Structural, Lithological, and Regional Tectonic Controls from P-Wave Azimuthal Anisotropy: Casper Arch, Wyoming

R. Duke Cooley and Eric A. Erslev
Geosciences, Colorado State University, Fort Collins, CO

Subsurface fracture networks can enhance/retard subsurface fluid flow; therefore characterization of fractures is important to predict permeability anomalies in petroleum reservoirs. Direct fracture characterization is currently limited to core/logs from wells or outcrop data. New seismic processing methods that calculate seismic velocity anisotropy may help to characterize fractures if their signatures can be deciphered from that of other aligned crustal heterogeneities. This project compares an anisotropic 3D seismic data set with field data on structural fabrics from nearby analog structures to help interpret seismic anisotropy.

A 3D seismic volume of an anticlinal structure was mapped for variations in azimuthal anisotropy at multiple stratigraphic and structural positions. Azimuthal variations are observed at two distinct lithological boundaries. A regional anisotropy (110-130°) is observed with maxima in undeformed areas above the Frontier. The Frontier-Tensleep section exhibits a general 120° azimuth and the Tensleep-Basement section exhibits a general 20° azimuth in the backlimb and crest. Azimuths vary, more E-W, near the fault and in the synclinal hinge.

In nearby analog structures, fracture orientations at different stratigraphic levels within the same structural position were similar indicating anisotropy variations at lithological boundaries are related to sedimentary structures/composition and not fracturing. Azimuthal deviations at structural features, anisotropy magnitude variations paralleling faults, and surface fractures indicate that structural mechanisms related to Laramide compression are responsible for local variations in anisotropy azimuths. The regional anisotropy (110-130°) indicates a regional cause of anisotropy. This azimuth matches a secondary joint orientation observed in the field and the present day stress orientation.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas