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ARTICLE

Quantifying the effects of micro-cracks on velocity anisotropy in lacustrine shales with variable sedimentary structures

Wenhui Tan1,2 ,  Weihua Liu1,2 ,  Yang Wang1,2* ,  Hui Shen1,2
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1 SINOPEC Geophysical Research Institute Co., Ltd., Nanjing, Jiangsu, China
2 SINOPEC Key Laboratory of Geophysics, Nanjing, Jiangsu, China
JSE, 025340060 https://doi.org/10.36922/JSE025340060
Submitted: 18 August 2025 | Revised: 12 September 2025 | Accepted: 17 September 2025 | Published: 23 October 2025
© 2025 by the Author(s). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International License ( https://creativecommons.org/licenses/by/4.0/ )
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Abstract

Understanding the relationship between micro-cracks and elastic anisotropy is crucial for characterizing subsurface flow pathways, optimizing hydraulic fracturing, and enhancing seismic interpretation in unconventional shale reservoirs. Although clay content and total organic carbon (TOC) are recognized primary controls on anisotropy, the specific influence of sedimentary structures on micro-crack parameters (such as crack porosity, crack density, and aspect ratio) and their contribution to anisotropic behavior have not been fully quantified, particularly in lacustrine shales with varied sedimentary architectures. In this study, 17 shale samples were categorized into three sedimentary structural types: laminated, bedded, and massive, based on their microstructure characteristics. Ultrasonic velocity measurements were performed on 17 paired shale plugs under confining pressures to quantify the relationship between micro-crack parameters and elastic anisotropy. Experimental results reveal a clear difference in stress sensitivity of bedding-normal velocities: Laminated shales > bedded shales > massive shales, which are attributed to varying degrees of micro-crack alignment and density. Laminated shales exhibit the strongest anisotropic properties, followed by bedded shales, while massive shales show weak anisotropy. Velocity predictions from the Mori-Tanaka effective medium model are in good agreement with the measurements, validating its applicability for shales with diverse structures. Micro-crack analysis indicates a positive correlation between crack density/porosity and anisotropy magnitude. Notably, laminated shales are characterized by the highest crack porosity (0.012–0.015%), high clay content (average 40%), and moderate TOC, indicating a combined effect of composition and microstructure on anisotropy. This study highlights that sedimentary structure plays a key role in controlling micro-crack development and related anisotropy in lacustrine shales, with laminated shales exhibiting the most significant combined effect, thus improving the accuracy of minimum-horizontal-stress prediction and hydraulic-fracture design.

Keywords
Lacustrine shale
Micro-cracks
Thomsen anisotropic parameters
Ultrasonic experiment
Mori-Tanaka model
Funding
This research was supported by the National Natural Science Foundation of China (grant number: 42430810).
Conflict of interest
The authors declare that they have no conflicts of interest.
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