1162 / 2019-08-03 10:04:41

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

6、海洋地球科学 > 专题7：深海极端环境与生命过程

A sediment layer with high gas hydrate saturations and abnormally high pore-water chlorinity concentrations has been identified from the logging-while-drilling (LWD) data and core samples in the Shenhu area, South China Sea. A buried trough-like sediment feature as interpreted from the high-resolution three-dimensional (3D) seismic data, and the architecture of gas hydrate-bearing reservoirs at various sites in the Shenhu area have been delineated from available seismic and well log data. However, not all gas hydrate-bearing sediments (GHBSs) form preferentially in low gamma ray log inferred coarser-grained units. The gas hydrate saturations calculated from resistivity log data using the Archie equation and from core derived pore-water chlorinities in gamma ray inferred coarse grained sedimentary layers (Sites W19, W18, SC-01 and Site SC-02) reach a maximum value of 72% at Site W19. The sedimentary units with high gas hydrate saturations exist mostly above an interpreted erosional surface (ES) at all of the sites examined in this study except for Site SC-02. Moreover, a striking increase in pore-water chlorinity values, with a maximum value exceeding 816 mM at Site SC-02, at the depth of 158 m, indicates an active or recently-active system. To understand this system, a simplified one dimension chemical concentration decay model is used to estimate the time when the gas hydrate formed based on the analyses of gas hydrate saturation, the thickness, and porosity of gas hydrate unit at each site. The results of these calculations show that the gas hydrates at Sites SC-02 and W18 are about 19.16 thousand and 26.40 thousand years old, respectively. To further analyze the observed highly saturated gas hydrate-bearing units and the apparent young age of these accumulations, the seismic derived variance, and frequency attributes as extracted along different sedimentary layers are used to display the evidence for vertical and lateral gas migration along normal faults and gas chimneys. The reported high pore-water chlorinities and the evidence for fluid migration from deeper sedimentary sections suggests that a possible pulse of migrating methane gas from below into the overlying hydrate stability zone may have contributed the formation of the recently formed, highly concentrated, gas hydrate occurrences identified in the Shenhu area.