鄂尔多斯盆地东南部长7油页岩时空分布及控制因素——来自沉积环境和沉积速率的制约

Time-space distribution of Chang 7 oil shale in southeastern Ordos Basin: Controlled by sedimentary environments and deposition rates

陈宇航 朱增伍 王喆 黄薇 李长春 杨治国 许锋 李卫波

CHEN Yuhang ZHU Zengwu WANG Zhe HUANG Wei LI Changchun YANG Zhiguo XU Feng LI Weibo

陕西省地质调查中心, 西安 710068 2. 西安石油大学 地球科学与工程学院, 西安 710065

Shaanxi Center of Geological Survey, Xi'an, Shaanxi 710068, China 2. School of Earth Sciences and Engineering, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China

鄂尔多斯盆地东南部延长组长7段油页岩资源丰富。早期研究主要集中在油页岩特征及形成环境描述上，对油页岩富集规律及控制因素的不明确，制约了油页岩选矿及开发。在综合分析露头、岩心和古生物资料的基础上，建立层序地层格架，并确定长7油页岩的沉积环境。结合有机地球化学和元素地球化学分析，从沉积相、有机质来源及沉积速率等方面分析油页岩时空分布规律及控制因素。鄂尔多斯盆地东南部延长组长7段油页岩形成于三角洲-湖泊环境中，有机地球化学分析表明油页岩有机质为湖泊低等生物和陆源高等植物混源，陆源有机质通过重力流输送到深湖中。沉积相和微量元素Th/U分析表明，湖侵域时期（长73）湖泊扩张，水深加大，可容纳空间增大和还原环境为有机质富集及油页岩形成提供了有利条件；在湖侵晚期，水深过大，陆源有机质供给不足，且有机质易降解，同时由于烃类向浊积岩夹层运移，导致油页岩含油率降低。稀土元素（La/Yb）N分析表明沉积环境和沉积速率控制了油页岩的分布：在靠近三角洲-浊积扇前缘的深湖区，沉积速率适中，距物源较近，有利于有机质输入和保存，为油页岩形成的理想场所；在三角洲-浊积扇沉积区，水体较浅，水动力较强，沉积速率过高，有机质易被稀释；在远离三角洲-浊积扇的深湖区，距物源较远，沉积速率较低，陆源有机质输入不足且易降解。

Abundant oil shale resources are hosted in the seventh member (Chang 7) of Triassic Yanchang Formation in the southeastern Ordos Basin. However, the previous studies mainly focused on the characteristics and generation environment of oil shale. However the enrichment regularities and controlling factors of oil shale were not clear, which restricted mining and subsequent production. A sequence stratigraphic framework was established based on the comprehensive analyses of outcrops, cores and fossil specimen to determine the sedimentary environment of Chang 7 oil shale. The controlling factors of oil shale distribution were discussed by the comprehensive analysis of sedimentary facies, organic matter sources and deposition rates. The Chang 7 oil shale in the southeastern Ordos Basin was deposited in delta and lacustrine sedimentary environment. Organic geochemical analyses showed that the organic matter in the Chang 7 oil shale originated from both terrestrial and lacustrine plants. The terrestrial organic matter was probably transported to deep lakes by gravity flows. Sedimentary facies analysis and Th/U value showed that during lake expansions, water depth increasing and associated accommodation space increasing in the transgressive system tract (TST) (Chang 73) provided a favorable environment for organic matter enrichment. However, during the late TST, the terrestrial organic matter input was lack and the organic matter was degraded in deep lakes, which led to oil yield decreasing. Additionally, hydrocarbon migrations to adjacent turbidites also led to oil yield decreasing. (La/Yb)N analysis showed that sedimentary environments and deposition rates controlled the spatial distribution of oil shale. In the deep lake close to delta-turbidite fan deposition systems, terrestrial and lacustrine organic matter inputs were bounteous and the deposition rate was moderate, which were beneficial to organic matter input and preservation, and provided a favorable environment for oil shale formation. In the area where delta-turbidite fan deposition systems developed, high deposition rates led to organic matter dilution, which was not beneficial to oil shale formation. In the deep lakes far away from delta-turbidite fan deposition systems, low deposition rates and the lack of deposit supplies were not beneficial to the input and enrichment of organic matter.

陕西省公益地质专项科技研究项目（20150301）资助。

https://doi.org/10.11781/sysydz201802200