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.
|Journal||Marine and Petroleum Geology|
|Early online date||15 Oct 2020|
|Publication status||Published - 31 Jan 2021|
Bibliographical noteFunding Information:
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 Environment (DCCAE), Ireland, for providing access to released borehole, seismic and potential field datasets. The authors would also like to thank Schlumberger for providing academic licenses of Petrel to University College Dublin. We are grateful to IHS Markit for providing the academic licence for the KINGDOM software package to iCRAG. This paper contains Irish Public Sector Data (INFOMAR) licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) licence. The authors acknowledge Dr. Matthew Service and Rory O'Loughlin (both Agri-Food and Biosciences Institute, Northern Ireland) for releasing the MBES data for Queenie Corner, and for support during the AFBI surveys. The pinger line shown was acquired as part of NERC project NE/H02431/1. The authors acknowledge Rosie Jebb (GSI) and Andy Trafford (UCD) for assistance processing MBES and sub-bottom profile data respectively. The authors also acknowledge the crew and scientists onboard all surveys listed for their work, co-operation and skill in collecting the data. The authors would like to thank 3 reviewers for their feedback and comments which greatly improved this manuscript.
Copyright 2020 Elsevier B.V., All rights reserved.
- Ecological conservation
- Fluid seepage
- Mud diapir
- Offshore infrastructure
- Seabed mounds