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The AAPG/Datapages Combined Publications Database
AAPG Special Volumes
Fracture Control of P-wave Azimuthal Anisotropy in a Laramide Basement-cored Anticline at Casper Arch, Wyoming: Insights from Correlations with Surface Analogs and Curvature Analyses
Ursa Resources Group II, 1050 17th St. Suite 2400, Denver, Colorado, 82065, U.S.A. (e-mail: [email protected])
Department of Geology and Geophysics, University of Wyoming, Dept. 3006 1000 E. University Ave., Laramie, Wyoming, 82071 (e-mail: [email protected])
In sedimentary basins not currently undergoing primary compaction (e.g., Rocky Mountain Basins), P-wave velocities noticeably vary with azimuth, yet the mechanism(s) controlling the anisotropy remain uncertain. Possible geologic causes for azimuthal anisotropy include but are not limited to sedimentary fabrics, steep bedding, changes in local in-situ or residual stress, and open or mineralized fractures. To test these hypotheses, P-wave velocity azimuths (Vfast) from a proprietary seismic survey of a NNW-trending Laramide Anticline on Casper Arch in central Wyoming were compared to image log data from the seismic coverage area and fracture orientations from nearby analog structures.
In general, Vfast azimuths are E-W above the Frontier Formation, NW-SE from the Frontier to the Tensleep Formation, and NE-SW from Tensleep Formation to basement. They exhibit abrupt changes at these lithologic boundaries. Vfast azimuths also parallel faults imaged in the seismic data, perhaps due to increased fracturing parallel to the fault or a local azimuthal stress shift. Vfast azimuth (090–110°) in the vicinity of a horizontal Mowry well parallels open fractures, suggesting open fractures control the P-wave anisotropy azimuth.
To further test these hypotheses, joints and minor fault orientations were collected from two NW-trending en echelon surface anticlines exposed south of Douglas, Wyoming. This data, and additional fracture measurements on anticlines in the Casper Arch area, reveal two regional joint sets oriented NW-SE (130°) and NE-SW (030°). These two joint sets parallel NW-SE Vfast azimuths in the Frontier to Tensleep interval and NE-SW Vfast azimuths in the Tensleep to basement interval, suggesting fracture orientation may control P-wave anisotropy. Curvature analysis on the seismic data also calculated well-defined NW-SE and NE-SW oriented lineations likely related to faults and/or fractures paralleling the Vfast azimuths.
Correlations between image log data, joints measured on exposed analogs, curvature analysis, and P-wave Vfast azimuths indicate that fracture orientations are a main control of the Vfast azimuth. Changes in Vfast azimuths at lithologic boundaries correlate with changes in fracture array strikes in the analog structures, possibly due to depth-dependent fracture mechanisms, timing, and/or stresses within the sedimentary section.
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