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Abstract
A post-stack inversion of 2D seismic data was conducted to estimate the spatial distribution of acoustic impedance associated with gas and hydrates in the Ulleung Basin, East Sea, Korea constrained by logs from three boreholes drilled on its continental margin. A model-based inversion was applied to a Plio-Quaternary succession composed of alternations of unconsolidated mass-flow deposits/turbidites. A comparison of seismic reflections and synthetic data computed from impedance logs is shown for two zones. An upper (steep) slope zone contains a moderately continuous, possibly bottom-simulating reflector feature along the corresponding section. This feature may be associated with a lithology boundary near a drill site in addition to, or instead of, a stability boundary of gas hydrates (i.e., gas below and hydrates above). The lower (gentle) slope zone has locally cross-cutting reflection patterns that are more likely to be attributed to gas- and hydrate-related physical phenomena than to spatiotemporal changes in lithology. This seismic inversion is informative and useful, making a contribution to enhance the interpretability of the seismic profiles for a potential hydrate recovery.
Highlights
Gas hydrates are crystalline solids containing gas molecules, usually methane, entrapped within a rigid cage of water molecules
This paper presents an application of model-based poststack seismic inversion to estimate acoustic impedance and its variations associated with the Plio-Quaternary gas and hydrate-bearing lithology in the Ulleung Basin
The P-wave velocity at UBGH04 in the expected hydrate zone shows a positive correlation with the corresponding density log in the predicted gas hydrate stability zone; the density increase is interpreted to be associated with the strength and density of the debris which are caused by mass-transport flows eroding the underlying turbidites/hemipelagic sediments as indicated in the mean grain size variation
Summary
Gas hydrates are crystalline solids containing gas molecules, usually methane, entrapped within a rigid cage of water molecules. The maximum depth of the GHSZ is a phase boundary between the lower limit of gas-hydrate stability and the possible upper limit of free gas; the corresponding seismic reflection has reverse polarity to that of the seafloor. This may be clearly observed in seismic profiles (if there is a sufficiently strong acoustic impedance contrast), where it parallels the seafloor and typically cuts across stratigraphic boundaries. This paper presents an application of model-based poststack seismic inversion to estimate acoustic impedance and its variations associated with the Plio-Quaternary gas and hydrate-bearing lithology in the Ulleung Basin. The approach in the present study is to demonstrate that inversion of seismic data from the gas- and hydrate-bearing deep-water sediments is a useful tool for seismic interpretation, especially when integrated with and constrained by borehole data
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