Liwen Chen

Geological controls on the gas hydrate system of Formosa Ridge, South China Sea

Formosa Ridge is one of many topographic ridges created by canyon incision into the eastern South China Sea margin. The northwestern termination of the ridge is caused by beheading of the ridge due to a westward shift of the canyon that originally formed to the eastern flank of Formosa Ridge. Below Formosa Ridge a bottom simulating reflector (BSR) exists. Its depth below sea floor coincides with the theoretical base of the gas hydrate stability zone and the reflection has reverse polarity suggesting that it is caused by free gas below gas hydrate accumulations. The BSR is ubiquitous but shows significant variations in depth below sea floor ranging from 150 ms TWT (or approximately 180 m) underneath the incised canyon in the north to up to 500 ms (or approximately 460 m) underneath the crest of Formosa Ridge. Predominantly this depth variation is the result of topography on subsurface temperature, but comparison with the average BSR depth underneath the surrounding canyons suggests that recent canyon incision in the north has perturbed the thermal state of the sediments. Formosa Ridge consists of a northern half that is dominated by refilled older canyons and a southern half that consists mainly of contourite deposits. However, judging by the reflection seismic data this difference in origin seems to have little effect on the distribution of gas hydrate.

Processes affecting the depth of the gas hydrate stability zone in the accretionary prism offshore Taiwan

Several geological processes introduce a discrepancy between the geothermal gradient derived from heat probe measurements on the seafloor and gradients derived from gas hydrate-related bottom-simulating reflectors (BSRs) at a few hundred meters below the seafloor. We use a wide-spread BSR offshore SW Taiwan in 3D seismic data and an in-situ thermal probe dataset, in addition to 3D finite element modeling, to study these processes, including topographic effects, fluid advection, and landslides. Topographic effects make the geothermal gradient lower on the ridge and higher under the flanks. Fluid advection from depth warms up the shallow crust through some conduits like faults, fissures, and mud diapirs. Landslides reset the seafloor temperature and generate temperature pulses that will take thousands of years to propagate to the BSR depth. To study regional crustal thermal structures we need to correct these effects. On the other hand, we could use these effects to better select gas hydrate prospects.