Coupling of diagenetic alterations and mechanical properties of Lower Permian siliciclastic sandstones: a pilot study

Initial detrital composition and authigenic alterations during diagenesis of three sandstone types are related to their mechanical properties. Sandstones were prepared for geotechnical standard tests [density, uniaxial compressive strength (UCS), Young's modulus (E), strain at failure (epsilon)] and thin sections for petrographic analyses (point counting). UCS ranges from 3 to 62 MPa and positively correlates with density (1.75-2.35 g/cm(3)) and E (0.3-12.7 GPa). Optical porosity is controlling these mechanical parameters and was linked to diagenetic alterations. Diagenetic alterations affecting porosity reduction are the abundance of clay minerals, and the intensity of mechanical and chemical compaction. The latter is controlled by clay mineral coatings on contacts between detrital grains, and the occurrence of authigenic quartz and dolomite. Horizontal contact lengths of grains normalized to their respective particle diameter (effective contact ratio, ECR) and porosity are identified as a control on the mechanical properties UCS and E, reflected by the rock strength index S-R. The results of this pilot study suggest that S-R is able to predict UCS and E based on petrographic information obtained from the studied samples. These results enhance the understanding of the coupling between mineralogy and geomechanics and highlight the impact of diagenesis on geomechanical behavior.

Automated summary

The mechanical properties of sandstones are influenced by the initial detrital composition and authigenic alterations during diagenesis, with porosity controlling key parameters such as density, UCS, and E. The presence of clay minerals and mechanical and chemical compaction during diagenesis are linked to porosity reduction and affect the mechanical properties. The horizontal contact lengths of grains normalized to their diameter and porosity play a significant role in controlling UCS and E, as reflected by the rock strength index S-R.