沉积环境对细粒沉积岩微观结构的影响——以鄂尔多斯盆地延长组7段页岩为例

Effects of sedimentary environment on microstructures of fine-grained rocks:a case study of the Yanchang Formation 7 Member shale in the Ordos Basin as an example

细粒沉积岩微观结构特征是岩石的基本属性,影响细粒沉积岩的物理化学行为。以鄂尔多斯盆地延长组7段页岩为例,根据多尺度多视域扫描电镜拼接和图像识别技术,分析细粒沉积颗粒的几何形貌特征,探讨沉积环境对细粒沉积颗粒几何形状和排列结构特征的影响。结果表明:研究区半深湖—深湖相细粒沉积颗粒为细颗度(2~3μm)、扁平和棱角形,颗粒沿某一段连续区间呈定向分布,强度分布呈扁平的仿锥状,定向排列程度好;浅湖相细粒沉积颗粒为中细颗度(2~8μm)、扁平和棱角形,颗粒沿某一区间呈尖状分布,规律性不明显,定向排列程度中等;三角洲相细粒沉积颗粒为细粒度(8~12μm)、似圆形,颗粒分布杂乱无章,强度分布呈似圆形或椭圆形。颗粒周长棱角性指数和扁平度越好,越有利于定向排列结构的形成;颗粒各向异性率和粒度越大,越不利于定向排列结构的形成。该结果有助于了解细粒沉积岩的微观结构特征,建立岩石微观结构和宏观结构的联系。

The microstructure characteristics of rocks are the internal basic properties of fine-grained rocks,which affect their physicochemical behavior.This paper takes an example as fine-grained grains in the Yanchang Formation 7 Member of Ordos Basin.First,the geometric morphology characteristics of fine-grained grains are analyzed quantitatively by field scanning electron microscope and image recognition technology.Then,we characterized the influence of the sedimentary environment on the geometric shape and orientation arrangement of fine-grained grains.Results show that the grains in the deep-semi-deep lacustrine facies take the characteristics of fine grain size(23μm),flat and angular shape.The grains are directionally distributed along with a continuous interval.The intensity of the distribution shows a flat quasi-cone shape and well-directional orientation.The fine grains in shallow lake facies are medium-fine grains(28μm),flat and angular type.Besides,the grains show a sharp distribution along with a certain interval with no obvious regularity,which has a moderate orientation arrangement.The fine grains in delta facies are fine-grained(812μm)and quasi-circular type.This type of fine-grained rock has an irregular distribution.The intensity distribution of fine grains is circular or elliptical.The circumferential angularity index and flatness of grains can promote the formation of oriented arrangement structure of grains.By contrast,the anisotropy rate and grain size are unfavorable for the formation of an oriented structure.The results are helpful to understand the microstructural characteristics of fine-grained rocks and establish the relationship between microstructures and macrostructures of rocks.