The link between permeability and velocity anisotropy of organic-rich chalks from laboratory measurements

We investigate the linkage between ultrasonic wave velocity and permeability anisotropy of organic-rich chalk (ORC) formations, based on laboratory measurements. These rocks have a wide range of porosity (up to 45 PU), high kerogen content (5–20 wt% TOC) and varying degrees of thermal maturation, from immature to early maturation stage. The ultrasonic P and S wave velocities were measured assuming Vertical Transverse Isotropy (VTI) using the three-plug method, and the Klinkenberg-corrected gas permeability were also measured using the same three plugs in order to understand the linkage between the permeability and the velocity anisotropy. Our results show significant scattering of the Klinkenberg-corrected permeability with depth. The permeability ranges between two millidarcy down to tens of nanodarcy, generally decreasing with burial and compaction and often displaying strong anisotropy. Using velocity hysteresis measured during cycles of pressurization and depressurization of the confining pressure, we found that pressure-sensitive microcracks (or soft porosity) were absent in the immature stage, whereas they are present in the early-mature stage. The development of microcracks at this stage of maturation has a major influence on both the matrix permeability and velocity anisotropy at low effective stresses and may be useful in seismic exploration.