Petrophysical and acoustic characteristics of Jurassic and Cretaceous rocks from Central Lebanon

Abstract

We conducted several petrophysical and acoustic measurements on 49 core samples collected from the Jurassic and Cretaceous strata exposed at west central Lebanon to evaluate their petrophysical and elastic properties and study the effects of the depositional conditions, sediment composition, and subsequent diagenetic processes on the measured parameters. First, these rocks were petrographically studied to identify their facies, porosity, and the main diagenetic features. In addition, these rocks were investigated under the scanning electron microscope (SEM) and by the X-ray diffraction (XRD) analysis to identify their mineralogy. The petrophysical measurements were performed on the core samples where porosity, permeability, bulk, and grain densities were first determined, followed by measuring the seismic wave velocities under dry and water-saturation conditions. Both carbonates and siliciclastics are encountered in the studied formations. The SEM and XRD analyses revealed that the main constituting minerals are quartz, calcite and dolomite. The studied rocks have generally low to moderate porosity and very low permeability with averages of 0.05, and 0.31 mD, respectively. The bulk density is moderate to high and varies narrowly between 2.03 and 2.79 with an average of 2.64 g/cm3, whereas the average grain density is 2.77 g/cm3. The average primary and secondary wave velocities (Vp and Vs) are 4263, and 2323 m/s, respectively, with an average Vp/Vs of 1.83. Water-saturation has significantly impacted the elastic properties of the studied rocks. From the obtained measurements, we further calculated the elastic coefficients of the studied rocks and constructed several relationships between the measured properties to investigate their mutual interdependence and evaluate the effects of porosity, rock composition, depositional and diagenetic processes on the rock characteristics. We found that some samples, mainly carbonates, deviate significantly from the expected porosity-velocity and density-velocity trends. Originally present micro- and intercrystalline pores and characteristic diagenetic processes in these carbonate rocks, and possibly coring-induced microcracking in few samples, may account for the observed outliers. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.