Oil & Gas Geology ›› 2024, Vol. 45 ›› Issue (4): 1079-1088.doi: 10.11743/ogg20240413

• Petroleum Geology • Previous Articles     Next Articles

Micromechanical characteristics and classification of the grains of lacustrine fine-grained sedimentary rocks: A case study of shales in the 7th member of the Upper Triassic Yanchang Formation, Ordos Basin

Xinhui Xie1,2(), Hucheng Deng3, Lanxiao Hu3, Yong Li1, Jinxin Mao3, Jiajie Liu1, Xin Zhang1, Boyang Li1   

  1. 1.College of Geophysics,Chengdu University of Technology,Chengdu,Sichuan 610059,China
    2.Postdoctoral Mobile Station,College of Geophysics,Chengdu University of Technology,Chengdu,Sichuan 610059,China
    3.College of Energy,Chengdu University of Technology,Chengdu,Sichuan 610059,China
  • Received:2023-12-01 Revised:2024-08-01 Online:2024-09-05 Published:2024-09-05

Abstract:

Fine-grained sedimentary rocks contain abundant hydrocarbon resources. To examine the microscopic structural and micromechanical characteristics of their grains, we explore such rocks in the 7th member of the Upper Triassic Yanchang Formation (also referred to as the Chang 7 Member) in the Ordos Basin. Using scanning electron microscopy (SEM) and nanoindentation experiments, we delve into the geometric characteristics and micromechanical properties of the grains of these rocks. The results indicate that based on five characterization parameters, namely grain size, flatness, angularity, hardness, and modulus of elasticity, the fine-grained sedimentary rocks in the Chang 7 Member can be categorized into six major grain types: grain size and angularity predominating; flatness and angularity predominating; flatness, angularity, modulus of elasticity, and hardness predominating; modulus of elasticity and hardness predominating; angularity, modulus of elasticity, and hardness predominating, and grain size, angularity, and modulus of elasticity predominating. Five types of grain assemblages can be further identified based on the relative contents of the six types of grains. Additionally, the coupling relationships between the grain types and mineral types of the fine-grained sedimentary rocks are determined. The proposed scheme for microscopic grain classification of fine-grained sedimentary rocks assists in understanding the microscopic structural characteristics of fine-grained sedimentary rocks and predicting the spatial distributions of geological sweet spots, thus holding great theoretical and scientific significance for the hydrocarbon resource assessment and efficient hydrocarbon exploitation of fine-grained sedimentary rocks.

Key words: scanning electron microscopy (SEM), nanoindentation, classification scheme, grain of fine-grained sedimentary rock, shale, the 7th Member of Yanchang Formation (Chang 7 Member), Ordos Basin

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