Lithologic and Structural Controls on Natural Fracture Characteristics Teapot Dome, Wyoming

Teapot Dome is an asymmetric, doubly plunging, basement-cored, Laramide-age anticline. A systematic study of natural fractures within the Cretaceous Mesaverde Formation at Teapot Dome, Wyoming indicates that lithology and structural position control outcrop fracture patterns. Lithology controls fracture, deformation band and fault patterns in the following ways: 1) fracture intensity increases with increased cementation, 2) fracture spacing increases proportionally with bed thickness within two sandstone facies, but not in carbonaceous shales where fracture spacing is inversely proportional to bed thickness, 3) coal cleats are generally oblique, by up to 20 degrees, to fractures in sandstones, 4) most fractures in sandstone units terminate at contact with shale layers, 5) deformation bands occur almost exclusively in a poorly cemented, high porosity, beach-sand facies, 6) normal faults within well cemented sandstones are generally expressed as fracture zones, whereas the same faults within poorly cemented sandstones are diffuse zones of subparallel deformation bands. Three primary throughgoing fracture sets were documented within this context. The oldest fracture set is oblique to the hinge of the anticlinal fold. The vast majority of these fractures strike NW to WNW. A small number of these oblique fractures strike roughly NNE. Fractures that strike oblique to the fold hinge appear to predate folding. The other two fracture sets are related to folding. The most common of these fractures, which are found throughout the fold, are bed-normal extension fractures striking subparallel to the fold hinge. A third set consists of bed-normal extension fractures striking perpendicular to the fold hinge. In many areas this fracture set is spatially related and subparallel to NE-striking, normal oblique-slip faults. The normal oblique-slip faults are common along the eastern limb, but more than 90% of these faults terminate before intersecting the western limb. Conjugate fractures, deformation bands and faults, oriented such that they have a vertical bisector to the acute angle and striking subparallel to the axis of the anticline, are common in the southwestern limb and southern arc of