Azimuthal anisotropy analysis of P-wave seismic data and estimation of the orientation of the in situ stress fields: An example from the Rock-Springs uplift, Wyoming, USA

Estimating the orientation and magnitude of maximum and minimum horizontal in situ stress is important for characterizing naturally fractured, unconventional, and carbon-sequestered reservoirs. For naturally fractured reservoirs, they are needed to guide directional drilling; for unconventional reservoirs, they are used for optimal placements of hydraulic fractures; and for carbon-sequestered reservoirs, they are used to avoid fracturing of overlying seal rocks. In addition, a knowledge of stress fields can be used to induce fractures within the target reservoirs and enhance additional storage for carbon-sequestration experiments. The orientation and magnitude of in situ stress can be calculated at the well locations. For locations, away from the wells, analysis of the azimuthal dependence of the amplitude-variation-with-angle gradient or azimuthal angle stacks are used to quantify anisotropy, which are then related with well data and other geologic information for stress estimation. Such azimuthal analysis requires accurate conversion of offset-domain seismic data into angles. We use isotropic prestack waveform inversion for an accurate offset-to-angle transformation along different source-to-receiver azimuths followed by azimuthal analysis. Applying our method to the real seismic data from the Rock-Springs uplift, Wyoming, USA, and relating the results to the well data, we find that our results are favorably related to the orientation of the maximum in situ horizontal stress field measured at the well location.