Shallow bottom-simulating reflectors on the Angola margin, in relation with gas and gas hydrate in the sediments

Author(s): Nouzé, Hervé; Baltzer, Agnès
Author Affiliation(s): Primary:
Centre IFREMER de Brest, Département Géosciences Marines, Plouzane, France
University of Adelaide, Australia
Aberswyth University of Wales, United Kingdom
University of Brunei Darussalam, Brunei
Université Caen, France
Volume Title: Subsurface sediment mobilization
Volume Author(s): van Rensbergen, Pieter, editor; Hillis, Richard R.; Maltman, Alex J.; Morley, Christopher K.
Source: Geological Society Special Publications, Vol.216, p.191-206; Subsurface sediment mobilization, Ghent, Belgium, Sept. 2001, edited by Pieter van Rensbergen, Richard R. Hillis, Alex J. Maltman and Christopher K. Morley. Publisher: Geological Society of London, London, United Kingdom. ISSN: 0305-8719. ISBN: 1-86239-141-6 CODEN: GSLSBW
Note: In English. 35 refs.; illus., incl. sects., 2 tables, sketch map
Summary: Acoustic facies interpretation, high-resolution velocity analysis and amplitude versus offset modelling have been performed on high resolution seismic data acquired on the West African margin offshore Angola, in water depths of about 2000 m. The area has a complex structural, thermal and fluid-flow setting, in which sediments are affected by salt diapirism and faulting associated with sediment compaction. A discontinuous bottom-simulating reflector (BSR) at a depth of about 200 m below sea floor could mark the base of the gas hydrate occurrence zone, which does not always coincide with the top of the free gas zone. Within the gas hydrate stability zone, a shallow bottom-simulating reflector is observed at a depth of about 75 m below seafloor. This shallow bottom simulating reflector, that is termed "sheep back reflector" (SR), correspond to a small amount of gas being trapped in the sediments. It could mark the top of the gas hydrate occurrence zone, where gas hydrate dissociation may occur. A reversed polarity reflector (R1) is also observed about 25 m below the sea floor. This reflector could correspond to a limit between normally compacted and underconsolidated sediments, possibly related to a permeability change in the sediments. Thus, the occurrence of excess pore pressure generated during gas hydrate dissociation could explain some subsurface sediment mobilization processes.
Year of Publication: 2003
Research Program: ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; 20 Geophysics, Applied; AVO methods; Acoustical methods; Aliphatic hydrocarbons; Alkanes; Angola Basin; Atlantic Ocean; Body waves; Bottom features; Bottom-simulating reflectors; Diapirs; Echo sounding; Elastic waves; Gas hydrates; Gases; Geophysical methods; Geophysical profiles; Geophysical surveys; High-resolution methods; Hydrocarbons; Leg 175; Marine sediments; Methane; ODP Site 1076; Ocean Drilling Program; Ocean floors; Organic compounds; P-waves; Physical properties; Porosity; Reflection methods; S-waves; Saturation; Sediments; Seismic methods; Seismic migration; Seismic profiles; Seismic stratigraphy; Seismic waves; South Atlantic; Surveys; Velocity; Well logs
Coordinates: S060000 S043000 E0120000 E0093000
Record ID: 2004078149
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