南海神狐海域天然气水合物系统中沉积物有机地球化学特征

Organic geochemical characteristics of sediments in the natural gas hydrate geo-system in Shenhu area,South China Sea

南海神狐海域是中国最重要的天然气水合物(以下称为水合物)调查研究区之一,为了解水合物赋存沉积体系中有机质的特征和对下一步的开发提供理论支持,对神狐海域W01B、W02B和W03B共3个站位的沉积物样品进行TOC、碳氮含量和可溶有机质的色谱—质谱分析。研究表明:物源指标结果显示W01B和W02B站位有着相似的垂向物源特征,在水合物赋存层位有陆源物质增多的现象;W03B站位则表现为随深度变浅,海源物质贡献增多;陆坡峡谷迁移体系的不对称剥削与横向沉积被认为是造成物源组成差异的重要原因。在分析不同物源指标时发现沉积物有机质遭受了不同程度的生物降解作用,并优先改变了正构烷烃的分布特征。进一步研究表明W01B和W02B站位生物降解作用更为明显,并且在水合物层位最为强烈,各项微生物降解指标都同步增大,与水合物垂向分布一致,都具有类似三角形分布的趋势,并且代表产甲烷菌的特征化合物——角鲨烷的增多,表明微生物降解作用与产甲烷作用在水合物层位内是同步进行的,为水合物中生物气的来源提供了充足保障。

Shenhu area of South China Sea is one of the most important natural gas hydrate survey areas in China. In order to understand the characteristics of organic matter in the sedimentary system with gas hydrates and provide theoretical support for further development,TOC,carbon and nitrogen content,and gas chromatographic-mass spectrometric analysis of sediment samples’ soluble organic matter from Wells W01B,W02B and W03B have been completed. The results of the source indicators showed that the W01B and the W02B had similar vertical mass-source characteristics,with an increase of terrestrial source material in the hydrate reservoir layers;while W03B showed an increase of marine source material contribution with depth upward. The asymmetric erosion and lateral deposition of channel-levee depositional system is considered as to be the most important reason for the differences in source composition. Analysis of the different source indicators revealed that the sediment organic matter was subjected to varying degrees of biodegradation and preferentially altered the distribution characteristics of n-alkanes. Through further analysis of biodegradation indicators,it is found that biodegradations at W01B and W02B were more pronounced,and the biodegradation effect in hydrate layers was the strongest,with all the microbial degradation indexes increased simultaneously. This was consistent with the vertical distribution of hydrates vertical distribution,both with a triangle-like trend. The increasing in squalane,a characteristic compound representing methanogens bacterial,suggests that microbial degradation and methanogenesis were synchronized within the hydrate layers,providing an adequate source of biogas for hydrate.