over the MNSH, as the barrier restricted marine-water ingress onto the MNSH and allowed the creation of a ‘crystal-starved basin’, the evaporitic equivalent of a sediment-starved basin.