Application of AVO Analysis for Gas Detection to Seismic Data Acquired in Nankai Trough Area, offshore Japan

The drilling in Nankai Trough, offshore Japan in 1999, confirmed the occurrence of methane hydrate (hereafter called _eMH_f). It gave Japan so big impact for the future energy source. Since 2001, Japan_fs national research project, named MH21, has been conducted by METI for technology development of geology, geophysics for MH exploration. When the drilling, we never obtained any direct evidence of gas occurrence below MH-bearing zone except circumstantial evidence such as very low P-wave velocities. This result gave us hope that there is so much gas that we could recover as an energy source. In our research, for the purpose of confirming the gas occurrence below MH-bearing zone, we applied Amplitude Versus Offset (AVO) analysis to the seismic data acquired in Nankai Trough area in connection with the well data. The procedure of AVO analysis includes AVO attribute analysis, cross-plot analysis and primitive offset-amplitude analysis. After integration and detailed analysis of the results from each analysis, we concluded that there are so few gas occurrences below MH-bearing zone, which never become energy resources. On the contrary, a little gas localization in obliquely stratified sediment above BSR is found in spite of the MH stability zone. To understand this result, we need more study by using other new geological information.