Stress-Dependent Anisotropic Rock Physics Modelling in Organic Shales of the Inoceramus Formation, Austral Basin, Argentina

We present an original anisotropic stress-dependent rock physics model for the organic rich shales of the Inoceramus formation, the main source rock and unconventional reservoir in the Austral Basin, Argentina. We implement a novel combination of anisotropic poroelastic theories which take into account organic matter content, lithologic description, fluid type, saturation and stress state. In this approach, we model the infill as a mixture of solid organic matter and interconnected pore fluids, using total organic carbon analysis and petrophysical data from two wells. The compliance of the matrix is considered to be stress-dependent following the porosity deformation approach (PDA). The elasticity and density of the multiminerallic saturated rock is obtained using the mineral fractions obtained from X-ray diffraction information, porosity analysis and fluid properties. This allows us to compute synthetic acoustic velocities. The calibration of the model also involved the inversion of several unknowns (the set of PDA parameters, clays and kerogen physical properties) by minimizing the misfit between modelled and ultrasonic measured velocities. Due to the lack of oblique velocity data, to complete the compliance tensor, a static-to-dynamic ratio was built for each sample, and constant anellipticity was assumed with increasing stress. The model calibrated with this innovative procedure demonstrated its usefulness to predict stiffness, compliance, and compressional and shear wave velocity variations under variable applied stress. It is also useful for the estimation of stress-related changes of porosity and Biot’s effective stress coefficients, which can be difficult to measure in shales, and therefore there are few values reported in the literature.