Sedimentology and Geochemistry of Gas Hydrate-Bearing Sands in the Greater Prudhoe Bay Area, Alaska North Slope: Insights from HYDRATE-02 Geo Data Well Core Analyses

An extended-duration gas production test from a gas hydrate-hosting sand layer was conducted in the JOGMEC–DOE–USGS Collaborative Gas Hydrate R&D Project in Alaska. Thick gas hydrate-bearing sand layers within the Prudhoe Bay Unit, sampled as part of the project, were studied to obtain fundamental data relevant to reservoir characterization and insights into the evolution of gas hydrate-hosting sand layers. Geochemical and petrophysical data were collected from pressure-cored sediments, gas hydrates, and interstitial water in approximately 15-m thick D1 and B1 sands (699 and 844 m in top, respectively) and from the overlying clay-rich layers. The proportion of sand-sized particles increased upward within the B1 sand of the progradation deposit; quartz content also increased upward. These parameters correlated with the electrical resistivity profile. The degree of gas hydrate saturation could be linked to the grain size pattern of the host sediments. Grain size and mineralogical data indicated a less smooth progradation process for the D1 sand than for the B1 sand. Scattered authigenic pyrite contents in the D1 sand suggest that geochemical processes within the sediments were accompanied by fluctuations. Distributions of spherical aggregates of fine siderite particles were restricted to the upper part of the B1 sand, where the highest hydrate saturation was observed. The local occurrence of carbonate-saturated brine was probably linked to high gas hydrate formation in the past. In situ chloride concentrations of interstitial water were ∼70 mM in the clay-rich layer above the D1 sand and ∼35 mM above and within the B1 sand. The downward freshening trend is consistent with diffusion following brine formation caused by thickening permafrost during the most recent glaciation 20 ky ago. The lower half of the D1 sand may have been influenced by present freshwater input, then exhibit unexpectedly low in situ chloride concentrations.