Publication Date:
2020-06-08
Description:
Downhole electrical resistivity measurements can be exploited for gas hydrate concentration estimates. However, to do so requires that several assumptions be made, in particular about in situ pore water salinity and porosity. During Integrated Ocean Drilling Program Expedition 311, electrical resistivity was measured in four boreholes along a transect across the northern Cascadia margin, offshore Vancouver Island, Canada. Logging-while-rilling and conventional wireline logging data of resistivity, density, and neutron-porosity are used together with measurements of porosity
and pore water salinity of the recovered core to systematically estimate gas hydrate concentrations at these four sites. Using Ar-
chie’s law, empirical parameters a and m are determined from gas hydrate–free zones by
means of a Pickett plot. The in situ salinity
baseline trend for each site was estimated directly from the measured pore water salinity values, as well as indirectly by calculating the trend using Archie’s law and simultaneously solving for gas hydrate saturation and in situ salinity. Results showed that the in situ salinities at Sites U1325, U1326, and U1329 were well determined from a smooth trend through the highest measured salinity values of the recovered core. Only Site U1327 exhibitsstrongly decreasing pore water salinity, reaching 22‰ at the base of the gas hydrate stability field.
This regional low salinity requires special analyses to estimate gas hydrate concentration from resistivity and introduces a large uncertainty. It is suggested that the decreased salinity results largely (90%) from a deeper fresh water source with the remaining
freshening being the result of dissociation of pervasive gas hydrate (~3% of the pore space).
Considering estimates from density porosity to be the most accurate, gas hydrate saturations average ~9% ± 7% at Site U1326, ~10% ± 7% at Site U1325, and 11% ± 7% at Site U1327 over the entire range of gas hydrate occurrence. No significant gas hydrate is inferred at Site U1329, although small am ounts may be present
just above the bottom-simulating reflector. In two localized zones at Site U1326 (60–100 mbsf [meters below seafloor]) and Site U1327 (120–150 mbsf), significantly higher gas hydrate concentrations of 〉30% of the pore space were encountered.
Type:
Article
,
PeerReviewed
Format:
text
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