Effects of porosity and composition on seismic wave velocities and elastic moduli of lower cretaceous rocks, central Lebanon

Abstract

We collected 40 rock samples from the Cretaceous strata exposed at central Lebanon in order to study the effects of porosity and rock composition on their seismic wave velocities and elastic moduli. Several sedimentological and mineralogical studies were conducted to evaluate the rock composition, provenance, depositional conditions, and the diagenetic history of the studied rocks. Porosity, bulk and grain densities and seismic wave velocities were measured for 35 drilled core samples at ambient conditions in the laboratory. Velocity measurements were conducted on the dry core samples utilizing the pulse transmission technique. Petrographically, four lithofacies have been identified under the polarizing microscope. From oldest to youngest, these comprise arenitic sandstone, lithic limestone, oolitic limestone, and micritic limestone. Investigations of representative rock samples under the SEM revealed that a number of diagenetic processes have impacted the studied rocks, and thereby affected their petrophysical properties. The XRD analysis, on the other hand, revealed that quartz and calcite are the dominant minerals in the sandstones of the Chouf Formation and the limestones of the Abeih and Mdairej Formations, respectively. The measured porosity, bulk density, and compressional and shear wave velocities of the investigated rocks vary, respectively, between 2.14–10.05%, 2.41–2.67 g/cm3, 3885–6385 m/s and 2246–3607 m/s. The grain density was calculated from the measured porosity and bulk density data and varies narrowly between 2.64 and 2.78 g/cm3. We further calculated the Poisson’s ratio and the moduli of shear, bulk, and Young from the measured bulk density and seismic wave velocities. Calculated values of these parameters vary between 0.18–0.28, 1.23–3.43 × 1010 Pa, 2.03–6.18 × 1010 Pa and 3.06–8.69 × 1010 Pa, respectively. The generalized mixture rule is used to provide a unified description of the physical properties of the studied rocks regarding their component properties, volume fractions, and microstructures. We constructed a number of relationships between the measured petrophysical and elastic properties to evaluate the mutual interdependence of these parameters and assess the effects of porosity and rock type on these important rock characteristics.