吴庐山, 杨胜雄, 梁金强, 苏新, 程思海, 付少英, 龚跃华, 祝有海. 南海北部神狐海域沉积物中烃类气体的地球化学特征[J]. 海洋地质前沿, 2011, 27(6): 1-10.
    引用本文: 吴庐山, 杨胜雄, 梁金强, 苏新, 程思海, 付少英, 龚跃华, 祝有海. 南海北部神狐海域沉积物中烃类气体的地球化学特征[J]. 海洋地质前沿, 2011, 27(6): 1-10.
    WU Lushan, YANG Shengxiong, LIANG Jinqiang, SU Xin, CHENG Sihai, FU Shaoying, GONG Yuehua, ZHU Youhai. GEOCHEMICAL CHARACTERISTICS OF HYDROCARBON GASES IN SEDIMENTS IN SHENHU AREA, THE NORTHERN SOUTH CHINA SEA[J]. Marine Geology Frontiers, 2011, 27(6): 1-10.
    Citation: WU Lushan, YANG Shengxiong, LIANG Jinqiang, SU Xin, CHENG Sihai, FU Shaoying, GONG Yuehua, ZHU Youhai. GEOCHEMICAL CHARACTERISTICS OF HYDROCARBON GASES IN SEDIMENTS IN SHENHU AREA, THE NORTHERN SOUTH CHINA SEA[J]. Marine Geology Frontiers, 2011, 27(6): 1-10.

    南海北部神狐海域沉积物中烃类气体的地球化学特征

    GEOCHEMICAL CHARACTERISTICS OF HYDROCARBON GASES IN SEDIMENTS IN SHENHU AREA, THE NORTHERN SOUTH CHINA SEA

    • 摘要: 烃类气体是形成天然气水合物的物质基础,浅表层沉积物顶空气中烃类气体的异常特征可用来探测深部是否存在天然气水合物藏。对南海北部神狐海域浅表层沉积物以及钻探采获的含或不含天然气水合物的沉积物的顶空气样品中烃类气体组分和同位素组成进行了测试分析,结果表明:291个1 mbsf(bsf,below sea floor)沉积物顶空气样品中的烃类气体以甲烷为主,甲烷含量经过归一化处理后的变化范围为2.49~77.23 μL/kg,平均值为15.22 μL/kg。根据趋势面分析共圈出12个异常区,这些异常区受NE向断裂构造控制呈NE向展布;4PC和23PC两个站位的14个顶空气样品中甲烷的δ13C值为-74.3‰~-46.2‰(PDB),平均值为-60.9‰(PDB),C1/(C2+C3)比值为5.78~49.45,平均值为16.23,烃类气体主要来源于微生物气,同时混入少量热解气;钻探采获的4个顶空气样品中甲烷的δ13C值为-62.2‰~-54.1‰(PDB),δD值为-225‰~-180‰(SMOW),C1/(C2+C3)比值为575~1 668,它们为微生物气或以微生物气为主的混合气,甲烷主要为二氧化碳还原型,同时还有少量来自深部热成因。浅表层沉积物顶空气中的烃类气体主要来源于深部地层中天然气水合物的分解以及更深地层中的烃源岩或天然气藏,这些烃类气体沿断裂或底辟构造等通道向上运移,前者以微生物气为主,后者为热解气。

       

      Abstract: The anomalous distribution of the gases in headspace of shallow sediments could be applied as an indicator to explore whether there are gas hydrate accumulations underneath. Comprehensive analyses of hydrocarbon gas composition and isotopic composition of methane in headspace gas in shallow sediments and related drilling cores in the Shenhu area are studied in this paper. Results show that CH4 is the major component of gases from the total 291 samples 1 mbsf (bsf, below sea floor) with trace C2H6 and C3H8, and the normalized concentrations of CH4 range from 2.49 to 77.23 μL/kg with an average value of 15.22 μL/kg. CH4 concentration from 12 anomalies were determined using tendency analysis method, and they extend in a NE direction as a whole, that is obviously controlled by NE trend fractures. The δ13C values of CH4 from 14 headspace gas samples from Sites 4PC and 23PC range from -74.3‰ to -46.2‰ with an average value of -60.9‰(PDB), and the ratios of C1/(C2+C3) range from 5.78 to 49.45 with an average of 16.23. This indicates that hydrocarbon gases of those samples are mainly microbial gas with small fraction of thermogenic gas. Isotopic anaylses were applied to 4 headspace gas samples collected from the drilling cores. It is found that δ13C and δD values of CH4 are -62.2‰ to -54.1‰ (PDB) and -225‰ to -180‰ (SMOW), respectively, and the ratios of C1/(C2+C3) was ranging from 575 to 1 668, suggesting that CH4 seems to be microbial gas or mixed gases with microbial gas dominating, and CH4 was mainly generated from microbial CO2 reduction and there is a little contribution from thermogenic gas. Our study indicates that the headspace hydrocarbon gases in shallow sediments were from gas hydrates, hydrocarbon source rocks or gas accumulations underneath. The hydrocarbon gases migrated upward along faults and diapir structures. Hydrocarbon gases originated from the dissociation of gas hydrates are mainly microbial gas, and those from hydrocarbon source rocks or gas accumulations are thermogenic gases in origin.

       

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