Influence of seafloor topography on gas hydrate occurrence across a submarine canyon-incised continental slope in the northern margin of the south china sea

In a submarine canyon-incised continental slope, the observed bottom simulation reflection (BSR) frequently appears not to match with the predicted base of the gas hydrate stability zone (BGHSZ). However, the mechanism behind this discrepancy is often debated due to the lack of in-situ borehole measurements. In the Shenhu area, a submarine canyon region on the northern continental slope of the South China Sea, a suite of datasets, including 3D seismic data, well logs and in-situ borehole temperature measurements enable us to study the gas hydrate stability zone. In-situ borehole temperature measurements reveal a nonlinear geothermal gradient within shallow 200 m-thick marine sediments. The geothermal gradient fit with the uppermost sediment interval (<50 m) neglected produces a better match between the depths of predicted BGHSZ and observed BSR. The gas hydrate stability zone (GHSZ) modeling using in-situ borehole measurement information at Site W07 shows that the predicted BGHSZ agrees well with the imaged BSR located atop an inter-canyon ridge, demonstrating that the gas hydrate system is in a steady state. Two-dimensional GHSZ modeling along seismic profiles transecting the ridge shows that the observed BSRs along the western margin of the ridge are deeper than the predicted BGHSZ. We interpret this apparent discrepancy to be attributed to topographic effects. Two scales of topographic effects, i.e. an alternating distribution of canyons and inter-canyon ridges, and localized crests and troughs over the crest of inter-canyon ridges, are revealed to influence gas hydrate system in the study area. The curved BGHSZ resulted from the topographic effect could facilitate the concentration of gas hydrate by enlarging the accumulation space and sealing free gas immediately below BGHSZ.