Carbonate strength classification based on depositional textures and fossil content of the Lower Eocene Drunka Formation, Assiut Area, central Egypt

Abstract The present study reports petrographic and physico-mechanical analyses conducted on the lower Drunka Formation in the Assiut Area of Central Egypt. The study investigated the percentage of carbonate content and percentage of allochems “Ach” and their impact on the petrophysical features using correlation and multivariate regression analyses. This study analyzes the petrophysical characterization of Drunka Formation rocks and the effect of fossil content on them. The carbonate rock samples derived from the lower Drunka Formation were studied to evaluate the compressive strength, dry and saturated unit weight, porosity, and acoustic wave velocities. The petrographic examination of the rock samples was conducted to understand the facies characteristics, size and percentage of grains, matrix type and extent of diagenesis. Various microfacies associations were also detected within the examined samples, such as lime-mudstone, echinoidal bioclastic wackestones, nummulitic/miliolid wackestones, bioclastic peiodal packstone, echinoidal packstone and nummulitic grainstone. Petrophysical analyses displayed a wide range of porosity and dry bulk density values from 1.3 to 2.75% and 2.31–2.48 g/cm3, respectively. The wet bulk density and grain densities are between 2.33˗2.50 and 2.35˗ 2.57 g/cm3, respectively. The dry ultrasonic wave velocity “Vp” was observed between 2926 and 5549 m/s, whereas the saturated ultrasonic wave velocity “Vp “was estimated between 3067 and 5875 m/s. The Unconfined Compressive Strength (UCS) of the tested samples was 22 ≤ UCS ≤ 82 MPa while the slake durability index was recorded as 94.37 ≤ SDI ≤ 98.7%. The Brazilian test indices characterized the studied rock samples as highly porous rocks and showed the values of 1.47 ≤ Brazilian ≤ 5.13 MPa. Finally, for all tested facies, the Bohme test data have a minimum, maximum, and an average of 12.98, 25.95, and 19.04 cm3/50 cm2, respectively. The study's findings showed that, except for the mineral grain scale, the physico-mechanical properties are highly dependent on the petrographic characteristics. The study also revealed that the Ach% and the rock's carbonate content significantly affect the rock's petrophysical and mechanical properties under investigation. The mechanical properties such as UCS, tensile and point load test. increase with increasing carbonate content and decreasing with increasing Ach% of the rock. This paper presents an extensive dataset of physical parameters of a carbonate formation in Egypt; many empirical relationships are presented for predicting mechanical properties. This provides site-specific predictive value; the relationships can be used for all carbonate rocks worldwide with broader international significance.