Gas Hydrate Occurrence Inferred from Dissolved Cl− Concentrations and δ18O 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,Wen Yan,Duofu Chen,Hongpeng Tong,Linying Chen

doi:10.3390/en7063886

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 δ18O 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 δ18O 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 δ18O values are attributed to gas hydrate dissociation instead of clay mineral dehydration. Moreover, the lower δ18O 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 18O-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 δ18O 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.

Highlights

Pockmarks form when seabed fluid discharges through the seafloor by suspending and winnowing the fine-grained sediments
When gas hydrates crystallize in the sediment pore space from water and either the biogenic methane bacterially generated in the methangenic zone or the thermogenic methane advected from greater depth, dissolved ions are excluded from the cage-like structure as water molecules are incorporated into gas hydrate, increasing the salinity, and the chlorinity of the pore waters
Na+ and K+ concentrations as well as pore water δ18O values change little in the whole core (Figure 2B–D)

Summary

Introduction

Pockmarks form when seabed fluid (generally methane-enriched) discharges through the seafloor by suspending and winnowing the fine-grained sediments. Methane-enriched fluid is likely to crystallize as gas hydrate when it is transported into the gas hydrate stability zone [1]. Fresh water and 18O-enriched fluid is released and mixes with surrounding pore water when gas hydrates dissociate due to the shift of temperature and pressure or sediment coring [7,8]. Excursions to low chlorinity in the pore waters, concomitant with the positive anomalies in the oxygen isotopic composition of the water provide compelling evidence for gas hydrate dissociation. These coupled anomalies are likely to give reliable quantitative estimates of gas hydrate concentration in the sediments [9,10]

Methods

Results

Discussion

Conclusion