Coupling mechanical properties in porous siliciclastic rocks with diagenetic alterations – a case study on Rotliegend outcrop analogs

The detrital composition and the authigenic alterations formed during diagenesis of three sandstone types are related to their mechanical properties. Sandstone samples originating from three studied quarries in Bebertal (Saxony-Anhalt), Cornberg (N Hesse), and Bowscar (NW England) were prepared to cylinders for geotechnical standard tests (rock density, uniaxial compressive strength (UCS), Young’s modulus, strain at failure). Corresponding thin-section across the bedding plane (stained in blue epoxy resin, thickness of 30 µm) were analyzed by optical microscopy, point-counting (300 counts), and clay mineral coverage analyses after Busch et al. (2017) and Monsees et al. (2020).
First results show UCS from 46-62 MPa. Strain at failure ranging from 0.55-0.77% correlates with bulk densities (2.20-2.35 g/cm³). The sandstones were classified as lithic arkoses to litharenites (Bebertal), sublitharenites to quartzarenites (Cornberg), and subarkoses to quartzarenites (Bowscar). The preservation of optical porosity is mostly dependent on the intensity of compaction quantified by the compaction index (Lundegard, 1992), whereas chemical compaction is controlled by the abundance of clay mineral coatings on detrital grain contacts.
... mehr
Samples with higher clay contents (2-14%) negatively correlate with UCS, while samples with higher amounts of quartz (grains and cements) and rigid rock fragments (63-79%) positively correlate with UCS and Young’s modulus. Higher bulk densities were observed with lower optical porosities (r²=0.85), while higher clay mineral coverages on detrital grain contacts correlated with higher densities (r²=0.60), driving chemical compaction. Furthermore, higher strain at failure positively correlated with higher clay mineral coverages and compaction indices, while showing a negative correlation with optical porosity. The relationship between geotechnical results and petrography is interpreted as an interplay between initial detrital composition demonstrated by the impact of clay and rigid components on UCS and Young’s modulus, while the impact of diagenesis is demonstrated by the clay-mineral controlled compactional behavior and resulting reduction in porosity, which in return can be linked to higher Young’s modulus and strain at failure. These results enhance the understanding of the coupling between mineralogy and geomechanics, and highlight the impact of diagenesis on geomechanical behavior. One possible applications would be the use as input parameters for geo-modelling in reservoir applications.