The influence of kerogen content on some brittleness indicators in a rich organic shale - comparative analysis of different approaches

In this work, we model some accepted brittleness indicators in rich organic shales. The rock is modeled as an effective poroelastic vertical transversely isotropic medium (VTI), composed by clay minerals and kerogen. The elastic properties of this composite medium are computed using the anisotropic theory of Ciz and Shapiro. By calibrating this model for the black shales of the Bakken formation (Williston basin, EEUU), we analyze the behavior and sensitivity to kerogen content of Poisson ratio, Young's modulus and its product with bulk density. Those coefficients are computed in normal (vertical) and parallel to bedding (horizontal) directions, using apparent isotropic definitions, the exact anisotropic formulation and the approximate anisotropic expressions recently derived by Thomsen for weak VTI anisotropy. While for Young's modulus we observed an excellent agreement between the different approaches, we found that Poisson ratios derived from the apparent and approximate isotropic approaches can lead to significant errors.