Multi-layered salt-prone stratigraphy and its influence on rift-basin development; The Slyne and Erris basins, offshore NW Ireland

Abstract

While salt-influenced rift basins have been identified across the European Atlantic margin, the Irish Atlantic margin remains the exception despite salt being proven in several boreholes. Using an extensive seismic database coupled with data from exploration wells and shallow boreholes, this study maps the distribution and composition of salt layers and investigates their role in the structural evolution of the Slyne and Erris basins offshore west of Ireland. Two salt-prone intervals have been proven: The Upper Permian Zechstein Group and the Upper Triassic Uilleann Halite Member. The Zechstein Group is present throughout the Slyne and Erris basins, while the Uilleann Halite Member is only developed in the Northern Slyne and Southern Erris sub-basins. Where salt-prone, both layers mechanically detach pre-, post-, and intra-salt stratigraphy. Both layers underwent halokinesis during basin development, creating a variety of salt-related structures; The Zechstein Group forms salt pillows and salt rollers, causing folding and rafting in the overlying Mesozoic section, driven by active faulting within the pre-salt basement. The Uilleann Halite Member caused thinskinned crestal collapse of the overlying Jurassic section above anticlines cored by Zechstein salt. Where both Triassic and Permian salt are present, unique structural geometries are formed when two stratigraphically discrete but kinematically linked halokinetic structures develop. The most common structural configuration consists of a Zechstein salt pillow and an Uilleann Halite salt wall separated by Lower Triassic sandstones. The fold-axis of the salt pillow trends parallel to the strike of the salt wall. The results of this study provide a framework for the evolution of halokinetic structures in other basins on the Irish Atlantic margin, greater insight into the Permian and Triassic paleogeography of the region, as well as having more general implications for the evolution of salt-related structures in rift basins with multiple stratigraphically discrete salt layers. This research is funded in part by a research grant from Science Foundation Ireland (SFI) under Grant Number 13/RC/2092 and is co-funded under the European Regional Development Fund, and by the Petroleum Infrastructure Programme (PIP) and its member companies. Efstratios Delogkos is thanked for thoughtful discussions regarding the links between the Slyne and Erris basins and the Bróna and Pádraig basins. Neil Jones and Kristian Lomas are thanked for engaging discussions on the structural evolution of the Slyne and Goban Spur basins respectively. The authors would like to thank the Petroleum Affairs Division (PAD) of the Department of Communications, Climate Action and Environment (DCCAE), Ireland, for providing access to released well, seismic, and potential field datasets. Europa Oil & Gas are thanked for providing access to the Inishkea 2018 reprocessed seismic volume and allowing a section from the volume to be shown. Shell Exploration & Production Ireland Ltd. are thanked for providing acc...