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.
Highlights
Greenhouse gases, such as methane (CH4) and carbon dioxide (CO2) may be naturally or artificially sequestered within porous and permeable subsurface reservoirs (Bachu, 2000; Benson and Cole, 2008; Ringrose and Meckel, 2019; Global CCS Institute, 2020)
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. 20 Keywords: Chimneys, Pipes, Overburden, Pockmarks, Fluid flow, North Sea, CO2 sequestration, 21 Glacial stratigraphy
3) we aim to synthesise our findings into a schematic model of pockmark genesis and chimney formation, and discuss how our findings can be used to improve our understanding of focused fluid conduit and pockmark formation within the shallow overburden, for applications to subsurface geohazard assessment and CO2 storage
Summary
Greenhouse gases, such as methane (CH4) and carbon dioxide (CO2) may be naturally or artificially sequestered within porous and permeable subsurface reservoirs (Bachu, 2000; Benson and Cole, 2008; Ringrose and Meckel, 2019; Global CCS Institute, 2020). Where fluid-escape pathways extend to the seabed, understanding the rate of gas release from offshore seeps is required to quantify more accurately the input of greenhouse gases into the atmosphere and hydrosphere (Ligtenberg & Connolly, 2003; Leifer & Boles, 2005; Greinert et al, 2010; Shakhova et al, 2010). Annual global methane emissions sourced from natural geological sources are estimated as 18-63 Mt, with offshore seeps contributing 541 10 Mt and considerable uncertainty in the estimates (Etiope et al, 2019a; Etiope et al, 2019b; Saunois 42 et al, 2020)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
