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doi:10.2204/iodp.proc.311.213.2010

Gas hydrate concentration estimates

Gas hydrate concentrations were determined during Expedition 311 using mainly downhole logging-while-drilling (LWD) electrical resistivity data as well as wireline electrical resistivity log data (see the "Expedition 311 summary" chapter). Procedures followed standard Archie analyses (Archie, 1942; Collett and Ladd, 2000), and a detailed description of the methods and assumptions involved can be found in the "Methods" chapter. More detailed analyses of gas hydrate concentrations from the downhole resistivity log data were conducted after the expedition by Malinverno et al. (2008) and Chen et al. Malinverno et al. (2008) presented a method to calculate gas hydrate concentrations from the direct comparison of core-derived salinity and downhole log data from Site U1325 that honors the spatial uncertainty in the measurements from different boreholes located ~25 m apart. The technique was also applied to all other sites drilled along the transect. Chen et al. analyzed in detail the effect of porosity uncertainty on gas hydrate concentration estimates by comparing results obtained from density and neutron porosity calculations for all transect sites.

Seismically derived velocities (mainly P-wave velocity, V_P) can also be used to calculate gas hydrate concentrations. Two studies were carried out by Goldberg et al. (2008) and Chen (2006) using wireline and LWD data to determine velocity log–based gas hydrate concentrations. Although both methods show generally comparable results, gas hydrate concentrations calculated from acoustic velocities are slightly higher than resistivity-based estimates. For the purpose of understanding geologic controls on the occurrence of gas hydrates, these differences were disregarded.

Additional constraints on gas hydrate concentrations were obtained from pressure core degassing experiments, as originally outlined by Dickens et al. (1997). A total of 16 PCS cores were recovered under in situ pressure conditions and yielded estimates of gas hydrate concentrations (Fig. F4). All of the results of the PCS degassing experiments are superimposed on the logging-derived results shown in Figure F5.

All data from Expedition 311 confirm that gas hydrate concentrations at the drill sites along the transect are generally relatively low (<5% of the pore space) but locally can exceed 50% of the sediment pore space (especially at Site U1326, 50–120 mbsf, and Site U1327, 120–140 mbsf). A summary of resistivity data and derived gas hydrate concentrations (S_h) from all five sites drilled during the expedition are shown in Figure F5. The results demonstrate that the highest gas hydrate concentrations are not found near the BGHSZ (i.e., just above the BSR), as was predicted by the preexpedition Hyndman and Davis (1992) pore fluid expulsion model.

The two key variables for estimating concentrations of in situ pore fluid constituents (i.e., not settings with gas hydrate in fractures) using the Archie calculations (in addition to porosity) are pore fluid salinity and geothermal gradient. Geothermal gradients were successfully determined from individual downhole temperature probe deployments at all sites except Site U1326 (where only one downhole deployment succeeded in a reliable measurement). Detailed results of each deployment can be found in the individual site chapters of this volume. Pore fluid salinities and chlorinity were carefully determined onboard to (a) establish the background trend in pore fluid salinity and (b) capture any gas hydrate present and determine the local gas hydrate concentration from the pore fluid freshening relative to the background trend.

Figure F6 shows the entire data set available for pore water chlorinity analyses at all sites. Sites U1325 and U1326 both exhibit increasing pore water chlorinity with depth, as opposed to the other three sites (U1327, U1328, and U1329), which all decrease in pore water chlorinity with depth. Previously, the entire freshening trend observed at Site 889 was attributed to the dissociation of gas hydrate upon recovery (Hyndman et al., 1999). In contrast, Kastner et al. (1995a) suggested some component of mixing with a deeper, fresher fluid source to account for the combined observations of the pore fluid geochemical profiles, thus resulting in reduced gas hydrate concentrations compared to the study by Hyndman et al. (1999). However, more recent data from Expedition 311 clearly show that only the discrete outliers can be attributed to gas hydrate recovered in the core because they were coincident with IR-inferred gas hydrate occurrences. In addition to the IR data acquired from cores within the first few minutes after recovery, a second IR camera was used in the onboard geochemistry laboratory to allow further detailed discrimination of gas hydrate in the recovered core (Fig. F5; see discussion in the next section for further details).

Despite evidence from IR imaging, some doubts remained about the cause of the freshening trend, as originally argued by Hyndman et al. (1999). Therefore, Chen et al. carried out the same analyses originally conducted by Hyndman et al. (1999) to simultaneously solve for the in situ pore water salinities as well as gas hydrate concentrations. The new application of the Archie analyses by Chen et al. using LWD and wireline data from Expedition 311 verified that the measured pore water salinities generally follow the assumed background freshening trend.

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