Ultrasonic velocity and anisotropy of organic-rich chalks

Based on the assumption of Vertical Transverse Isotropy (VTI), we investigated the ultrasonic velocity and anisotropy of organic-rich chalks (ORC). These chalks, from the late Cretaceous Ghareb and Mishash formations in Israel, have a wide range of porosity (0-45 PU), high kerogen content (5-15 wt% TOC) and include sections with varying degrees of thermal maturation. Cores were extracted both from the Shefela basin of central Israel and two wells in the southern Golan Heights. We measured ultrasonic velocities of P and S waves in different directions using the ultrasonic transmission method, whilst adjusting effective (confining) pressure levels. The elastic constants were then calculated from the measured velocities and densities, as well as Thomsen anisotropy parameters. Compared to previous studies of organic-rich shale source rocks, which exhibit strong acoustic anisotropy, these organic-rich chalks display weak, yet still measureable transverse anisotropy. Our results present significant differences between the immature and early-maturation stage samples. In our samples, it had become evident that microcracks are present during the early-maturation stage, yet are absent throughout the immature stage. An additional distinction between the maturation stages reveals itself while examining the elastic constants, showing a much stiffer nature in the early-mature sample, which could also be attributed to compaction and lower porosity.