Role of Cenozoic fault reactivation in controlling pre-rift plays, and the recognition of Zechstein Group evaporite-carbonate lateral facies transitions in the East Orkney and Dutch Bank basins, East Shetland Platform, UK North Sea

Abstract

The East Orkney and Dutch Bank basins are located to the north of the Moray Firth rift arm in the UK North Sea. The East Orkney Basin measures approximately 60 by 30 km and is bounded to the west, north and east by large normal faults. The West Fladen High consists of a series of horsts and grabens, and separates the East Orkney Basin from the Dutch Bank Basin, which is situated immediately to the north of the Witch Ground Graben and to the west of the Fladen Ground Spur. Although exploration well results for the limited penetrations of structural highs on the margins of the East Orkney Basin have been disappointing, geophysical and geochemical analyses all appear to indicate the presence of a mature source in the region. Integration of wireline data, core samples, and the interpretation of 2D seismic data within the area allow mapping of the main structures in the basins and correlation to the adjacent well penetrations, thus not only enabling a tectonic and stratigraphic history, but also a framework for prospectivity to be established. A pillow-shaped sedimentary package at approximately 3 seconds two-way traveltime within the centre of the East Orkney Basin represents the downdip equivalent of stratigraphy on the West Fladen High, and is interpreted to be a Zechstein Group salt body, underlain by a thick sequence of Lower Permian (Rotliegend Group) and Devonian strata. Cored intervals from well 14/2-1 on the West Fladen High contain carbonates, evaporite dissolution breccias and anhydrites, implying a lateral change in sedimentary facies between structural highs and lows. Rifting took place through the Triassic and Jurassic, followed by relative tectonic quiescence in the Cretaceous, but as in adjacent areas, the Cenozoic witnessed structural inversion related to plume-generated uplift in the North Atlantic and the activation/reactivation of underlying faults coupled with deformation of the basin fill adjacent to these structures. Regional tilting from the Paleocene to early Eocene led to an influx of siliciclastics into the Dutch Bank Basin. The main play type is thought to consist of Rotliegend Group and Devonian sandstone reservoirs on the West Fladen High, sealed by Zechstein Group anhydrites or Cretaceous/Tertiary mudstones and charged by a deeply buried Devonian (Orcadian) lacustrine source. Although hydrocarbons are interpreted to have utilized normal faults as migration pathways, comparison with adjacent areas and observations of numerous oil seeps on the sea surface suggest that reservoir breach through reactivation of these faults to the seabed poses the main exploration risk. This fault reactivation has resulted from differential uplift combined with synchronous extensional faulting that affected adjacent areas, including the Inner Moray Firth.