The Characterization of Shale Differences Based on Petrophysical Properties and Pore Structure: A Case Study of the Longmaxi Formation in Northern Guizhou Province and the Yanchang Formation in the Ordos Basin

Abstract

The pore structure and the petrophysical properties are essential parameters during the marine and continental shale reservoir evaluation. In order to the reveal the mechanism of the pore parameters differences and discuss whether the controlling factors of pore structure have extensive influence in different lithologies. Based on the analysis of N2/CO2 isotherm adsorption, FE-SEM, and geomechanical test for the marine shale samples of Longmaxi Formation and continental shale samples of Yanchang Formation are undertaken. The results indicate that (1) Continental shale is a typical transversely isotropic, while marine shale can be considered a homogeneous rock on a centimeter scale. The lithological stratification is the main reason for the difference in petrophysical properties. (2) The thermal evolution degree has significant influence on the pore structure in shale samples except for TOC content, particularly for the micropores structure. (3) Since the matrix strength of shale is much greater than the strength of bedding planes, the continental shale tends to crack and fail along the weak surface of the bedding during the axial compression, showing a lower peak strength, while the marine shale presents main crack along the principal stress direction without activating the bedding crack.

Anisotropic Characterization of Mechanical Behavior and Fracture Morphology in Marine and Continental Shales under Uniaxial Compression: A Comparative Analysis

“…(c) MS SEM and (d) MS AmicSCAN, showing Illite without a clear orientation. Reproduced with permission from ref . Copyright 2022 by Applied Sciences.…”

“…It is also significantly influenced by the depositional and diagenetic conditions. The distinct alignment of lamellar minerals (illustrated in Figure 15 24 ) and the microscale-stratified structures confer noticeable directionality and regionality to the original microvoids. This observation accentuates the more marked microanisotropy in CSs relative to MSs.…”

“…22,23 Nevertheless, traditional hydraulic fracturing methods encounter challenges in achieving industrial production in CSs due to their high clay mineral content, pronounced stratification, and significant sedimentary heterogeneity. 24 Therefore, a comparative analysis of the macromechanical anisotropic differences and mechanisms between these two shale types is essential for the efficient exploitation of shale oil and gas resources.…”

“…Sedimentarily, MSs in China are predominantly found in deep-water continental shelves, whereas CSs are deposited along lake margins. , The Longmaxi Formation in the Sichuan Basin and adjacent areas is known for its high content of brittle minerals, low porosity, and low permeability. , Conversely, the CSs, exemplified by the Yanchang Formation in the Ordos Basin, display a compositionally lower content of brittle minerals and a higher content of clay minerals. This formation is characterized by its developed lamellar texture, intricate layer combinations, and enhanced heterogeneity. , Nevertheless, traditional hydraulic fracturing methods encounter challenges in achieving industrial production in CSs due to their high clay mineral content, pronounced stratification, and significant sedimentary heterogeneity . Therefore, a comparative analysis of the macromechanical anisotropic differences and mechanisms between these two shale types is essential for the efficient exploitation of shale oil and gas resources.…”

“…In recent years, alongside the development of unconventional shale oil and gas resources, comparative studies on MS and CS have been increasingly reported . These studies primarily concentrate on the disparities under hydrocarbon storage conditions and pore structure between the two types of shale. ,, The findings reveal that the storage conditions of hydrocarbons in shales from different depositional environments vary significantly, accompanied by clear variations in pore evolution, which are influenced by multiple factors. However, research on the mechanical differences between MS and CS remains sparse.…”