Estimated gas hydrate saturation from the reservoir of clayey silt with sandy interlayers at Site U1517, Tuaheni landslide complex on the Hikurangi margin, New Zealand

Accurate assessment of natural gas hydrate saturation and reservoir distribution in the Tuaheni landslide complex (TLC) area on the Hikurangi margin, New Zealand plays a critical role in explaining the creeping TLC phenomenon and its forming mechanisms. Gas hydrates are discovered in the interval from 104 mbsf to the BGHS at 160 mbsf based on the logging- while-drilling (LWD) and coring data from the site U1517 located on the extensional and creeping part of the TLC. Elevated P-wave velocity (＞1.7 km/s), electrical resistivity (＞1.5 Ω·m), and the high saturation intervals occur in 112～114 mbsf, 130～145 mbsf and 150～160 mbsf respectively. The mineral components of different well intervals are used to calculate the P-wave velocity, to estimate the gas hydrate saturation of the 104～160 mbsf interval with the simplified three-phase equation (STPE) and the biot- gassmann theory by Lee (BGTL) models, and the average saturation was 5.2% and 6.0% and the highest was 22.7% and 21.6%, respectively. Adoption of the new method made it more efficient to get BGTL model parameters. Compared with the hydrate saturation estimated by the Expedition 372 from the Archie equation was used, the average gas hydrate saturation is similar in the 104～160 mbsf interval (about 6.0%) and about 8.5% in the interval of 130～145 mbsf. In this study, two sound velocity models were used to estimate the hydrate saturation of U1517, making the results more reliable. The accurate estimation of the gas hydrate saturation and distribution at site U1517 will provide basic data for simulation of the creeping TLC phenomenon on the Hikurangi margin.