Rock-physics model for unconventional shales

Abstract

An analytic model was adopted to calculate bedding-normal, dry elastic moduli of rocks enriched with solid organic matter — kerogen. Elastic stiffnesses in the model are functions of solid matrix moduli, total porosity, pore-shape factor, and crack density. Crack density is taken to be an exponential decay function of stress, which was calibrated using ultrasonic core measurements, whereas pore-shape factor was calibrated using sonic logs. Bedding-normal, fluid-saturated moduli were computed by means of Gassmann's equation, i.e., ignoring anisotropy. As a result, a model to calculate elastic moduli of organic shales was constructed and used for sonic-velocity prediction and pore-pressure calibration. Among other things, the model established the fact that the saturating fluid effect on elastic properties in organic shales can be noticeable, especially at extremely low effective stress. However, it is the kerogen content that is largely responsible for the anomalous deviation of log data points from conventional shale trend toward the hydrocarbon-charged sandstone trend in the acoustic- versus shear-impedance (AI–SI) domain.