Azimuthal Velocity Analysis of 3-D Pre-Stack Seismic Data for Fracture-Induced Anisotropy in the Altamont-Bluebell Field

摘要

Altamont-Bluebell field is located in northeastern Utah within the Uinta basin. The Uinta basin is an asymmetric east-west trending basin with a south flank that has a gentle slope. The north flank is steep and bounded by east-west trending Uinta Mountains. The field is located in the northern-central part of the basin. In 2010, a 3D surface seismic survey was acquired over 35 square miles area within Bluebell Field, the eastern portion of Altamont-Bluebell field. The Bluebell field is considered an unconventional reservoir in the sense that natural fractures act as fluid storage and conduits in the tight sandstones, shales, 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 information related to fractures. Therefore, this paper utilizes Velocity Variations with Azimuth (VVAZ) to estimate the direction and intensity of fracture-induced anisotropy within Upper Green River formation. Upper Green River formation is the shallowest reservoir of the three main Tertiary reservoirs which includes Lower Green River and Wasatch/Colton formations. A method for VVAZ inversion, based on the elliptical NMO equation for Transverse Isotropy (TI) media that was derived by Grechka and Tsvankin (1998), is applied. The method has been tested on a 3D physical modeling dataset. The results of the physical modeling test are found to be adequate (Al Dulaijan et. al., 2015). For Bluebell field 3D seismic data, isotropic velocities are used along with azimuthally variant time residuals to estimate fast and slow NMO velocities and their directions for the Upper Green River formation. Along with fast and slow NMO velocity maps, maps of fracture-induced anisotropy orientation and intensity were created. Pre-stack and post-stack seismic attributes are also calculated for the Upper Green River formation in Bluebell field and compared to WAZ results. Intensity and orientation maps of seismic anisotropy are compared to post-stack coherency attributes and to geomechanical attributes obtained by seismic pre-stack elastic inversion to estimate brittle zones of the unconventional reservoir.