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

Abstract

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-drilling 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 Archie'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 exhibits strongly 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 amounts 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.

Presence and Consequences of Coexisting Methane Gas With Hydrate Under Two Phase Water‐Hydrate Stability Conditions

“…The value of m was determined by fitting the initial resistivity for the known initial brine saturation, found to be 2.825, which is within the range of 1.3 to 4 reported by Jackson et al (). As hydrate forms, the value of m increases (Chen et al, ; Spangenberg, ), but the exact form of this increase is not known. To account for this change, we arbitrarily assumed a slightly higher value of m = 3.1 for hydrate saturations above 5% (Chen et al, ; Spangenberg, ).…”

“…As hydrate forms, the value of m increases (Chen et al, ; Spangenberg, ), but the exact form of this increase is not known. To account for this change, we arbitrarily assumed a slightly higher value of m = 3.1 for hydrate saturations above 5% (Chen et al, ; Spangenberg, ). Spangenberg () modeled the variation of n with water saturation, during hydrate formation.…”

P-wave and S-wave velocity structure of northern Cascadia margin gas hydrates

“…(1999) estimated 25–30 per cent pore space hydrate saturation in this same interval. However, recent interpretations of log resistivity data from IODP X311 by Chen et al . (2008) yielded saturations of 11(±7) per cent over this range of gas hydrate occurrence.…”

Effectiveness of 2D Induced Polarisation & Electrical Resistivity Tomography in Mapping Fracture Heterogeneity in Foundation Soils.

“…Porosity is a major influencer of effective resistivity especially when the resistivity of porewater is tending towards unity as can be deduced from Equation (2) (Archie, 1942;Jackson et al, 1978;Ioannidis et al, 1997;Keary et. al, 2002;Chen et al, 2008;Reynolds, 2011). Bulk resistivity has been observed to be approximately equal to the inverse of the square of porosity as shown in equation 3 below.…”