Migration of methane associated with gas hydrates of the Shenhu Area, northern slope of South China Sea
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ORIGINAL RESEARCH PAPER
Migration of methane associated with gas hydrates of the Shenhu Area, northern slope of South China Sea Rui Yang • Ming Su • Shaohua Qiao • Xiaorong Cong • Zheng Su • Jinqiang Liang Nengyou Wu
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Received: 31 March 2014 / Accepted: 22 January 2015 Ó Springer Science+Business Media Dordrecht 2015
Abstract Shenhu Area is located in the northern slope region of the South China Sea. Pre-drilling reservoir prediction by the analysis of both 2D and 3D seismic data indicates the presence of hydrate-bearing sediments of approximately 70 m thickness. Data also suggests a strong spatial correlation between gas hydrates and gas chimneys. Evidence suggests that these gas chimneys created pathways for the migration of gas and fluids into the gas hydrate stability zone (GHSZ). In this study, we analyze reflection seismic data collected in the vicinity of a well where an abundance of gas hydrate is known to exist. Between the base of the gas hydrate stability zone and the top of the gas chimney, a thin layer of fine grained sediments is observed. A number of small faults (and possibly additional micro-fractures) breach the stratum and connect the gas chimney and GHSZ directly. Fluids, including free gas, may migrate upwards through these fractures.
R. Yang M. Su S. Qiao X. Cong Z. Su N. Wu Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China e-mail: [email protected] R. Yang M. Su S. Qiao X. Cong Z. Su N. Wu (&) Laboratory of Gas Hydrate Formation Mechanism, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China e-mail: [email protected] R. Yang Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, Guangdong, China J. Liang Guangzhou Marine Geological Survey, Guangzhou 510075, Guangdong, China
Additionally, small faults within the flanks of the gas chimney may effectively promote gas migration to the GHSZ. Amplitude variation with offsets analysis and instantaneous frequency analysis provided evidences of fluid pathways. A low-frequency anomaly over the center of the gas chimney indicates high seismic attenuation. This is interpreted to be caused by the migration of fluids (probably methane). Based on the results of this study, we propose an exploration for the origin of formation of Shenhu Area gas hydrate formation through the migration of methane. Keywords Shenhu Area Gas hydrate Gas chimney AVO analysis Instantaneous frequency
Introduction Gas hydrates are crystalline solids that are formed when water molecules trap gas, commonly methane, within a crystal lattice cage (Sloan 1990). They are formed when pore fluids are saturated with methane at appropriate low temperature and high-pressure conditions. They commonly occur in marine sediments where water depths exceed 500 m. The most common indicators for marine gas hydrates are bottom simulating reflections (BSRs),
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