Abstract

The mechanisms and timing of fracturing are important parameters to predicting intensely fractured trends with enhanced reservoir permeability and production potential. Fracture analyses were used to test hypotheses for the structural development of the Wind River Basin in Wyoming that include: 1) pre-Laramide regional compression; 2) Laramide ENE-WSW horizontal compression and left-slip faulting; and 3) multiple post-Laramide hypotheses: a) near-surface mechanisms; b) regional extension due to a variety of causes; and/or c) localized extensional reactivation and backsliding of thrust faults during release of compression and collapse of basin-bounding arches.

Fracture data collected from Cambrian- to Eocene-aged formations throughout the basin included 1,900 joints and minor faults measured at 45 outcrop stations and 14,775 fractures compiled from 39 micro-resistivity image log interpretations. Inferred stress axes were calculated using eigenvector averaging and show two distinct stages of deformation: 1) Laramide ENE-WSW horizontal shortening followed by 2) post-Laramide extension consistent with modern stresses. Post-Laramide fractures in the basin parallel NW-SE striking joints observed across the Rocky Mountain foreland, except in the vicinity of E-W trending basin-bounding arch margins where these fractures closely parallel arch-bounding thrust faults.

Fracture analyses are consistent with Laramide shortening followed by localized extension in proximity to both basin-bounding master thrusts and smaller productive structures in the basin. Seismic-based models show backsliding on these thrusts related to arch/anticline collapse. Recent wells at Frenchie Draw gas field have high initial production rates and probably intersected off-structure, intensely fractured trends along the margins of what appears to be a collapsed anticline.