中上扬子地区中二叠世缺氧环境成因模式研究

In the recent decade years, anoxic deposits are one of the hot issues in sedimentology, and it is closely related with the source rocks, so the researches on anoxic environments are of great theoretical and practical significance. Currently, international studies focus on the global retaceous oceanic anoxic events, while the domestic researches on the anoxic deposits of the Permian in south China are relatively tenuous. In response to this phenomenon, this project takes the Middle Permian in Middle and Upper Yangtze Area as the research object. And the theoretical foundations of this project are the theories of modern sedimentology and related marine geology. The methods adopted by this project are the combination of geological field work and lab researches, the combination of macroscopic and microscopic researches, the combination of modern and ancient deposition researches, the combination of routine and advanced analytical tests and the combination of inorganic geochemical and organic geochemical analysis. The types, lithological and geochemical features, distribution and sedimentary.environments of the anoxic deposits of the Middle Permian in Middle and Upper Yangtze Area will be studied in detail. On the basis of the studies above, their identification criteria will be perfected and their genetic models will be established. This research will not only be beneficial to the reconstruction of the palaeogeography of the Permian in the Middle and Upper Yangtze Area, but also be of important practical significance to the hydrocarbon exploration of the Permian in the study area.

本项目选取中上扬子地区中二叠统为研究对象，以现代沉积学理论为指导，结合相关的海洋地质学的理论，采取野外地质工作和室内综合研究相结合，宏观和微观相结合，现代沉积和古代沉积研究相结合，常规分析与先进的分析测试相结合，无机地球化学分析和有机地球化学分析相结合的方法开展此项研究，对中上扬子地区中二叠世缺氧沉积的类型、岩石特征、地球化学特征和沉积环境进行详细研究。在此基础上，完善其辨别标志，建立其成因模式。该项研究不仅有利于中上扬子地区二叠纪的古地理重建，同时对研究区二叠系的油气勘探也具有重要的现实意义。

中上扬子地区中二叠统缺氧沉积较为发育，岩石类型主要有黑色泥页岩、黑色薄层硅岩，深灰色—灰黑色中—薄层泥晶石灰岩和含泥石灰岩。前三种岩石类型常形成暗色泥页岩—薄层硅岩—泥晶石灰岩组合，为Ⅰ类缺氧沉积；而含泥石灰岩为眼球状石灰岩的“眼皮”部分，为Ⅱ类缺氧沉积。分别对这两类缺氧沉积的分布规律、岩石特征、古生物特征及地球化学特征进行了研究，并分析了各自的沉积环境及缺氧环境的成因。结果表明Ⅰ类缺氧沉积发育于茅口组中上部，平面上主要分布于北部地区及鄂西地区；Ⅱ类缺氧沉积广泛发育，垂向分布于栖霞组下部和茅口组中下部，分布具阶段性。Ⅰ类缺氧沉积，生屑含量低，生物以深水型为主；Ⅱ类缺氧沉积呈波状起伏的纹层状；生屑含量多为30~50%，种类以介壳类较丰富，缺乏藻类；生屑较破碎，显定向排列，多为原地堆积；Ⅱ类缺氧沉积遗迹化石Zoophycos较为发育。研究区Ⅰ类缺氧沉积和Ⅱ类缺氧沉积微量元素含量差异较明显，表明二者成因不同。REE研究表明Ⅰ类缺氧沉积Ce、Eu负异常明显；Ⅱ类缺氧沉积Eu负异常明显，而Ce负异常不太明显，表明Ⅰ类缺氧沉积的缺氧程度应更高。沉积环境研究认为Ⅰ类缺氧沉积为盆地、盆地边缘或斜坡环境的产物，其缺氧环境主要为大规模的海侵形成；Ⅱ类缺氧沉积应为浅水碳酸盐岩台地的产物，其缺氧环境应为上升流作用形成。在上述基础探讨了研究区中二叠统缺氧沉积的石油地质意义。分析认为研究区中二叠统具有发育优质烃源岩的良好条件。沉积环境控制着烃源岩的岩石类型和有机质丰度。盆地、盆地边缘或斜坡是优质烃源岩发育的良好环境，表明中上扬子地区Ⅰ类缺氧沉积是良好的烃源岩。Ⅱ类缺氧沉积不仅有机质丰度较高，而且横向、纵向分布广泛，具有较大的体积，就有机质数量而言，其应是良好的烃源岩。因此研究区中二叠统烃源岩的发育与沉积环境和上升流作用的关系密切。在油气勘探工作中对Ⅱ类缺氧沉积应加以重视。