Gas hydrates in Nankai Trough

                                                       Gang-Yu Chan

                                                 Abstract:

                        BSRs are distributed widely in the Nankai accretionary prism and
                      associated forearc basins and 90%of them have migration and recycling origins.
                      The widest distribution of the BSRs can be seen at the prism. A correlation
                      between the BSR distributions and prism size shows that the BSRs tend to be
                      more well-developed in a prism of large size. This suggests that a large prism
                      may prouduce much amount of gas-bearing fluids that migrate to the BGHS and
                      form the BSRs (tectonic control). In the forearc basins, the BSRs are identified
                      at topographic highs, anticlines and basin margins (structural control).
                        The upward migration of gas-bearing fluids is carried out through
                      permeable sand layers and as a result, the distribution of BSRs is confined to
                      alternating beds of sand and mud facies (sedimentary control). However, if
                      there is enough time for upward migration and accumulation of gas to the
                      BGHS, the BSRs can be generated widely in low-permeable mud facies(time control).
                        The phenomena of double BSRs could have followed a recent migration
                      of the base of the methane hydrate stability zone from the lower BSR to upper
                      one. As possible causes for the migration this paper discusses sea bottom 
                      warming and tectonic uplift.
                        Those results imply that structural, sedimentary tectonic and time controls
                      are primary factors to decide the distribution of BSRs in the Nankai Trough 
                      area.

                                                 References

                      Yasuhiro Yamada et al., (2004) BSRS and associated reflections as an indicator 
                      of gas hydrate and free gas accumulation: An example of accretionary prism and 
                      forearc basin system along the Nankai trough, off central Japan. Resource 
                      Geology, 54, 11-24           
                                                   (Abstract)(Full text)

                      Jean-Paul Foucher et al., (2002) Observation and tentative interpretation of a 
                      double BSR on the Nankai slope. Marine Geology 187, 161-175

                                                  (Abstract)(Full text)