Gas hydrates (clathrates) causing pore-water freshening and oxygen isotope fractionation in deep-water sedimentary sections of terrigenous continental margins

Isotopic Composition of Interstitial Fluids and Origin of Methane in Slope Sediment of the Middle America Trench, Deep Sea Drilling Project Leg 84

“…The correlation between increasing δ 18 θ and decreasing chlorinity or salinity of pore water in sediments on the continental slope off Guatemala was first observed on Leg 67 (Harrison et al, 1982;Hesse and Harrison, 1981). The pore-water δ 18 θ analyses reported in Table 1 and shown in Figure 3 for samples from Holes 568 and 570 confirm the Leg 67 observations.…”

“…The water molecules that form the solid clathrate hydrate are known to exclude salts and concentrate 18 O, relative to the coexisting liquid water (Davidson et al, 1983 The question of what happens to the salts and 18 Odepleted H 2 O excluded during gas hydrate formation has not been resolved, but loss to overlying seawater or adjacent sediments seems to be required. An earlier interpretation (Hesse and Harrison, 1981) was that the gas hydrate probably is in contact with pore water of normal salinity and δ 18 θ. In this view, the observed "freshening" and 18 O-enrichment is due to decomposition of gas hydrates during sampling and dilution of the pore water with water from the hydrate.…”

Observed correlation between the depth to base and top of gas hydrate occurrence from review of global drilling data

“…Data used for our inventory are from offshore drilling campaigns that span the time from as early as 1983 with DSDP Leg 76 [Sheridan et al, 1983] to IODP Expedition 353 off India conducted in 2015 [Clemens et al, 2016]. The proxies used in this study include core-based proxies such as pore water freshening in the form of reduced chlorinity as shown in Figure 3a and described first by Hesse and Harrison [1981], significant cold spots in IR images of cores just after recovery (Figure 3b) [e.g., Long et al, 2010], soupy or mousse-like textures of sediments [e.g., Kastner et al, 1995;Pinero et al, 2007] (Figure 3b), logging-based proxies compiled by Collett and Lee [2012] such as formation-responses above a (site-specific) background trend in either electrical resistivity ( Figure 3c), P and S wave velocity, or sonic wave attenuation or pressure-core-derived methane concentrations ( Figure 3d) .…”

Review of exploration and production technology of natural gas hydrate

“…During gas hydrate formation process dissolved ions such as Na + and Cl − are excluded from the hydrate structure and only water molecules crystallize into cubic lattice structure (Hesse and Harrison, 1981). Surrounding pore waters initially become more saline during the process of hydrate formation.…”