Tectonosedimentary Framework And Hosting Of Gas Hydrate

The margins of the northern sector of the South China Sea (SCS) displays a complex, tectonosedimentary framework. The NW margin of the SCS is a passive tectonic margin forming the boundary to the broad Chinese continental shelf. The eastern margin of the SCS is an active collisional margin of a prong of the circum-pacific plate. The Heng-Ch'un Ridge to the south of Taiwan (Fig. 1) is the southern continuation of the Japanese Ryukyu arc which passes through Taiwan.


Figure 1. Simplified bathymetric and location map. Contours in m, 50, 100, 200, 400, 600, 800, 1000, 2000, etc. Locations of hydrate from interpretation of seismic reflection lines. A (Bochu, 1998), B (Liu, et al., 1993); C, D, E, F (Chi, et al., 1998).

Sediments along the NW margin of the South China Sea are draped on the continental slope and thickened in complex extensional basins whose linear trends are both parallel and at high angles to the slope trend. Some minor open folding seen mainly at the eastern end of this margin are probably related to compression emanating from the impingement of the collisional arc in Taiwan. Folds appear to be asymmetrically west-vergent, indicating an obducting tectonic framework toward the collisional margin. Indications of hydrate occur in the passive margin sediments of the NW SCS

margin (Fig. 1) (Bochu, 1998). Passive margin sediments are likely to be good hosts for hydrate. Similar sediments in the Blake Ridge along the SE U.S. coast retain a porosity on the order of 40-50%, even at more than 500 m depth below the seafloor and below the base of hydrate where hydrate dissociation may have taken place (Dickens et al., 1997).

Sediment thickness contours (Hayes, 1978) show thin sediment in the Heng-Ch'un Ridge. Reflection seismic records (Liu et al., 1993; and Chi, et al., 1998) suggest that the sedimentary succession along the collisional margin is thicker than along the passive margin, which is probably due to tectonic thickening rather than revealing the position of a sediment depocenter.

Regional tectonics along the eastern margin of the South China Sea south of Taiwan are dominated by collisional structures along a structurally young accretionary prism outboard from a volcanic arc. Sediments and the subjacent oceanic crust of the Philippine Sea plate are being actively obducted over the older oceanic crust of the South China Sea basin. Overthrusts and related folding in the accretionary prism are separated from the volcanic arc by a more structurally complex zone containing sediments possibly transported from the deep subduction zone along with possible ophiolites (Huang, 1993) as part of a polycyclic plate history that has not been fully elucidated. Apparent depression of the South China Sea oceanic crust, owing to impingement of the subduction zone and its consequent loading, is associated with a forearc trench lying upon oceanic crust of the SCS. The forearc trench is filled with relatively thick sediment, on the order of several kilometers thick just west of the deformation front thrust.

Sediments within the accretionary prism of the Heng-Ch'un Ridge appear to be only moderately deformed (Liu et al., 1993), with west verging asymmetrical anticlinal closures between east-dipping imbricate thrusts. Although compaction would be anticipated from the presence of compressional tectonics, the acoustic character of the sediments appears similar where they are folded and unfolded, although this may not be significant with respect to porosity. This similarity could, however, suggest that these dominantly fine-grained sediments contain significant porosity and probably permeability throughout. These conditions would allow methane to migrate into the hydrate stability zone (HSZ) and be sequestered in gas hydrate.

Gas hydrate has been identified on one multichannel seismic survey line in the area (Liu et al., 1993), but we identify probable hydrate on another four lines where hydrate was not identified by the authors. Blanking, which is expressed as a transecting of strata by an acoustically blanked zone conforming to the lower HSZ, and discontinuous BSRs indicating concentrations of free gas below hydrate, appears to be common in this region. Identification of clear evidence of hydrate on four of the five reflection seismic lines available to us suggests that considerable gas hydrate may occur throughout this area.

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