ALBERT

Description: No abstract available. doi:10.2204/iodp.sd.3.04.2006

Description: Gas hydrate beneath the N. Cascadia continental slope off Vancouver Island occurs as a regional diffuse layer above the BSR and as local high concentrations in large vent or upwelling structures. Regional concentrations of gas hydrate beneath the N. Cascadia continental slope off Vancouver Island have been estimated earlier using multichannel seismic, seafloor electrical, and IODP Leg 146 downhole data. The concentrations of between 15 and 30% of pore saturation in a 100 m thick layer above the BSR are much higher than estimated elsewhere where there is good data, especially the Blake Ridge and central Cascadia off Oregon on ODP Leg 204. Although both of these other studies involved different sediment environments, a careful re-evaluation of the N. Cascadia estimates seemed desirable. We have re-evaluated the methods used to calculate the gas hydrate concentrations from pore-water chlorinity (salinity), electrical resistivity, and seismic velocity, describing in detail the assumptions and uncertainties. Use of the pore-water chlorinity/salinity and electrical resistivity directly have low reliability because of the effect on the no-hydrate reference of hydrate formation and dissociation, and the effect of pore fluid freshening by clay dehydration. At ODP Site 889/890 hydrate concentrations range from 5–10% to 30–40%, depending on the no-hydrate reference salinity used. Use of core salinity data along with the downhole and seafloor electrical resistivity data allows calculation of both the in situ reference salinity and the hydrate concentrations. The most important uncertainty in this method is the relation between resistivity and porosity, i.e., Archie’s Law parameters. Significantly different relations were determined from the ODP Leg 146 core and downhole log data, the log data resistivity-porosity relation giving much lower concentrations. Finally, seismic velocities from sonic-logs and multichannel data can be used to calculate gas hydrate concentrations, if an appropriate no-hydrate velocity-depth profile can be estimated. A velocity-hydrate concentration relation is also required. Depending on which no-hydrate/no-gas velocity baseline is used, estimated hydrate concentrations range from as low as 5% to above 25% saturation. In spite of having three nearly independent methods of estimating hydrate concentrations, it is concluded that the data allow regional concentrations in the 100 m layer above the BSR from less than 5% to over 25% saturation (3-13% of sediment volume). ODP drilling in the region scheduled for the fall of 2005 should help resolve the uncertainties.

Description: Industry 3D seismic covers the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate well. Interpretation supported by biostratigraphy indicates a normally-faulted anticline with 5L-38 occupying the northern downthrown crest. This EW-striking fault, near 5L-38, has 800 m throw interpreted 700 m below gas hydrate levels. Fault offset at 5L-38 gas hydrate levels is probably several hundred metres less, because syn-depositional fault movement during or after Late Eocene time is suggested. Down hole velocity surveys at Imperial Mallik wells, P-59, J-37 and A-06, infer 105, 225, and 135 m gas hydrate, respectively, compared to 116 m known at 5L-38. Gas hydrate seismic characterization includes: multiple, generally weak, near-horizontal events; amplitude features on geologic reflectors; amplitude “blanking” and; chaotic vertical reflective zones surrounding faults. Recognized data contamination include amplitude-frequency degradation beneath lakes and reverberated energy. Noted are spatial correspondence of some subsurface faults with position/orientation of surface lakes, suggesting a possible genetic link.