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.

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 (2-3 μm), 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 (2-8 μm), flat and angular type. Besides, the grains show a sharp distribution a-long with a certain interval with no obvious regularity, which has a moderate orientation arrangement. The fine grains in delta facies are fine-grained (8-12 μm) 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. [ABSTRACT FROM AUTHOR]