HR: 0800h
AN: OS41C-0497    [Abstracts]
TI: 3D-Seismic and Acoustic Imaging of Gas Migration and Gas Hydrate Accumulations Beneath Pockmarks in Hemipelagic Sediments off Congo, SW Africa
AU: * Spiess, V
EM: vspiess@uni-bremen.de
AF: Dept. of Geosciences, University of Bremen, P.O. Box 330440, Bremen, 28334 Germany
AU: Z�hlsdorff, L
EM: lzuehls@uni-bremen.de
AF: Dept. of Geosciences, University of Bremen, P.O. Box 330440, Bremen, 28334 Germany
AU: Seifert, A
EM: aseifert@uni-bremen.de
AF: Dept. of Geosciences, University of Bremen, P.O. Box 330440, Bremen, 28334 Germany
AU: Hirsch, K
EM: khirsch@uni-bremen.de
AF: Dept. of Geosciences, University of Bremen, P.O. Box 330440, Bremen, 28334 Germany
AB: Seismic and acoustic imaging is a major tool to study basic geological parameters controlling the occurrence of gas hydrate as well as of free gas within the gas hydrate stability field. For sufficient upward fluxes of hydrocarbon gases, gas hydrates grow within pore spaces or gases are trapped beneath efficient seals, thereby revealing information about the nature and efficiency of fluid flow pathways and about the time scale on which fluid transport occurs. In the hemipelagic sedimentary sequences off the Congo, where layering and uniform properties exist at the time of deposition, modification of sediment physical properties due to mixing between water, gas and hydrates within pore spaces affects amplitude and phase properties of seismic reflections. Furthermore, fluid flow and gas or hydrate accumulations are often associated with sediment deformation or faulting on different scales. Thus, an integrated interpretation of seismic, acoustic, and surface mapping data sets was used to optimize lateral and vertical resolution at each depth level and to connect deeper processes to their surface expressions. 3D seismic data across seafloor pockmarks indicate that the typical low-amplitude signature of opal-rich and water-rich sediments is superimposed by high amplitude zones near faults and potential fluid pathways. A high amplitude patch observed in 40-50 m depth is interpreted as a gas hydrate cap that plugs the feeder channel of a pockmark and initiates hydrate growth parallel to the bedding. The upflow zone at greater depth is characterized by amplitude blanking, indicating free gas bubbles that scatter seismic energy. A package of high amplitude reflector elements at 150-200 m sub-bottom depth suggests the presence of trapped gas beneath a low permeable layer. This package is bent upwards at the vicinity of the pockmark, probably indicating a deeper salt diapir, that is associated with faulting and probably higher permeability above the diapir. However, the creation of pathways beneath the pockmarks is not yet completely understood. Preliminary results based on 3D mapping of fault plane orientations suggest that faulting due to diapirism may be superimposed by zones of weakness within a regional fault pattern that probably is of polygonal structure.
DE: 3022 Marine sediments--processes and transport
DE: 3025 Marine seismics (0935)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting