1193 / 2019-08-06 15:33:01

Complex gas hydrate systems and its influence on seeps in the South China Sea

6、海洋地球科学 > 专题7：深海极端环境与生命过程

Six targeted drilling expeditions (GMGS1- GMGS-6) have been conducted to confirm and characterize the occurrence of gas hydrates in the different basins of South China Sea. In addition, researchers have collected three-dimensional (3-D) seismic data, which can be used to infer geological controls on the distribution of gas hydrate within various parts of the reservoir. The expedition GMGS5 indicates that gas hydrate was identified from the chimney zone where mass transport deposits are widely developed. The chimney structures with pull-up reflections were caused by high velocity of the occurrence of gas hydrate in the sediments. The BSR is absent or parallel to the seafloor. Enhanced reflections are widely distributed below the base of gas hydrate stability for methane, which indicates the occurrence of structure II gas hydrate or free gas in this region. However, P-wave velocity and resistivity are higher comparing to the background sediment indicating the presence of gas hydrate. The dead mussels, shells and others are observed at the seafloor, which indicates an inactive seep. Gas hydrate showed the heterogeneous distribution. The coherence, dip, and RMS amplitude attributes are used to delineate spatial range of chimneys related with gas hydrate. The exposed gas hydrate, carbonate and flourishing chemosynthetic communities are detected using ROV with a high amplitude seismic reflection at the east of the gas hydrate drilling expedition GMGS2. Active and inactive cold seeps indicate the different activity stages of seepage. Double BSRs also occur in some places, an effect of different hydrate stability fields for gases of different compositions. Three drilling expeditions (GMGS3&4) were conducted in the Pearl River Mouth Basin, where the BSRs occur at depths inconsistent with predicted pressure-temperature conditions due to the complex processes of sedimentation, erosion and slope failures of migrating canyons. BSRs also shoal in some places, perturbed by the upward migration of fluids. Therefore, gas hydrate systems show different geological controls and geophysical anomalies identified from the well log data and seismic data in the South China Sea.