Probabilistic Mechanical Stratigraphy from Seismic Inversion - A Case Study from the Vaca Muerta Shale, Argentina

This work presents a methodology and workflow to deal with situations of limited data availability in the characterization of unconventional shale plays. The study analyzes the variability in the geomechanical properties of the Vaca Muerta Formation and their relationships with lithology, stress state and total organic content.

The uncertainties inherent from limited well control in the estimation of elastic properties were investigated using blind well testing. Sensitivity and error analysis was conducted on post-stack vs pre-stack derived P-impedance. The choice of the inversion algorithm and the definition of the low frequency model were examined. Also, the use of isotropic AVA equations to approximate the anisotropic (VTI) behaviour of the reservoir was evaluated, using estimates of Thomsen parameters and simple AVA modelling.

The integration of the inversion results with petrophysical analysis and mechanical stratigraphy, suggests that the rock composition has the largest influence on the geomechanical behaviour of the reservoir. Overpressure is also a major driving factor in that it controls changes in elastic properties. Cluster analysis was used to identify good quality rock classes. The probabilistic interpretation of these rock classes from seismic inversion provides a quantitative measure of uncertainty and guides the selection of potential drilling targets.