Open Fracture Prediction and Detection at the Bluebell - Altamont Field, Uinta Basin, Utah
Norman Harthill and C. Richard Bates
Production in the Bluebell-Altamont field is controlled by fracture permeability, and to date, intersecting open fractures has been a hit-or-miss process. By integrating knowledge of basin formation, structural history, and specific geophysical characteristics, we will show that the fractures in the producing region of the Uinta Basin occur systematically, and that the open fractures can be defined using reflection seismic AVO anomalies. The Uinta basin is markedly asymmetrical, reaching its deepest point of -20,000 feet bsl. The interpreted mechanism of basin formation is left-lateral strike-slip faulting in the basement along the Duschesne and South Flank fault systems, created by the indentation of the Colorado Plateau into the North American craton during the Laramide orogeny. Fracture directions in the sedimentary reservoir rocks predicted using this concept are N15°-50°W, which agree with core and joint analyses. Blackhawk Geosciences shot two orthogonal 3C reflection seismic lines, and a 9C VSP at the Bluebell-Altamont field under DOE Contract No. DE-RP21-91MC28135; the 9C VSP defined N35°W as the open fracture direction. Amplitudes at far offsets on the N70°E line were significantly greater than amplitudes on the N30°W line, which shows that the N70°E crosses open fractures at approximately right angles, while the N30°W line is approximately parallel to the open fractures. These AVO anomalies are consistent with the results of the 9C VSP, surface shear wave seismic anomalies, and geologic structural analysis.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California