Abstract In 2008, the Rocky Mountain Association of Geologists (RMAG) publications committee, under the leadership of Jerry Cuzella, chose to focus on the genreal topic of structural geology as a publication theme. Many earlier RMAG publications pertained to specific oil and gas basins or regions, whereas some relatively recent RMAG publications related to special topics such as, coalbed methane, gas shale, and so forth. Until this collection of articles was compiled, structural geology as a standalone publication topic had not been addressed. In the current climate of deeloping tight “unconventional” reservoirs, and in the face of dinding and producing hydrocarbons from “forgotten” or underdeveloped reservoirs, the role of structural geology is of utmost importance. Geoscientists and engineers are adressing challenges involving fracture-system/stress characterization issues on a daily basis. Geoscientists are unraveling the geologic histories of basins and of individual structures. The editors hope that this collection of papers will provide additional tools for oil and gas scientists and engineers to help in their search for and development of oil and gas resources.

Abstract The generation of one or more three-dimensional (3-D), freehand drawings, based on integrated analysis of a two-dimensional (2-D) geologic database (e.g., borehole data, seismic profiles, surface geology, etc.), is proposed here as a rewarding exercise in the development of a final interpretation of subsurface geologic structures. A freehand 3-D drawing based on integration of 2-D interpretive structural contour maps (of at least two horizons) and structural cross sections can clarify and verify the 3-D details of complex subsurface geologic structures, check on the internal consistency of the interpretation, uncover untenable, interpretive, geologic configurations, and highlight possible obscure trap geometries. In some cases freehand 3-D drawings can aid in the visualization of impenetrable 3-D images produced by computer software programs. Isometric projection or linear perspective drawings are generally the most useful kinds of 3-D renditions, but strict adherence to these disciplines is not a requirement in the generation of an initial 3-D sketch. Generating a 3-D image using computer software is dominantly the functional domain of the left hemisphere of the brain (left brain), whereas the generation of freehand 3-D drawings is dominantly the functional domain of the right brain and requires penetrative visualization in the conversion of 2-D data to 3-D imagery. The right brain excels in intuitive, creative, imaginative structural interpretation. Examples of freehand 3-D drawings of complex subsurface and surface geologic structures, both self-generated and from literature, are presented along with some auxiliary 3-D analog modeling methods.

Abstract Low-temperature thermochronometers can be used to measure the timing and the rate at which rocks cool. Generally, rocks cool as they move towards Earth’s surface by erosion or normal faulting (tectonic exhumation of the footwall), or warm as they are buried by sediments and/or thrust sheets, or when they are intruded by magma and association hydrothermal fluids. Changes in heat flow or fluid flow can also cause heating or cooling. Apatite fision-track and apatite (U-Th)/He dating have low closure temperatures of ~120° C and ~70° C respectively, and are used to date cooling in the upper ~3–4 km (~1.8–2.4 mi) of Earth’s crust. Age-elevation relationships from samples collected from different elevations along vertical transects or from wellbores are used to calculate exhumation rates and the time of onset of rapid exhumation. The spatial distribution of cooling ages can be used to map faults in basement or intrusive rocks where faults can be difficult to recognize. Cooling ages from detrital minerals in sedimentary rocks can be used to constrain provenance. If sedimentary samples reached temperatures high enough to reset the thermochronometers, then ages may provide information on the cooling history of the basin. Forward thermal modeling can be used to test proposed thermal history models and predict thermochronometer ages. Inverse thermal modeling finds a best-fit thermal history that provides a good statistical match to measured thermochronometer ages. Both types of thermal modeling may help contrain maximum temperature of the sample and time spent at that temperature. Thermochronometer ages can be used as constraints in basin modeling. Maturation of kerogen to petroleum in a sedimentary basin is controlled by the maximum temperature reached by the kerogen and the amount of time it spends at or near that temperature (i.e., the thermal history of the basin). The timing of tectonics and the formation of structures in a region influence the generation, migration, entrapmet, and preservation of petroleum. Techniques such as low-temperature thermochronology that illuminate the relationship between time and tempearture during basin evolution can be valuable in understanding petroleum systems. These techniques are especially powerful when multiple dating techniques (such as apatite fission-track, zircon fission-track, and apatite (U-Th)/He dating) are applied to the same sample and when they are combined wiht other thermal indicators such as vitrinite reflectance data.

Abstract A synthesis of low-temperature thermochronologic results throughout the Laramide foreland illustrates that samples from wellbores in Laramide basins record either (1) detrital Laramide or older cooling ages in the upper ~1 km (0.62 mi) of the wellbore, with younger ages at greater depths as temperatures increase; or (2) Neogene cooling ages. Surface samples from Laramide ranges typically record either Laramide or older cooling ages. It is apparent that for any particular area the complexity of the cooling history, and hence the tectonic history interpreted from the cooling history, increases as the number of studies or the area covered by a study increases. Most Laramide ranges probably experienced a complex tectono-thermal evolution. Deriving a regional timing sequence for the evolution of the Laramide basins and ranges is still elusive, although a compilation of low-temperature thermochronology data from ranges in the Laramide foreland suggests a younging of the ranges to the south and southwest. Studies of subsurface samples from Laramide basins have, in some cases, been integrated with and used to constrain results from basin burial-history modeling. Current exploration for unconventional shale-oil or shale-gas plays in the Rocky Mountains has renewed interest in thermal and burial history modeling as an aid in evaluating thermal maturity and understanding petroleum systems.This paper suggests that low-temperature thermochronometers are underutilized tools that can provide additional constraints to burial-history modeling and source rock evaluation in the Rocky Mountain region.

Abstract Dating of detrital zircons from well cuttings is a useful technique to constrain stratigraphic ages and structural interpretations in complexly deformed terranes. This technique was applied in the Helena Salient of the Montana Disturbed Belt to determine whether the Norcen, Kimpton Ranch 1-11 and Buckhorn, Federal 2-24 wells penetrated Phanerozoic strata beneith the allochthonous Precambrian rocks carried on the Lombard thrust. Since some Phanerozoic strata have reservoir potential, their presence below the Lombard thrust has important implications for the oil and gas potential of this structural province.

Abstract The consistent low porosity and permeability of reservoirs in the Bakken petroleum system, in the Williston Basin, have increased the need for fracture studies. Although situated in an intracratonic setting, the Williston Basin displays evidence of deformation enabling the presence of regional and local fracturing. In this study, applicable fracture models are utilized to delineate regional and local fracture orientations within the Williston Region. Northwest and northeast regional fracture trends have been determined by integrating results from previous fracture studies, collecting field data at outcrop locations in the Williston Basin Region, and from subsurface three-dimensional (3-D) seismic data in the Williston Basin. A right-lateral wrench fault strain ellipse model is offered to explain these regional trends. Fracture orientations acquired from outcrop sites (Little Rocky Mountains, Big Snowy Mountains, and Beartooth Mountains) also reveal local, structurally controlled, conjugate fracture trends that are parallel or perpendicular to the structural axis. Using curvature analysis on the interpreted 3-D seismic data, local fracture patterns are also observed within the Williston Basin. When regional and local trends are compared, overlap occurs in fracture orientation showing preference to fractures produced from local structures. Regional and local trends are also incorporated into a mechanical stratigraphy study using field observations of outcropping Bakken age equivalent and lithologically similar strata from the Bighorn Basin. Dense fracturing occurs within the middle Bakken equivalent member of the Cottonwood Canyon Formation. Extensive fractures that are perpendicular to bedding are also observed and cut through the lower bounding Three Forks Formation, Cottonwood Canyon Formation, and overlying Lodgepole Formation.

Abstract Study of a regional three-dimensional seismic data set by Cumella and Ostby (2003) indicated the potential existence of wrench faults in the southern Piceance Basin, Colorado. Although the faults could be inferred to cut through the productive interval, no direct observation was possible until the Reservoir Characterization Project (RCP) conducted a multicomponent seismic study at Rulison Field. This study confirms the existence of faults and coduments their importance in creating fracture zones critical to higher expected ultimate recovery (EUR) well production within the field. Three-dimensional seismic data were acquired at Rulison Field by RCP to investigate whether zones of high fracture density within the Mesaverde reservoir interval could be detected. Three time-lapse, multicomponent seismic surveys were acquired in 2003, 2004, and 2006. The study confirmed the existence of wrench faults, documented zones of high fracture density, and observed pressure depletion within these zones. Wrench faults and fracture zones play an important role in the creation of “sweet spots” associated with wells of high EUR. Sweet spot identification with multicomponent seismic data can improve the economics of tight gas exploration and production.

Abstract In sedimetary basins not currently undergoing primary compaction (e.g., Rocky Mountain Basins), p-wave velocities noticeably vary with azimuth, yet the mechanism(s) controlling the anisotropy remain uncertain. Possible geologic causes for azimuthal anisotropy include but are not limited to sedimentary fabrics, steep bedding, changes in local in-situ or residual stress, and open or mineralized fractures. To test these hypotheses, P-wave velocity azimuths (Vfast) from a proprietary seismic survey of a NNW-trending Laramide Anticline on Casper Arch in central Wyoming were compared to image log data from the seismic coverage area and fracture orientations from nearby analog structures.

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 A study of fracture characteristics in sandstones from the Tensleep Formation on a spectrum of thrust-related Laramide Anticlines in Wyoming suggests that an idealized fracture model can be constructed for these reservoirs but that important local variations are common. A conceptual model for fracture genesis and distributions in folded Tensleep reservoirs was derived from a comparison of fracture characteristics in the various anticlines, with key relationships being documented by an important surface and subsurface data set at Teapot Dome. Knowledge of the most likely fracture patterns in different parts of the folded Tensleep reservoirs across Wyoming can be used to predict and model fracture-related fluid flow, but this requires a solid understanding of the structural development of a given anticline.

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.

Abstract Detachment folds basinward of Laramide Rocky Mountain arches are relatively poorly known, partially due to coverage by synorogenic strata that may conceal undiscovered anticlinal fields. This study documents the geometry and kinematics of the Beaver Creek Detachment system (BCD), which is located west of a series of NW-trending thrust faults and folds defining the Beaver Creek reentrant on the western edge of the Bighorn Arch. Possible origins for this proposed detachment include syn-Laramide detachment rooted in mountain-front faulting, syn-Laramide gravity slinding during mountain-front folding, and post-Laramide gravity sliding.

Abstract 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.