Numerical modeling of gas hydrate saturation for the Shenhu area, South China Sea

The Shenhu area is located in the Pearl River Mouth Basin. It is a critical testing area for gas hydrate drilling and pilot production. Comprehensive studies of core samples and logging and seismic data suggest that gas hydrate saturation, thickness of gas hydrate layer and gas source conditions are different from sites to sites. Based on the geological model established by integrating the well log and seismic data from both gas hydrate and deep-water oil and gas drilling sites, we simulated the temperature field, organic matter maturity, hydrocarbon generation of source rocks, fluid migration pathways and gas hydrate saturation related to different source rocks with the PetroMod software. The results suggest that biogenic gas is mainly distributed in the immature organic strata 1 500 m below the seafloor, while thermogenic gas is distributed in the matured and over matured deposits over a depth of 2 300 m. Gas hydrate cannot be formed by in-situ biogenic gas within the gas hydrate stability zone. Therefore, the gases, which may form gas hydrate are mainly the biogenic and thermogenic gases moving up from the deep strata. The comparison between the modeling results and the log-derived saturation data suggest that the simulated saturation is around 10% for biogenic gas to become gas hydrate in the lower part of stability zone, while the value is higher at some areas such as canyon ridges. Higher saturation (＞40%) for hydrate formation is closely related to deep source thermogenic gas from the Wenchang and Enping Formations released in an episodic manner along the fluid migration channels such as sand layers, faults and gas chimneys. In addition, the methane contents from biogenic and thermogenic gases are calculated based on the modeling gas hydrate saturation. It shows that the thermogenic gas content is about 80% at Site W19 and 73% at Site W17, and nearly no thermogenic gas is found at Site SH2.