Gas Hydrate Occurrence Inferred from Dissolved Cl ? Concentrations and ? 18 O Values of Pore Water and Dissolved Sulfate in the Shallow Sediments of the Pockmark Field in Southwestern Xisha Uplift, Northern South China Sea

Min Luo, Linying Chen, Hongpeng Tong, Wen Yan and Duofu Chen
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Min Luo: Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
Linying Chen: Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
Hongpeng Tong: Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
Wen Yan: Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
Duofu Chen: Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

Energies, 2014, vol. 7, issue 6, 1-14

Abstract: Deep-water pockmarks are frequently accompanied by the occurrence of massive gas hydrates in shallow sediments. A decline in pore-water Cl ? concentration and rise in ? 18 O value provide compelling evidence for the gas hydrate dissociation. Mega-pockmarks are widely scattered in the southwestern Xisha Uplift, northern South China Sea (SCS). Pore water collected from a gravity-core inside of a mega-pockmark exhibits a downward Cl ? concentration decrease concomitant with an increase in ? 18 O value at the interval of 5.7–6.7 mbsf. Concentrations of Cl ? , Na + , and K + mainly cluster along the seawater freshening line without distinct Na + enrichment and K + depletion. Thus, we infer that the pore water anomalies of Cl ? concentrations and ? 18 O values are attributed to gas hydrate dissociation instead of clay mineral dehydration. Moreover, the lower ? 18 O values of sulfate in the target core (C14) than those in the reference core (C9) may be associated with the equilibrium oxygen fractionation during sulfate reduction between sulfate and the relatively 18 O-depleted ambient water resulting from gas hydrate formation. The gas hydrate contents are estimated to be 6%–10% and 7%–15%, respectively, according to the offset of Cl ? concentrations and ? 18 O values from the baselines. This pockmark field in southwestern Xisha Uplift is likely to be a good prospective area for the occurrence of gas hydrate in shallow sediments.

Keywords: gas hydrate; pore water; chloride; ? 18 O; southwestern Xisha Uplift (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2014
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