Petrographic, physico-mechanical, and geochemical characteristics of Eocene sedimentary rocks from the Tbilisi Area (Georgia)

The Tbilisi Area is located within the eastern segment of the Achara-Trialeti fold-and-thrust belt, which is itself a part of the Kura foreland basin system. The region is characterized by an active convergent tectonic regime resulting from the ongoing collision between the Eurasian and Arabian plates. The convergence has induced extensive deformation features, including large-scale folding, faulting, and uplift, which have influenced the stratigraphy and lithological compositions of the Eocene sedimentary rocks of the study area.

A detailed, interdisciplinary study of these Eocene sedimentary rocks was implemented, integrating petrographic, geochemical, geomechanical, and petrophysical analyses to comprehensively understand physico-mechanical properties, and their interrelation. For this purpose, samples taken from the field were analysed at the Institute of Applied Geosciences, KIT. Petrographic examinations reveal a heterogeneous lithological assemblage comprising predominantly fine-grained clay mineral matrix-rich arkosic wackes, lithic wackes, arkosic arenites, and lithic arenites often containing foraminifera and glauconite, implying marine deposition. Geochemical analyses indicate the most prominent elements of the rocks are Si, Al, and Ca. Their content ranges 23.2-29.6%, 7.1-10.1%, 1.7-11.6%, respectively.

The rocks exhibit notably low permeability, generally in the range of 10^-4 to 3*10^-1 millidarcies (mD), with permeability strongly dependent on porosity metrics (ranging from 1.1% to 10.3%). This low permeability is primarily controlled by clay matrix content and to a lesser extent on cementation processes. The heterogeneity and complexity of these formations are further confirmed by the wide range of uniaxial compressive strength (UCS), from 36.9 MPa to 208.8 MPa, reflecting variations in lithology, degree of cementation, and diagenetic modifications across different sections.

This work presents an initial effort to showcase the diverse rock properties from the Tbilisi Area, as the Eocene sedimentary rocks show distinct lithological heterogeneity and complex mineralogical and petrophysical characteristics, strongly influenced by their depositional and tectonic history with implications for engineering utilization of the lithologies.

Acknowledgement: This work was supported by the Joint Rustaveli-DAAD fellowship programme, 2025. We thankfully acknowledge assistance in the lab by Martin von Dollen (KIT).