Fracture Characteristics in Core from Tensleep Reservoirs Across Wyoming

John Lorenz
University of Wyoming, Laramie, WY

Cores from 25 wells across Wyoming show that fractures are common in the anticlinal reservoirs of the Tensleep Formation, and that most of them are vertical extension fractures. The most intense fracturing is typically found along crests of the folds. Where orientation data are available, most of the fractures trend approximately normal and parallel to the hinge of the anticlines, although oblique fracture orientations have also been reported. Shear fractures are present in the more tightly folded anticlines. Tests on fractured plugs from core usually show that the fractures have less permeability than the associated matrix, and these test results are commonly used to erroneously suggest that fractures have little effect or may even degrade system permeability. This is because typically only the tightest fractures are tested since they are the only ones that will keep their integrity during plugging. Fracture populations typically include a range of sizes, and it is the more open fractures, those that don’t remain intact during plugging and that get discarded, that control permeability. In fact, well tests suggest that fracture-enhanced permeability is present in most of the fields studied. Tensleep fractures are variously mineralized with anhydrite, quartz, calcite, dolomite bitumen, yet most fractures are not completely occluded, retaining up to 80% of the original aperture as remnant porosity. In contrast, although the tight dolomites between the Tensleep sandstone reservoirs commonly act as seals for pressure compartments even though they tend to be highly fractured, probably because the fractures in this facies are short and poorly interconnected.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado