Lithological control of gas hydrate saturation determined by pressure core analysis in sub-permafrost reservoirs of the Alaska North Slope (HYDRATE 02 Geo Data Well)

Abstract

Quantitatively degassing pressure cores by controlled depressurization is a key approach to characterizing gas hydrate reservoirs by measuring the gas hydrate saturation (S h ) of sediments. We measured S h for 53 core sections from the Alaska North Slope during the JOGMEC-DOE-USGS Collaborative Gas Hydrate R&D Project in Alaska, from subpermafrost sediments that were later subjected to an extended-duration gas production test. We characterized lithologic influences on S h using physical property core scans (P-wave velocity, gamma ray attenuation density, and X-ray computed tomography) and grain size measurements. Pressure cores were recovered from the Hydrate-02 Geo Data Well (GDW) at the Kuparuk State 7-11-12 site through two hydrate-bearing sand reservoirs, the B1 and D1 sands, and their finer-grained bounding sediments. S h reaches very high values (68 to 91% of the pore space) in silty fine sands within the B1 sand, which also contains two lower P-wave velocity intervals (one measured at 30% S h ) within the main reservoir. S h and grain sizes of the B1 sand are generally higher than the D1 sand, which hosts high S h (53 to 73%) within its sandy silt to silty fine sands. The heterogeneous silty clay to silt “seal” sediments directly overlying both reservoirs have low S h (0 to 13%) even in thin, coarser beds, suggesting minimal methane transport between the reservoir and seal. S h decreases with depth downward toward the base of both the D1 and B1 sands, corresponding to decreasing grain size. These transitions are underlain by silty clay to clayey silt sediments with low S h (<10%), similar to the seal sediments. Slow depressurization was used to estimate a brackish salinity based on the pressure and temperature of the onset of hydrate dissociation for two core sections within the B1 sand. We show that S h increases with median grain size, sand content, and porosity but exhibits some variation within the sand beds that is not directly explained by lithology.