Mapping Rocky Mountain fractures: Interplay of Laramide thrust and transpressional zones
Karen Aydinian, Laura Kennedy, Eric Erselv, and Scott Larson
The lobate form of the Laramide orogen and locally en echelon geometry of Laramide arches predicts zones of transpression at orogen margins and within zones of arch reorganization. These transpressive zones had, and may still have, different stress orientations and fracture systems, which may significantly impact resource plays. Areas of laterally continuous Laramide deformation are dominated by thrust faults with subhorizontal opening fractures, which can be detrimental to reservoir development as they may not connect strata vertically. Transpressive zones are dominated by strike-slip minor faults and vertical opening fractures which can connect strata more favorably in a vertical direction. This project uses new GIS fracture mapping techniques to investigate the interplay of thrust and transpressive fracturing. The Laramide foreland has two distinct settings where transpression occurred: oblique orogen margins and internal reorganization zones. Oblique orogen margins in Montana and New Mexico are the boundaries between shortened Laramide arches and relatively unaffected continent to the NE and SE, respectively. In the Bighorn and Pryor mountains of northern WY and MT, the effect of transpression is apparent in the proportion and orientations of minor faults. Thrusts make up over 50% of minor faults in the central Bighorn Arch, whereas strike-slip faults comprise 82% of minor faults at the northern end of the arch. This broad transpressional zone extends from the WY-MT border to Billings, MT. Internal reorganization zones typically were either east-west-striking zones (Owl Creek and Granite arches) of left-lateral transpression or NE-striking zones (Front Range-Laramie Range transition at WY-CO border) of right-lateral transpression. For instance, structures along the southeastern margin of the Wind River Basin trend in a multitude of directions, but generally record ENE-WSW compression as WSW-directed slip on Casper Arch was transferred southward to ENE-directed slip on the Laramie Range. This area is dominated by strike-slip minor faults (78% strike-slip, 22% thrust) and 3D balancing confirms the need for left-lateral transpression. Zones of transpression may enhance resource play productivity by adding fracture permeability or degrade it by allowing hydrocarbon escape. The project postulates that mapping zones of transpression and thrusting could enhance delineation of sweet spots in resource plays that depend on natural fracturing.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012