Gas hydrate concentration estimates from chlorinity, electrical resistivity and seismic velocity

Abstract

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 porewater 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 2 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 nohydrate/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.

Gas hydrate on the northern Cascadia margin: regional geophysics and structural framework

“…The estimated concentrations of between 15% and 30% of pore saturation in a 100 m thick layer above the BSR are much higher than estimated elsewhere, especially the Blake Ridge (Paull et al, 1996) and central Cascadia off Oregon during Leg 204 (Tréhu, Bohrmann, Rack, Torres, et al, 2003). Although both of these other studies involved different sediment environments, a careful reevaluation of the northern Cascadia estimates and their uncertainties was carried out by Riedel et al (2005). They concluded that the data and model uncertainties allowed a wide range of concentrations.…”

“…E. Gas hydrate concentrations from the Jarrad et al (1995) reference with a slightly revised empirical porosity-velocity function. F. Gas hydrate concentrations using the Yuan et al (1996) reference profile for accreted sediments (from Riedel et al, 2005). …”

Cascadia Margin gas hydrates

“…Historical work with data from Sites 889/890 has shown that the concentration of gas hydrate in sediments has been estimated by using (1) deviations in interstitial water chlorinities from measured baseline conditions, (2) electrical resistivity measurements by applying Archie's relation, and (3) both seismic-and downhole logging-derived P-and S-wave velocities. But results from Leg 146 left many questions unanswered (Riedel et al, 2005), such as:…”

“…An earlier approach by Ussler and Paull (2001) for interpreting chlorinity data from Leg 146 by using a smooth chlorinity baseline suggested lower gas hydrate concentrations at Sites 889/890 but apparently contradicted other geophysical results using electrical resistivity and seismic velocity. In preparation for Expedition 311, the gas hydrate concentrations along the northern Cascadia margin were recalculated using Leg 146 acoustic/electrical resistivity logs and pore water chlorinity/salinity data (Riedel et al, 2005). New estimates show that the concentrations could alternatively be between 5% and 10% on average of the pore volume from ~130 mbsf to the BSR (~230 mbsf).…”

Data report: a downhole electrical resistivity study of northern Cascadia marine gas hydrate

“…An emphasis is taken on the salinity background trend and its influence on the regional gas hydrate concentration estimates. Concentration estimates have been a long-debated topic, especially at Ocean Drilling Program (ODP) Leg 146 Site 889 prior to Expedition 311 (e.g., Hyndman et al, 1999;Riedel et al, 2005), as this location exhibits a strongly decreasing trend in pore water salinity down to bottom-simulating reflector (BSR) depths and remains constant below that level, which is unique compared to all others sites drilled during Expedition 311.…”