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

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 seeprelated 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.

Chapter 5: Ambient Bubble Acoustics -Seep, Rain and Wave Noise

“…The continuous passage of gas through the same area may cause the development of open channels (or chimneys) in the sediment which direct the flow of gas to a single localised point on the seabed, forming a seep [29][30][31] .…”

“…A subsea gas seep is broadly defined by the continuous release of gas from the seabed into the water column. There is no set magnitude for the flux of gas from a seep [29][30][31] , meaning the term encompasses seeps that release tens to millions of bubbles per minute. Similarly, there is no strictly defined time scales for being continuous, some seeps are born and die within a few hours, some are only active for certain times of the day or year while others have been active for centuries 30,32,33 .…”

“…There is no set magnitude for the flux of gas from a seep [29][30][31] , meaning the term encompasses seeps that release tens to millions of bubbles per minute. Similarly, there is no strictly defined time scales for being continuous, some seeps are born and die within a few hours, some are only active for certain times of the day or year while others have been active for centuries 30,32,33 . It is also worth noting a seep does not have to be in sediment, it may also come from exposed bedrock or even manmade features such as leaking pipes.…”

Distributed acoustic sensing for active offshore shear wave profiling

“…The WISMB covers an extensive area with water depths up to 100 m that is filled with substantial thicknesses (up to 40 m) of marine, Holocene sediments 22 . Previous investigations 23 – 25 in the area have identified four stratigraphic units in the WISMB consisting of a basal subglacial (lodgement) till (Unit 4) emplaced by the Irish Sea Ice Stream (ISIS) as it advanced across the area. Unit 4 overlies irregular bedrock.…”

Controls on shallow gas distribution, migration, and associated geohazards in the Yangtze subaqueous delta and the Hangzhou Bay

“…Seismic and acoustic explorations have been widely used for investigating gas-related anomalous reflections and mapping gas distribution in sub-bottom sediments (Ye et al, 2003;Hu et al, 2012;Cukur et al, 2013;Schneider von Deimling et al, 2013: Hu et al, 2016Yang et al, 2019). Because gas-charged sediments can effectively absorb and scatter the energy of sound waves, which rapidly attenuate inner gas-charged sediments along the vertical direction (Hovland and Judd, 1988;Ye et al, 2003;Coughlan et al, 2021;Toker and Tur, 2021;Yang et al, 2022b), stratal structures usually display acoustic anomalous reflections including acoustic turbidity, acoustic blanking, enhanced reflection, bright spot, gas chimney, and pockmark (Ye et al, 2003;Visnovitz et al, 2015;Coughlan et al, 2021;Toker and Tur, 2021). Shallow gas has been surveyed to be widely distributed in coastal zones, such as the Gulf of Mexico, the Baltic Sea, and the East China Sea (Hovland and Judd, 1988;Zhang et al, 2004;Zhang et al, 2008;Lin et al, 2015;Liu et al, 2019;Chen et al, 2020).…”