Gas-Hydrate System of the Shenhu Area, Northern South China Sea

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20485, ’Gas Hydrate System of Shenhu Area, Northern South China Sea: Wire-line Logging, Geo chem ical Results and Preliminary Resources Estimates,’ by Nengyou Wu, Shengxiong Yang, Haiqi Zhang, Jinqiang Liang, Hongbin Wang, and Jing'an Lu, China Geological Survey, originally prepared for the 2010 Offshore Technology Conference, Houston, 3-6 May. The paper has not been peer reviewed. The Shenhu area, one of the most favorable locations for gas hydrates in the north slope of the South China Sea, is in the Pearl River mouth basin. The geological setting and temperature and pressure conditions are favorable for the formation of gas hydrates. Drilling recovered high concentrations of hydrate (maximum 26 to 48%, with gas composition of greater than 99% methane) in a disseminated form in foram-rich clay sediments. Introduction Gas hydrate is an ice-like solid substance formed by the combination of low-molecular-weight gases (e.g., methane, ethane, and carbon dioxide) with water. Gas hydrate mainly occurs naturally in sediments beneath the permafrost and in the sediments of the continental slope in water depths greater than 300 m. The reason that marine gas-hydrate systems are important economically, socially, and the environmentally is not only that gas hydrate in some places may be sufficiently concentrated to be an economically viable fossil-fuel resource, but also because gas hydrate can cause geohazards through large-scale slope destabilization and can release methane, a potential greenhouse gas, to affect the global climate. Gas hydrate and its associated sediments also have become an important focus for biogeochemical study of the deep biosphere. The stability of gas hydrate fundamentally depends on four factors: temperature, pressure, gas composition, and pore-water salinity. The nucleation and growth of gas hydrate also depend on sediment grain size, shape, and mineral composition. These factors, which control gas-hydrate formation and stability, are affected by a series of physical and chemical processes in the marine sediments and result in a variation of gas-hydrate dynamics on different time scales. Therefore, gas hydrate is not distributed continuously all over the world in the vertical and horizontal scales. Because the geological setting and the factors controlling gas-hydrate formation are different in various locations, scientists have established different geological models on the basis of the formation mechanisms, gas sources, and dynamics. Gas-hydrate deposits can be classified into the two end-member regimes of the high-flux gas-hydrate deposit and the low-flux gas-hydrate deposit, on the basis of the mechanisms that control gas transport into the gas-hydrate-stability zone (GHSZ), although both regimes operate simultaneously in many regions. One investigator grouped the hydrates in the marine environment into these same two categories, calling them diffusion gas hydrates and vent gas hydrates. The diffusion gas hydrates occur widely in areas where bot-tom simulating reflectors (BSRs) are recorded in seismic profiles, and they reflect a thermodynamic equilibrium system of hydrates and water with dissolved methane within a GHSZ. The hydrates are buried within the sediment, significantly deeper than the sediment/water interface, and are characterized by low concentrations. The vent gas hydrates occur in an area where gas vents out of the seafloor. This is a thermodynamic-disequilibrium system of hydrate, water, and free gas, occurring in a zone that extends from the base of GHSZ to the seafloor, and is characterized by high concentration. However, these conceptual models should be verified by actual evidence from sites around the world.