Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration

Callow, Ben; Bull, Jonathan M.; Provenzano, Giuseppe; Böttner, Christoph; Birinci, Hamza; Robinson, A. H.; Henstock, T.; Minshull, T. A.; Bayrakci, Gaye; Lichtschlag, Anna; Roche, Ben; Yilo, Naima; Gehrmann, Romina; Karstens, Jens; Falcón-Suarez, Ismael; Berndt, Christian

doi:10.1016/j.marpetgeo.2021.105301

Cited by 17 publications

(15 citation statements)

References 62 publications

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“…This complicates the analysis of vertical seismic anomalies (chimneys), which are defined by their disturbed seismic character, and differentiates these structures from imaging artefacts. Seismic artefacts, including acoustic masking, velocity pull‐down and pull‐up effects and bright spot multiples can generate vertical seismic anomalies, which can be misinterpreted as focused fluid conduits (Callow et al, 2021).…”

Section: Discussionmentioning

confidence: 99%

Distribution and geological controls of the seabed fluid flow system, the central‐western Bohai Sea: A general overview

et al. 2022

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Thorough understanding of seabed fluid flow system is of great significance to geohazard identification and hydrocarbon exploration as it can reshape the seabed and act as an indicator of subsurface hydrocarbon resources. For the first time, an integrated study of side‐scan sonar, single‐ and multi‐channel seismic data and magnetic data reveals a complex fluid flow system composed of various seafloor expressions (i.e. pockmarks and mounds) and shallow fluid migration pathways in the central‐west Bohai Sea off northeast China. Gas chimneys, mud diapirs and a dense network of Quaternary faults are the main fluid migration pathways in the shallow subsurface. The gas chimneys can be classified into three categories (Type A formed by relatively rapid gas escape, Type B formed by episodic fluid expulsion and Type C formed by fluid escape from mud diapirs), based on their distribution and seismic character, implying variability in the formation processes. Sediment remobilization and basement‐involved faults contribute to deep fluid migration into shallow depths. As a seal for up‐moving fluids, the nature and thickness of Holocene marine sediments generally decide the permeability and overburden pressure that may control the distribution of pockmarks and mounds since they are almost distributed above relatively thin Holocene deposits (thickness <20 m) and localized coarse surface sediments. The results of the interpretation gain an improved understanding of the geological processes controlling the genesis and spatial distribution of gas chimney formation and show the significance of gas chimney classification. The distribution pattern of different types of gas chimneys may signify the difference of geological background and fluid flow process, like fluid migration through faults or flow of mobilized sediments, that is crucial for the evaluation of global petroleum systems and Carbon Capture and Storage studies.

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Section: Discussionmentioning

confidence: 99%

Distribution and geological controls of the seabed fluid flow system, the central‐western Bohai Sea: A general overview

et al. 2022

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“…They are vertical to sub-vertical seismic anomalies with circular or elliptical planforms that hydraulically connect deeper stratigraphic layers with the overburden (Karstens and Berndt, 2015). Within seismic chimney structures, seismic amplitude blanking and chaotic reflections are commonly observed (Callow, et al, 2021). Seismic chimneys have been observed extensively on the Western slope of the Okinawa Trough, and they may act as a pathway for methane fluids, allowing upward migration to the seafloor and eventually into the water column.…”

Section: Seismic Chimneysmentioning

confidence: 99%

“…Topographic expressions at the seafloor range from build-ups (e.g. mud volcanoes (Milkov, 2000;Tsunogai, et al, 2012;Magalhaes et al, 2019), mounds [Chun et al, 2011;Somoza et al, 2014;Benjamin and Huuse, 2017) or domes (Koch et al, 2015)] to depressions [e.g., submarine pockmarks (Cathles et al, 2010;Dondurur et al, 2011;Callow et al, 2021)]. Furthermore, many studies revealed that fluid migration pathways were associated with buried underlying structures such as mud diapirs (Rovere et al, 2014;He et al, 2016;Wan et al, 2017), fault systems (Mohammedyasin et al, 2016;Hillman et al, 2020), and seismic chimneys (Kempka et al, 2016;Callow, et al, 2021).…”

Section: Introductionmentioning

confidence: 99%

Geophysical evidence for submarine methane seepage on the Western slope of Okinawa Trough

Luo

Cai

et al. 2022

Front. Earth Sci.

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Identifying seafloor methane seepage efficiently has important implications for assessing environmental impact, reducing the uncertainty of top seal integrity, understanding the petroleum system, and mitigating the drilling hazards due to shallow gas influx. Pore water geochemistry analyses suggest that the study area has an extremely high methane seepage flux and active methane anaerobic oxidation processes. However, geochemical data cannot provide details about the internal seepages. The geophysical dataset from the Western slope of Okinawa Trough, including 2D high-resolution seismic, sub-bottom profiles, and bathymetry, gives us a good opportunity to understand the detailed characteristics of methane seepages in this study. Geophysical data have revealed numerous methane seepage-related features such as seismic chimneys, pockmarks, submarine domes, and amplitude anomalies, including bright spots and enhanced reflections. Pockmarks and domes are often associated with seismic chimneys, indicating that fluid migration is important in their formation. The various geophysical expressions may represent different stages of methane seepage. Fluid quickly drains, causing severe sediment deformation and forming pockmarks, whereas domes may indicate the early stages of fluid discharge. Chimneys that do not extend to the seafloor may indicate that the venting is gradual and focused. Flares linked to domes or pockmarks may indicate that the fluid migration is active. Several factors triggered the existence of methane seepages on the Western slope of the Okinawa Trough, including tectonic setting, overpressure and rapid sedimentation.

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“…The formation of the 500 m-wide and 18 m-deep Scanner Pockmark in the British North Sea is characterized by an initial expulsion of overpressured gas from a gas pocket ~50 m below seafloor (Figure 7C; Böttner et al, 2019;Callow et al, 2021). This was followed by prolonged moderately intense, capillary-dominated seepage over a long period from at least 13 to 26.6 ka and tidally controlled bubble release at present-day (Böttner et al, 2019;Callow et al, 2021). The depth of the Scanner Pockmark base coincides with the boundary between consolidated glacial tills and the overlying unconsolidated, glaciomarine sediments (Böttner et al, 2019).…”

Section: Reconstruction Of Crater Formation During the B1 Blowoutmentioning

confidence: 99%

Formation of the Figge Maar Seafloor Crater During the 1964 B1 Blowout in the German North Sea

Karstens

Deimling²,

Berndt

et al. 2022

Earth Sci. Syst. Soc.

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In 1964, exploration drilling in the German Sector of the North Sea hit a gas pocket at ∼2900 m depth below the seafloor and triggered a blowout, which formed a 550 m-wide and up to 38 m deep seafloor crater now known as Figge Maar. Although seafloor craters formed by fluid flow are very common structures, little is known about their formation dynamics. Here, we present 2D reflection seismic, sediment echosounder, and multibeam echosounder data from three geoscientific surveys of the Figge Maar blowout crater, which are used to reconstruct its formation. Reflection seismic data support a scenario in which overpressured gas ascended first through the lower part of the borehole and then migrated along steeply inclined strata and faults towards the seafloor. The focused discharge of gas at the seafloor removed up to 4.8 Mt of sediments in the following weeks of vigorous venting. Eyewitness accounts document that the initial phase of crater formation was characterized by the eruptive expulsion of fluids and sediments cutting deep into the substrate. This was followed by a prolonged phase of sediment fluidization and redistribution widening the crater. After fluid discharge ceased, the Figge Maar acted as a sediment trap reducing the crater depth to ∼12 m relative to the surrounding seafloor in 2018, which corresponds to an average sedimentation rate of ∼22,000 m3/yr between 1995 and 2018. Hydroacoustic and geochemical data indicate that the Figge Maar nowadays emits primarily biogenic methane, predominantly during low tide. The formation of Figge Maar illustrates hazards related to the formation of secondary fluid pathways, which can bypass safety measures at the wellhead and are thus difficult to control.

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Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration

Cited by 17 publications

References 62 publications

Distribution and geological controls of the seabed fluid flow system, the central‐western Bohai Sea: A general overview

Distribution and geological controls of the seabed fluid flow system, the central‐western Bohai Sea: A general overview

Geophysical evidence for submarine methane seepage on the Western slope of Okinawa Trough

Formation of the Figge Maar Seafloor Crater During the 1964 B1 Blowout in the German North Sea

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