Sedimentary environmental controls on petrology and organic matter accumulation in the upper fourth member of the Shahejie Formation (Paleogene, Dongying depression, Bohai Bay Basin, China)
Sedimentary environmental controls on petrology and organic matter accumulation in the upper fourth member of the Shahejie Formation (Paleogene, Dongying depression, Bohai Bay Basin, China)
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沉积环境对沙四上段岩石学和有机质富集的控制（渤海湾盆地东营凹陷古近系）
DOI：
10.1016/j.coal.2017.11.016
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发表时间：
2018-02
影响因子：
5.6
通讯作者：
Song Guoqi
中科院分区：
文献类型：
--
作者：
Liang Chao;Wu Jing;Jiang Zaixing;Cao Yingchang;Song Guoqi
始新统沙河街组上四段是渤海湾盆地典型的湖相页岩。这些沉积单元的矿物组成与海相硅质碎屑页岩明显不同。因此，了解湖相页岩特征和沉积环境，有助于分析其成因、有机质富集、富有机质页岩分布以及页岩油气勘探目标。通过岩心薄片和场发射扫描电镜观察，结合x射线衍射和地球化学分析，分析了沙四期页岩的矿物学、岩相特征、有机地球化学特征以及沉积环境。结果表明，整个研究区沙四系页岩以方解石、粘土矿物、石英为主，次要有白云石、长石、黄铁矿、硬石膏等。根据矿物组成、总有机碳（TOC）含量和沉积构造，确定了8个岩相。样品TOC含量在0.15% ~ 11.4%之间，平均为2.27%，有机质主要由I型干酪根组成。根据古气候、古盐度资料以及氧化还原条件和初级生产力，将沙四期页岩的沉积环境进一步划分为I、II、III和IV四个层段，这四个层段的差异较大，而不是持续的暖湿气候和还原条件。详细分析了这些层段的演化过程，并讨论了其对岩相和有机质聚集的控制作用。研究结果表明，沉积环境的旋回演化对矿物组成的发育和频繁的垂向岩相组合具有强烈的控制作用。因此，有机质的积累是许多相互关联的因素的结果，包括初级生产力、陆地输入和氧化还原条件的变化。在给定的时间间隔内，这些因素可以一起工作，或者其中一个可以发挥主导作用。
The upper fourth member of the Eocene Shahejie Formation (Es4s) is a typical example of a Bohai Bay Basin lacustrine shale. These sedimentary units are characterized by markedly different mineral compositions compared to marine siliciclastic shales. Thus, understanding the characteristics and sedimentary environment of lacustrine shales enables analysis of their origin, enrichment of organic matter (OM), distribution of organic rich shales, and exploration targets for shale oil and gas. Based on thin sections and field emission scanning electron microscope observations from well core samples, combined with X-ray diffraction and geochemistry, the mineralogy, lithofacies, organic and geochemical characteristics as well as the sedimentary environment of the Es4s shale are analyzed in this paper. The results show that, across the study area, the Es4s shale is mainly composed of calcite, clay minerals, and quartz, but also contains subordinate dolomite, feldspar, pyrite, and anhydrite. On the basis of mineral composition, total organic carbon (TOC) content and sedimentary structures, eight lithofacies are identified. The TOC content of samples ranges between 0.15% and 11.4%, with an average of 2.27%, while OM primarily consists of Type I kerogen. Based on paleoclimate and paleosalinity data as well as redox conditions and primary productivity, the sedimentary environment of the Es4s shale can be further divided into four intervals: Intervals I, II, III, and IV, and these four intervals have large differences, rather than persistently warm-damp climate and reducing condition. The evolution of these intervals is also analyzed in detail in this study, and the controls on lithofacies and OM accumulation are also discussed. The results of this study suggest that the development of mineral compositions and frequent vertical lithofacies associations are strongly controlled by the cyclic evolution of the sedimentary environment. Thus, the accumulation of OM occurs as the result of a number of inter-connected factors including primary productivity, terrestrial inputs, and variation in oxidizing-reducing conditions. Across a given interval, these factors can either work together, or one can play a leading role.
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