塔里木盆地西缘下寒武统玉尔吐斯组沉积地球化学及有机质富集机制研究

对塔里木盆地西缘下寒武统玉尔吐斯组3个典型的剖面开展岩石学及地球化学研究,探讨玉尔吐斯组沉积时期水体氧化-还原环境及初级生产力,为玉尔吐斯组烃源岩分布及资源潜力评价提供理论依据。玉尔吐斯组下部较高含量的Ba (>1000μg/g)说明其沉积时期表层水体初级生产力较高,产出的大量有机质埋藏于沉积物中,与玉尔吐斯组产出的大量浮游植物化石及较高的TOC值相一致。与之对应层位中,较高的δ15N (>8‰)说明水体中发生反硝化作用和/或厌氧氨氧化作用,表明有机物在从表层向深层水体沉降过程中发生降解作用,不断地消耗水体中的氧气,使得海洋内部形成偏还原的水体环境。同时,玉尔吐斯组下部硅质岩具有极低的Ge/Si值,表明正常海水是硅质的主要来源。硅质过饱和的水体促进沉积物快速胶结,可能对有机物的保存具有重要作用。

Paleoenvironment Interpretation of Early Cambrian Yurtus Formation,Tarim Basin,and Its Mechanism for Organic Carbon Accumulation

This study scrutinizes the petrological and geochemical signatures by systematically sampling three sections of Yurtus Formation, which deposited during the early Cambrian in the western margin of Tarim Basin. The redox conditions together with the intensity of primary productivity during Yurtus Formation deposition are reconstructed, which provide theoretical basis for the distribution of source rocks and evaluation of reservoir potentials. The extremely high Ba content (>1000 μg/g) and appearance of barite in the lower part of Yurtus Formation implies high primary productivity, which explains the substantive organic matter in the surface ocean and is also consistent with the abundant phytoplankton fossil record and high TOC content. The large positive excursion of δ¹⁵N (>8‰) in the lower parts of Yurtus Formation probably is resulted from denitrification or annamox, both of which occur only when O₂ is depleted. It may indicate that organic matter transporting from the surface ocean to deeper ocean could consume dissolved oxygen by the process of degradation, which would drive the condition of deeper ocean more reduced. Meanwhile, low Ge/Si values of cherts in the lower part of Yurtus Formation indicate normal seawater origin of Si input. Oversaturation state of silica promotes the organic matter preservation due to relatively fast cementation.