Geological controls on the occurrence of recently formed highly concentrated gas hydrate accumulations in the Shenhu area, South China Sea

The Shenhu area is an important gas hydrate exploration and production test zone in the northern slope of South China Sea. A sediment layer with high gas hydrate saturations and abnormally high pore-water chlorinity concentrations has been identified from recently acquired logging-while-drilling (LWD) data and core samples. Data from four sites (W19, W18, SC-01 and SC-02) indicates that most gas hydrate-bearing sediments (GHBSs) form preferentially in low gamma ray log inferred coarser-grained units, except at SC-02. At this site, a striking increase in pore-water chlorinity values, with a maximum value exceeding 816 mM at a depth of 158 mbsf, indicates an active or recently-active system. A one-dimensional (1D) diffusion model is used to estimate the time when the gas hydrate formed based on the saturation, thickness, and porosity of gas hydrate-bearing units at each site. The results show that gas hydrates at sites SC-02 and W18 respectively formed 19-29 thousand and 26-28 thousand years ago (assuming a closed system). To further investigate the apparently young age of these highly saturated gas hydrate accumulations, three-dimensional (3D) seismic variance and frequency attributes extracted along different sedimentary layers are shown to provide evidence for vertical and lateral gas migration along normal faults and gas chimneys. The high pore-water chlorinities and evidence for fluid migration from deeper sedimentary sections suggest that upward migration of methane gas into the hydrate stability zone may have contributed to the recent formation of the highly concentrated gas hydrate occurrences identified in the Shenhu area.