[J08-3] Imaging and interpreting lithospheric structures using seismic and geodetic approaches III

[J08-3-03] Hydrocarbon accumulation controlled by tectonic activity in the subduction zone: Insight from advanced seismic velocity analysis

Chanmaly Chhun¹, ^○Takeshi Tsuji^{1, 2}, Arata Kioka³ (1.Earth Resources Engineering Department, Kyushu University, Fukuoka, Japan, 2.International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, Fukuoka, Japan, 3.Institute of Geology, University of Innsbruck, Innsbruck, Austria)

The presence of gas hydrates is widespread in the Kumano Forearc Basin which is situated above the Nankai accretionary prism off the Kii peninsula, Japan. Bottom-Simulating Reflectors (BSR) at the base of the gas hydrate stability zones has imaged as strong acoustic impedance contrast on the seismic profiles. When we applied seismic velocity analysis to 3D seismic data via automatic velocity picking, the results reveal the high velocity anomaly zone (1900-2500 m/sec) above BSR interpreted as Gas hydrate bearing sediments, and the low velocity anomaly zone (1300-2000 m/sec) beneath BSR interpreted as free gas bearing sediments. Based on the results, we suggest that the gas hydrates accumulated due to the free gas influx which migrated upward through the steeply dipping strata and faults or fractures cutting through the basin. The gas (or hydrate) accumulated area is further controlled by the large faults in the accretionary prism. Therefore these factors generated by intensive tectonic movements control the distribution and saturation pattern of gas hydrate and free gas formation. When we characterize the features of gas hydrate (i.e., double BSR caused by variation in temperature and pressure), we could discuss the history of tectonic and seismic activity in the accretionary prism.

Key words: gas hydrates, seismic velocity analysis, automatic velocity picking, free gas influx