Low-amplitude BSRs and gas hydrate concentration on the northern margin of the South China Sea |
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| Authors: | Shiguo Wu Xiujuan Wang How Kin Wong Guangxue Zhang |
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| Institution: | (1) Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China;(2) Key Laboratory of Marine Geology and Environment, Chinese Academy of Sciences, Qingdao, China;(3) Institute of Biogeochemistry and Marine Chemistry, University of Hamburg, 20146 Hamburg, Germany;(4) Guangzhou Marine Geological Survey, MLR, Guangzhou, 510760, China |
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| Abstract: | The passive northern continental margin of the South China Sea is rich in gas hydrates, as inferred from the occurrence of
bottom-simulating reflectors (BSR) and from well logging data at Ocean Drilling Program (ODP) drill sites. Nonetheless, BSRs
on new 2D multichannel seismic reflection data from the area around the Dongsha Islands (the Dongsha Rise) are not ubiquitous.
They are confined to complex diapiric structures and active fault zones located between the Dongsha Rise and the surrounding
depressions, implying that gas hydrate occurrence is likewise limited to these areas. Most of the BSRs have low amplitude
and are therefore not clearly recognizable. Acoustic impedance provides information on rock properties and has been used to
estimate gas hydrate concentration. Gas hydrate-bearing sediments have acoustic impedance that is higher than that of the
surrounding sediments devoid of hydrates. Based on well logging data, the relationship between acoustic impedance and porosity
can be obtained by a linear regression, and the degree of gas hydrate saturation can be determined using Archie’s equation.
By applying these methods to multichannel seismic data and well logging data from the northern South China Sea, the gas hydrate
concentration is found to be 3–25% of the pore space at ODP Site 1148 depending on sub-surface depth, and is estimated to
be less than values of 5% estimated along seismic profile 0101. Our results suggest that saturation of gas hydrate in the
northern South China Sea is higher than that estimated from well resistivity log data in the gas hydrate stability zone, but
that free gas is scarce beneath this zone. It is probably the scarcity of free gas that is responsible for the low amplitudes
of the BSRs. |
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| Keywords: | Bottom simulating reflectors Gas hydrates Free gas Saturation South China Sea |
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