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

Abstract

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.

Shallow-water hydrothermal venting linked to the Palaeocene–Eocene Thermal Maximum

“…Similar feeder systems have been described in outcrops in the Karoo Basin (South Africa), where they consist of fragmented sedimentary rocks altered due to fluid migration and high temperatures 13 . The funnel shape and aspect ratio of the Modgunn Vent crater is similar to diatreme-crater systems in maar volcanoes 22,23 and blow-out craters due to drilling accidents [24][25][26] (Extended Data Fig. 1), indicating rapid initial formation by fluid overpressure release.…”

Circular structures on the seabed: differentiating between natural and anthropogenic origins—Examples from the Southwestern Baltic Sea

“…Blowouts are the result of an uncontrolled increase in borehole pressure, which causes an uncontrolled upward flow of formation fluids and may result in the creation of pathways that reach the seafloor Karstens et al (2022).…”

“…B1 blowout (1964, North Sea) formed the Figge Maar crater. Major axis: ~550 m wide and up to 38 m deep Karstens et al (2022).…”

Fate of Methane from the Nord Stream Pipeline Leaks

“…Massive point-source releases of gas in shallow seas generate strong bubble plumes comprising gas bubbles and entrained seawater, which rapidly convey entrained water toward the sea surface. − In addition to the plume advection, the size of the generated gas bubbles as well as gas composition largely control the extent of dissolution during ascent. − For example, the extent of methane dissolution during gas bubble ascent in the water column can range from very little in very shallow settings up to almost 100% in deep settings. ,,− …”