FOLD-FRACTURE RELATIONSHIPS IN THE SHEEP MOUNTAIN ANTICLINE, BIG HORN BASIN, WYOMING

The study of fold-fracture relationships has two motivations: 1) to acquire a better knowledge of how fractures organize and participate in the deformation of a folded structure; and 2) to understand which rules can help us predict fractures in the subsurface for application to hydrocarbon production. In Wyoming, the Laramide compression created several NW-SE trending anticlines where fractures distribution have been studied, offering a good framework for the present study in terms of regional fractures sets. Moreover, several of these structures have been exploited for oil and gas production. Some reservoirs of the oil and gas fields are located in Paleozoic formations such as the Madison limestone, Tensleep sandstone, and Phosphoria formation. These formations crop out in the Sheep Mountain anticline, located near the eastern border of the Big Horn Basin. This is an asymmetric doubly plunging basement cored anticline, trending NW-SE. This study includes systematic mapping using GPS and GIS technology of fractures in all exposed parts of the fold in two different lithologies: the upper sandstone of the Tensleep formation and the basal limestone of the Phosphoria. The Madison limestone also is investigated but only using outcrops in limited areas. Both flanks (one very steep compared to the other), the hinge region, and the northern fold termination were mapped. This methodology allows us to study how pre-existing fractures evolve in such different structural positions and how the local stress state in the fold might generate new fracture sets. Several fracture sets are observed trending from N10°E, N45°E, N110°E to N140°E. The field observations show that near the fold termination at least 4 fracture sets can be found while only 2 are generally observed on the flanks and the hinge region. The dominant fracture set trends N45°E and is observed in many outcrops throughout Wyoming, and can be explained by Laramide compression. The other fracture sets are linked to structural position on the fold, normal curvature of the layers, and their mechanical properties.