2014 Rocky Mountain Section AAPG Annual Meeting

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Post-Laramide Thrust Backsliding and Arch Collapse: a Mechanism for Creating Intensely Fractured Trends and High Off-Structure Production; Wind River Basin, Wyoming


The mechanisms and timing of fracturing are important parameters to predicting intensely fractured trends with enhanced formation permeability and production potential. Fracture analyses were used to test hypotheses for the structural development of the Wind River Basin in Wyoming. These hypotheses include: pre-Laramide regional compression or forebulge migration; syn-Laramide ENE-WSW horizontal compression and left-slip faulting; and multiple post-Laramide hypotheses: 1) elastic strain release following Laramide shortening; 2) near-surface mechanisms; 3) regional extension due to transtensional plate interactions or epierogeny; and/or 4) localized extension due to backsliding on thrust faults during collapse of basin-bounding arches. Fracture data from Cambrian to Eocene strata throughout the basin included 1,900 joints and minor faults measured at 45 outcrop stations and 14,775 previously interpreted fractures from 39 micro-resistivity image logs. Inferred stress axes, calculated using eigenvector analysis, show two distinct clusters indicating two stages of deformation: syn-Laramide ENE-WSW horizontal shortening followed by post-Laramide extension consistent with variation in the modern stress. 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 and seismic data indicate localized extension in proximity to basin-bounding thrust faults and backsliding on these thrusts that is probably due to arch collapse. Recently completed wells at Frenchie Draw gas field probably intersected off-structure, intensely fractured trends that parallel the margins of what appears to be a collapsed anticline. These wells have uncommonly high production rates.