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

https://doi.org/10.1016/j.marpetgeo.2020.104762Get rights and content

Highlights

  • An integrated methodology is used to assess fluid flow in the north Irish Sea.

  • Characterisation of an accumulation of shallow gas in Quaternary sediments, 17 new pockmarks and an area of seabed mounds.

  • New mechanisms proposed for pockmark and seabed mound formation for this location.

  • Cenozoic faulting and re-activation of older faults generates pathways for fluids to migrate to the shallow sub-surface.

  • At the seabed, sediment properties play a strong role in the morphological expression of fluid seepage structures.

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.

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). It represents an important tool for frontier hydrocarbon exploration, while also posing a significant geohazard, affecting sediment engineering properties (Andreassen et al., 2007; Hovland et al., 2002; Sills and Wheeler, 1992). The impacts of shallow gas and seepage on seabed ecology has also gained importance over the recent years (Jordan et al., 2019; Kiel, 2010; Rathburn et al., 2000). 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. Habitats Directive (National Parks and Wildlife, 2015). These can form two described habitat types: Bubbling Reefs and Structures within Pockmarks. In the Irish Sea, the SAC areas are predominantly related to Methane-Derived Authigenic Carbonates (MDAC) and are known locally as the Codling Fault Zone (CFZ) SAC and Croker Carbonate Slabs (CCS) SAC (Fig. 1). Further north, Queenie Corner is an offshore site within the Western Irish Sea Mud Belt (WISMB) that was designated as a UK Marine Conservation Zone (MCZ) in 2019 for its subtidal mud habitat and sea-pen and burrowing megafauna communities (Clements and Service, 2016).

Shallow gas in unconsolidated marine sediments can have a biogenic or thermogenic origin. Bulk isotopic analysis on samples from the CFZ by O'Reilly et al. (2014) indicate a biogenic origin of the seeping gas, with some possible thermogenic contribution from underlying Carboniferous coal deposits. Methanogenesis of organic-rich Quaternary sediments has been proposed as a source for shallow gas in Bantry Bay (Jordan et al., 2019) and Dunmanus Bay (Szpak et al., 2015) elsewhere in Irish waters. Evidence for shallow gas accumulations and seepage in the Irish Sea has been detected from geophysical observations on seismic lines as gas chimneys, enhanced reflectors and acoustic turbidity (e.g. Judd and Hovland (1992)). Where fluids (e.g. methane gas) emanate from the seabed, morphological features such as mounds and pockmarks have formed in the Western Irish Sea (Croker et al., 2005).

Mounds are elevated bathymetric features which can form due to upward migrating fluids exerting pressure on overlying relatively impermeable layers or precipitation of carbonates due to prolonged methane gas seepage. Owing to their different formation mechanism, they are known as seabed domes, mud diapirs, and carbonate mounds, all of which have been found in the Irish Sea (Croker et al., 2005). Hovland and Curzi (1989) documented seabed domes and mud diapirs in the Adriatic Sea offshore Italy, where gas bubbles concentrating in plastic clay caused local density reversals, resulting in the upward buoyant flow of the clay and deformation of overlying unlithified layers, thus forming elevated bathymetric features at the seafloor and associated gas seepages. Such seabed domes and mud diapirs have also been found offshore India (Ramprasad et al., 2011), in Norwegian Arctic fjords (Roy et al., 2014), and offshore New Zealand (Koch et al., 2015). Croker et al. (2005) previously mapped mounds (referred to as “seabed doming”) in the WISMB, and suggested that they may have formed due to the replacement of water in the pore space with gas causing an increase in sediment volume in the upper sediment layers. For this to occur, fine-grained, relatively impermeable sediments are required. Croker et al. (2005) also suggested that seabed doming might be an initial stage of pockmark formation. Mounds can also form when prolonged methane gas seepage at the seabed chemically interacts with surrounding minerals to form a carbonate precipitate cement (MDAC), binding the sediment matrix and forming hard, resistive rocks (Judd et al., 2019). With continued seepage over time, MDACs can continue to precipitate and grow into sizeable features up to 10 m high and 250 m in length, as found at the CFZ in the western Irish Sea (O'Reilly et al., 2014).

Pockmarks are the most common manifestations of fluid seepage on the seafloor and are formed by fluids escaping through the seafloor sediments (Hovland and Judd, 1988). Unconsolidated sediments at the seafloor are lifted and winnowed by the escaping fluids (pore water or gas) forming crater-like depressions. Their shapes are typically circular to sub-circular, however, asymmetric, elongated and trough-like pockmarks have also been documented (Judd and Hovland, 2007; Roy et al., 2015). Pockmark diameters range from < 5m (unit-pockmarks) to > 1500m (mega-pockmarks) (Hovland et al., 2010; Sun et al., 2011). Pockmarks found in Irish waters vary in size with smaller features typically 2–3 m in diameter (unit-pockmarks) and tens of centimetres deep. Relatively larger pockmarks offshore Ireland are approximately 20 m in diameter and up to 2 m in depth (Croker et al., 2005; Games, 2001; Szpak et al, 2012, 2015). What is imperative for their formation is a fine-grained, clay to silt, substrate at the seafloor (Croker et al., 2005).

Seafloor and sub-seabed evidence for shallow gas and fluid migration in the Irish Sea, specifically the CFZ and WISMB, has been previously documented (e.g. Croker et al. (2005)). Geochemical analysis of the seep and mound locations suggest mixed biogenic and thermogenic signatures (Judd et al., 2019; O'Reilly et al., 2014). However, factors such as structural and stratigraphic features responsible for the migration of fluids responsible for a thermogenic signature are still poorly understood. Furthermore, models applicable to the formation mechanisms of the seep-related seafloor features in the Irish Sea are lacking. With this in mind, the aims of this study are:

  • (i)

    To spatially map and characterise geophysical evidence for shallow gas, fluid migration and seafloor seepage in the north Irish Sea;

  • (ii)

    To establish a geological framework incorporating bedrock geology, hydrocarbon source rocks, structural geology (faults), Quaternary geology and seafloor morphology in the Irish Sea which will facilitate further studies into subsurface fluid flow mechanisms;

  • (iii)

    To suggest theories of seabed mound and pockmark formation in the WISMB.

To achieve this, we provide an integrated analysis of shallow high-resolution datasets (sub-bottom acoustic, multibeam echosounder bathymetry and backscatter data) and deep 2D multichannel seismic datasets from the north Irish Sea. Inferences are made on the formation mechanisms of seep-related seabed features which can be used to better predict their distribution elsewhere in the region. Finally, the implications of shallow gas and fluid-seepage at the seafloor are considered in the context of marine infrastructure siting and ecological conservation.

The bedrock geology of the Irish Sea is characterised by a series of rift basins with several kilometres of Carboniferous, Permian and Triassic sedimentary fill. These basins formed through a series of extensional events in the Carboniferous, Permian and Jurassic, punctuated by episodes of uplift during the Late Carboniferous Variscan Orogeny and more recently the Alpine Orogeny during the Cenozoic. 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). Similar exploration activities took place in the western Irish Sea, primarily in the Kish Bank Basin, with four wells drilled between 1977 and 1997. 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).

The bedrock in the Irish Sea has largely been blanketed with Quaternary sediments, collectively referred to as the Brython Glacigenic Group (Fig. 2). Subglacial sediments deposited by the Irish Sea Ice Stream (ISIS) during the Last Glacial Maximum are referred to as the Upper Till (UT) member (Fig. 2), and comprise a till containing stiff or hard clay with clasts ranging in size from sand-grade to boulders up to 1 m (Jackson et al., 1995). Overlying the UT are a series of units deposited in a glaciomarine to marine environment as the ISIS retreated, referred to as the Western Irish Sea Formation (WISMF) (Fig. 2) (Jackson et al., 1995). Included in this formation, at the base, is the Chaotic Facies (CF). This unit consists of ice-proximal sediments, dominated by gravels with silts, sands and cobble-grade components (Coughlan et al., 2019; Jackson et al., 1995). The overlying Prograding Facies (PF) is composed of fine-to medium-grained sands that are tabular stratified, having been deposited in a marine environment in front of the retreating Irish Sea Ice Stream (ISIS) (Coughlan et al., 2019; Jackson et al., 1995). The Mud Facies (MF) is characterised by stratified grey-brown muddy sands with silts and clays and is interpreted as being deposited in a fully marine environment (Coughlan et al., 2019; Woods et al., 2019). The organic-rich sediments of the MF have been identified as a potential source of shallow gas (biogenic-origin) in the north Irish Sea in the Western Irish Sea Mud Belt. The anaerobic decomposition of the organic-rich sediments followed by rapid burial under high sedimentation rates during marine transgression in the Early Holocene produced biogenic gas in the shallow sediments (Yuan et al., 1992). The UT and WISF deposits have been reworked during marine transgression and sea-level rise in the Holocene forming a complex distribution of sediments and bedforms, collectively referred to as the Surface Sands Formation (SSF) (Fig. 2) (Jackson et al., 1995; Ward et al., 2015).

Section snippets

Data and methods

This study uses a variety of shallow and deep geophysical datasets. The shallow datasets used in this study include multibeam echosounder (MBES) bathymetry and backscatter data as well as shallow sparker and pinger seismic data from a variety of surveys (Table 1). They were acquired primarily as part of the Integrated Mapping for the Sustainable Development of Ireland's Marine Resource (INFOMAR) programme, delivered by the Geological Survey of Ireland (GSI) and Marine Institute of Ireland. Data

2D multichannel seismic data

A 2D multichannel reflection seismic dataset, consisting of several discrete surveys, was used to investigate the bedrock geology of the region, structural lineaments and gas related features. 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) Base-Quaternary; (iii) Base-Cenozoic; (iv) Top Lower Triassic; (v) Top Permian; (vi) Top Basement

Revised geological model with inferences on gas origin and controls on fluid migration

Structural lineaments (i.e. faults) and the properties of Quaternary sediments in the Irish Sea play a significant role in fluid migration from deep seated hydrocarbon source rocks to the shallow sub-seafloor stratigraphic layers, and eventually in subsequent seepage at the seafloor. In this section we discuss an individual, revised geological model for the CFZ and WISMB to elucidate the potential origins for hydrocarbon fluids in both areas and the pathways that would allow for the migration

Conclusions and future work

High-resolution geophysical datasets from the Irish Sea reveal sub-seabed shallow gas accumulations in Quaternary sediments and a range of seafloor expressions of fluid seepage. Based on the integrated geophysical investigation of seafloor geomorphologies, shallow sub-surface sediments and deeper geological and tectonic features, this study generated a geological framework from which the following can be made summarised.

In both the Codling Fault Zone and Western Irish Sea Mud Belt, there is

CRediT authorship contribution statement

Srikumar Roy: Conceptualization, Investigation, Formal analysis, Writing - original draft, Visualization, Conceptualization, Formal analysis, Methodology, Writing - original draft, Visualization. Conor O'Sullivan: Methodology, Formal analysis, Writing - original draft, Visualization. Annika Clements: Investigation, Writing - original draft. Ronan O'Toole: Investigation, Data curation, Supervision, Writing - review & editing. Ruth Plets: Investigation, Supervision, Data curation, Writing -

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

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. SR is funded by the Irish Research Council Government of Ireland Postdoctoral Fellow Award (GOIPD/2018/17). The authors would like to thank the Petroleum Affairs Division (PAD) of the Department of Communications, Climate Action and

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