Gas Hydrate System of Shenhu Area, Northern South China Sea:Wire-line Logging, Geochemical Results and Preliminary Resources Estimates

摘要

Shenhu Area, one of the most favorable locations for gas hydrates in the north slope of South China Sea, is located in the Pearl River Mouth Basin. The geological setting, temperature and pressure conditions are favorable for the formation of gas hydrate. Analysis of the geochemistry of sediments from large piston cores showed that the sulfate-methane interface (SMI) is from 10 to 27 mbsf in the research area. Methane was the dominant hydrocarbon gas in all gas analyses. The δ~(13)C_(CH4) values of headspace gases from the gravity piston core sediments ranged from -74.3‰ to -46.2‰, with the majority being less than -55‰. It is likely that the methane of gas hydrate originated from in-situ microbial activity. The drilling recovered high concentrations of hydrate (maximum 26-48%, with the gas composition of >99% CH_4) in a disseminated form in foram-rich clay sediments. The wire-line log showed a very distinctive positive anomaly in resistivity and sonic velocity in the interval of the hydrate layer. It was also interesting that the caliper log was smoother through this interval and the density log also showed a small decrease. From the combined well-log and geochemical data, the hydrate-bearing sediments were determined to range from 10 meters to 25 meters in thickness, and to concentrate just above the base of the gas hydrate stability zone (BGHSZ). The estimate of the total amount of methane trapped in the drilling-confirmed 15 km~2 extent of the Shenhu hydrate deposit is about 160×10~8m~3at a 50% probability. Given the size of this accumulation, the Chinese government and industry have a keen interest in the commercial development potential of the resource.