琼东南盆地与甲烷渗漏有关的早期成岩作用和孔隙水化学组分异常

Early diagenesis records and chemical composition abnormalities in pore water for methane-seep in sediments from the southern Qiongdong Basin

X-射线衍射和扫描电镜观察表明,采自南海北部琼东南盆地的沉积物样品中有天然气水合物和甲烷渗漏指示意义的自生碳酸盐、硫酸盐和草莓状framboids黄铁矿,自生矿物组合和显微结构特征与冷泉沉积物类似,属微生物成因。沉积物孔隙水化学组分分析结果显示,随着埋藏深度加深,SO42-,Ca2+,Mg2+和Sr2+浓度明显降低,Mg2+浓度与Ca2+浓度和Sr2+浓度与Ca2+浓度的比值急剧增加,这些地球化学特征与世界上天然气水合物产区的浅表层沉积物孔隙水中离子浓度异常吻合较好,暗示采样站位深部可能有油气或天然气水合物藏。

Complicated authigenic minerals were identified by XRD and SEM in the study sediment samples from the southern Qiongdong Basin, the northern South China Sea, such as miscellaneous carbonate, sulphate and frambiodal pyrite, which consistently indicate the gas hydrate and methane seep to occur in the area. The assemblage and fabric features are similar to those being found in cold-seep sediments, which are thought to be related to microorganisms fueled by dissolved methane. Results of chemical composition analysis in the pore water of the sediment samples show that the concentrations of SO4^2- , Ca^2＋ , Mg^2＋ and Sr^2＋ decrease clearly, and the concentration ratios of Mg^2＋ to Ca^2＋ and Sr^2＋ to Ca^2＋ increase sharply as the depth increases. These geochemical characteristics are similar to chemical composition abnormalities in the pore water of the shallow sediments where the gas hydrate occurs in the world. Those results strongly indicate that there should be gas hydrates or deep water oil （gas） reservoirs underneath.