鄂尔多斯盆地长7段细粒沉积岩特征与古环境——以铜川地区瑶页1井为例

三叠系延长组长7段是鄂尔多斯盆地中生界重要的烃源岩层系。为探究长7段细粒沉积岩发育特征、沉积环境及有机质富集控制因素，以盆地南缘瑶页1井为例，运用X射线衍射、偏光显微镜、扫描电镜、XRF成像等方法，对长7段细粒沉积岩沉积特征及其纵向演化规律开展系统分析。研究表明，长7段主要发育三种细粒沉积岩类型:块状泥岩、水平纹层状页岩和透镜纹层状页岩。其中块状泥岩主要位于长7段的上部和下部，发育块状构造，石英长石含量较高，黄铁矿多以八面体自形晶产出，判断块状泥岩是高能环境中的产物，水体富氧，不利于有机质的生成和保存，有机质丰度较低。透镜纹层状页岩主要发育于长7段的中部，黄铁矿异常丰富，含有大量火山灰组分；草莓状黄铁矿粒度分布指示水体缺氧但具有一定波动；在较好保存环境和火山营养物质注入的双重作用下，透镜纹层状页岩的平均有机质丰度最高。水平纹层状页岩则多位于长7段中上部，以"有机质+黏土"或"有机质+黏土+火山灰"的结构产出，黄铁矿粒度分析指示沉积环境安静，水体缺氧，有利于有机质保存，有机质丰度介于块状泥岩和透镜纹层状页岩之间。瑶页1井长7段有机质丰度与黄铁矿含量的高相关性指示铜川地区长7期水体可能具有较高的硫酸盐浓度，硫酸盐的来源可能与湖底热液、海侵、火山喷发等事件相关。

Sedimentary Characteristics of Fine-grained Sedimentary Rock and Paleo-environment of Chang 7 Member in the Ordos Basin: A case study from Well Yaoye 1 in Tongchuan

The Chang 7 member of the Triassic Yanchang Formation is one of the critically important source rocks in the Ordos Basin. In order to reveal the characteristics of fine-grained sedimentary rock, sedimentary environment and controlling factors of organic matter accumulation, X-ray diffraction(XRD), polarizing optical microscopy, scanning electronic microscopy(SEM)and X-ray fluorescence(XRF)imaging were implemented for the fine-grained sedimentary rock in well Yaoye 1 located at the southern margin of the Ordos Basin. The study found three kinds of finegrained sedimentary rock in the Chang 7 member:massive mudstone, lenticular laminated shale and horizontally laminated shale.The massive mudstone mainly developed during the early and late periods of the Chang 7 member. It is characterized by a massive structure, and has a high content of both quartz and feldspars from a terrestrial source. Octahedral crystals and pyrite aggregation predominantly occur in the massive mudstone, indicating that the water column was rich in oxygen; from this it is inferred that the massive mudstone was mainly formed in a turbulent and oxidizing environment, with a consequent negative impact on organic matter product and preservation. The lenticular laminated shale predominantly occurs in the middle part of the Chang 7 member. The pyrite and volcanic ash content are extremely high. The framboidal pyrite distribution indicates an anoxic-dysoxic environment with limited hydrodynamic fluctuation. In this depositional environment, large quantities of volcanic ash input triggered algal blooms and increased primary productivity in the water column, resulting in organic matter enrichment.The horizontally laminated shale is mostly found in the middle and upper parts of the Chang 7 member. It is characterized by a combination of clay laminae, organic-matter laminae and volcanic ash beds. Analysis of the pyrite morphology indicated that the oxygen content in the bottom water was relatively low. Consequently, well preserved total organic carbon(TOC)content is much higher than in the massive mudstone. The high correlation between TOC content and pyrite suggest an abundance of sulfates in the water column, probably derived from either hydrothermal activity, transgression or volcano eruption.