Seismic indications for free gas within the gas hydrate stability zone in the Yaquina Basin off Peru

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Hübscher, C. P. , Netzeband, G. L. , Gajewski, D. , Grobys, J. , Kukowski, N. , Wagner, M. and Bialas, J. (2004): Seismic indications for free gas within the gas hydrate stability zone in the Yaquina Basin off Peru , European Geosciences Union, 1st General Assembly, 25 - 30 April, Nice, France. .
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MCS data from the Yaquina forearc basin off Peru reveal a complex distribution of gas and gas hydrate related reflections. Intricate lateral variations of the reflection pattern at the assumed base of the GHSZ in terms of continuity, reflection amplitude, and signal attenuation underneath are observed, as well as the occurrence of paleo-BSR. Phase reversed reflections at an erosional unconformity above the BSR indicate free gas within the GHSZ. In order to further constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the MCS line from high-resolution ocean bottom hydrophone recordings with two independent methods. The results from 2D-forward modelling and interactive velocity analysis show consistentresults. They exhibit a low velocity layer almost directly beneath the seafloor. Another low velocity layer with less than 1.5 km/s is present between the unconformity and the BSR.. In the vicinity and beneath prominent chemoherms, high velocities have been observed between the BSR and seafloor. Heat flux values calculated on the basis of the velocity-depth functions increase with decreasing amplitude of the bottom simulating reflector and peak near chemoherms. These results suggest a model of the Yaquina Basin where free gas is present under parts of the BSR, and within the hydrate stability zone, particularly under the sea floor and under the erosional unconformity. The higher interval velocities near and beneath the chemoherms are suspected to be caused either by thick gas hydrate lenses or a significant amount of precipitated carbonate within the sediment or a combination of both. The hypothesis of a paleo-BSR that reflects the uplift of the base of the GHSZ caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values.

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