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

Abstract

Abstract 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 d13CCH4 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% CH4) 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 km2 extent of the Shenhu hydrate deposit is about 160×108m3 at 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. 1. Introduction Gas hydrate is an ice-like solid substance formed by the combination of low-molecular-weight gases such as methane, ethane, and carbon dioxide with water. Gas hydrate mainly occurs naturally in sediments beneath the permafrost and the sediments of the continental slope in water depths more than 300 m. The reason why marine gas hydrate systems are important to economic society and the environment is not only that gas hydrate in some places may be concentrated enough to be an economically viable fossil fuel resource, but also because gas hydrate can cause geo-hazards through large-scale slope destabilization (Maslin et al., 2004) and can release methane, a potential greenhouse gas, to impact the global climate (Dickens et al., 1995; Hesslebo et al., 2000; Maslin and Thomas, 2003; Kennett et al., 2003; Sowers, 2006; Bowen et al., 2006). Gas hydrate and its associated sediments have also become an important focus for biogeochemical study of the deep biosphere (Wellsbury et al., 2000).