Fracture Analysis Across Derby and Dallas Domes, West-Central Wyoming: Implications for the History of Hydrocarbon Migration Along Bleached Fracture Sets 

Chris Brocka, University of Missouri-Columbia, Geological Sciences Department, Columbia, Missouri, [email protected]

Fractures cutting the Mesozoic strata folded across Derby and Dallas Domes region are a result of a complex stress history.  The fractures record regional NE-SW compressional stress and locally variable stresses associated with folding.  The stress-fracture relationships can be determined using regional and local fracture sets, along with relative age relationships and surface geology.  The purpose of this study is to analyze the fracture sets associated with the formation of the domes and to determine their role in hydrocarbon migration.

Steeply dipping fractures striking ~240° define the most prominent regional fracture set, and slickenlines on minor thrust fault planes trending ~240° define the NE-SW σ₁ orientation.  Bleaching that occurs along many of the fractures, a result of Fe (III) reduction, is related to the migration of hydrocarbons through units of the Chugwater group and the Nugget Sandstone.  Evidence for this migration includes hydrocarbon residues and the formation of authigenic Magnetite.  Thin sections cut from samples of bleached fractures, fault zones, and cataclasites will be examined using Magnetite identification procedures and ultraviolet hydrocarbon film detection methods.  Bleached fractures cutting sandstone units of the Chugwater group are locally curved as a result of layer-parallel slip associated with folding, thus constraining the timing of hydrocarbon migration between fracture formation and the last stages of folding.  Pooling of hydrocarbons where fractures in the porous sandstone intersect less porous limestone cause bleaching in the sandstones to spread laterally along the base of tight formations, such as the Alcova Limestone.