The South China Sea is the largest marginal sea in the western Pacific, and it is well known for its abundant oil and gas reserves. A broad and wide continental slope 210,000 km2 in area extends to the northern parts of the South China Sea, and it is a good site for gas hydrate formation and conservation (Zhu et al., 2001). Furthermore, BSRs have been found on the northern slope (Fig. F1) (Yao, 1998; Chi et al., 1998; Song et al., 2001; Zhang and Chen, 2000; Chow et al., 2000).
Site 1146 is located at 19°27.40´N, 116°16.3´E at a water depth of 2092 m within a small rift basin (Jianfengbei Basin) on the southern slope of the Dongsha Islands (Fig. F1) (Wang et al., 2000). Three holes (Holes 1146A, 1146B, and 1146C) drilled at Site 1146 penetrated to the maximum depths of 607, 245.1, and 603.5 meters below seafloor (mbsf), respectively, and ended in lower Miocene, Pliocene, and lower Miocene sediments, respectively, with a maximum basal age of ~19 Ma (Shipboard Scientific Party, 2000b). Hole 1146C is ~30 m east of Hole 1146A.