A seepage gas hydrate system in northern South China Sea: Seismic and well log interpretations

• Geophysical data and core samples demonstrate gas hydrate occurrences.
• Pore fill, fracture fill, nodule and massive hydrates were cored.
• Hydrate morphology is affected by buried depth and fluid flow.
• A local model of hydrate formation in South China Sea is present.

The second gas hydrate expedition of the Guangzhou Marine Geological Survey drilled 13 sites and recovered a large amount of gas hydrate in the east of the Pearl River Mouth basin of the South China Sea. In this study, we examine three of these sites (sites 05, 08, and 16) using multichannel seismic data, LWD logging data, and multi-beam echo sounder data. The geophysical data suggest and core samples confirm that gas hydrates occur near the BSR at all three sites, whereas near-surface gas hydrates exist at sites 08 and 16 with high fluid flux.We present a conceptual model for the formation and accumulation of gas hydrates in our study area, controlled by the activity of dissolved and free gas-rich fluid. Interpreted abundant gas sources below the BSR contribute to the generation of overpressure, resulting in the movement of gas-rich fluid. Dissolved gas-rich fluid forms pore filling hydrates near the BSR, and the relatively deep hydrate layer thickens over time by making its way through the growing hydrate layer and adding to the top, if the fluid is supplied sufficiently. Free gas-rich fluid is more favorable to generate near-surface nodules or massive hydrates, which could be stable in natural conditions. As sediments deposit, the relatively shallow hydrates are buried deeper and keep their morphologies. Furthermore, gas hydrate formation and stability imply that gas hydrates may be increasingly considered as a potential energy resource.