南海北部琼东南海域HQ-48PC站位地球化学特征及对天然气水合物的指示意义

摘要/Abstract

摘要：

南海北部陆坡琼东南海域是中国天然气水合物勘探最具潜力的区域之一。对该海域的HQ-48PC站位沉积物样品的顶空气甲烷含量、孔隙水阴、阳离子及微量元素含量等地球化学特征进行综合分析，结果显示：在硫酸盐-甲烷界面(SMI，SulfateMethane Interface)(推算深度约为6.05 mbsf)发生了强烈的甲烷厌氧氧化反应(AMO，Anaerobic Methane Oxidation)，主要表现为SO₂^-4含量线性降低接近于0、CH₄含量发生骤增、有机碳和黄铁矿含量达到最大值及形成一个“钡锋”等特征。在SMI之上，HCO^-3浓度随深度的增加呈明显上升的趋势， Ca ²⁺ 、Mg²⁺ 、Sr ²⁺等离子浓度随深度的增加呈降低的趋势，Mg²⁺/Ca²⁺比值随深度的增加呈明显增加的趋势，自生碳酸盐矿物以方解石为主；在SMI之下，HCO^-3浓度随深度的增加呈缓慢下降的趋势，Ca²⁺浓度变化不大，Mg²⁺、Sr²⁺浓度和Mg²⁺/Ca²⁺比值随深度的增加呈降低的趋势，自生碳酸盐矿物以白云石为主。沉积物孔隙水的PO3-4和NH+4含量较高，它们随深度的增加呈明显升高的趋势，且这种变化趋势与 SO₂^-4含量的下降趋势大致呈镜像关系。这些地球化学异常特征与国际上已发现有天然气水合物地区的地球化学特征相类似，暗示该采样站位深部沉积物中可能赋存有天然气水合物藏。

关键词: 南海北部陆坡, 琼东南海域, 天然气水合物, 地球化学, 甲烷厌氧氧化反应, 硫酸盐-甲烷界面

Abstract:

The Qiongdongnan area is one of the most prospective areas of the gas hydrate exploration in the northern continental slope of the South China Sea. Headspace gas and pore water samples were collected from sediment core in Site HQ-48PC in this area. Comprehensive studies on methane concentrations in headspace gas, anions, cations and trace element contents in pore water indicate that strong anaerobic methane oxidation(AMO) occurres at the sulfatemethane interface(SMI), with the calculated depth of 605 mbsf by Linear Regression. For example, the concentration of dissolved sulfate(SO2-4) decreases linearly to zero, while that of methane increases abruptly; the concentrations of pyrite and organic carbon in sediments reach maximum value, and that of dissolved Ba2+ behaves as a Ba front character. The concentrations of alkalinity(HCO^-3) increase clearly with the increasing of the depth above the SMI; those of Ca ²⁺ ,Mg²⁺,Sr²⁺show a decreasing trend, and the ratio of Mg²⁺/Ca²⁺increases sharply. The main product of authigenic carbonates is calcite above the SMI. The concentration of HCO^-3 decreases slowly with the increasing of depth below the SMI; and concentration of Mg²⁺,Sr²⁺show the same trend with the increasing of depth above the SMI, but the concentration of Ca²⁺ shows little fluctuation, which results in a decreasing ratio of Mg²⁺/Ca²⁺. The main product of authigenic carbonates is dolomite in this region. The concentrations of phosphate(PO3-4) and ammonium(NH+4) in pore water of sediment core in Site HQ-48PC are high and increase clearly with depth;the increasing trend has an enantiomorphous relation to the decreasing trend of the concentration of SO₂^-4. These geochemical characteristics are similar to those of the shallow sediments where the gas hydrate occurs in the world, strongly indicating that there should be gas hydrate reservoirs underneath.

Key words: northern continental slope of the South China Sea, Qiongdongnan area, gas hydrate, geochemistry, Anaerobic Methane Oxidation(AMO), SulfateMethane Interface(SMI)

中图分类号:

P736.21

吴庐山, 杨胜雄, 梁金强, 苏新, 杨涛, 张欣, 程思海, 陆红锋. 南海北部琼东南海域HQ-48PC站位地球化学特征及对天然气水合物的指示意义[J]. 现代地质, 2010, 24(3): 534-544.

WU Lu-Shan, YANG Sheng-Xiong, LIANG Jin-Qiang, SU Xin, YANG Tao. Geochemical Characteristics of Sediments at Site HQ-48PC in Qiongdongnan Area, the North of the South China Sea, and Their Implication for Gas Hydrates[J]. Geoscience, 2010, 24(3): 534-544.

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