The Occurrence, Saturation and Distribution of Gas Hydrate Identified from Three Dimensional Seismic Data in the Lw3 Area: The Northern Slope of the South China Sea

Abstract Gas hydrates with high saturation in clayey silt sediments have been identified from several gas hydrate drilling expeditions in the South China Sea (SCS). However, the spatial distribution is not well delineated due to the limitation of two dimensional or pseudo-three dimensional seismic data. The three dimensional (3D) seismic data acquired by oil & gas exploration was reprocessed using high-density velocity analysis, which can be used to show the detailed mapping and characterization of high concentration gas hydrate in the LW3 area, SCS. The well log data and core samples have indicated that pore-filling gas hydrates have various saturations and thickness in the study area, but the spatial variations are still poorly defined. High-resolution broadband inversion without well constraint was conducted based on the high-resolution velocity analysis, which dramatically increased the accuracy in detecting thin gas hydrate-bearing layers. Rock physical models were involved to quantitatively predict gas hydrate saturation, which has been used to describe the gas hydrate-bearing layers with a thickness ranging from 5 to 6 m. Multi-attribute analysis extracted along the base of gas hydrate stability zone (BGHSZ) was used to predict the gas hydrate distribution. Gas hydrate distribution identified from the reprocessed high-resolution 3D seismic data indicate that gas hydrate-bearing layers are mainly located at four ridges of the migrating canyons in the Liwan 3 area. The thickness and saturation of gas hydrate distribution at the third and fourth ridges are better than the other two ridges. The gas hydrate saturation is over 40 % with a thickness exceeding to 70 m. Moreover, the thin gas hydrate layer is interbedded with thick layer at the forth ridge. However, the gas hydrate saturation is over 70 % with a thickness of 30 m at the third ridge. The gas hydrate distributions exhibit obvious heterogeneous at each canyon zone. The accuracy prediction and delineation of gas hydrate distribution are very important for assessing gas hydrate test production responses and scenarios.