STRESS FIELD VARIABILITY AROUND SALT STRUCTURES EVALUATED THROUGH A COMPARISON OF NATURAL FRACTURE SETS WITH MECHANICAL MODELS, PARADOX BASIN, UTAH

Due to its exceptionally low mechanical strength, salt deforms under minimal differential stress loading conditions. As a result, regional stresses are significantly reoriented in sediments adjacent to salt structures as the salt deforms under applied differential stresses. The spatial extent of this effect, and the sensitivity to salt geometry may be inferred from mechanical models, which have in some cases been tested against drilling results, but generally, the validity of these modeling approaches has not been interrogated through empirical correlation with spatially-dense data from natural structures. In this study, we aim to do this through a comparison of extensive observations of fracture sets from the northwestern Paradox Basin, with boundary-element mechanical models of stress field variations under different loading conditions. Thousands of fractures were mapped using a combination of image processing and machine learning approaches on aerial photographs to extract information about fracture orientations, lengths, and abutting relationships. A 3D structural model of the basin geometry, with a focus on the salt-sediment interface, was constructed using surface geology, well data, and published and original cross-sections. This model was then subject to different stress conditions representative of past geologic events in the basin during which the fractures might have formed (for example, associated with recent exhumation, peak burial, and Laramide tectonics). The optimal fracture orientations in the models were then compared with the empirically observed fractures. Though the subsurface geometry of the basin is not tightly constrained, permissible changes in the model geometry and assumed mechanical properties led to improved matching of observed and modeled fracture orientations, demonstrating the utility of this kind of modeling approach to representing natural systems. A better understanding of near-salt stress conditions is of practical importance to drilling safety in petroleum applications. Furthermore, the contextualization and better understanding of paleostress environment during fracture formation has the potential to improve interpretation of natural fractures, which is of importance to applications in petroleum systems and hydrology.