204 / 2023-07-15 16:33:15

鄂尔多斯盆地延长组长73段细粒重力流沉积特征及其成因机制

细粒重力流,延长组,岩相

摘要

作为深湖环境的重要沉积作用过程之一，细粒重力流沉积作用可以把浅水的细粒碎屑和有机质搬运到深湖，形成优质烃源岩和页岩油甜点储层，因此，开展湖盆细粒重力流沉积特征及其成因的研究对陆相盆地页岩油的勘探有着重要的意义。通过岩心描述、薄片观察、X射线衍射和微量元素分析等手段，对鄂尔多斯盆地西南部某研究区块延长组长7₃段细粒重力流的沉积特征及其成因机制展开了研究。结果表明，鄂尔多斯盆地上三叠统延长组长7₃段为温暖湿润的深湖还原环境，这套地层内主要发育了7种细粒沉积岩相组合，分别为：细粒静水空落沉积岩相组合（LA1），细粒泥质湍流-泥流沉积岩相组合（LA2），细粒过渡流-泥流岩相组合（LA3），细粒滑塌碎屑流-泥流沉积岩相组合（LA4），细粒浪涌状浊流沉积岩相组合（LA5）、细粒异重流沉积岩相组合（LA6）和细粒浓密度流沉积岩相组合（LA7）。这些岩相组合分别发育在非限定水道湖底扇体系和细粒水道-堤岸-朵叶体系内。细粒重力流体系的发育主要受控于湖平面的变化、沉积物的补给和湖床古地貌等因素。鄂尔多斯盆地长7₃段沉积期为晚三叠世最大湖泛期,构造运动活跃，挠曲坡折发育，火山喷发频繁，多重因素导致坡折之下发育大量细粒重力流沉积；与此同时，凝灰质的加入为水体提供了养分，也促进形成了缺氧还原环境，有利于有机质的保存。以上多重作用导致长7₃段形成了多套源储组合，而细粒过渡流、细粒碎屑流-泥流、细粒异重流、细粒浪涌状浊流等流体形成的粉砂质和泥质沉积物则具有形成页岩油“甜点段”的潜力。



Abstract

As one of the important depositional processes of the deep lake environment, fine-grained gravity flow can transport fine-grained particles and organic matter from shallow water to deep lakes, forming high-quality hydrocarbon source rocks and shale oil desert reservoirs. Therefore, it is of great significance to study the sedimentary characteristics and genesis of fine-grained gravity flow in the lacustrine basin for shale oil exploration.

Through core description, thin section observation, X-ray diffraction, and trace element analysis, the sedimentary characteristics and genetic mechanism of fine-grained gravity flow in Chang 7₃ member of the Yanchang Formation of a research block in southwest Ordos Basin are studied. The results show that the Yanchang 7₃ member of the Ordos Basin, deposited during the Upper Triassic, was formed in a warm and humid deep lake reduction environment, in which 7 kinds of fine-grained sedimentary facies assemblage are mainly developed, respectively: Fine-grained hydrostatic and fallout lithofacies association (LA1), fine-grained mud turbulent and mud flow lithofacies association (LA2), fine-grained transitional plug flow and mud flow lithofacies association (LA3), fine-grained debris flow and mud flow lithofacies association (LA4), Fine-grained surge-like turbidity flow lithofacies association (LA5), fine-grained hyperpycnal flow lithofacies association (LA6) and fine-grained dense fluid lithofacies association (LA7). These lithofacies associations are mostly developed in the unrestricted channel substring fan system and fine-grained channel-bank-lobe system. The development of the fine-grained gravity flow system is mainly controlled by lake level changes, sediment recharging speed, and the palaeogeomorphology of the lake bed. The depositional period of Chang 7₃ Member of Ordos Basin was the maximum lake-flooding period of the Late Triassic, with active tectonic movement, flexion slope break development, and frequent volcanic eruption. Meanwhile, the addition of tuff provides nutrients for the water and promotes an anoxic reduction environment, which is conducive to the preservation of organic matter. The above multiple actions lead to the formation of multiple source-reservoir combinations in Chang 7₃ Member, and the silty and muddy sediments formed by fine-grained transitional plug flow, fine-grained debris flow and mud flow, fine-grained hyperpycnal flow, fine-grained surge-like turbidity flow, and other flows have the potential to form shale oil dessert segment.