Azimuthal Anisotropy: Distinguishing Between Unequal Horizontal Stress and Vertical Aligned Macro-Fractures, as Demonstrated in Thirty Years of Field Data Analysis

Lynn, Heloise Bloxsom (Lynn Inc.)



In the past, the anisotropy community has asserted that we are unable to distinguish between the effects of stress-aligned vertical micro-cracks (arising from unequal horizontal stresses), and vertical aligned macro-fractures that flow fluids. This assertion is one manner of phrasing the hypothesis that we are unable to determine the scale (size) of the fracture causing the azimuthal anisotropy. This paper asserts: 1) that the result of unequal horizontal stress is azimuthal traveltimes, best quantified as P-P azimuthal interval velocities following azimuthal prestack depth migration, and 2) that the information concerning vertical aligned macro-fractures that flow fluids is best evident in azimuthal amplitudes (P-P, etc.) of said processed data, with the caveat that all the standard issues of bed thickness, removal of noise, preservation of signal, temporal resolution, and spatial resolution constrained by the proper bin size, given a geologic dip on the beds, a fold, and offsets equal to target depths, are present and operative. Having looked at azimuthal reflection seismic data for more than thirty years, and having seen a consistent pattern in field data, I offer the above two assertions as hypotheses to be tested against current and future datasets, both field data and model data, provided that these model data come from algorithms using orthorhombic, monoclinic and/or triclinic symmetries.