Natural Fractures in Folded Sandstones of the Tensleep Formation, Wyoming
John C. Lorenz, Scott P. Cooper, 2013. "Natural Fractures in Folded Sandstones of the Tensleep Formation, Wyoming", Application of Structural Methods to Rocky Mountain Hydrocarbon Exploration and Development, Constance N. Knight, Jerome J. Cuzella, Leland D. Cress
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Several types of meter-scale structures accommodated strain during folding of the Tensleep sandstones at Flat Top Anticline, a compound fold overlying an east-northeast to west-southwest striking Laramide thrust fault in southeastern Wyoming. The suite of structures includes (1) syn-depositional hydraulic injection fractures that were reactivated in shear and extension during folding, (2) early-formed hinge-oblique extension fractures, (3) later-formed hinge-parallel extension fractures concentrated on the crest and forelimb, (4) scattered small shear planes oriented both parallel and oblique to the large-scale eolian cross-bed foresets, (5) larger-scale bedding-parallel shear between sedimentary units on the steeper forelimbs, (6) faults, and (7) rare scattered deformation bands. The hinge-oblique extension fractures strike parallel to the direction thrusting, which was not normal to the basement fault. Most of these structures formed due to extension of he strata parallel and oblique to the anticlinal hinge folding developed over the thrust fault. The degree and type of extension fracturing vary by structural position: hinge-oblique fractures dominate the unfolded backlimb, whereas both hinge-oblique and hinge-parallel fractures developed on the forelimb and anticlinal crest. Paradoxically, extension fracturing is minimal where folding is most acute at the westernmost Pine Butte substructure, where small faults, bedding-parallel shear, and reactivation of the preexisting, well-developed suite of hydraulic injectites accommodated most of the strain. Many of these structures record more than one structural event.
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With increasing industry emphasis on developing “unconventional” tight gas reservoirs and on enhancing recovery from existing fields, geologists are facing diverse challenges in the applications of structural geology. Identifying fracture characteristics within petroleum systems is essential. Understanding the timing of tectonics and the formation of structures is important, as these factors strongly influence hydrocarbon generation, migration, entrapment, and preservation. The purpose in publishing this collection of key papers is to aid future workers in addressing complex interrelationships between structural geology and hydrocarbon exploration and development. The first four chapters of this book focus on structural concepts and techniques. The second part of this book is a collection of Rocky Mountain fault and fracture studies. These well documented studies are valuable reference materials for all petroleum geologists.