MECHANICAL CHARACTERIZATION AND FAILURE DEVELOPMENT WITHIN JURASSIC SANDSTONES PROXIMATE TO SALT STRUCTURES, PARADOX BASIN, SOUTHERN UT

Numerous and well-exposed systems of faults and fractures throughout the Paradox basin has motivated mechanical investigations into the structural development and stress conditions required to produce the observed failure. This research has been encouraged by the presence and abundance of high angle conjugate faults within sandstones near salt structures throughout the basin. High angle faults can develop due to nonlinear failure criterion at low differential stress, either under the Hoek and Brown model with mode II shear failure or under the Griffith model with bimodal (trans-tensile) failure. Both types of failure indicate distinct paleostress conditions that require low differential stress, presumably caused by circumstances related to the underlying Pennsylvanian salt. The specific stress conditions required to produce the observed failures can be determined by a precise analysis of the mechanical properties of fractured Jurassic Sandstones. In order to quantify this, we carried out a mechanical characterization of reservoir quality sandstones that dominate the Jurassic section, including the Curtis, Wingate, Navajo, and Entrada formations. This characterization has involved a series of geomechanical analyses including Brazilian, seismic velocity, uniaxial and triaxial testing to determine the elastic constants and the complete failure envelope. Our results indicate that the tensile strength of the sandstones is too low to reasonably produce widespread bimodal failure. Our samples exhibit distinct nonlinear failure envelopes under low confining stress and the observed parabolic geometry can be used to explain the steep failure angles seen near the salt structures. We hypothesize that because the salt has essentially no strength and deforms readily, its presence will decrease differential stress in nearby sandstones, producing the required stress conditions for high angle failure. With detailed field mapping, we expect that normal faults further from the salt-sandstone interface will be less steeply dipping, conforming to the Coulomb criteria with higher differential stress. We can then combine the observed failure angle with the established failure envelopes to determine what control the salt presence has on differential stress and the encompassing area of stress field influence.