Geological settings and controls of fluid migration and associated seafloor seepage features in the north Irish Sea

Mark Coughlan,Srikumar Roy,Conor O'Sullivan,Annika Clements,Ronan O'Toole,Ruth Plets

doi:10.1016/j.marpetgeo.2020.104762

Abstract

Shallow gas accumulation in unconsolidated Quaternary sediments, and associated seepage at the seafloor, is widespread in the north Irish Sea. This study integrates high-resolution seafloor bathymetry and sub-surface geophysical data to investigate shallow gas accumulations and possible fluid (gas and/or liquids) migration pathways to the seafloor in the northern part of the Irish Sea. Shallow gas occurs broadly in two geological settings: the Codling Fault Zone and the Western Irish Sea Mud Belt. The gas has been recognised to accumulate in both sandy and muddy Quaternary marine near-surface sediments and is characterised by three characteristic sub-bottom acoustic features: i) enhanced reflections, ii) acoustic turbid zones, and iii) acoustic blanking. The seepage of shallow gas at the seafloor has resulted in the formation of morphological features including methane-derived authigenic carbonates, seabed mounds and pockmarks. In many instances, the evidence for this gas as biogenic or thermogenic in origin is inconclusive. Two distinct types of pockmarks are recorded in the Western Irish Mud Belt: pockmarks with a relatively flat centre, and pockmarks with a central mound. Based on our observation and existing models, we infer that the formation of a carbonate crust at the seabed surface is needed as a precursor for the creation of such mounds within pockmarks. The formation processes are interpreted to be different for sandy versus muddy sediments, due to variability in erodibility and sealing capacities of the substrate. We suggest that the origin of these features is linked to the presence of deeper hydrocarbon source rocks with existing and reactivated faults forming fluid migration pathways to the surface. This in turn could indicate a mixed thermogenic-biogenic origin for seep-related structures in the study area. These features have significant implications for the future development of offshore infrastructure including marine renewable energy as well as for seabed ecology and conservation efforts in the Irish Sea.

Highlights

The accumulation of gas in shallow, unconsolidated marine sediments is aa global phenomenon (Andreassen et al, 2007; Dondurur et al, 2011; Ergün et al, 2002; Hovland and Judd, 1992; Karisiddaiah and Veerayya, 1994; Mazumdar et al, 2009)
To date in the Irish Sea (Fig. 1), a number of areas associated with shallow gas and fluid seepage have been designated as Special Areas of Conservation (SAC) due to the unique habitats they form as “Submarine structures made by leaking gases”, according to the Annex I / II of the E.U
Six key horizons were mapped in the vicinity of the Lambay Deep and Kish Bank Basin where formation tops from four hydrocarbon exploration boreholes provided stratigraphic control: (i) Seabed; (ii) BaseQuaternary; (iii) Base-Cenozoic; (iv) Top Lower Triassic; (v) Top Permian; (vi) Top Basement (Carboniferous & older) (Fig. 1 and Fig. 3)

Summary

Introduction

The accumulation of gas in shallow, unconsolidated marine sediments is aa global phenomenon (Andreassen et al, 2007; Dondurur et al, 2011; Ergün et al, 2002; Hovland and Judd, 1992; Karisiddaiah and Veerayya, 1994; Mazumdar et al, 2009). During the Cenozoic event, the Irish Sea experienced kilometre-scale uplift resulting in the present-day configuration of erosional outliers, which are remnants of a much larger rift system (Jackson and Mullholland, 1993) These rift basins include the Kish Bank Basin and Peel Basin, both of which have been the focus of hydrocarbon exploration during the last fifty years (Fig. 1) (Dunford et al, 2001; Newman, 1999). Lithologies capable of generating hydrocarbons have been encountered in the Carboniferous, including the gas-prone Pennine Coal Measures Group and the oil-prone Bowland Shale Formation (Fig. 2) These source rocks have generated significant quantities of hydrocarbons, with an estimated 1.8 BBOE (Billion Barrels of Oil Equivalent) discovered in the East Irish Sea Basin (Bunce, 2018). While no commercial discoveries were made, the presence of the Pennine Coal Measures Group was proven in the 33/22-1 well on the southern margin of the Kish Bank Basin (Thomas, 1978)

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Conclusion