The offset VSPs, used for this paper, were acquired within Bluebell Field, the eastern portion of Altamont-Bluebell field in northeastern Utah. Altamont-Bluebell field is within the Uinta Basin, and is considered an unconventional reservoir in the sense that natural fractures act as fluid storage and conduits in the tight sandstones and carbonates. Information related to fracture orientation and intensity is vital for the development of such reservoirs. Azimuthal variations of P-wave velocities can be a valuable tool for fracture information. Therefore, this paper utilizes Velocity Variations with Azimuth (VVAZ) to estimate the direction and intensity of fractured-induced anisotropy within the three main reservoirs using offset VSPs seismic data.
VVAZ inversion method is applied based on the elliptical NMO equation for TI media that was derived by Grechka and Tsvankin (1998). Here, a VVAZ workflow is developed for offset, workaround, or walkaway VSPs using a method for surface seismic. Interval anisotropy properties are calculated for each receiver.
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