OS4A-2:DISTRIBUTION AND QUANTIFICATION OF GAS HYDRATE WITHIN KUMANO FOREARC BASIN IN NANKAI TROUGH, SOUTHEAST JAPAN

Jihui JIA 1, Takeshi TSUJI 2, Toshifumi MATSUOKA 3,
1. Department of Urban Management, Kyoto University JAPAN; 2. International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, JAPAN; 3. Department of Urban Management, Kyoto University, JAPAN

    Kumano Forearc Basin is located to the southeast of Kii Peninsula of Honshu, Japan, overlying the accretionary prism in Nankai Trough. Over the last three decades it has been the focus area of gas hydrate studies and of accretion tectonics and earthquake prediction. Bottom Simulating Reflectors (BSR) are widely identified on the seismic profiles acquired in the Nankai Trough area, indicating much amount of methane hydrate there. The pilot project which is first offshore production test of gas hydrate in human history was also operated in this area. Using the three dimension seismic data and log data from two boreholes (well C0002A and well C0009A) within research area, acoustic impedance inversion was carried out converting the seismic data to rock properties (i.e. acoustic impedance; AI). Based on the estimated acoustic impedance profile, we could understand that gas hydrate is spatially corresponding to BSR and reveal the distribution of gas hydrate in the area.

    The AI inversion indicates a zone of comparatively high impedance along BSR approximately ~15 km with distribution from outer-ridge towards land. We could observe abnormal layer where acoustic impedance values decrease abruptly below BSR. The preceding phenomena represent hydrate-bearing sediments above the BSR and free gas-bearing sediments beneath the BSR. In addition, it has been proved that there is plenty of free gas occurs at deeper intervals in well C0009A by logging data through Integrated Ocean Drilling Program (Mai-Linh Doan et al., 2011 and Expedition 319 Scientist, 2010). The well location is in the Kumano forearc basin ~20 km northwest (landward) of Site C0002 where many gas hydrate samples have been recovered (Expedition 314 Scientist, 2009). As there was no evidence for in situ forming gas hydrate in well C0002A, the gas is probably derived from the organic carbon rich layers deposited in the lower part of basin and migrate obliquely along permeable fracture and/or landward-dipping sand layers to shallower and more seaward parts of the basin. The results firstly map the pattern of the pathways, such as the length, the depth, dip angle of each channel and its relation with strata. These information could not be obtained only from drilling and original seismic profiles.

    Using logging data, saturation of gas hydrate was estimated based on (1) Wood’s equation, (2) Achie equation (using resistivity), and (3) Gassmann equation in well C0002A to investigate the concentration. By using each method, the mean value of gas hydrate saturation is obtained: (1) 0.447 from Wood’s equation, (2) 0.225 from Achie equation and (3) 0.3 from Gassmann equation. Then we apply the constructed relationship derived from log data to 3D seismic data, and obtain the spatial distributions of both gas hydrate saturation and free gas in Kumano Forearc Basin.

Reference
1. Doan, M.-L., M. Conin, P. Henry, T. Wiersberg, D. Boutt, D. Buchs, D. Saffer, L.C. McNeill, D. Cukur, and W. Lin (2011), Quantification of free gas in the Kumano fore-arc basin detected from borehole physical properties: IODP NanTroSEIZE drilling Site C0009, Geochem. Geophys. Geosyst., 12, Q0AD06, doi:10.1029/2010GC003284
2. Expedition 319 Scientists, 2010. Site C0009. In Proc. IODP, 319. Tokyo: IODP Management International, Inc.
3. Expedition 314 Scientists 2009 Expedition 314 site C0002. In Proc. IODP vol. 314/315/316. Washington, DC: IODP Management International, Inc.