DTI Strategic Environmental Assessment Area 6, Irish Sea, seabed and surficial geology and processes

Hydrocarbons prospectivity • The East Irish Sea Basin is at a mature exploration phase. • The hydrocarbons-prospective sedimentary basins are characterised by source rocks, an abundance of structured regional petroleum reservoir and seal rocks and by suitable timing of geothermal events for generation and transfer of petroleum products from the source rocks to the reservoir rocks. • SEA6 has regionally diverse seabed habitats which vary significantly from area to area in currently licensed acreages in the eastern Irish Sea. The variations in seabed habitats have been systematically related to the sedimentary processes driven by quantifiable patterns of mean seabed stress. Sedimentary Processes • Open shelf seabed sedimentary processes are driven by seabed stress originating from interaction of the seabed with strong currents generated by tidal streams and by waves. • The scale of the stress imposed on the seabed and the related seabed habitat variability varies from regional (between mainlands, varying shelter around headlands) to macroscopic (around boulders and pebbles). • During the fair to moderate weather conditions characteristic of the late spring, summer and early autumn seasons, the seabed sediment types on the open continental shelf are dominated by the stress imposed on the seabed by the strengths and flow directions of the peak tidal currents. In this setting most of the regional variations in seabed sediment types are dominated by the effects of the coastal configurations on the tidal streams. • In areas where the seabed stress from waves is dominant, the seabed sediments coarsen with exposure to the increasing seabed stress generated when the waves interact with the seabed. Seabed stress from waves is dependent on wave power that varies with weather, wave fetch, seabed slope, wave direction and water depth. • There are knowledge gaps on possible regional variations of seabed properties when the seabed is stressed during extreme weather events associated with storm surge and storm waves. • In the most highly stressed seabed environments, exposed bedrock and strongly cohesive unsorted gravelly, sandy and muddy sediments are often swept clean of unconsolidated muds, sands, granular gravel and pebbles. Parts of the seabed in these areas may consist of cobbles and boulders. Environments of least seabed stress are characterised by fine-grained muddy sediments. Mobile sandwaves are characteristic of areas where sediments are being transported along the seabed in environments that are situated between the areas of extremely high seabed stress and very low seabed stress. The sense of regional seabed sediment transfer is from and across areas of high seabed stress to areas of lower seabed stress. The observations summarised above indicate that if large-scale disruptions to the natural seabed habitat are to be avoided, new development scenarios should avoid barriers that could have a significant effect on the regional patterns of seabed stress. • As elsewhere on the UKCS, glacigenic sediments and relict static glacigenic bedforms have had significant regional and local effects on the patchiness of the distribution patterns of seabed sediments and seabed habitats. • There is a knowledge gap in the research evidence required to securely link subregional increases in the percentage of biogenic carbonate in the sand fraction of the Irish Sea Mud Belt with increased biological productivity of surface waters, with methane expulsion from shallow and seabed sediments or with processes of bedload carbonate transport. • Investigations of shipwrecks and artificial continuous barriers indicate that the amount of seabed scour is much larger than the profile of large seabed obstacles presented to near-bed current flow. The observations reveal patterns of scour asymmetry consistent with model predictions of mean peak tidal current speeds and the interpretations of the directions of regional sediment transport based on the geometries of seabed bedforms. Wreck studies could therefore be used to calibrate modelling on the likely long term effects of future seabed development scenarios. Shipwrecks have also contributed to seabed diversity. • Active pockmarks, bioherms, banks in less than 20m water depth and some shipwrecks are already regulated by conservation measures. The following geological features are also worthy of consideration for preservation because they are irreplaceable: Static bedforms — sarns, pingos, upstanding rock outcrops in mud belts Mobile bedforms — banner banks, estuary banks and spits • A gateway for sand exchange between the open shelf and the eastern Irish Sea coast off England appears to be defined by a zone situated between North Wales and the southern limit of the Eastern Irish Sea Mudbelt. Although the current prospects for large oil and gas developments in this area are very small, any developments that could the patterns of sand exchange through this environmentally sensitive area should be avoided.