Abstract The Pennsylvanian-age Tensleep Formation in south-central Wyoming is comprised of repeated limestones, sandy limestones, and sandstones. Strata of these varied lithologic units are folded over Beer Mug Anticline and cut by numerous intersecting fractures. The anticline, with a near-vertical forelimb and backlimb dip up to 50 degrees, provides an ideal analog for fracture systems in tightly folded Paleozoic hydrocarbon reservoirs. Fracture type and degree of development vary systematically with lithology, structural position, and degree of folding. Fracturing is most intense towards the core of the anticline, which locally consists of folding. Fracturing is most intense towards the core of the anticline, which locally consists of brecciated, oil-stained rock with large-scale vuggy porosity. Most of these strata exhibit inherited (F0) fracture patterns that predate folding, as well as fold-related extension fractures that trend approximately normal (F1) and parallel (F2) to the axis of folding.

Abstract 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.