Field Evidence and Elastic Dislocation Modeling of Stress Field Alteration in the Rock Mass Adjacent to Salt

Abstract

ABSTRACT: We present a geomechanical analysis of stress and fracture development in the rock mass adjacent to a salt body under tectonic loading. This is accomplished with 3D elastic dislocation modeling to evaluate stress conditions adjacent to a salt wall in the Paradox Basin, southern Utah, USA. Detailed field work has documented stress indicative strain patterns adjacent to an exhumed salt wall that suggest a rotation of principal stresses and a decrease in differential stress magnitude in proximity to salt relative to regional conditions. A geomechanical model of the area was constructed to solve for 3D stress field perturbations using the elastic dislocation method to simulate rock deformation due to fault slip and salt creep under tectonic loading. The model results suggest both faulting and salt deformation may have contributed to the local stress field that produced the observed strain pattern. These findings help constrain geologic and geomechanical parameters used in elastic dislocation modeling and enhances ability to predict stress field perturbations in locations with limited data. This work aims to improve the accuracy of predicted stress conditions while drilling near salt, as well as assessing fracture development with fluid flow implications adjacent to salt bodies. 1. INTRODUCTION The formation of natural fracture systems in brittle rocks and the state of stress under which they form have long been a subject of interest as fractures are important conduits for subsurface fluids in many applications in petroleum systems, carbon capture and storage, induced seismicity, hydrology, and ore forming systems. The state of stress can locally vary where stress and strain perturbations are present. These perturbations are often associated with geologic deformation such as faults and fractures or in proximity to salt bodies or volcanic intrusions. Changes in stress state due to slip or dilation along faults and fractures have been modeled using elastic dislocation theory (Comninou and Dundurs, 1975; Maerten et al., 2005; Meade, 2007; Okada, 1992). We employ a proprietary geomechanics code (Busetti, 2021b) to solve for 3D stress and strain fields on a target surface following the triangular elastic dislocation method (Meade, 2007), which has been used to model the stress and strain effects of faulting and fracturing in previous studies (Busetti, 2019, 2021a). This project uses the capabilities of the software to model stress perturbations in the rock mass adjacent to a salt body. Due to the weakness of salt, it will internally deform under small amounts of differential stress. Because the stress field must equilibrate across welded salt-sediment boundaries, the stress field in the rock mass adjacent to a salt body is also altered from regional stress conditions. Stress anomalies in formations adjacent to salt have been extensively observed and modeled in subsurface applications (Bradley, 1978; Dusseault et al., 2004; King et al., 2012; Seymour et al., 1993), but field-based analog studies, such as this, permit laterally extensive observations that are otherwise limited in subsurface applications. Detailed field work examining stress indicative strain patterns adjacent to an exhumed salt wall in the Paradox Basin has documented altered stress patterns in a deformed homogeneous sandstone under regional tectonic loading. Utilizing this field case study to constrain geomechanical models, we predict locally perturbed stress and strain patterns in the rock mass adjacent to salt under regional tectonic stress.