Seismic velocities from the Yaquina forearc basin off Peru: evidence for free gas within the gas hydrate stability zone

Abstract

Multichannel seismic (MCS) data from the Yaquina forearc basin off Peru reveal a complex distribution of gas and gas hydrate related reflections. Lateral variations of the reflection pattern at the assumed base of the gas hydrate stability zone in terms of continuity, amplitude, and signal attenuation underneath are observed, as well as the possible occurrence of paleobottom simulating reflectors (BSRs). Phase reversed reflections above the bottom simulating reflector point to free gas within the gas hydrate stability zone (GHSZ). To 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. Heat flux values estimated on the basis of the velocity-depth functions increase with decreasing amplitude of the BSR and peak near chemoherms. These results suggest a model of the Yaquina Basin where free gas is trapped under parts of the BSR, and within the GHSZ, particularly under the seafloor and under an erosional unconformity. The hypothesis of a paleo-BSR that reflects the uplift of the base of the hydrate stability zone caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values.

Gas hydrate saturation at Site C0002, IODP Expeditions 314 and 315, in the Kumano Basin, Nankai trough

“…Free gas coexisting with gas hydrate in the gas hydrate stability zone above the BSR has been reported elsewhere (e.g. Milkov et al 2004;Netzeband et al 2005;Lee & Collett 2006), and Lee and Collett (2006) suggest that gas hydrates may coexist with free gas throughout the gas hydrate-bearing zone in the absence of a double BSR within the Hydrate Ridge area, offshore Oregon, USA, with free gas produced either by the dissociation of gas hydrate during drilling and/or gas transport along fractures and faults. Although dissociation of gas hydrate during drilling may have occurred at Site C0002, any dissociation is assumed to be minor as logging data were obtained with LWD immediately after drilling, and both bit resistivity and Vp measurements were undertaken deep in the formation (Expedition 314 Scientists 2009b).…”

Widespread active seepage activity on the Nile Deep Sea Fan (offshore Egypt) revealed by high-definition geophysical imagery

“…Based on regional knowledge (e.g. Napoli mud volcano area, Robertson et al, 1996) and background values for normal pelagic and hemipelagic sediments Woodside et al, 2003;Lee et al, 2005), gassy sediments (Netzeband et al, 2005), carbonates (Netzeband et al, 2005) and stiff mud breccia, an average velocity of 1700 m/s has been assumed for estimating the sediment thicknesses and depths derived from the Chirp profiles discussed in this paper. Commonly observed are small faults in the sediments, seafloor irregularities (pockmarks and mounds), sediments containing gas, and hard substrates corresponding to authigenic carbonates.…”

Indications for the Occurrence of Gas Hydrates in the Fram Strait from Heat Flow and Multichannel Seismic Reflection Data

“…The features, which characterise the BSR in those seismic profiles, are phase-reversals, reflector cutting horizons, sudden increases in amplitudes, and increases in wavelength ( Figure 5(a)). A BSR is also commonly marked by a decrease in P-wave velocity below the GHSZ [39]. This, however, could not be validated along our seismic profiles, because the velocity analyses for the seismic data do not resolve shallow velocity variations in that detail.…”