Improving gas hydrate saturation estimates using P-wave velocity log data by incorporating XRD-data for detailed matrix-mineralogy definition

Logging-while-drilling (LWD) and wire-line (WL) velocity data from the second Korean Ulleung Basin Gas Hydrate Drilling Expedition in 2010 (UBGH2) were used to predict gas hydrate saturations using the effective medium theory (EMT) at four representative sites: We used Sites UBGH2-10 and UBGH2-6 where gas hydrate occurs mostly within strata-bound sandy turbidite layers. Sites UBGH2-2_1 and UBGH2-2_2 are used to investigate a typical cold vent setting and fracture-dominated systems. We first use a new technique to correct the LWD velocity data within the shallower portions of the borehole (typically upper 60–100 m below seafloor, mbsf) where the nature of soft sediments often results in erroneous low velocity readings of the LWD tool around values between 1350 m/s and 1400 m/s. Using the Vertical Seismic Profile (VSP) data, a smooth background velocity trend is defined, onto which the LWD data is projected. After this correction, the LWD velocity data of all four sites were used to estimate gas hydrate saturation values using the EMT for a 2-phase mineral-assemblage of 90% clay and 10% quartz only, as well as a detailed mineralogy based on the core XRD data. Both these estimates are then compared to the estimates from the electrical resistivity data using an empirical Archie-analysis. The results from EMT and Archie are on average the same, when gas hydrate occurs as a pore-filling medium within thick sediment layers of homogenous nature, such as that encountered at Site UBGH2-10 where saturations are about 60% of the pore space in an interval of 124–128 mbsf. If gas hydrate occurs in thin sand layers where the thickness of the layer is at or smaller than the vertical resolution of the velocity logging tool, Archie-based results are higher than the EMT-based results due to the higher resolution of the resistivity logging-tool. In cold vent settings with fracture-dominated gas hydrate settings (such as that investigated at Sites UBGH2-2_1 and 2-2_2). Archie-based results are non-reliable and strongly over-estimated due to the anisotropic effect of near-vertical hydrate-filled fractures. In such cold vent settings, velocity-based techniques using the EMT are the preferred method to estimate the gas hydrate saturations. At Site UBGH2-2_1, hydrate saturations are on average 5–10% of the pore-space and even lower at Site UBGH2-2_2 (less than 5% on average).