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doi:10.2204/iodp.proc.311.203.2008

Introduction

Gas hydrate is a solid icelike substance consisting of a water lattice in which hydrocarbon molecules (most often methane) are embedded (Sloan, 1998). Marine gas hydrate generally occurs in the top few hundred meters of sediments in continental margins worldwide, especially in subduction zone accretionary prisms (e.g., Kvenvolden, 1993). Because gas hydrate has different physical properties than the pore fluid it commonly replaces (notably, its electrical resistivity and seismic velocity are significantly greater), its inclusion in the pore space of marine sediments can significantly affect the bulk physical properties of the sediment. Measurements of such properties can therefore provide a means to estimate gas hydrate concentration (e.g., Collett and Ladd, 2000; Yuan et al., 1996).

Gas hydrate is electrically resistive, to first approximation similar to the sediment matrix and in strong contrast to the conductive saline pore fluid it replaces. Therefore, as measured by resistivity, the effect of gas hydrate occurrence is, to first order, similar to that of replacing the pore fluid with matrixlike material (i.e., reducing the porosity). Based on this approximation, measurements of sediment resistivity and porosity can be used to estimate gas hydrate concentration. This assumption clearly breaks down if gas hydrate occurs massively in veins and fractures or sheets but may still provide an adequate approximation. The result of treating log resistivity with the porosity reduction in such cases is that the gas hydrate concentration is projected into the pore space and vertically smeared out because of the limited resolution of the logging tools used (as discussed later).

Several studies have made use of resistivity measurements (downhole and field) to estimate gas hydrate concentration, generally by employing Archie's (1942) relation to relate resistivity, porosity, and gas hydrate saturation (e.g., Pearson et al., 1983; Hyndman et al., 1999; Collett and Ladd, 2000; Yuan and Edwards, 2000; also see the "Expedition 311 summary" chapter). In this study, high-quality downhole log and core measurements are used to constrain gas hydrate occurrence, distribution, and concentration from electrical resistivity. The data were collected during Integrated Ocean Drilling Program (IODP) Expedition 311 (see the "Expedition 311 summary" chapter), offshore Vancouver Island, Canada, along a transect of four sites across the northern Cascadia accretionary prism (Fig. F1).

The estimates of gas hydrate concentrations reported in this paper are accompanied by a detailed uncertainty analyses. These have been achieved by systematically exploring the different data measurements available and by comparing results from different formulations of Archie's law. 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.

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