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 recog-nition technology.Then,we characterized the influence of the sedimentary environment on the geomet-ric 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 distribu-tion 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 ellipti-cal.The circumferential angularity index and flatness of grains can promote the formation of oriented ar-rangement 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 characteris-tics of fine-grained rocks and establish the relationship between microstructures and macrostructures of rocks.
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