Gas hydrate system comparison between active and passive margin of the NE South China Sea

Abstract

1. Introduction: Presence of gas seeps at active and passive continental margin adjacent to each other in the NE South China Sea provides a rare opportunity to study and to compare gas migration and hydrate formation at both systems under two different tectonic settings. The accretionary wedge offshore SW Taiwan is characterized by a series of semi-parallel elongate ridges and slope basins which receive huge quantity of terrigenous sediments from a number of small rivers on island of Taiwan. Another feature of this accretionary wedge is the frequent occurrences of earthquakes and thrust faults in and among ridges, which may become best conduits favoring gas migration to the surface. Further offshore is the broader passive margin receiving, also, large amount of river particles from larger rivers of the Asian continent but spread further onto the larger and wider passive margin. Earlier studies indicated existences of gas hydrate in both systems with commonly observed BSR at depth and gas seep on the seafloor. However, early studies with samples mostly short piston cores made it difficult to evaluate, in depth, type(s) of gas migration and hydrate formation/dissociation in these two different tectonic settings, nor possible to clear define system differences in between active and passive margin hydrate, if any. The objectives of this study are to locate evidences and define gas migration and formation of gas hydrate at these two different environments with the Mebo drill core. 2. Study methods: Tools of the SO266 study include gravity core and Mebo drill core, CTD/rosette sampler, heat probe, OFOS sea floor visual survey vehicle, together with borehole loggings and on-board analyses of IR scan, MSCL, gas and pore water extraction and analyses, as well as core description, paleontology and microbial sampling on board r/v Sonne. Also available are earlier 3 D seismic data and high resolution AUV maps. Study sites include: 1) the Formosa Ridge at the passive margin; 2) the Four Way Closure Ridge and the Southern YuanAn East Ridge at the active margin. Multiple CTD sampling were conducted on top of each seeps together with heat flux measurements, as well as OFOS, a tow camera system, surveys to cover spatial surface differences. Duration of SO266 were 34 days with works covering 6 sites, 11 holes, with results of 834 m drill length, 350 m cores with logging data, and ~4200 km multibeam tracks. 3. Results: Up to 140 m long Mebo cores were obtained, a record long core in this area. Gas flares, life seep communities, rapid sulfate depletion, shallow SMTZ, low chloride concentrations, and authigenic carbonate mounds were found at both environments. Major findings include 1) presences of gas hydrate at various layers in gravity and drill cores at both systems; 2) extremely high concentrations of gas under carbonate cap rock; 3) record breaking long cores (>100 m) in the area; 4) 3-5 m turbidite sand layers; 5) methane plume in the water column; 6) record length of total drill; 7) 6 m long core of authigenic carbonate; 8) different seeps communities; 9) volcanic ash layers. Detail results will be presented.

AAPG Datapages/Search and Discovery Article #90348 © 2019 AAPG Asia Pacific Region Geosciences Technology Workshop, Gas Hydrates – From Potential Geohazard to Carbon-Efficient Fuel?, Auckland, New Zealand, April 15-17, 2019