Detection and evaluation of gas hydrates in the eastern Nankai Trough by geochemical and geophysical methods

Interstitial waters extracted from the sediment cores from the exploration wells, BH-1 and MITI Nankai Trough, drilled similar to60 km off Omaezaki Peninsula in the eastern Nankai Trough, were analyzed for the chloride and sulfate concentrations to examine the depth profiles and occurrence of subsurface gas hydrates. Cored intervals from the seafloor to 310 mbsf were divided into Unit 1 (similar to70 mbsf, predominated by mud), Unit 2 (70-150 mbsf, mud with thin ash beds), Unit 3 (150-250+ mbsf, mud with thin ash and sand), and Unit 4 (275-310 mbsf, predominated by mud). The baseline level for Cl- concentrations was 540 mM, whereas low chloride anomalies (103 to 223 mM) were identified at around 207 mbsf (zone A), 234-240 mbsf (zone B), and 258265 mbsf (zone C) in Unit 3. Gas hydrate saturation (Sh %) of sediment pores was calculated to be 60 % (zone A) to 80 % (zones B and C) in sands whereas only a few percent in clay and silt. The total amount of gas hydrates in hydrate-bearing sands was estimated to be 8 to 10 m(3) of solid gas hydrate per m(2), or 1.48 km(3) CH4 per 1 km(2). High saturation zones (A, B and C) were consistent with anomaly zones recognized in sonic and resistivity logs. 2D and high-resolution seismic studies revealed two BSRs in the study area. Strong BSRs (BSR-1) at similar to263 mbsf were correlated to the boundary between gas hydrate-bearing sands (zone C) and the shallower low velocity zone, while the lower BSRs (BSR-2) at similar to289 mbsf corresponded to the top of the deeper low velocity zone of the sonic log. Tectonic uplift of the study area is thought to have caused the upward migration of BGHS. That is, BSR-1 corresponds to the new BGHS and BSR-2 to the old BGHS. Relic gas hydrates and free gas may survive in the interval between BSR-1 and BSR-2, and below BSR-2, respectively. Direct measurements of the formation temperature for the top 170 m interval yield a geothermal gradient of similar to4.3 degreesC/ 100 m. Extrapolation of this gradient down to the base of gas hydrate stability yields a theoretical BGHS at similar to230 mbsf, surprisingly similar to35 m shallower than the base of gas hydrate-bearing sands (zone C) and BSR-1. As with the double BSRs, another tectonic uplift may explain the BGHS at unreasonably shallow depths. Alternatively, linear extrapolation of the geothermal gradient down to the hydrate-bearing zones may not be appropriate if the gradient changes below the depths that were measured. Recognition of double BSRs (263 and 289 mbsf) and probable new BGHS (similar to230 mbsf) in the exploration wells implies that the BGHS has gradually migrated upward. Tectonically induced processes are thought to have enhanced dense and massive accumulation of gas hydrate deposits through effective methane recycling and condensation. To test the hypothetical models for the accumulation of gas hydrates in Nankai accretionary prism, we strongly propose to measure the equilibrium temperatures for the entire depth range down to the free gas zone below predicted BGHS and to reconstruct the water depths and uplift history of hydrate-bearing area.