Research paperVelocity structures imaged from long-offset reflection data and four-component OBS data at Jiulong Methane Reef in the northern South China Sea

- Normal faults dipping SE beneath the continental slope provided conduits for gas migration at Jiulong Methane Reef (JMR).
- Highest hydrate saturation (11%) with high Vp (1.83 km/s) and relatively high Vp/Vs ratio (3.48) at SE of JMR.
- Highest gas saturation (2%) with low Vp (1.52 km/s) and relatively low Vp/Vs ratio (4.5) below the anticline at NW of JMR.

In this study, P- and S-wave velocity models were built based on two pre-stack depth migration (PSDM) profiles of long-offset reflection data and 25 four-component ocean-bottom seismometers (OBS) data at Jiulong Methane Reef off SW Taiwan in the passive margin of the northern South China Sea (SCS). According to the velocity models, the average P-wave velocity and Vp/Vs ratio of the free gas beneath the bottom-simulating reflector (BSR) are 1.52-1.58 km/s and 4.5-5.10, respectively. The depth of the BSR is found at 80-300 m below the sea floor and the sedimentary thickness of the hydrate and the free gas are about 50-100 m and 70-100 m, respectively. P-wave velocity of about 1.75 km/s in the hydrate-bearing sediment is southeastward increased to about 1.85 km/s above the BSR. Similarly, Vp/Vs ratio of about 3.06 in the hydrate-bearing sediment is increased southeastward to about 3.48 above the BSR. Based on the pseudo-3D map of gas-hydrate saturation estimated from the PSDM and OBS models, the average saturations of hydrate and free gas at Jiulong Methane Reef are about 7% and 0.9-2.4%, respectively. The highest hydrate saturation (11%) is located at 5-20 m above the BSR in the SE portion of the Jiulong Methane Reef. On the other hand, the highest gas saturation of about 2% is observed at 10-70 m below the BSR in the NW portion of the Jiulong Methane Reef. We suggested that several normal faults dipping southeastward beneath the continental slope provided conduits for gas migrating northwestward at Jiulong Methane Reef. Therefore, the highest gas saturation is observed below the anticline in the NW portion of the Jiulong Methane Reef and the highest hydrate saturation (high P-wave velocity and high Vp/Vs ratio) is identified above the BSR in the SE portion of the Jiulong Methane Reef.