鄂尔多斯盆地延长组长7黑色页岩岩相分类与沉积环境恢复

Lithofacies characteristics and sedimentary environment of Chang 7 black shale in the Yanchang Formation,Ordos Basin

鄂尔多斯盆地延长组长7黑色页岩分布面积广、厚度大、有机质含量高,具有广阔的页岩油勘探前景。在大量岩心、薄片和地化数据分析的基础上,对长7黑色页岩的岩相类型、地质特征与分布规律进行剖析,利用微量元素指标对其形成期古环境和页岩成因机制进行研究。按照“有机质丰度—岩矿组成—沉积构造”分类标准,将长7划分出3类岩相,其中长7_(3)底部发育极富有机质纹层状页岩,向长7_(2)过渡为富有机质纹层状黏土/粉砂质页岩,长7_(1)发育贫有机质块状粉砂质泥岩。自长7_(3)到长7_(1),随着湖盆萎缩和三角洲推进,黏土矿物、有机质和黄铁矿含量以及纹层发育程度逐渐降低。亚热带温暖潮湿气候和长期稳定的深水湖泊环境为藻类大规模发育创造了条件。微量元素研究表明,长7_(3)极富有机质页岩形成于湖盆生产力极高、水体缺氧甚至硫化的强还原环境,因受火山活动和湖底热液的影响而富含胶磷矿和Ni、Cu、Mo等营养元素,是国内罕见的富铀页岩。结合古地貌、气候、水文条件综合分析,认为长7黑色页岩的成因机制为强烈构造运动下火山活动、热液活动叠加拗陷湖盆的水体分层模式。

Chang 7 black shale of the Yanchang Formation,Ordos Basin,has a wide distribution area,large thickness and high organic matter content,which has a broad shale oil exploration prospect.Based on the analysis of a large number of core,thin section and geochemical data,the lithofacies types,geological characteristics and distribution laws of Chang 7 black shale are analyzed,and the palaeoenvironment and shale genetic mechanism during its formation period are studied by using trace element indicators.According to the classification standard of“organic matter abundance-rock mineral composition-sedimentary structure”,Chang 7 is divided into three types of lithofacies,in which the bottom of Chang 7_(3) is developed with very rich organic matter laminated shale,and Chang 7_(2) is transformed into organic matter rich laminated clay/silty shale,and Chang 7_(1) is developed with poor organic matter massive silty mudstone.From Chong 7_(3) to Chang 7_(1),as the lake basin shrinks and the delta advances,the content of clay minerals,organic matter and pyrite and the development of laminae gradually decrease.The subtropical warm and humid climate and long-term stable deep-water lake environment create conditions for the large-scale development of algae.The trace element study shows that Chang 7_(3) extremely rich organic shale was formed in a strong reducing environment with extremely high productivity in the lake basin,oxygen deficiency and even sulfurization in the water body,and was affected by volcanic activity and lake bottom hydrothermal solution.Therefore,it is rich in collophanite,Ni,Cu,Mo and other nutrients,and is a rare uranium rich shale in China.Based on the comprehensive analysis of palaeogeomorphology,climate and hydrological conditions,it is considered that the genetic mechanism of the Chang 7 black shale is the water stratification model of the depressed lake basin under the strong tectonic movement of volcanic and hydrothermal activities.