Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration. (November 2021)

• Record Type: Journal Article
• Authors: Callow, Ben; Bull, Jonathan M.; Provenzano, Giuseppe; Böttner, Christoph; Birinci, Hamza; Robinson, Adam H.; Henstock, Timothy J.; Minshull, Timothy A.; Bayrakci, Gaye; Lichtschlag, Anna; Roche, Ben; Yilo, Naima; Gehrmann, Romina; Karstens, Jens; Falcon-Suarez, Ismael H.; Berndt, Christian
• Is Part Of: Marine and petroleum geology. Volume 133(2021)
• Publication Date: 2021-11
• Subjects: Chimneys -- Pipes -- Overburden -- Pockmarks -- Fluid flow -- North sea -- CO2 sequestration -- Glacial stratigraphy; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2021.105301
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: Fluid-escape structures within sedimentary basins permit pressure-driven focused fluid flow through inter-connected faults, fractures and sediment. Seismically-imaged chimneys are recognised as fluid migration pathways which cross-cut overburden stratigraphy, hydraulically connecting deeper strata with the seafloor. However, the geological processes in the sedimentary overburden which control the mechanisms of genesis and temporal evolution require improved understanding. We integrate high-resolution 2D and 3D seismic reflection data with sediment core data to characterise a natural, active site of seafloor methane venting in the UK North Sea and Witch Ground Basin, the Scanner pockmark complex. A regional assessment of shallow gas distribution presents direct evidence of active and palaeo-fluid migration pathways which terminate at the seabed pockmarks. We show that these pockmarks are fed from a methane gas reservoir located at 70 m below the seafloor. We find that the shallow reservoir is a glacial outwash fan, that is laterally sealed by glacial tunnel valleys. Overpressure generation leading to chimney and pockmark genesis is directly controlled by the shallow geological and glaciogenic setting. Once formed, pockmarks act as drainage cells for the underlying gas accumulations. Fluid flow occurs through gas chimneys, comprised of a sub-vertical gas-filled fracture zone. Our findings provide an improved understanding of focused fluid flow and pockmark formation within the sediment overburden, which can be applied to subsurface geohazard assessment and geological storage of CO2 . Graphical abstract: Image 1 Highlights: Seismic chimneys/pipes underlie active pockmarks in North Sea Witch Ground Graben. High resolution 2D and 3D seismic reflection images characterise gas chimneys. Gas chimneys identified as series of sub-vertical fractures in fine-grained sediments. Glacial stratigraphy controls fluid migration pathways and accumulation zones in sedimentary overburden. New mechanisms for pockmark formation and spatial distribution proposed.